Fuel Consumption

The relative importance of aerodynamic design to reducing truck fuel consumption can be seen from an overview of the various factors involved in fuel consumption. It is important here to distinguish between different types of truck. The chart below shows the components that use energy in an articulated vehicle expressed as losses. As the chart shows, 15% of the fuel is used to overcome mechanical friction in the engine, gearbox and drive shaft. 45% of the fuel is used to overcome the rolling resistance. Drag is responsible for 40% of fuel consumption.

Friction versus drag

Vehicle velocity versus drag

Drag, indicated by the letter D, is the force that the air exerts on the vehicle, which the engine has to overcome. The power needed to overcome this drag equals D · V. As the drag itself increases with the square of the travelling speed V, the power needed is proportional to the cube of the speed.

The rolling resistance, indicated by Drol, is caused by the contact between the tyres and the road. The rolling resistance can be expressed as the product of the friction coefficient μ of the tyres and the normal force N, the actual weight of the vehicle. The correlation between rolling resistance and drag depends on the speed of the vehicle and the vehicle configuration.

The correlation between fuel consumption and travelling speed is similar to the curve of overall drag. At a constant speed of 50 kmph less than 40% of the engine power is used to overcome drag, as against 60% at a speed of 80 kmph. Crosswind, interference with other traffic and weather conditions all have an effect on the aerodynamic forces that ultimately need to be overcome: these constantly changing factors increase drag.

Friction, rolling resistance and drag on different road types

Friction, rolling resistance and drag on different road types

If a truck accelerates it uses more fuel. This factor depends above all on the driving style, but also on the traffic situation and the nature of the journey. A large change in speed per unit of time yields a high acceleration a, ultimately resulting in higher energy consumption.

Tip: Driving style has a major influence on fuel consumption, so drivers should be sent on an ‘economical driving’ course.