Global energy consumption due to friction in trucks and buses

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Global energy consumption due to friction in trucks and buses
Kenneth Holmberg a,n, Peter Andersson a, Nils-Olof Nylund a, Kari Mäkelä a, Ali Erdemir b
a VTT Technical Research Centre of Finland, P.O. Box 1000, FI-02044 VTT, Finland
b Argonne National Laboratory, Argonne, IL 60439, USA
a r t i c l e i n f o
Article history:
Received 15 March 2014
Received in revised form
28 April 2014
Accepted 1 May 2014
Available online 10 May 2014
Keywords:
Friction
Energy
Trucks
Buses
a b s t r a c t
In this paper, we report the global fuel energy consumption in heavy-duty road vehicles due to friction in
engines, transmissions, tires, auxiliary equipment, and brakes. Four categories of vehicle, representing an
average of the global fleet of heavy vehicles, were studied: single-unit trucks, truck and trailer combinations,
city buses, and coaches. Friction losses in tribocontacts were estimated by drawing upon the literature on
prevailing contact mechanics and lubrication mechanisms. Coefficients of friction in the tribocontacts were
estimated based on available information in the literature for four cases: (1) the average vehicle in use today,
(2) a vehicle with today's best commercial tribological technology, (3) a vehicle with today's most advanced
technology based upon recent research and development, and (4) a vehicle with the best futuristic technology
forecasted in the next 12 years. The following conclusions were reached:
� In heavy duty vehicles, 33% of the fuel energy is used to overcome friction in the engine, transmission,
tires, auxiliary equipment, and brakes. The parasitic frictional losses, with braking friction excluded,
are 26% of the fuel energy. In total, 34% of the fuel energy is used to move the vehicle.
� Worldwide, 180,000 million liters of fuel was used in 2012 to overcome friction in heavy duty
vehicles. This equals 6.5 million TJ/a; hence, reduction in frictional losses can provide significant
benefits in fuel economy. A reduction in friction results in a 2.5 times improvement in fuel economy,
as exhaust and cooling losses are reduced as well.
� Globally a single-unit truck uses on average 1500 l of diesel fuel per year to overcome friction losses;
a truck and trailer combination, 12,500 l; a city bus, 12,700 l; and a coach, 7100 l.
� By taking advantage of new technology for friction reduction in heavy duty vehicles, friction losses
could be reduced by 14% in the short term (4 to 8 years) and by 37% in the long term (8 to 12 years). In
the short term, this would annually equal worldwide savings of 105,000 million euros, 75,000 million
liters of diesel fuel, and a CO2 emission reduction of 200 million tones. In the long term, the annual
benefit would be 280,000 million euros, 200,000 million liters of fuel, and a CO2 emission reduction
of 530 million tonnes.
� Hybridization and electrification are expected to penetrate only certain niches of the heavy-duty
vehicle sector. In the case of city buses and delivery trucks, hybridization can cut fuel consumption by
25% to 30%, but there is little to gain in the case of coaches and long-haul trucks. Downsizing the
internal combustion engine and using recuperative braking energy can also reduce friction losses.
� Electrification is best suited for city buses and delivery trucks. The energy used to overcome friction
in electric vehicles is estimated to be less than half of that of conventional diesel vehicles.
Potential new remedies to reduce friction in heavy duty vehicles include the use of advanced low-
friction coatings and surface texturing technology on sliding, rolling, and reciprocating engine and
transmission components, new low-viscosity and low-shear lubricants and additives, and new tire
designs that reduce rolling friction.
& 2014 Elsevier Ltd. All rights reserved.
1. Introduction
During the past two decades, global awareness of the need for
more fuel-efficient and environmentally benign transportation
systems has increased tremendously, mainly because of limited
petroleum reserves, skyrocketing fuel prices, and much tougher
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/triboint
Tribology International
http://dx.doi.org/10.1016/j.triboint.2014.05.004
0301-679X/& 2014 Elsevier Ltd. All rights reserved.
n Corresponding author. Tel.: þ358 40 544 2285; fax: þ358 20 722 7069.
E-mail address: kenneth.holmberg@vtt.fi (K. Holmberg).
Tribology International 78 (2014) 94–114
environmental regulations to combat greenhouse gas emissions.
Accordingly, researchers have been exploring new strategies to
improve fuel economy and environmental compatibility of future
transportation systems. While alternative ways to power future
transportation systems with low-carbon energy resources, includ-
ing biofuels, natural gas, electricity, etc., are under development,
more advanced materials and lubrication technologies are also
being explored to cut down parasitic energy losses due to friction
in the moving parts of modern engines [1,2].
In a recent comprehensive study focused on passenger cars,
Holmberg et al. [3] determined that nearly one-third of the fuel's
energy is spent to overcome friction in passenger cars. The same study
advocated that, with the adaptation of more advanced friction control
technologies, parasitic energy losses due to friction in engines could be
reduced by 18% within the next 5 to 10 years, which would result in
global fuel savings of 117,000 million liters annually, and by 61%within
the next 15 to 25 years, which would result in fuel savings of 385,000
million liters annually. These figures equal world-wide economic
savings of 174,000 million euros in the next 5 to 10 years and
576,000 million euros in the next 15 to 25 years. Such a fuel efficiency
improvement in passenger cars would, furthermore, reduce CO2
emission by 290 million and 960 million tons per year, respectively.
The estimation of the global saving potential is in agreement with
detailed energy calculations carried out for passenger cars in Japan by
Nakamura [4]. This level of savings should have a significant positive
impact on the global efforts to reduce the greenhouse effect and
control global warming.
Most passenger cars are privately owned and not used for
commercial services. While cars are important to individual
transportation, commercial heavy duty vehicles, such as buses
and trucks, are critical to society at large because they are used for
mass transportation of people, products, and services. Indeed,
buses are the backbone of most public transportation systems in
the world. Interestingly, only 12% of world freight is carried on
some form of road vehicle, while 13% is by rail, 75% by ships, and
0.3% by aviation [5,6]. However, the picture looks very different in
terms of total transport energy consumption, for which 73% is
consumed by road transport, 3% by rail, 10% by ships, and 10% by
aviation [7], as shown in Fig. 1. Heavy duty vehicles represent 36%
of the road transport oil consumption [8].
In terms of energy consumption, ranking second is heavy duty
vehicles, comprising both trucks and buses (21%). Therefore, the
energy consumption of this segment deserves close attention for
the following reasons:
� Despite the relatively low numbers of such vehicles, their share
of the energy use is high.
� They have strategic importance to society (see above).
� They rely heavily on diesel fuel, and internal combustion engines
will still be used for a long period of time, especially in the case of
long-haul heavy duty trucks, since the electrification of trucks is
more challenging than that of light duty vehicles.
� Their driving range and load profiles differ significantly from
those of passenger cars.
� Commercial heavy duty vehicles are often part of fleets: it is
thus easier to influence decision-making concerning these
vehiclescompared to passenger cars.
� The fuel economy ratings and carbon footprint of heavy duty
vehicles are rather dismal and need urgent improvement.
For heavy duty vehicles, the power-to-weight ratio, and thus the
average relative load, is quite different compared to passenger cars. In
the case of passenger cars, significant fuel savings can be achieved by
downsizing and choosing less powerful vehicles. Commercial vehicles
are, in most cases, more tailored for their purpose than passenger cars;
hence, the potential for fuel savings by downsizing is not as obvious as
in the case of passenger cars [9,10].
Buses, and especially city buses, constitute a relatively homo-
geneous vehicle category in terms of energy consumption. For a
city bus, operated on low average speed, with a frequent stop-and-
go pattern, a major portion of the fuel energy is used for
accelerating the vehicle. Consequently, without hybridization, a
large amount of energy is lost when decelerating the vehicle by
using the brakes. Coaches, by contrast, are operated at much higher
and constant cruising speeds, at which aerodynamic drag becomes
far more important than the weight and rolling resistance.
For trucks used for goods transporting, the gross weight and
configuration of the vehicle vary significantly. The gross vehicle
weight range is from some 3.5 t up to 60 t or even more. Duty
cycles vary from start-and-stop type driving typical of urban
settings to the constant high-speed cruising of long-haul trucks.
Our earlier paper reviewed the global energy consumption due to
friction in passenger cars [3]. This review presents calculations of the
global energy consumption due to friction and potential savings
through the adaption of advanced friction control technologies in
trucks and buses. We focus on four categories of heavy duty vehicles:
single-unit trucks, truck and trailer combinations, city buses, and
coaches. The vehicles were chosen to represent an average of the
world heavy vehicle fleet. We base our calculations on vehicles with
diesel engines. We discuss the effect of future change to electrical
motors separately. Other expected changes, such as improvements in
aerodynamics and a more extensive use of light-weight materials, and
related predictions, are not included in the present analysis.
2. Methodology
The present analysis is carried out according to a methodology
developed by Holmberg et al. [3,11]. It is based on the combination
of analyses on several physical phenomena resulting in the energy
consumption in vehicles. The methodology includes five parts:
1. The global energy consumption of heavy duty vehicles.
2. The distribution of the friction and energy losses in four
categories of heavy duty vehicle (single-unit truck, truck and
trailer combination, city bus, and coach).
3. Driving cycle effects in the four vehicle categories.
4. Tribocontact friction levels today and in the future.
5. The global fuel consumption today due to frictional losses and
potential savings.
The calculations of friction energy loss proceed in the following
way: (1) The global fleet of heavy duty vehicles is estimated. (2) Its
Fig. 1. Global breakdown of the energy consumption by transportation vehicles [7].
The 52% share by the light duty vehicles includes 37% passenger cars and 15% vans,
pick-ups, and sport utility vehicles.
K. Holmberg et al. / Tribology International 78 (2014) 94–114 95
total energy consumption is calculated by using fuel consumption
statistics, fleet data, and traffic information from the open litera-
ture. (3) An average vehicle in average operation globally is
statistically defined for each of four categories of heavy duty
vehicles. (4) Subsequently, the average mileage, driving speed,
driving conditions, and average fuel consumption are balanced
with available fuel consumption statistics for each category.
(5) The total energy loss of the average vehicle in each category
is subdivided into frictional and other energy losses, using the best
available estimates in published friction loss studies for vehicle
engines, transmissions, and entire vehicles. (6) The friction losses
are further subdivided into losses on the component level and the
tribocontact level. The friction loss sources of the average vehicles
are identified and classified according to the prevailing tribological
contact and lubrication mechanisms. (7) The engine sub-systems,
transmission parts, and other locations of energy consumption in
the vehicles are analyzed with regard to lubrication and friction.
(8) The coefficient of friction and the friction energy are calculated
or estimated for each friction loss source. Finally, the total friction
loss is summarized.
To assess the friction reduction potential of today's best commercial
solutions, the coefficient of friction at each friction loss source is
replaced by a lower value representing the best commercial tribolo-
gical solution found in the literature. The procedure is repeated for the
best tribological solution reported from research laboratories world-
wide and for estimates of what the coefficient of friction values could
be after 10 years of intensive and focused tribology research.
The savings on the global level are calculated from the savings
for a single average vehicle in each category multiplied by the total
global number of vehicles within this category, and the total
savings are determined as the sum of the savings within the four
categories. The energy and economical savings as well as the
potential emission reduction are calculated for various regions and
countries based on the fuel use of those areas.
These calculations were carried out on the basis of publically
available statistical data, scientific publications, and the authors'
own experience. There are detailed energy statistics for the U.S.
and Europe, and these data were taken as a starting point for the
estimations on a global level [2,12–16].
3. Analyses of global heavy duty vehicles
3.1. Statistics on the global heavy duty vehicles and their energy
consumption
Trucks (including their combination with trailers) and city
buses and coaches represent the main body of the global fleet of
heavy duty vehicles. In addition, there is a multitude of special
vehicles, such as road trains, double-decker buses, fire engines,
road maintenance vehicles, and cement and building material
trucks. Because their proportion of the entire fleet is small they are
not analyzed separately but embedded in the four main categories
studied. In the present study, trucks are defined as goods freight
vehicles with a minimum total weight of 3.5 t, which is consistent
with the definition of the European Environment Agency (EEA)
[17].
