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Cars

Car engines come in different sizes (capacities) and power ratings. For example, a small family car may have a 1.2 litre engine while a sports car may have a 3 litre engine. In general, engines with larger capacities are more powerful.

More powerful engines in cars can work more quickly than less powerful ones. As a result they usually travel faster and cover the same distance in less time but also require more fuel. Increased fuel consumption costs more and has a bigger impact on the environment.

The fuel consumption and range of all vehicles also depends on factors such as whether the vehicle is:

• driven uphill a lot
• carrying large loads
• driven at high speed
• driven over rough road surfaces
• driven with underinflated tyres

Aerodynamic losses

The shape of a vehicle is very important. Vehicles can be streamlined to produce less air resistance when they move, which decreases their fuel consumption and increases their range.

• Sports cars are wedge-shaped. Smoother vehicle shapes have already reduced drag significantly, but further reductions of 20-30 % are possible.
• Lorries and caravans may be fitted with deflectors at the front.
• On the other hand, roof boxes on cars increase air resistance, as does driving with the windows open. Increased air resistance increases fuel consumption and decreases the range.

Frictional losses

As well as air friction due to the shape and size of the vehicle, fuel consumption is affected by the friction between the car tyres and the road surface, and the friction between the wheels and the axles of the car. This is often called the rolling resistance of the car.

In petrol-powered vehicles, over 62 % of the fuel's energy is lost in the internal combustion engine because it is very inefficient at converting the fuel's chemical energy to mechanical energy. Energy is lost to engine friction, pumping air in and out of the engine, and heat loss.

In urban driving, significant energy - about 17 % - is lost to idling at stoplights or in traffic. Technologies such as integrated starter/generator systems help reduce these losses by automatically turning the engine off when the vehicle comes to a stop, and restarting it instantaneously when the accelerator is pressed.

Rolling resistance

Rolling resistance is a measure of the force necessary to move the tyre forward and is directly proportional to the weight of the load supported by the tyre. A variety of new technologies can be used to reduce rolling resistance, including improved tyre tread and shoulder designs and materials used in the tyre belt and traction surfaces.

For passenger cars, a 5-7 % reduction in rolling resistance increases fuel efficiency by 1 %. However, these improvements must be balanced against traction, durability, and noise.