General information on fuel consumption
Summary
Summary
This document summarises and explains the factors that affect a vehicle’s fuel consumption.
• Air resistance
• Power take-off
Air resistance
The air resistance is one of the most important factors that affect how much energy
is used to move a vehicle forwards.
The air resistance is formed by two factors:
• Friction, when the air follows a surface
• Pressure, when the air hits a surface
The size of the two factors are affected by the design of the cab and bodywork. As
regards high and wide vehicles, the pressure forces dominate, as the air hits a large
frontal area which must press away the air to reduce the pressure forces.
Bodywork can be designed in many ways to reduce the air resistance. Some examples:
•
•
•
•
The bodywork must have smooth sides and covering side skirts
The distance between cab and bodywork must be as small as possible
Protruding equipment must be avoided
Air deflectors on the cab roof and sides if the bodywork is larger than the cab
External accessories and components may affect the fuel consumption negatively.
Scania’s cabs and genuine accessories are tested in wind tunnels and are designed to
create minimal air resistance and minimal fuel consumption.
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General information on fuel consumption
Air resistance
Air resistance effect on fuel consumption
The air resistance factor (CD) varies between 0.5 and 1.0 for a complete vehicle
Air resistance factor (CD)
Fuel consumption (l/100 km)
1,0
40 - 45
35 - 40
0,9
30 - 35
0,8
25 - 30
20 - 25
0,7
15 - 20
10 - 15
0,6
5 - 10
5
15
25
35
45
55
65
75
85
95
105
115
Vehicle speed (km/h)
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375 688
0-5
0,5
General information on fuel consumption
Air resistance
Speed
The air resistance is quadratically dependent on the speed. This means that a doubling of the speed increases the air resistance by 4.
Height and width
The following factors increase the surface which creates pressure forces:
• Vehicle height
• Vehicle width
• Protruding objects such as roof rack
Example: By reducing the vehicle height by 0.4 m you can save approx. 2 litres of
fuel/100 km at a speed of 80 km/h.
Smooth surfaces for covers and loads
Smooth and fixed surfaces create less air resistance. If the transport needs require a
cover over the platform and trailer, the cover must be stretched and of high quality to
make the air resistance as low as possible.
Example: Stretched cover of high quality with smooth surfaces in combination with
smooth side skirts can save fuel in the order of 2 litres per 100 km.
Air deflectors
Roof air deflectors together with side air deflectors can in certain cases reduce the air
resistance by approx. 30%.
It is important that air deflectors are installed correctly.
• A correctly adjusted roof air deflector can reduce air resistance by 15%.
• A vertical error of 50 mm can increase the air resistance by 2%.
• It is better to set the roof air deflector a little too high than too low.
Roof-mounted equipment such as auxiliary lamps and roof signs can reduce the positive effect of the air deflector.
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General information on fuel consumption
Air resistance
Further information about roof air deflectors is available in the document Roof Air
Deflectors.
Side skirts
If a vehicle or trailer lacks side skirts, the air flow is disturbed by irregular surfaces
and protruding equipment.
Example: The opening between the fuel tank and the rear wheel mudguard.
Side skirts prevent side winds from affecting the air flow under the vehicle and trailer
negatively. It is important that also the trailer is provided with side skirts.
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General information on fuel consumption
Air resistance
Accessories that increase the air resistance
Much of the extra equipment fitted on the outside of the cab increases the air resistance. The illustration shows how much the fuel consumption can be affected by different accessories.
If there must be extra equipment, it is important that it is fitted correctly and in suitable locations. For example, the air resistance is less affected by auxiliary lights fitted
on the front grille panel compared with roof-mounted.
The X axis shows speed in km/h, the Y axis shows examples of fuel savings in
litres per 100 km.
Y
0,6
1. Light bar on roof
1
2. Roof sign
0,5
0,4
2
3. Roof-mounted air conditioning
3
4. Michelin man
4
6. Hub caps on driving axle
5. Horn on roof
0,3
0,2
7. Scania’s aerodynamic rear view mirrors compared to conventional mir-
rors
5
0,1
0
6
-0,1
X
-0,2
20
40
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60
80
100
120
375 702
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General information on fuel consumption
Air resistance
Air resistance and distance between the cab, bodywork and trailer
Within many areas of use the distance between the parts in a road train is controlled
by legal requirements and axle weight limitations. It is important that the distance is
as short as possible as a large distance will result in higher fuel consumption.
For tractors, Scania’s side air deflectors are optimised for a distance of 600 mm between the rear of the cab and the front of the trailer.
Example: If the distance can be reduced from 1.5 m to 0.6 m, the fuel consumption
can be reduced by up to 2.4 litres per 100 km.
Increased fuel consumption (l/100 km)
Distance (mm)
Y
3
1600
2,5
2
1200
1,5
1000
800
0,5
The air currents between the tractor and trailer are affected by the distance between the cab and trailer.
X
0
20
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30
40
40
60
70
80
90
100
110
375 725
375 736
1
Vehicle speed
(km/h)
General information on fuel consumption
Power take-off
Power take-off
Select power take-offs and hydraulic pumps so that the engine speed is between 800
and 1,000 rpm.
General rule:
• Select a power take-off with low rotational speed for pumps and other units that
are used for the bodywork while driving.
• Select a power take-off with high rotational speed for bodywork that is used while
the vehicle is stationary.
At Scania Truck Bodybuilder, under Tools and services, there is a calculator for selecting power take-offs and hydraulic pumps.
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