Key figures representing the operational profiles of the four
main categories of heavy duty vehicle are shown in Table 1.
Determining the worldwide number of trucks with a weight over
3.5 t and mainly used for freight transportation was challenging,
because the term “truck” is used with varying meanings in
different parts of the world. Even more challenging was the
subdivision into single-unit trucks and truck and trailer combina-
tions, since not all fuel and traffic statistics distinguish between
these two categories.
According to the Transportation Energy Data Book [14], in the
year 2010 the total number of cars, trucks, and buses worldwide
was 1015 million. In the U.S., the number of single-unit trucks and
truck and trailer combinations was 10.7 million [14], which is 4.5%
of all road vehicles in the U.S. Of all road vehicles, the proportion of
single-unit trucks and truck and trailer combinations was 2.1% in
the 27 European Union (EU) countries [22] and 3.2% in Finland
[23].
Looking through the statistics from many other sources, we
concluded that the worldwide proportion of single-unit trucks and
truck and trailer combinations is 3.6% of all road vehicles, whichequates to 36.5 million vehicles. The share of truck and trailer
combinations in relation to all the trucks is 24% in the U.S. [14],
26% in the 27 EU countries [22], and 28% in Finland [23]. The above
figures represent countries with well-developed heavy transporta-
tion logistics. For the entire world, we found that the truck and
trailer combinations constitute 20% of all the trucks. Thus we
conclude that the number of single-unit trucks worldwide is 29.2
million, and the number of truck and trailer combinations,
7.3 million.
The worldwide number of buses in the year 2010 has been
estimated as 3.6 million units [18]. In this study, the global fleet of
buses was subdivided into city buses and intercity, long-distance
coaches. In Europe, the share of coaches was estimated as 37% of
all buses [24]. Based on these data, we concluded that in 2010
there were 2.3 million buses and 1.3 million coaches worldwide.
In the U.S., the estimated average annual mileage is 22,000 km
for a single-unit truck and 111,000 km for a truck and trailer
combination [14]. For the entire world, we estimate the corre-
sponding figures to be 20,000 km and 100,000 km, respectively.
Table 1
Global key parameters for the four categories of heavy duty vehicle.
Total
number
globally
(million)
Average annual
mileage (km)
Percent operating in
urban (urb) and
highway (hgw) regions
Average
speed (km/h)
Average
weight with
load (kg)
Idlinga (%) Engine
power
outputb (%)
Single-unit trucks 29.2 20,000 50 urb 60 10,000 15 40
50 hgw
Truck and trailer
combi-nations
7.3 100,000 5 urb 80 30,000 5 50
95 hgw
City buses 2.3 80,000 100 urb 20 14,000 30 20
Coaches 1.3 100,000 10 urb 80 16,000 5 35
90 hgw
The data in the second column originates from global statistics, the data in the last column is based on Table 7 and the Figs. 4–7, and the data in the other columns are
estimations based on information from operators [2,12,14,16,18–21].
a Average idling time as percentage of the total running time.
b Average power used given as percentage of the maximum output power of the engine.
K. Holmberg et al. / Tribology International 78 (2014) 94–11496
According to Davis [14], the average fuel economy in the U.S. is
32 l per 100 km for single-unit trucks and 40 l per 100 km for
truck and trailer combinations. The International Energy Agency
(IEA) estimates that the fuel economy is 42 l per 100 km for heavy
trucks and 29 l per 100 km for medium trucks [25]. For the entire
world, we estimate an average fuel consumption of 25 l per
100 km for single-unit trucks and 40 l per 100 km for truck and
trailer combinations. Based on these figures, the annual average
consumption of diesel fuel is 5000 l for single-unit trucks and
40,000 l for truck and trailer combinations (Table 2).
With these figures, we calculated the total annual energy use of
the global fleet of single-unit trucks to be 5.2 EJ, and that of truck
and trailer combinations to be 10.5 EJ in 2010. These numbers are
in good agreement with data presented by the World Energy
Council (WEC), which reports a total energy consumption in the
transport sector of 2200 Mtoe, which equals 92 EJ, in the year
2010, and a 17% share of energy consumed by trucks, which equals
energy consumption of 15.6 EJ by the global fleet of trucks [26].
The heavy road transportation statistics and the energy con-
sumption globally as well as average values are summarized in
Table 2. The average annual mileage was estimated to be
80,000 km for city buses and 100,000 km for coaches. The IEA
estimates the average fuel consumption for the global fleet of
buses and coaches to be 24 l per 100 km [25]. We estimate that the
fuel economy is 38 l per 100 km for city buses and 25 l per 100 km
for coaches. Using these estimates, we calculated the annual
consumption of diesel fuel to be 30,400 l for a city bus and
25,000 l for a coach. The total annual energy consumption is thus
2.5 EJ for city buses and 1.2 EJ for coaches. According to the WEC,
the share of energy consumed by the buses and coaches of the
total energy consumption by the transport sector (2200 Mtoe) is
4%, which is in agreement with the numbers presented above. In
the EU, the buses and coaches account for 15% of the total energy
consumed by the transport sector [22]. Applying this figure on the
total global energy consumption of the heavy vehicles, i.e., 19.3 EJ
[26], yields an annual energy consumption of 2.9 EJ for all buses
and coaches. This is in fair agreement with our results (3.7 EJ),
bearing in mind that Europe's transportation system differs from
the global system.
3.2. Four global average heavy duty vehicles
Two types of trucks and two types of buses were selected for
detailed analysis (see Fig. 2). They are considered as typical
representatives for the four major categories that cover the
majority of all heavy duty vehicles in commercial and public use.
These four typical vehicles are called the “global average vehicle”
in each vehicle category:
� Single-unit truck
� Semi-trailer truck, consisting of a truck unit and a semi-trailer
� City bus
� Coach (or intercity bus)
Based on the available global statistics, the average age of all
studied heavy duty vehicles was estimated to be about 13 years,
which means that they were manufactured in the year 2000. The
technical details of the global average vehicle in each category are
specified in Table 3.
3.3. Operating profiles
When a vehicle is moving, the fuel energy delivered to the
engine is consumed for the following reasons:
� to overcome the rolling resistance,
� to overcome the aerodynamic drag,
� to accelerate the mass of the vehicle, which increases the
kinetic energy, which, in turn, is consumed by friction when
using the brakes,
� to move the vehicle uphill, which increases the potential
energy, which is returned when the vehicle moves downhill,
� to energy losses due to friction and viscosity in the mechanical
components of the vehicle, and
� to energy losses in the auxiliary equipment.
Depending on how the vehicle is operated, the relation
between these six types of energy losses will vary significantly,
from the highly transient low-speed operation of city buses and
delivery trucks to the mainly constant high-speed operation of
coaches and truck and trailer combinations. Single-unit trucks
operate in urban conditions as well as on highways, with a large
variety in the operating pattern [10,27–36].
In the low-speed transient operation that is typical of city
buses, the energy needed to accelerate the vehicle dominates the
energy use, and since the deceleration is largely based on the use
of brakes with friction linings, most of the kinetic energy accu-
mulated in the vehicle during the acceleration is lost by conversion
into thermal energy. When operating in urban areas for goods
distribution, the operating patterns of delivery trucks resemble, to
Table 2
Global energy consumption and the energy consumption for an average heavy duty vehicle within each category.
Number of vehicles
globally (million)
Energy use
globally (EJ/a)
Annual mileage (km
per vehicle)
Fuel economy (liters
per 100 km)
Annual average fuel
consumption (l)
Annual average energy
consumption (MJ)
Single-unit trucks 29.2 5.2 20,000 25 5000 180,000
Truck and trailer
combinations
7.3 10.5 100,000 40 40,000 1,440,000
Trucks, in total 36.5 15.7
City buses 2.3 2.5 80,000 38 30,000 1,080,000
Coaches 1.3 1.2 100,000 25 25,000 920,000
Buses, in total 3.6 3.7
Trucks and buses,
in total
40.1 19.4
Fig. 2. Representatives of the four main categories of heavy duty vehicles chosen
for detailed analysis.
K. Holmberg et al. / Tribology International 78 (2014) 94–114 97
some extent, those of city buses. However, for the delivery trucks
the stops are not as frequent, and the share of idling might, in
some cases,be higher than that of the city buses. For the city buses
and the single-unit delivery trucks, the energy needed for accel-
eration and the energy needed to overcome rolling resistance
dominate, while the air drag is of lesser importance.
Air drag is a dominating factor within the energy use in
highway driving, which is typical of the intercity coaches and
long-haul truck-and-trailer combinations. When operating on
mainly flat topography at almost constant speed, some variation
in speed and elevation always occurs in practice. However, the
energy losses due to such minor changes in speed and altitude
typically cause only some 75% to 15% variations in the kinetic and
potential energy levels, with a long-term sum effect close to zero
[37] and will, hence, not be considered in this study.
4. Friction and energy losses in trucks and buses
4.1. Global average energy losses in heavy road vehicles
In Section 3.1, the global average energy consumption was
calculated for the four heavy duty vehicle categories (Table 2):
� Single-unit trucks 180,000 MJ/a
� Trucks and trailer combinations 1,440,000 MJ/a
� City buses 1,080,000 MJ/a
� Coaches 920,000 MJ/a
In the following discussion, these values are used as representative
values for the four global average vehicles defined in Section 3.2. In
the heavy-duty road vehicles, the mechanical energy generated by
the combustion process in the engine is primarily consumed by
friction losses in the engine, transmission, and other mechanical
components; by energy losses in auxiliary equipment; and by the
running resistance acting on the vehicle during operation. The
running resistance consists of a set of factors that cause forces on
the vehicle opposite to its running direction. The dominating types
of running resistance are the aerodynamic drag, the tire rolling
resistance, the grade resistance, and the acceleration resistance [38].
Average values for the breakdown of the fuel energy in all heavy
duty vehicles are shown in Fig. 3, based on the calculations in
Appendix A, Table A1.
Published data and experience from fuel consumption, exhaust
emissions, duty cycles, and energy flow measurements performed
by the VTT Technical Research Centre were used as complemen-
tary information when no other reliable data in the literature were
available [20,28,33,39].
4.2. Energy distribution in trucks and buses
The internal combustion engine converts part of the chemical
energy of the diesel fuel into useful mechanical work from the
engine crankshaft. The thermal losses, including the heat losses
from engine friction, through the exhaust and cooling energies are
significant, as they form a total of more than 50% of the total
energy input, as shown in Fig. 3. Some mechanical work is lost
through the internal friction in the engine, to the gas exchange of
the engine, and to the power uptake of auxiliaries such as the
water pump, generator, and air compressor.
The net mechanical work at the drive end of the engine
crankshaft is partly lost due to frictional power losses in the
transmission – the gearbox and the final drive – while the major
proportion of the energy is consumed after the transmission by
the running resistances of the vehicle (see Table A1).
The fuel energy in heavy duty vehicles is dissipated through the
following mechanisms, as estimated based on collected informa-
tion from published studies [29,30,40–54]. These studies were
used in the calculations in Appendix A. Below is the energy
distribution given as average values for the global fleet of trucks
and buses, with the range of values appearing in the literature
given in parentheses:
� 30% (22–31%) goes to exhaust gases, mainly in the form of
thermal energy that disappears by convection.
� 20% (20–25%) goes to cooling in the form of heat disappearing
by conduction through the engine structure, the cooling
Table 3
Characteristics of the average heavy vehicles in the four categories, with reference to the year 2000.
Feature Single-unit
truck
Semi-trailer truck City bus Coach
Manufactured (year) 2000 2000 2000 2000
Average total weight (kg) 10,000 30,000 14,000 16,000
Number of axles 2 5 2 2
Engine capacity (dm3) 7 12.5 9 12.5
Maximum engine power (kW) 150 300 200 250
Type of engine Turbo diesel,
6-in-line
Turbo diesel, 6-in-line Turbo diesel, 6-in-
line
Turbo diesel,
6-in-line
Engine oil grade (age 1 year) 10 W–40 18cSt @
90 1C
10 W–40 18cSt @ 90 1C 10 W–40 18cSt @
90 1C
10 W–40
18cSt@90 1C
Gearbox Manual or
robotized
Manual or robotized Automatic with
torque converter
Manual or
robotized
Gearbox oil grade (age 5 years) SAE 80 W–90
12cSt@80 1C
SAE 80 W–90 12cSt@80 1C ATF 29cSt@801C SAE 80 W–90
12cSt@80 1C
Final drive Hypoid Hypoid Hypoid Hypoid
Final drive oil grade (age 5 years) SAE 80 W–90
60cSt@60 1C
SAE 80 W–90 60cSt@60 1C SAE 80 W–90
60cSt@60 1C
SAE 80 W–90
60cSt@60 1C
Drag coefficient 0.75 0.8 0.62 0.51
Projected frontal area (m2) 8 9 7 7.5
Tires on the front axle Single tires 315/
70R22.5
Single tires 315/80R22.5 Single tires 295/
80R22.5
Single tires 315/
80R22.5
Tires on the other axles Dual tires 315/
70R22.5
Truck: Dual tires 315/80R22.5 Trailer:
Dual tires 385/65R22.5
Dual tires 295/
80R22.5
Dual tires 315/
80R22.5
Coefficient of rolling resistance of tires (age 2 years, with
average tire pressure on average road)
0.01 0.01 0.01 0.01
K. Holmberg et al. / Tribology International 78 (2014) 94–11498
radiator (and oil cooler), and occasionally the heating element,
and heat further dissipated to the environment.
� 3% (1–6%) goes to non-frictional auxiliary losses.
� 47% (36–54%) is converted into mechanical power. This part
can be sub-divided into:
○ 13.5% (5–26%) goes to overcome air drag, including external
and internal air flow resistance, as well as losses in the
electrical and indoor cooling system.
○ 33.5% (30–37%) goes to overcome friction in any part of the
vehicle, including brakes and tires.
The part of the fuel energy that is used as mechanical power to
overcome friction can be subdivided in groups based on the
published studies [21,27,29,30,37,40–46,49,53,55,56]. The energy
to overcome friction is distributed as follows:
� 42% (17–52%) in the tire-road contact (TR),
� 18% in the engine system,
� 13% in the transmission system,
� 18% in the brake contact (BC), and
� 9% in the auxiliary equipment.
4.3. Energy losses from engine friction
The engine systems in heavy vehicles have many different
designs but basically they comprise a piston assembly, a crankshaft
mechanism, and a valve train. The energy needed to overcome
friction in the engine subsystems has been analyzed in detail by
Pinkus and Wikcock [40] and Taylor and Coy [57]. Their findings
have been discussed in many papers [41–43,45,49,51,53,55,57–
64]. Based on these sources, we estimate the further engine
friction energy loss distribution as follows:
� 45% (45–55%) in the piston assembly,
� 30% (20–40%) in bearings and seals (HD),
� 15% (7–15%) in the valve train (ML), and
� 10% by pumping and hydraulics viscous losses (VL).
The main tribological contact mechanism is marked in par-
entheses above. For engine bearings and seals, the dominant
mechanism is hydrodynamic lubrication (HD), while that in the
valve train is mixed lubrication (ML). In mixed lubrication,
combined effects from hydrodynamic (HD), elastohydrodynamic
(EHD), and boundary lubrication (BL) are present. Boundary
lubrication is normally regarded as a regime where the nominal
fluid film thickness is much less than the average composite
surface roughness of the contacting bodies. Hence, frequent
asperity collisions or direct solid-to-solid contacts occur and lead
to rather high friction. In this analysis, the EHD contacts are
divided into three groups: EHD in sliding contacts (EHDS), such
as in piston-cylinder contacts,with interfacial “Stribeck-type”
friction; EHD in rolling contacts (EHDR), such as in rolling bear-
ings, where friction originates from Poiseuille flow of lubricant and
elastic hysteresis; and EHD in sliding-rolling contacts (EHDSR),
such as in gears, with a combination of sliding and rolling [65].
The piston assembly, with the piston skirt, piston rings, and
gudgeon pin as the frictional components, is more complex and
needs an additional level of subdivision. The tribocontacts in the
piston assembly are estimated to be represented by the following
tribological contact mechanisms [57,58,62]:
� 40% is HD lubrication, including the squeeze film lubrication
effect at the top and bottom dead centers,
� 38% is EHDS lubrication,
� 11% is ML, and
� 11% is BL.
4.4. Energy losses from transmission friction
Coaches and heavy trucks are normally equipped with manu-
ally operated or automated mechanical gearboxes with helical
spur gears in order to minimize the transmission friction losses.
The number of gears may vary from six in a simple gearbox up to
16 in a four-speed gearbox combined with additional splitter and
range-change units [66]. For ergonomic reasons, a city bus is
usually equipped with an automatic gearbox with planetary gears
and a torque converter.
The energy losses from gearbox friction occur in the rolling
bearings, gears, gear synchronizers, and shaft seals; oil churning
also contributes. For manual or automated mechanical gearboxes
with helical spur gears, typical of commercial vehicles, the
efficiency of the entire gearbox lies in the range 90–97%. For
automatic gearboxes of city buses, the efficiency varies between
90% and 95% under optimal conditions of operation, while at non-
optimized driving cycles the efficiency of automatic transmissions
can be significantly lower due to the low efficiency of hydrody-
namic torque converters at low speed ratios across the converter
[66].
Fig. 3. Breakdown of the average energy consumption in heavy duty vehicles based on average values from the four categories of trucks and buses.
K. Holmberg et al. / Tribology International 78 (2014) 94–114 99
The energy losses from friction in the bevel gear of the final
drive occur in the rolling bearings, the hypoid gear, and the shaft
seals; oil churning in the gear contacts also contributes. In road
bends, some energy is lost due to the action of the differential gear
of the final drive.
The friction torque in the wheel bearings was included in the
analysis on frictional power losses in the transmission. For
simplicity, the wheel bearings for the vehicles and the semi-
trailer truck combination were analyzed as a single bearing loaded
by its portion of the total weight of the vehicle or vehicle
combination. A coefficient of rolling friction of m¼0.002 was used
for the bearings [67,68].
Since the wheel bearings are grease-lubricated and do not
contain significant amounts of superfluous lubricant, the propor-
tion of additional frictional torque from any squeezing or churning
of the lubricant was regarded as insignificant. The energy losses
from friction in the bevel gear of the final drive occur in the rolling
bearings, the hypoid gear, and the shaft seals; oil churning also
contributes.
A great variety of different transmission system designs are
used in heavy duty vehicles. In general, the energy consumed to
overcome friction in a manual or robotized transmission system
is consumed for the following reasons [43,66,69–72]:
� 20% to overcome viscous losses (VL) in the oil tank, gear
contacts, synchronizers, and bearings;
� 55% to overcome friction in gears (EHDSR);
� 20% to overcome friction in bearings (EHDR); and
� 5% to overcome friction in seals, forks, etc. (ML).
4.5. Energy losses from air drag
The air drag, or the aerodynamic drag, is a major source of
energy loss in the operation of heavy duty vehicles. The air drag
originates from the internal friction in the air that flows around a
moving vehicle. In the present study, the air drag losses are not
included in the frictional losses, as air drag is normally not
considered as a tribological friction loss. The air drag force Fd is
calculated as the product of the dynamic air pressure (ρ�v2/2),
the projected frontal area, and the drag coefficient of the vehicle
[73]:
Fd ¼ A� Cd � ρ� v2=2
where A is the projected frontal area, Cd is the drag coefficient, ρ is
the density of the air, and v is the velocity of the vehicle.
The above formula is valid for driving in still air, whereas in
normal traffic the aerodynamic drag force on a vehicle is affected
by the wind speed and the yaw angle, or the angle between the
wind direction and the driving direction. In the present study, we
assumed an average wind speed of 5 m/s and an average max-
imum yawing angle of 131, and these can be taken into considera-
tion by the use of a wind-averaged drag coefficient Cd0, which is
about 10% higher than the drag coefficient Cd in still air [73]. The
drag coefficient includes the drag forces from the shape of the
wheels of the vehicle.
For semi-trailer trucks, a recent study on truck efficiency in the
Organisation for Economic Co-operation and Development (OECD)
countries assumed a frontal area of 9.5 m2 and a drag coefficient of
Cd¼0.6 to 0.8, depending on the aerodynamic details [74]. For this
category of trucks, Cd¼0.6 to 0.65 can be achieved by aerodynamic
improvements on the design of the truck and trailer combinations
[52]. Work by Leduc [50] reports Cd¼0.5 to 0.75 for semi-trailer
trucks, Cd¼0.58 to 0.66 for city buses, and Cd¼0.42 to 0.60 for
coaches. According to the same source, the drag area, which is the
frontal area A multiplied by Cd, is about 5 m2 for 60-ton semi-
trailer trucks, which equals a mean value of 8 m2 for the frontal
area. Correspondingly, Hucho [73] reports Cd¼0.48 to 0.75 for
semi-trailer trucks and Cd¼0.40 to 0.65 for coaches. The drag
coefficients and frontal areas chosen for the present analysis are
given in Table 3.
The power loss arising from the air drag is proportional to the
third power of the driving velocity, for which reason these power
losses increase substantially at higher speeds. For example, at a
steady velocity of 104 km/h, the total power loss from the running
resistances, comprising air drag and rolling resistance, of a typical
semi-trailer truck combination is 136 kWh, and of this, 85 kWh is
due to aerodynamic drag [2]. A velocity reduction from 104 km/h
to 80 km/h decreases the energy losses by some 35%, mainly
owing to the decrease in the drag losses. Another route for drag
loss reduction is to improve the aerodynamics of the vehicle, for
instance, by adding fairings, covers, and skirts to the roof, sides,
nose, tail, and chassis of the trucks and the trailers and the gap
between them [30,52].
4.6. Energy losses from tire rolling
The rolling resistance of the tires is the reason for a significant
part of the energy consumed in the operation of heavy vehicles
[30,73]. This energy loss is of the same order of magnitude as the
air drag.
In this work, the tire rolling resistance is studied separately
from the wheel bearing friction, which is regarded as part of the
overall transmission friction. Furthermore, we consider the drag
forces acting on the wheels as part of the sum drag force acting on
the vehicle.
The rolling friction of viscoelastic bodies like pneumatic tires
depends on the rotational speed because the relaxation of the
material at the trailing edge of the contact is slower than the com-
pression at the leading edge, and this difference is expressed by the
hysteresis factor. The rolling resistance, furthermore, depends on the
micro-slip at the rolling interface, which arises from different elastic
constants of the wheel and the road. As a third factor, the rolling
resistance depends on the roughness of the road [75].
The hysteresis loss in the viscoelasticrubber material of the
rolling tire is of particular importance. Under rolling contact, some
energy is stored as compression and released as relaxation of the
elastically deformed sections of the tire. The rest of the rolling
resistance energy is converted into heat by hysteresis losses due to
the viscous nature of the rubber material. The hysteresis losses are
determined by the tire materials, geometry, and construction in
combination with the load, velocity, wheel alignment, air pressure,
and temperature. The main geometry features are the outer
diameter, rim diameter, and tire width, all of which affect the
rolling resistance. The rubber material contains additives for
several purposes, and these additives are known to influence the
rolling resistance. The rubber material of the tread is of particular
importance, since more than one-half of the rolling resistance can
originate from the deformation that takes place in the tread. As a
consequence of this, tire wear reduces the rolling resistance [44].
The tire rolling friction is the sum of the tire-against-road
losses at all the wheels of the vehicle, and in this work it is
represented by a single coefficient of rolling friction, representing
all the wheels, and the load of the entire vehicle:
Fr ¼m� g � mr
where m is the mass of the vehicle, g is gravitational acceleration
(9.81 m/s2), and mr is the coefficient of tire rolling friction.
In a study on truck efficiency in the OECD countries, the rolling
resistance was calculated using a coefficient of rolling friction of
mr¼0.004 for single-tire axles and mr¼0.005 for dual-tire axles [74].
The coefficient of rolling friction for modern truck tires lies in the
K. Holmberg et al. / Tribology International 78 (2014) 94–114100
range mr¼0.004 to 0.008 for single-mounted tires, 0.005 to 0.008 for
dual-mounted tires, and 0.004 to 0.005 for the most modern tires
[52,76,77]. The ranges of the coefficient of rolling friction presented
above are representative for trucks operated under optimized condi-
tions. Under less than optimal conditions, such as running at low tire
pressure [50,52] or on rough or soft surfaced roads, higher coefficients
of rolling friction occur. For this reason, the average value for the
coefficient of rolling friction for the global fleet of heavy vehicles is
probably at the higher end of the range, or beyond it. In the present
analysis, we selected a coefficient of rolling friction of mr¼0.01.
4.7. Energy losses from auxiliary equipment
The energy for the auxiliary equipment is taken from the
crankshaft of the engine. The main sources of energy losses in the
auxiliary equipment comprise the cooling fan of the radiator, the
other electrical equipment, the air compressor of the pneumatic
system, the air-condition compressor, and the hydraulic oil pump for
the power steering. The energy consumed in the auxiliary equipment
does not move the vehicle but makes the driving more comfortable
and is partly even necessary for the operation of the vehicle.
In laboratory and field measurements carried out by VTT, the
following average power consumption was determined for the
auxiliary equipment of a delivery truck and a city bus:
– The cooling fan of the radiator is driven by an electrical motor
or a belt drive from the engine, and the power need greatly
depends on the driving conditions and the ambient tempera-
ture. A continuous average power consumption level of 0.3 kW
was measured for the delivery truck and 2.2 kW for the
city bus.
– In the pneumatic system, mechanical energy from the engine is
converted in a compressor to pneumatic energy, to be con-
sumed in components like actuators for the wheel brakes and,
possibly, bellows for the suspension and door actuators in
buses. The power consumption by the air compressor was
measured to be 0.72 kW for the delivery truck and 0.51 kW for
the city bus.
– The power to the air-condition circuit is consumed for trans-
porting thermal energy from the truck or bus cabin. The rate of
operation of the air condition unit depends on the ambient
conditions, and the power consumption was measured to be in
the range 0.04–0.16 kW.
– Power steering is based on hydraulics or on electrical actuators,
which assist the driver to turn the steering wheel. Whichever
power steering arrangement is chosen, some energy is con-
sumed for the steering. The power needed for the air com-
pressor was determined to be 0.01 kW for the delivery truck
and 0.03 kW for the city bus.
– Traditionally, the electrical energy that is consumed in a vehicle
during starting and operation is generated by an AC generator,
which is driven by a belt drive from the crankshaft of the
engine. The average generator power level was found to be
0.18 kW for the delivery truck and 0.14 kW for the city bus,
both equipped with mechanically driven cooling fans.
– The total power consumption of the most common auxiliary
equipment in use in trucks and buses was determined to be
about 2 kW as a global average.
The total energy loss in the auxiliary equipment has been
reported to be in the range 1–6% of the fuel energy input
[29,50,51,52], and in this work 3.5% is used as an average value.
Of this amount, approximately 15% is consumed due to frictional
power losses in the auxiliary equipment.
4.8. Energy losses from road inclination, acceleration, and braking
The running resistance due to road inclination equals the forces
in the direction opposite to the running direction and is deter-
mined by the mass of the vehicle, gravity, and the inclination of
the road. In the present work, however, we assumed that all
running resistance forces from uphill driving are compensated by
the corresponding forces during subsequent downhill driving, or
that the energy lost by the road inclination during uphill driving is
available as an energy reserve during subsequent downhill driving.
During acceleration the motion of the vehicle is counteracted
by the inertia force. In the present context, the time integral of the
inertia force times the velocity of the vehicle gives the amount of
energy needed for the velocity increase, which is equal to the
increase in the kinetic energy of the vehicle. When eventually the
vehicle is to be decelerated, i.e., slowed down or entirely stopped,
the kinetic energy corresponding to the higher velocity will
decrease to the level that corresponds to the lower velocity.
Since energy cannot be destroyed, the release of the kinetic
energy is associated with energy consumption due to the other
running losses than the rolling and air resistance which are
already separately calculated in our analysis. In the main part of
all traffic situations, the deceleration of a vehicle is controlled by
the brakes. Consequently, it can be assumed that the work for
acceleration equals the work for braking. The work that is
transferred into kinetic energy during the acceleration is trans-
ferred through brake friction into heat in the brakes during
subsequent retardation of the vehicle.
4.9. Energy loss during idling
During idling, the engine is loaded by the internal friction and
the torque from part of the gearbox, the oil pump, and accessories
like the generator, the compressors, and the pump for the steering
assistance. When a diesel engine is running at low torque, such as
during idling, the specific fuel consumption, or the amount of fuel
per produced unit of energy, is high.
According to a review by Ashrafur Rahman et al. [34], the diesel
fuel consumption during idling of heavy vehicles varies between
approximately 1 and 7 l per hour, depending on the type of vehicle
and the ambient temperature. The typical fuel consumption for a
semi-trailer truck in the U.S. is between 2.5 and 4.3 l per hour,
depending on the idling speed and the use of the air condition
compressor [14].
In the present work, the fuel consumption during idling of each
ofthe four categories of heavy vehicles has been approximated to
be 4 l per hour, as a global average. Through the idle percentages
Fig. 4. Annual energy flow and distribution in the global average single-unit truck
(model year 2000) corresponding to 20,000 km annual mileage.
K. Holmberg et al. / Tribology International 78 (2014) 94–114 101
presented in Table 1, the energy loss during idling has been taken
into account in the energy balance calculations.
4.10. Energy consumption in trucks and buses
Based on the assumptions and data presented above, the
energy flow in each type of global average vehicle was calculated
as shown in Appendix A and in Figs. 4–7.
A comparison of the distribution of energy losses in the four
vehicle categories is shown in Fig. 8 based on the calculation
results resented in Table A1 of Appendix A.
5. Tribocontact friction losses today and in the future
The sources of frictional losses in vehicles have largely been
studied during the last decade or so. In particular extensive
research has been conducted on contact mechanics and lubrica-
tion mechanisms of rolling and sliding surfaces, especially from
the last 40 years. As a result, we have a good understanding of how
various contacts can be classified, and what level of frictional
losses they typically represent [78–82]. In this study, we calculated
the amount of mechanical energy dissipated due to friction in
various tribocontacts in vehicles on the basis of the published data
for friction coefficients typical of each contact type.
Friction levels for different tribocontacts in four types of global
average vehicles were separately estimated: the typical 13-year-
old vehicle in use today (designated hereafter as “Truck & Bus
2000”), a vehicle that represents a combination of today's most
advanced commercial tribological solutions (“Truck & Bus 2013”),
a vehicle that represents the best tribological solutions demon-
strated in research laboratories today (“Lab 2013”), and a vehicle
that reflects estimations by the best experts in the field of what is
possible to achieve in the future after about 10–15 years extensive
R&D work (“Truck & Bus 2025”).
The coefficients of friction based on the above classifications
are given in Table 4 and Fig. 9. The references in the Table relate
mainly to the friction values for Truck & Bus 2013 and Lab 2013,
while the friction values for Truck & Bus 2000 and Truck & Bus
2025 are estimates by the authors based on available present
information and their own experience. Possible future technical
solutions for friction reduction are discussed in Section 7.
Table 4 presents friction coefficients for common tribological
contacts. The friction in such contacts can be controlled and
reduced by improved scientific knowledge and advanced techno-
logical solutions. The friction estimates in Table 4 are based on
commercial oil lubrication for Truck & Bus 2000 and 2013 and on a
new kind of lubrication, often non-petroleum-based (e.g., a water-
based lubricant such as polyalkylene glycol) for Lab 2013 and
Truck & Bus 2025.
Viscous losses in transmission systems can occur due to shear
and churning of oil in the transmission case. In engine systems,
pumping and hydraulic losses are related to viscous losses. Viscous
losses are included in this study because, even if not directly
friction losses, these losses can be reduced by tribological solu-
tions resulting in lubricants with lower viscosity and thus also
lower viscous losses. The reduction in viscous losses associated
with the different classifications of vehicle is shown by the bottom
row of Table 4.
6. Potential savings
The calculations in Table A3 of Appendix A show that, globally,
average annual friction losses for year 2000 models are 54 GJ for
single-unit trucks, 446 GJ for semi-trailer trucks, 454 GJ for city
buses, and 253 GJ for coaches. From Table A2 we can see how the
friction losses are distributed in various contact mechanisms in the
vehicles. The largest contributors are tire rolling contact (40%) and
inertia transformation to the brakes (21%). They are followed by
elastohydrodynamically lubricated contacts (13.5%), hydrodynami-
cally lubricated contacts (10.5%), viscous losses (8.5%), and mixed
lubricated contacts (5%).
In Table A3 we have used the estimated possibilities for friction
reduction calculated in Table 4 and further calculated the potential
annual energy reduction for the four vehicle types. The data show
that, by implementing the tribological solution in use in the
commercial vehicle of today in all heavy duty vehicles world-
wide, the energy consumption due to friction could be reduced by
Fig. 5. Annual energy flow and distribution in the global average semi-trailer truck
(model year 2000) corresponding to 100,000 km annual mileage.
Fig. 6. Annual energy flow and distribution in the global average city bus (model
year 2000) corresponding to 80,000 km annual mileage.
Fig. 7. Annual energy flow and distribution in the global average coach (model year
2000) corresponding to 100,000 km annual mileage.
K. Holmberg et al. / Tribology International 78 (2014) 94–114102
37%. If the best tribological solutions demonstrated in research
laboratories were in use, this factor would be reduced by 60%, and
if the new solutions forecasted for 2025 were in use, it would be
68%. Note that the savings in fuel energy can be larger than the
total energy used to overcome friction because reduced friction
results in reduced energy demand, and thus the energy going to
exhaust and cooling is also reduced, as shown in Fig. 3. A reduction
of 10% in friction results in a reduced fuel consumption of 7.4%.
Obviously, implementing today's advanced commercial solu-
tions in all trucks and buses would require an enormous effort and
would result in large implementation costs, which cannot be
commercially justified. Nonetheless, it would be realistic to esti-
mate that perhaps half of this level could be reached in the short
term, within four to eight years, as shown in Fig. 9. As shown in
Table 5, that improvement would result in a 13.8% reduction in
fuel consumption which corresponds to 2.7 million TJ/a energy
savings, equal to 104,500 million euros saved annually worldwide,
and 196 million tonnes reduction in CO2 emission [12,95]. Table 6
shows the energy and cost savings broken down by region.
We estimate that after 8 to 12 years of extensive and focused
research and development work and actions for implementation
of new technology, the level half way between Trucks and Buses
2013 and Lab 2013 could realistically be achieved, as shown in
Fig. 9. This would result in 36.8% reduction in fuel consumption,
which on the global scale corresponds to 7.2 million TJ/a energy
savings, equal to 280,600 million euros saved annually worldwide,
and 527 million tonnes reduction in CO2 emission (Table 5).
7. Means of reducing friction and energy use
Several reports have been published on methods and techni-
ques on how to improve fuel economy in heavy duty vehicles and
in transportation [1,2,26,36,52]. Below we will focus on the
techniques related to friction reduction in heavy-duty road vehi-
cles. Some of the non-tribological means of improving fuel
economy of trucks are currently being explored under the spon-
sorship of various government agencies (i.e. the Super truck
programs sponsored by the Department of Energy, United States;
the Heavy Duty Vehicle transport program at the Energy Technol-
ogies Institute in Europe, etc.). Collectively, these and the
Fig. 9. Trends in the coefficient of friction in the four truck and bus categories for
different lubrication mechanisms and for tire rolling friction.
Fig. 8. Breakdown of the global average energy consumption for single-unit truck, semi-trailer truck, city bus, and coach. Friction losses also shown.
Table 4
Tribological contact performance for thechosen four types of heavy vehicles [50,52,70,71,74,83–94].
Contact types acting as friction sources Coefficients of friction
Truck & Bus 2000 Truck & Bus 2013 Lab 2013 Truck & Bus 2025
Boundary lubrication (BL) (e.g., piston ring contact) 0.14 0.1 0.01 0.005
Mixed lubrication (ML) (e.g., piston ring contact) 0.10 0.05 0.01 0.005
HD lubrication (HD) (e.g., engine bearing) 0.025 0.01 0.002 0.001
EHD sliding (EHDS) (e.g., piston ring contact) 0.08 0.04 0.01 0.005
EHD sliding & rolling (EHDSR) (e.g., transmission gears) 0.06 0.03 0.005 0.0008
EHD rolling (EHDR) (e.g., transmission roller bearing) 0.01 0.002 0.001 0.0005
Tire rollinga (TR) 0.010 0.006 0.003 0.002
Resistance to viscous shearb (VL), ν (cSt at 80 1C) 35 20 15 5
a Average rolling friction coefficients for trucks and buses on average roads with average tire pressure.
b Estimated average of engine and transmission oils adjusted based on their part of energy losses.
K. Holmberg et al. / Tribology International 78 (2014) 94–114 103
tribological means of improvements discussed below may lead to
substantial improvements in fuel economy of trucks and buses.
In Section 5, the technical possibilities to mitigate various
friction loss sources in heavy duty vehicles are estimated both
for the short and long term. Below, we outline some of the known
new technical solutions that could be implemented now to
achieve such reductions. These solutions mainly correlate to what
has been called above today's best solution on the laboratory level
(Lab 2013).
It is important to notice that some of the new solutions for
friction reduction can be directly implemented by the end user to
existing vehicles, such as the change to new type of engine oil or
oil additives and an adjustment to the tire air pressure. However,
many of the new solutions need replacement of existing compo-
nents, like the introduction of new coatings or surface-textured
components. These improvements would need to be introduced
by the vehicle manufacturers and would come out on the market,
together with other new design solutions, when new vehicle
models are launched.
7.1. Low friction coatings for engine components, gears, and bearings
During the past two decades, research on low-friction materials
and coatings has intensified, mainly because the traditional solid
and liquid lubrication approaches would not alone meet the
increasingly more stringent operational conditions of modern
mechanical systems, including engines [96,97]. In recent years,
concerted effort to develop more advanced techniques for physical
and chemical vapor deposition (PVD and CVD) plasma, and
thermal spraying methods, etc., has made it possible to coat all
kinds of engine components with low-friction coatings reliably
and cost effectively. Some of the more advanced PVD technologies
are based on pulse DC, arc-PVD, high power impulse magnetron
sputtering (HIPIMS), and pulsed laser deposition (PLD). These
techniques seem to afford much superior chemical and structural
qualities to coatings and, hence, lower friction and wear even
under marginally lubricated sliding conditions [98]. Because of
their highly energetic nature, PVD techniques also afford much
stronger bonding between tribological coatings and underlying
Table 5
Global energy consumption, emissions, costs and potential global annual energy savings per year in short and long terms.
Present situation (2012) Annual fuel consumption
(million liters)
Annual energy
demand (TJ)
Annual CO2 emission
(million tonnes CO2)
Annual costsa
(million euros)
Single-unit trucks 145,600 5,200,000 382.9 203,800
Trucks and trailers 294,000 10,500,000 773.2 411,600
Trucks in total 439,600 15,700,000 1156.1 615,400
City buses 70,000 2,500,000 184.1 98,000
Coaches 33,600 1,200,000 88.4 47,000
Buses in total 103,600 3,700,000 272.5 145,000
Trucks and buses in total 543,200 19,400,000 1428.6 760,400
To overcome friction, trucks and
buses in total
180,900 6,460,000 475.7 253,200
Potential savings by new
technology
Savings/
reduction (%)
Reduction in fuel
consumption (million liters)
Energy demand
reduction (TJ)
CO2 emission reduction
(million tonnes CO2)
Economic savings
(million euros)
Single-unit trucks
– short term (4–8 yr) 12 17,500 624,000 45.9 24,400
– long term (8–12 yr) 32.5 47,300 1,690,000 124.4 66,200
Trucks and trailers
– short term (4–8 yr) 15 44,100 1,575,000 116 61,700
– long term (8–12 yr) 40.5 119,000 4,253,000 313.1 166,700
City buses
– short term (4–8 yr) 11.5 8100 288,500 21.2 11,300
– long term (8–12 yr) 30 21,000 750,000 55.2 29,400
Coaches
– short term (4–8 yr) 15 5000 180,000 13.3 7100
– long term (8–12 yr) 39 13,100 470,000 34.5 18,300
Trucks and buses
– short term (4–8 yr) 13.8 74,700 2,670,000 196.6 104,500
– long term (8–12 yr) 36.8 200,400 7,160,000 527.2 280,600
a Calculated based on average diesel fuel price in Europe, November 2013 (1.4 euros for 1 l).
Table 6
Estimated realistic energy savings by region, representing 50% of the total potential energy savings by using today's best commercial solution, after four to eight years of
concentrated actions to reduce friction in heavy duty vehicles worldwide, see Fig. 9. Also given are the corresponding cost savings, fuel savings, and CO2 reduction.
Energy savings (TJ/a) Cost savings
(million euro/a)
Fuel savings
(million l/a)
CO2 emission reduction
(million tonnes/a)
World 2,670,000 104,500 74,700 196.6
Industrialized countries (60%) 1,600,000 62,700 44,800 118.0
Industrially developing countries (35%) 935,000 36,600 26,100 68.8
Agricultural countries (5%) 130,000 5200 3700 9.8
EU (17.3%) 470,000 18,100 12,900 34.0
U.S. (21.7%) 580,000 22,700 16,200 42.7
China (10.4%) 280,000 10,900 7800 20.4
Japan (5%) 130,000 5200 3700 9.8
Finland (0.25%) 6700 260 190 0.490
K. Holmberg et al. / Tribology International 78 (2014) 94–114104
substrates and thus provide very long wear life, which is critically
important for most engine applications.
With the use of sophisticated computer codes and finite
element modeling [99–102], the tribological performance and
durability of such coatings were further improved. Specifically,
these techniques can help more closely match the coating proper-
ties with those of the substrate materials through predictive
interface engineering and better internal stress control, which
together ensure outmost film-to-substrate bonding and hence
high performance and longevity under severe operating condi-
tions. Modern tribological coatings may range in thickness from
half a micrometer to several millimeters. Mainly because of their
self-lubricating nature, they can act as a backup lubricant in oil-
lubricated contacts to provide much lower friction, even under
severe boundary and oil-out conditions.
Recent tribological experiments have confirmed that low-friction
coatings such as diamond-like carbon, MoS2, etc., can drastically
reduce the friction coefficients of dry and lubricated sliding contacts
by more than 90% [81]. Such impressive reductions are for
boundary-lubricated regimes, where direct metal-to-metal contacts
occur since in HD and EHD contacts, there are very few asperity
contacts, and shearing takes place within the fluid film itself. Besides
friction, coatings can extend the lifetime of tribological components.
For example, with the use of hard low-friction coatings, as much as
ten-fold increase in fatigue lifetime was reported under rolling
contacts. Furthermore, such coatings have reduced bearing wear
by seven-fold and increased gear lifetime by three-fold [81,103].
In engines, drive trains and transmissions, there are many coat-
ings that can improve the efficiency, performance, and durability.
The most important tribocontacts producing friction losses are
typically the piston ring and cylinder liners, gears, bearings, valves,
and cam and follower contacts. Fuel injection,commutators, ball
pivots, connecting rods, gear selection shafts, synchronizer rings,
clutch mechanisms, shifter forks, joints, shock absorber parts, steer-
ing system parts, and brake components [104,105] can also give rise
to friction but at much reduced levels. Even if the friction losses in
some of these parts may not be large, they may cause increased
wear and reduced lifetime in the long run and sometimes even
result in catastrophic failures due to gradual damage accumulation.
Among the many hard and low-friction coatings, the develop-
ment of diamond-like carbon (DLC) coatings has attracted the
greatest attention in recent years, since they provide the best
overall frictional performance under dry and lubricated conditions
[106]. Some of the components cited above are nowadays coated
with DLC and used in actual engines [107]. Systematic lubricated
studies by Kano et al. on DLC [108] have demonstrated that as
much as 90% reduction in boundary friction is feasible with certain
types of DLCs, provided that the additive package contains polar
additives like glycerol. In another similar study with different
types of DLCs (such as metal doped), more than 30% reduction in
friction compared with uncoated steel/steel contacts was reported
by Podgornik and Vizintin [109,110] compared to uncoated steel/
steel contacts. Likewise, Gåhlin et al. [111] achieved a 70-fold
increase in lifetime by applying WC/C coatings on gears tested in
an FZG test machine. Other hard and low-friction materials and
coatings developed for improved friction and wear performance in
engine components include Cr–N, TiN, Ni–SiC, AlMgB14, MoS2, WC/
Co, AlTiN, W–C:H, AlMgB14–TiB2, composite coatings with, for
example, TiN, TiC, or TiB2 particles embedded in Si3N4 or SiC
ceramic matrices, as well as various nanostructured and nano-
layered coatings [43,59,98,104,112–115].
7.2. Surface texturing of components in engines, gear boxes, and bearings
The surface texture or topography of sliding contacts has a
strong influence on friction, wear, scuffing, and fatigue
performance of both dry and lubricated tribological components.
Honing is a well-established practice for ring-liner assembly and
used routinely by industry. Originally, researchers were concerned
about formation of macro-scale dimples or deep scratches on
sliding surfaces causing higher friction and wear, but lately it has
become clear that when prepared properly, dimples, grooves, and
protrusions can have very beneficial effects even at the nanoscale
[116], although the underlying mechanisms become very complex
[63].
Specifically, recent systematic studies confirmed that micro-
and nano-scale dimples created by laser beams or lithographic
techniques can substantially improve the friction and wear per-
formance of sliding surfaces under lubricated conditions [117]. For
example, laser surface texturing of piston rings has been shown to
reduce fuel consumption of engines by as much as 4% [118–121].
Shallow crater-like dimples with about 100 μm diameter and
about 10 μm depth could produce a wedge flow effect and, hence,
hydrodynamic pressure build-up within the contact area that may,
in turn, reduce friction by about 25% [122]. Changing the surface
topography of gears to be more smooth by superfinishing has been
shown to reduce friction by typically 30% [123]. Fine particle
peening of the contacting surface that produces a surface with
microdimples was also found to reduce friction in lubricated
conditions by up to 50% [124].
In a study on frictional losses in heavy duty diesel engines, a
tribometer test indicated that significant reduction in hydrody-
namic friction can be accomplished by applying textures on
cylinder liner surfaces [63]. The results put forward a new surface
design, which includes an untextured cylinder liner surface in the
vicinity of the top and bottom dead centers of the piston ring
motion, where the contribution of the hydrodynamic friction on
the total friction is small, and the cylinder texture density
increases with the piston speed.
7.3. Lubricants
Almost all sliding or rolling interfaces in engines are lubricated
by oils. Besides providing easy slippage, the lubricant provides a
fluid film that separates opposing surfaces from one another,
especially in the mixed and hydrodynamic lubrication regimes.
The oil films assure that shearing occurs within the fluid film, and
therefore, low friction is attained and direct metal-to-metal con-
tacts are avoided. Lubricating oils can also transport frictional heat
from contact spots and thus prevent thermal and mechanical
degradation of contacting surfaces.
Most important, lubricants used in engines contain a range of
anti-friction, anti-wear, and extreme pressure additives that help
in the formation of a chemical boundary film that suppresses wear
and scuffing damages on sliding surfaces. Some of the additives
are designed to retard or prevent oxidation and corrosion as well,
and some improve viscosity under high heat and loading condi-
tions. However, the pumping and moving of oil from one place to
another in engines result in energy losses due to the viscosity of
the lubricant [43,57]; hence, the current trend is toward the
adaptation of low-viscosity lubricants in engines.
In the present analysis, we have seen that viscous losses and
shear in hydrodynamic contacts give rise to significant energy
losses. Therefore, if the lubricant viscosity is further reduced while
the anti-friction and anti-wear functions are maintained as in
viscous oils, then a very large energy saving in engines could be
achieved. For this to become reality, lubricant specialists and
additive manufacturers have suggested a number of approaches
that are still being explored worldwide.
Undoubtedly, with the use of lubricating oils of lower viscosity,
the energy losses due to oil shear, churning, and pumping will
decrease. For example, with a reduction of the engine oil viscosity
K. Holmberg et al. / Tribology International 78 (2014) 94–114 105
by approximately 25% (which would roughly correspond to a
change in viscosity class from SAE 40 to SAE 30 or from SAE 30
to SAE 20 at a reference temperature of 100 1C), the corresponding
fuel savings could be from 0.6% to 5.5%. The fuel savings from
lowering of the viscosity of a gear oil in the same fashion could be
in the order of 0.2–2.5% [41,57]. As a potential alternative to
mineral oils and synthetic-based hydrocarbon oils, polyalkylene
glycol (PAG)-based lubricants are being considered for engine
lubrication purposes. These lubricants have been around for a
long time but have rarely been used in the past. Due to their lower
viscosity and better environmental compatibility, they are now
gaining some attention, and several exploratory studies have been
in progress for many years [125]. A significant portion of the
research efforts on lubricants is devoted to base oil formulations
that provide much lower viscosity, yet good load-carrying capacity
in engines.
Attaining friction coefficients below 0.04 has proven difficult
when using the traditional oils and additives under boundary-
lubricated sliding conditions. By “traditional,” we mean well-
established mineral or synthetic oils with typical anti-friction,
anti-wear, and extreme pressure (EP) additives containing sulfur,
chlorine, and phosphorous.
New and more interesting research has demonstrated that, with
different types of additives in combination with low-friction coat-
ings like DLC, it is possible to achieve much lower friction. The
addition of friction modifier additives like glycerol mono-oleate
(GMO) to a polyalphaolefin (PAO) oil gave a friction coefficient of
0.05 in sliding contact with tetrahedral amorphous carbon, ta-C.
However, the same material combination had a coefficient of
friction of 0.005 when lubricated by pure glycerol [126–128]. This
friction coefficient is about one-tenth of whatcurrently can be
achieved with the best lubricating oils. Under conditions of hydro-
dynamic lubrication, recent research has shown that similar levels
of friction reductions are feasible by the use of organic friction
modifiers [129] or by liquid crystal mesogenic fluids [130]. Likewise,
using a series of novel additives, Li et al. [131] report coefficients of
friction as low as 0.001 under liquid-lubricated sliding surfaces.
Another emerging research topic within the area of lubricants
in recent years has been the possible use of ionic liquids and
nanomaterials as anti-friction and anti-wear additives in lubrica-
tion. Ionic liquids have existed for a long time and have been in use
in many other fields. These liquids are highly polar, mainly because
they consist of many types of cations and anions, in contrast to the
relatively benign or inert hydrocarbon molecules in conventional
oils. The ionic liquids represent a very large family of fluids and
possess high thermal stability, non-flammability, and high flex-
ibility in molecular design; they are also economically beneficial
and environmentally friendly [132,133]. The viscosity of ionic
liquids can be adjusted over a wide range from 50 to 1500 cP at
23 1C. In lubricated sliding experiments, they have been shown to
offer coefficients of friction of about 0.1 for hydrocarbon-based
oils. Some ammonium-based ionic liquids were shown to exhibit
friction coefficients down to 0.06. More systematic studies
reported 20–35% reductions in friction, as compared to conven-
tional engine oils in all lubrication regimes, and a 45–55% reduc-
tion in wear. In a piston ring on flat test, the coefficient of friction
was reduced by 55% by the use of ionic liquids [134–136].
In recent years, researchers have placed a strong emphasis on
potential uses of certain nanomaterials as anti-friction and wear
additives. Most of these are carbon-based and include onion-like
carbons, nano-diamonds, carbon nano-tubes, graphene, and gra-
phite. Some inorganic fullerenes of transition metal dichalcogen-
ides, such as MoS2 and WS2, as well as metallic, polymeric, and
boron-based nanoparticles have also been considered [137]. Sys-
tematic laboratory-scale studies have shown significant reductions
in friction and wear of sliding surfaces when nano-particulate
lubrication additives are present in lubricating oils, but the poor
shelf-life and relatively high cost of such nanomaterials appear to
hinder their uses in engine lubricants. When and if these short-
comings are overcome, it is feasible that some of the nanomater-
ials mentioned can be used to reduce friction and thus improve
fuel efficiency of future engines.
In an attempt to mimic lubrication by living organisms,
biomimetic approaches have also been explored in recent years.
As is known, in some human or animal joints, friction coefficients
as low as 0.001 are feasible; hence, the lowest friction values so far
are still provided by nature. The biomimetic approaches are
mainly based on water lubrication and lubricants that contain
polymeric or protein additives such as brushes of charged poly-
mers (polyelectrolytes), porcine gastric mucin, and glycoprotein
mucin. With such bio-based lubricants, the coefficients of friction
can be as low as 0.0006 [138–140]. Unfortunately, the applicability
of such approaches for engines is not yet clear, but they may in the
distant future offer some possibility to mimic lubricant molecules
that are equally effective in mechanical systems.
7.4. Tires
During the last five decades, road vehicle tires have been
subjected to advanced product development, of which, however,
little information is available in the open literature. There is
significant opportunity to reduce fuel consumption of current and
future vehicles through the development of new tire materials,
better tread designs, weight reduction, optimized pressure mon-
itoring, and new maintenance technologies. All of these innova-
tions can be quickly adopted by a large percentage of the existing
trucks and buses and, thus, lead to significant energy savings
worldwide. Part of the new technology development work has
been focused on reduction of the rolling resistance since a 10%
reduction in rolling resistance corresponds to a 2% reduction in the
energy demand, or a 2% reduction in the fuel consumption [44,141].
To be commercially reasonable, the tires need to have a good
balance among low rolling resistance, safety, and low wear [50].
Recently, means of reducing the rolling resistance for heavy
road vehicles have involved use of the following [35,50,51]:
� Tires with a design that leads to low rolling resistance.
� Super-single, wide-base tires instead of double wheels with
conventional tires.
� Correct tire pressure combined with equipment that continu-
ously maintains the pressure.
We estimated in Section 5 that the average global heavy duty
vehicle has a tire rolling resistance corresponding to a coefficient
of friction of 0.01; in new vehicles it would be 0.006. Significant
improvements in tire design have been achieved during the past
few decades to achieve rolling resistance to as low as a coefficient
of friction of 0.004 [52,142]. Misalignment between the tires on
the respective axles of a truck or bus may increase the level to
approximately 0.0045. These values are slightly higher than the
lowest coefficients of rolling friction for truck tires that have been
suggested by modeling calculations [77].
For the last two decades, and at increasing rate over the years,
the rolling resistance of truck and bus tires has been suppressed by
adding silica (silicon dioxide, SiO2) particles into the rubber
material of the thread surface. The addition of silica instead of
the traditional carbon black particles into the rubber has reduced
the rolling resistance by about 20% [50]. The reduction in the
rolling resistance is the consequence of an increase in the stiffness
of the rubber material from the increase in the proportion of the
silica filler, from increased rubber–filler interaction, and from an
increase in the crosslink density in the rubber elastomer [143].
K. Holmberg et al. / Tribology International 78 (2014) 94–114106
New computational finite element analyses and molecular-
dynamics-based nano-architecture design of tire compounds has
been reported to reduce the rolling resistance by 20% [150].
To prolong the operational life-time of tires for heavy vehicles,
they can be re-threaded after being worn to a certain thread
depth. The re-threading is a gain in terms of tire renewal cost
savings, while a drawback is that the rolling resistance is increased
by the deepening of the threads [52]. Consequently, for the
minimization of the rolling resistance, sufficient thread depth
must be maintained.
Wide-base single truck tires, which are wider and have a lower
profile than traditional tires, are a significant step on the route
toward low rolling resistance [10,14,52]. For the wide-base tires, a
typical range for the coefficient of rolling friction is 0.004 to 0.005,
while for traditional installations with dual tires on the non-
steering axles of trucks and buses, it is 0.005 to 0.008, or
significantly higher on an average [35,52,74]. Hence, without any
other modifications of a heavy vehicle, replacing the traditional
dual-tire installations with wide-base single tires leads to energy
loss reductions on the order of several percent. A partial benefit of
the change into single tires is that the conflicts between double-
mounted tires with slightly different dynamic diameters disap-
pear. A drawback of the single tires, and the higher tire pressure
associated with them, is that there may be a higher probability for
road damage [52].
The tire pressure has a considerable effect on the rolling
resistance. For truck tires a 20% reduction in the pressure results in
an increase of 5–8% in the rolling resistance anda 2–3% increase in
the energy consumption. In practice, maintenance of sufficient tire
pressure provides significant means for energy savings on the global
level [50,52]. As a practical tool, the monitoring of the inflation
pressure in the tires of heavy vehicles provides important means of
suppression of the rolling resistance. Several monitoring systems are
already available, and their adoption has been encouraged [141,144].
Furthermore, the use of nitrogen gas instead of air in tires gives less
reduction in the tire pressure due to leakage over time [52].
7.5. Powertrain components and design
The performance of the powertrains in heavy duty vehicles is
already now optimized for the maximum power output that is
needed for normal operation conditions. Because, there is currently
not much margin for engine or transmission downsizing, any clear
benefits in terms of reduced oil churning power losses due to such
size reductions are not likely. The following addresses some more
likely routes for fuel economy improvements of the powertrains.
Recent development in diesel engine technology for heavy duty
vehicles has been strongly driven by the increasingly tightened
regulations for the exhaust emissions. Experimental work on low-
friction coatings, as described in Section 7.1, has shown the
friction-reduction potential when applied to engine components.
Studies on surface engineering of piston rings by laser-texturing,
as described in Section 7.2, has indicated benefits in terms of
frictional power loss reduction in heavy-duty diesel engines. The
use of engine oils of lower viscosity and oils with improved
additives for reduced boundary friction, as presented in Section
7.3, has proven a most powerful tool for reducing frictional power
loss in heavy duty diesel engines. Work by Green et al. [31] shows
a reduction in fuel consumption of a single-unit truck of 1.3–2.1%,
depending on which driving cycles are applied, when changing
from “mainstream” lubricants in the engine, gearbox, and rear axle
to lower viscosity oils.
The tribological power losses in gearboxes can be divided into
load-dependent and load-independent gear losses, load-dependent
and load-independent bearing losses, and speed-dependent seal
losses, as well as pump and torque converter losses in the case of
automatic gearboxes [66]. Therefore, for friction reduction in
gearboxes, the following four features should be optimized:
� The gearing losses consist of torque-dependent friction losses
and torque-independent churning and squeezing losses, which
are related to splash lubrication. Neunheimer et al. [66] indicates
efficiencies of 99.0–99.8% for each spur gear pair and 90–97% for
complete manual gearboxes. Churning and splash losses in the
gear pairs can be reduced by the use of lower viscosity lubricants
and smaller oil volumes in the gearboxes. If a re-design is
possible, a reduction of the number of engaged gear pairs in
the gearbox should lead to lower power losses. The gear contacts
of the driveline, furthermore, are a prime candidate for the
application of low-friction coatings and other novel surface
engineering methods aimed at reducing the sliding friction forces
in the gear contacts (see the examples in Sections 7.1 and 7.2).
� Rolling bearings contribute to load-dependent frictional losses
and viscous losses that are related to the viscosity and amount
of the lubricant in the bearing. The change to an optimized
amount of a lubricant with a lower, albeit sufficient, viscosity
can reduce the frictional loss of a bearing in a powertrain.
Furthermore, rolling bearings with lower frictional losses can
make use of improved internal geometry, surface roughness,
and coated rolling elements. Use of new bearing technology in
the transmissions of heavy vehicles has the potential for
friction reduction [145]. For instance, Bandi [146] studied the
friction losses of tapered roller bearings in tests that simulated
the drivetrains of heavy vehicles, and the results indicated
friction torque reductions of approximately 20% when changing
from baseline bearings to re-designed ones with coated rollers.
� Another potential source of friction reduction available for
commercial vehicle transmissions is use of low-friction shaft
seals, in which the elastomeric sealing lips are pressed against
the shaft by the shape and properties of the lip material instead
of the traditional garter springs made from steel, or rather stiff,
spiral-grooved lips made from polytetrafluoroethylene (PTFE)
polymer. A reduction of more than 50% in the friction losses, by
changing the seal type to one with spiral-grooved elastomer
lips and a lowered radial force, has been reported [84].
� The viscous pumping losses in the automatic gearboxes are an
issue of fluid dynamics rather than tribology. If automatic gear
changewithout interruptions in the torque transmission is desired,
and design changes can be allowed, then the power losses can be
diminished: for instance, by replacing the low-efficiency torque
converter by a dual clutch for the drivetrain velocity accommoda-
tion between the engine and the gearbox [66,147].
The friction losses in the final drive and the differential gear of
the driving axle or axles comprise losses in gears, bearings, and seals
similar to those in the gearboxes. The beneficial effect of coated
rollers on the friction loss of rolling bearings for final drives has been
discussed by Bandi [146]. The final drive is, in most cases, a hypoid
gear characterized by a combination of rolling friction and sliding
friction in the gear contacts. According to work by Winter and Wech
[69], the efficiency of a hypoid gear is higher for smaller pinion shaft
off-sets, the absolute effect being dependent on the gear design and
the operating conditions. The amount of sliding friction can be
reduced by a hypoid design with a smaller off-set between the
centerlines of the pinion shaft and the crown gear shaft, and by the
use of a lubricant with additives that reduce the coefficient of sliding
friction. A drawback of reducing the off-set is that the torque
transmission capacity of the final gear will be reduced.
The friction losses in the wheel bearings and their seals, for a
given speed and total weight of the vehicle or semi-trailer truck
combination, can be reduced by the use of low-friction bearings
with improved geometry and surface finish.
K. Holmberg et al. / Tribology International 78 (2014) 94–114 107
8. Discussion
8.1. Energy consumption in trucks and buses
The present calculations show that as much as 33% of the total
annual energy use (19.4 EJ) for trucks and buses globally goes to
overcome friction. The direct frictional losses, excluding the
inertia-related brake friction, are 26% of the total energy use (see
Fig. 3). We conclude that in the short term 14% of the energy used
could be saved by implementing new available technology and
technology under development.
The calculations show that the friction losses are the highest in
city buses (42%) compared with the other truck and bus categories
(see Fig. 8). City buses are subjected to considerably lower air drag
because of low speed in urban traffic but much higher braking
friction and transmission friction due to multiple stop-start situa-
tions. Air drag is high especially for semi-trailers and coaches that
drive on highways, up to 18 to 20% of the energy use. The rolling
resistance is high for semi-trailer trucks, as high as 18%, because of
the heavy weight. The exhaust and cooling losses, which average
50%, are much lower for heavy duty engines compared to average
passenger cars due to the higher thermal efficiency of their
turbocharged diesel engines.
The data that the present calculations are based on are, in some
respects, more accurate compared to the data we used in our
previous study on passenger cars [3]. More detailed statistics are
now available on operatingconditions, technical performance,
number of vehicles in different categories, etc., as the total truck
and bus fleet is smaller and owned by a more limited group of
operators that are more organized than is the case for cars. This
data is well structured, especially in the U.S. and Europe, and form
the basis for our analysis. The data on friction in specific car and
engine subsystems, however, was more detailed for passenger
cars, and thus, this information was occasionally used and
adjusted to fit heavy-duty diesel engines. In cases when the data
available in the open literature was lacking or not so convincing,
we used information from operators or from the authors’ own
laboratory measurements of truck and bus performance.
Due to the smaller number of heavy duty vehicles in the global
fleet, the smaller number of vehicle owners, and their better
organization, compared to owners of passenger cars, we assumed
that changes to lower friction losses are easier to implement and
will thus have a more rapid effect. To be specific, we estimate a
short-term penetration time for the global fleet of heavy duty
vehicles to be 4–8 years, compared with the 5–10 years for
passenger cars we used in our previous study.
8.2. Energy consumption of road transport globally
To present the whole picture of energy consumption in road
transportation worldwide, Table 7 summarizes the key data from
this study along with data from our previous study on energy
consumption due to friction in passenger cars [3]. It reveals that
the more than 1000 million road vehicles in our planet use
annually about 22 EJ fuel energy to overcome friction. In the short
term (4 to 8 years), on average, 14% of the energy used could be
reduced by efficiently implementing new technological solutions.
On an annual global basis, this energy savings would result in
economical savings of 475,000 million euros and reduced CO2
emissions of 856 million tonnes.
The data shown for passenger cars in Table 7 is based on what
we previously reported for the year 2009 [3], but it has been
corrected and updated to correlate with the year 2011. The data for
the energy use and savings by friction reduction in passenger cars
contains lower values in our first study compared with the data in
Table 7. The difference is partially because in the first study we
used the global numbers for the energy production (TPSE¼total
prime energy supply) while in the present study the global
numbers for energy consumption (TFC¼total final consumption)
were used. Additionally, in the first study, there was vagueness in
the definition of passenger car when estimating the total number
of vehicles worldwide. In this study, we have clearly separated
passenger cars from other light vehicles such as vans, pick-ups,
and sport utility vehicles. With these corrections we believe that
the data in Table 7 more accurately represents the real situation
worldwide for passenger cars and other light vehicles.
8.3. Change to electrical heavy duty vehicles
Certain road vehicle segments are starting to move toward
electrification. Pure electric vehicles have the advantages of zero
local pollution and low noise. They also produce low overall
greenhouse gas emissions, especially if electricity for charging is
drawn from nuclear or renewable energy sources such as hydro-
power, solar, or wind. Electric vehicles are often claimed to deliver
superior efficiency compared to internal combustion engine vehi-
cles. This claim is certainly true when looking at end-use energy
efficiency only. However, in an overall efficiency assessment, one
has also to consider the efficiency of the utility power generation
in the case where the primary energy is coal, gas, or oil.
Heat and friction losses in an electric power line are signifi-
cantly lower compared to conventional power lines. With opti-
mum load, larger electric motors easily reach a maximum
Table 7
Key figures related to the annual friction losses and energy use for global average road vehicles in the year 2011.
Calculated annually Single-unit
trucks
Trucks and
trailers
City
buses
Coaches Passenger
carsa
Other light
vehiclesb
Road transport
total
Number of units worldwide, (millions) 29.2 7.3 2.3 1.3 700 300 1040
Energy use worldwide, (EJ) 5.2 10.5 2.5 1.2 34 14 67.5
Part of global energy consumption, (%) 1.4 2.8 0.7 0.3 9.1 3.7 18
Energy use for friction, (EJ) 1.6 3.3 1.0 0.3 11.2 4.6 22
Energy use for friction per vehicle, (GJ) 54 446 454 253 12 20 –
Short-term saving period, (years) 4–8 4–8 4–8 4–8 5–10 5–10 5–9
Short-term savings/reduction, (%) 12 15 11.5 15 18.5 18.5 17.5
Energy savings from reduced friction in short term, (EJ) 0.62 1.6 0.29 0.18 6.3 2.6 11.6
Cost savings from reduced friction in short term,
(1000�million €)
24.4 61.7 11.3 7.1 260 110 475
Fuel savings from reduced friction in short term,
(1000�million litres)
17.5 44.1 8.1 5 178 73 326
CO2 savings from reduced friction in short term, (million
tonnes)
45.9 116.0 21.2 13.3 468 192 856
a Data from Holmberg et al. [3] representing the year 2009 but corrected and updated to the situation in the year 2011.
b Vans, pick-ups, and sport utility vehicles.
K. Holmberg et al. / Tribology International 78 (2014) 94–114108
efficiency of more than 90% [148]. Fig. 10 shows that the motor
efficiency for two electric four-pole motors (IE1 and IE3) is above
85% at a motor nominal output of more than 10 kW and three load
levels (25, 50, and 100%), with only one exception (IE3 at 25%
load). Efficiency values in the range of 80 to 95% for electric motors
can be compared with a typical maximum efficiency of 43% for a
diesel engine in a heavy duty vehicle.
Frictional losses only constitute a minor part of the losses in an
electric motor (see Table 8). Friction and internal air drag losses
together account for less than 10% of the total losses, so improve-
ments in bearing technology cannot deliver significant efficiency
improvements.
One feature of the electric motor is full torque starting from
zero rotational speed. This means that the driveline for an electric
vehicle can, in most cases, be realized without a traditional
gearbox, a factor that reduces frictional losses. Depending on the
design, an electric vehicle may still need a reduction gearbox, an
angle transmission, or a final drive. In the case of hub-mounted
motors, the driveline can be realized completely without gears.
In an electric vehicle, in addition to the relatively small losses in
the motor itself, losses occur in the charging and discharging of
the batteries, as well as in the power electronics and battery
management system. Fig. 11 presents a breakdown of the energy
use of an electric bus for public transportation. Energy consump-
tion of the vehicle in the Braunschweig bus cycle is 4.3 MJ/km
(electrical energy supplied from the grid), compared with 15.1 MJ/
km (chemical energy as fuel) for a conventional diesel bus [33,37].
The losses in the electric power line, the including final drive,
are 50% of total energy input. Some 30% is lost in the battery-
related systems, and some 20% in the power electronics, the
electric motor, and the mechanical parts of the power line. An
electrical vehicle without a gearbox can be equipped with a
longitudinally mounted electric motor and a conventional rear
axle with a final drive ratio of about five. In this case, the auxiliary
systems consume 10% of total energy input. Only 3% of the energy
is lost in the mechanical brakes, as the greater part of braking
recovers energy in an electric regenerative mode.
In sum, the friction losses in an electric vehicle are much lower
than in a conventional vehicle with internal combustion engine.
The electric motor itself is highly efficient, with very low mechan-
ical losses. An electric vehicle can be built with gearless drive,
practically eliminating transmission losses.
Battery electric vehiclesare best suited for urban service. In the
case of commercial vehicles, the most obvious applications are city
buses and delivery trucks. The interest in electric city buses is
currently very high. Coaches and heavy long-haul trucks are not
suitable for electrification, unless systems for continuous power
supply (e.g., catenary or inductive) are developed.
An interim step toward electrification is hybridization. In the
case of city buses and some delivery truck applications, hybridiza-
tion typically saves 25 to 30% fuel [33,35,149]. The savings are
highly dependent on the duty cycle, with little to gain in constant
high-speed operation in a flat topography. The factors contributing
to fuel savings include regenerative braking, engine downsizing,
and optimized utilization on the internal combustion engine.
The relative breakdown of the energy use and losses of a
conventional city bus (Fig. 8) and that of an electric bus (Fig. 11)
differ significantly. As expected, the rolling friction work in
absolute terms is roughly the same for both bus designs. The
engine or motor friction practically disappears, and the transmis-
sion friction as well as the braking friction is considerably reduced.
In the electric motor there is no exhaust, and the cooling need is
significantly reduced. Based on these factors, we can conclude that
the frictional losses, not taking the rolling resistance into account,
are about one-third of those in diesel-engine-powered vehicles,
and with rolling friction included, they are only about 40%. On a
general level we estimate that for all heavy duty vehicles, when
rolling friction is included, the total energy to overcome friction in
electric vehicles is less than one-half of that in diesel vehicles.
9. Conclusions
We reached the following conclusions from our analyses:
� In heavy duty vehicles, 33% of the fuel energy is used to
overcome friction in the engine, transmission, tires, auxiliary
equipment, and braking. The parasitic frictional losses, with
braking friction excluded, are 26% of the fuel energy. In total,
34% of the fuel energy is used to move the vehicle.
� Worldwide, 180,000 million liters of fuel was used in 2012 to
overcome friction in heavy duty vehicles. This equals 6.5 million TJ/
a. Reductions in frictional losses provide additional advantages in
fuel economy. A reduction in friction has a 2.5 times benefit for fuel
economy, as exhaust and cooling losses are reduced as well.
Fig. 10. Efficiency for two electric motors as function of the nominal power output
and three load levels [148].
Table 8
Typical losses in electric motors [148].
Losses
(%)
Factors affecting the losses
Stator losses 30–50 Stator conductor size and material
Rotor losses 20–25 Rotor conductor size and material
Core losses 20–25 Type and quality of magnetic material
Additional load losses 5–15 Primary manufacturing and design
methods
Friction and internal air
drag
5–10 Selection/design of fan and bearings
Fig. 11. Breakdown of the energy consumption of an electric bus [37].
K. Holmberg et al. / Tribology International 78 (2014) 94–114 109
� Globally, one single-unit truck uses on average 1500 l of diesel
fuel per year to overcome friction losses; a truck and trailer
combination, 12,500 l; a city bus, 12,700 l; and a coach, 7100 l.
� By taking advantage of new technology for friction reduction in
heavy duty vehicles, friction losses could be reduced by 14% in
the short term (4 to 8 years) and 37% in the long term (8 to 12
years). In the short term, this would annually equal worldwide
savings of 105,000 million euros, 75,000 million liters of diesel
fuel, and a CO2 emission reduction of 200 million tonnes. In the
long term the annual benefit would be 280,000 million euros,
200,000 million liters of fuel, and a CO2 emission reduction of
530 million tonnes.
� Hybridization and electrification are expected to penetrate
only certain niches of the heavy duty vehicle sector. In the
case of city buses and delivery trucks, hybridization can cut
fuel 25% to 30%, but there is little to gain in the case of coaches
and long-haul trucks. The downsizing of the internal combus-
tion engine and use of recuperative braking would reduce
friction losses.
� Electrification is best suited for city buses and delivery trucks.
The energy to overcome friction in electric vehicles is estimated
to be less than half of that of conventional diesel vehicles.
Acknowledgments
We want to acknowledge Kimmo Erkkilä, Micke Bergman, and
Petri Laine of VTT for their contributions in the form of detailed
discussions on vehicle measurements and Jarkko Metsäjoki of VTT
for assisting in the literature search.
This study has been carried out as part of the Finnish joint
industrial consortium strategic research action coordinated by
FIMECC Ltd. within the program on Breakthrough Materials
called DEMAPP in the Friction and Energy Project. We grate-
fully acknowledge the financial support of Tekes, the Finnish
Technology Agency, the participating companies, and VTT
Technical Research Centre of Finland. Additional support was
provided by the U.S. Department of Energy, Office of Science
and, Office of Energy Efficiency and Renewable Energy, under
Contract DE-AC02-06CH11357.
Appendix A
Table A1–A3.
Table A1
Annual fuel energy consumption breakdown for four types of average global heavy vehicle.
Energy losses Vehicle, Single-unit truck 2000 Semi-trailer truck 2000 City bus 2000 Coach 2000 Total
Average speed 60 km/h 80 km/h 20 km/h 80 km/h
Energy loss source Friction mechanism % GJ % GJ % GJ % GJ % GJ
Engine friction 8 14.4 6 86.4 10 108.0 6 55.2 7.3 264
– piston assembly HD 18 2.59 18 15.55 18 19.44 18 9.94 18 47.52
EHDS 17 2.45 17 14.69 17 18.36 17 9.38 17 44.88
ML 5 0.72 5 4.32 5 5.40 5 2.76 5 13.20
BL 5 0.72 5 4.32 5 5.40 5 2.76 5 13.20
– bearings HD 30 4.32 30 25.92 30 32.40 30 16.56 30 79.20
– valve train ML 15 2.16 15 12.96 15 16.20 15 8.28 15 39.60
– pumping & hydraulics VL 10 1.44 10 8.64 10 10.80 10 5.52 10 26.40
Subtotal 100 14.4 100 86.4 100 108.0 100 55.2 100 264
Transmission friction 4 7.2 4 50.4 8.5 91.8 4.0 36.8 5.1 186
VL 20 1.44 20 10.08 55 50.49 40 14.72 41 76.73
EHDSR 55 3.96 55 27.72 25 22.95 35 12.88 36 67.51
EHDR 20 1.44 20 10.08 15 13.77 20 7.36 18 32.65
ML 5 0.36 5 2.52 5 4.59 5 1.84 5 9.31
Subtotal 100 7.2 100 50.4 100 91.8 100 36.8 100 186
Auxiliary, friction loss EHDR 0.5 0.9 0.5 7.2 0.5 5.4 0.5 4.6 0.5 18
Tire rolling TR 8 14.4 18 259.2 8 86.4 13 119.6 13.2 480
Braking contact BC 9.5 17.1 3 43.2 15 162.0 4 36.8 7.2 259
Friction total 30.0 54.0 31.0 446.4 42.0 453.6 27.5 253.0 33.3 1207.0
Air drag 12.0 21.6 18.0 259.2 2.0 21.6 20.0 184.0 13.4 486.4
Auxiliary, losses 3.0 5.4 2.0 28.8 5.0 54.0 2.5 23.0 3.1 111.2
Exhaust 32.0 57.6 29.0 417.6 31.0 334.8 30.0 276.0 30.0 1086.0
Cooling 23.0 41.4 20.0 288.0 20.0 216.0 20.0 184.0 20.1 729.4
Other losses total 70.0 126.0 69.0 993.6 58.0 626.4 72.5 667.0 66.7 2413.0
Energy losses total 100.0 180.0 100.0 1440.0 100.0 1080.0 100.0 920.0 100.0 3620.0
K. Holmberg et al. / Tribology International 78 (2014) 94–114110
Table A2
Annual friction energy breakdown for tribological contact mechanisms in four heavy vehicle categories.
Lubrication and contact mechanism Code Single-unit truck 2000 Semi-trailer truck 2000 City bus 2000 Coach 2000 Total
60 km/h 80 km/h 20 km/h 80 km/h
% GJ % GJ % GJ % GJ % GJ
Tire rolling TR 26.7 14.4 58.1 259.2 19.0 86.4 47.3 119.6 39.7 479.6
Hydrodynamic lubrication HD 12.8 6.9 9.3 41.5 11.4 51.8 10.5 26.5 10.5 126.7
Mixed lubrication ML 6.0 3.2 4.4 19.8 5.8 26.2 5.1 12.9 5.1 62.1
EHD lubrication, sliding EHDS 4.5 2.5 3.3 14.7 4.0 18.4 3.7 9.4 3.7 44.9
EHD, sliding and rolling EHDSR 7.3 4.0 6.2 27.7 5.1 23.0 5.1 12.9 5.6 67.5
EHD, rolling EHDR 4.3 2.3 3.9 17.3 4.2 19.2 4.7 12.0 4.2 50.8
Boundary lubrication BL 1.3 0.71.0 4.3 1.2 5.4 1.1 2.8 1.1 13.2
Viscous losses VL 5.3 2.9 4.2 18.7 13.5 61.3 8.0 20.2 8.5 103.1
Braking contact BC 31.7 17.1 9.7 43.2 35.7 162.0 14.5 36.8 21.4 259.1
Total 100.0 54.0 100.0 446.4 100.0 453.6 100.0 253.0 100.0 1207.0
Table A3
Friction losses and potential friction reduction for different tribological contact mechanisms and types of vehicles.
Single-unit truck Semi-trailer truck City bus Coach Heavy duty vehicle
MJ/a MJ/a MJ/a MJ/a MJ/a
T2000 T2013 L2013 T2015 TT2000 TT2013 L2013 TT2015 CB2000 CB2013 L2013 CB2025 C2000 C2013 L2013 C2015 HV2000 HV2013 L2013 HV2025
TR 14,400 8640 4320 2880 259,200 155,520 77760 51,840 86,400 51,840 25,920 17,280 119,600 71,760 35,880 23,920 479,600 287,760 143,880 95,920
HD 6910 2760 550 280 41 470 16,590 3320 1660 51,840 20,740 4150 2070 26,500 10,600 2120 1060 126,700 50,680 10,140 5070
ML 3240 1620 320 160 19,800 9900 1980 990 26,190 13 100 2620 1310 12,880 6440 1290 640 62,100 31,050 6210 3110
EHDS 2450 1230 310 150 14,690 7350 1840 920 18,360 9180 2300 1150 9380 4690 1170 590 44,900 22,450 6610 2810
EHDSR 3960 1980 330 50 27,720 13,860 2310 370 22,950 11,480 1910 310 12,880 6440 1070 170 67,500 33,750 5630 900
EHDR 2340 470 230 120 17,280 3460 1730 870 19,170 3830 1920 960 11,960 2390 1200 600 50,800 10,160 5080 2540
BL 720 520 50 30 4320 3090 310 150 5400 3860 390 190 2760 1970 200 100 13,200 9430 940 470
VL 2880 1650 1230 410 18,720 10,700 8020 2670 61,290 35,020 26,270 8760 20,240 11,570 8670 2890 103,100 58,910 44190 14,730
BC 17,100 17,100 17,100 17,100 43,200 43,200 43,200 43,200 162,000 162,000 162,000 162,000 36,800 36,800 36,800 36,800 259,100 259,100 259,100 259,100
Total 54,000 35,970 24,440 21,180 446,400 263,670 140470 102,670 453,600 311,050 227,480 194,030 253,000 152,660 88,400 66,770 1,207,000 763,290 481,780 384,650
Friction reduction 33% 55% 61% 41% 69% 77% 31% 50% 57% 40% 65% 74% 37% 60% 68%
Reduction in fuel Energya 24% 41% 45% 30% 51% 57% 23% 37% 42% 30% 48% 55% 27% 44% 50%
a Considering that reduced friction will result in reduced cooling and exhaust at same ratio¼410% reduction in friction results in 7.4% reduction in fuel consumption.
K
.H
olm
berg
et
al./
Tribology
International
78
(2014)
94
–114
111
Appendix B. Conversion factors
Energy conversions
1 kWs ¼1 kJ
1 kWh ¼3.6 MJ
1 Mtoe ¼41 868 TJ
1 Btu ¼1054 J
Diesel fuel conversions
1 l ¼0.832 kg
1 MJ ¼0.028 l
1 l ¼35.9 MJ
1 kg ¼43.1 MJ
1 kg ¼43.16 kg CO2 emission
1 l ¼42.63 kg CO2 emission
1 l ¼1.4 euro on an average in Europe, Nov 2013
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	Global energy consumption due to friction in trucks and buses
	Introduction
	Methodology
	Analyses of global heavy duty vehicles
	Statistics on the global heavy duty vehicles and their energy consumption
	Four global average heavy duty vehicles
	Operating profiles
	Friction and energy losses in trucks and buses
	Global average energy losses in heavy road vehicles
	Energy distribution in trucks and buses
	Energy losses from engine friction
	Energy losses from transmission friction
	Energy losses from air drag
	Energy losses from tire rolling
	Energy losses from auxiliary equipment
	Energy losses from road inclination, acceleration, and braking
	Energy loss during idling
	Energy consumption in trucks and buses
	Tribocontact friction losses today and in the future
	Potential savings
	Means of reducing friction and energy use
	Low friction coatings for engine components, gears, and bearings
	Surface texturing of components in engines, gear boxes, and bearings
	Lubricants
	Tires
	Powertrain components and design
	Discussion
	Energy consumption in trucks and buses
	Energy consumption of road transport globally
	Change to electrical heavy duty vehicles
	Conclusions
	Acknowledgments
	Appendix A
	Conversion factors
	References