Improving Energy Efficiency

Section 1 of 7Why efficiency matters

Useful and wasted energy

• Useful and wasted energy Only part of the chemical energy in a vehicle's fuel ends up as useful kinetic energy;
  • the rest is wasted, mostly as thermal energy transferred to the surroundings.
• Efficiency = useful energy output ÷ total energy input.

Benefits of improving it

• Benefits of improving it A more efficient vehicle uses less fuel for the same journey, so the journey costs less.
• Burning less fuel also releases fewer greenhouse gases and less pollution into the atmosphere.

Section 2 of 7The four main energy losses

Energy wasted by a vehicle

Aerodynamic losses

• Aerodynamic losses Air resistance (drag) opposes motion, so the engine must do work against it and the air and vehicle warm up.

Rolling losses

• Rolling losses Friction between the rotating tyres and the road, called rolling resistance, transfers energy as heat.

Idling losses

• Idling losses Fuel is burned while the vehicle is stationary but the engine is still running, for example at traffic lights.

Inertial losses

• Inertial losses Energy is needed to accelerate the mass of the vehicle and is wasted as heat when the brakes slow it down again.

Section 3 of 7Reducing aerodynamic losses

1Air resistance grows with frontal area

• Air resistance grows with frontal area A large flat front pushes a large volume of air aside, producing a big drag force.

2Streamline the shape

• Streamline the shape A low, smooth, rounded profile with a small front surface area lets air flow past more easily.

3Cover the gaps

• Cover the gaps Enclosed wheel arches and a lower ride height reduce turbulence underneath and around the car.

4Less drag, less fuel

• Less drag, less fuel A smaller resistive force means the engine does less work to keep the same speed, so less fuel is burned.

Section 4 of 7Reducing the other losses

Loss	What the manufacturer can do	What the driver can do
Rolling resistance	Tyre pressure sensors to warn of under-inflation; tyre materials and narrower tyres that heat up less while still gripping safely	Keep tyres inflated to the recommended pressure
Idling	Automatic stop-start system that switches the engine off when the car is stationary	Switch the engine off in long waits
Inertia	Lighter materials such as carbon fibre; regenerative braking that converts kinetic energy back to electrical energy	Remove unnecessary heavy loads and avoid harsh acceleration and braking
Air resistance	Streamlined body, low profile, enclosed wheel arches	Close windows and remove roof boxes at speed

Section 5 of 7Exam technique

Answer the question asked

• Answer the question asked If a question asks what manufacturers can do, describe design features such as stop-start systems.
  • Actions a driver could take will not earn marks.

Link feature to loss

• Link feature to loss Name the feature, name the loss it reduces and say why: for example, lighter materials lower the mass,
  • so less force is needed to accelerate and less fuel is used.

Section 6 of 7Key terms

Term	Meaning
Efficiency	The fraction of the total energy input that is transferred usefully, found as useful output ÷ total input.
Air resistance	The frictional force of the air that opposes the motion of an object moving through it.
Rolling resistance	The friction between a rotating tyre and the road surface.
Idling	Running a vehicle's engine while the vehicle is not moving.
Inertia	The tendency of an object to resist any change in its motion; it increases with mass.
Regenerative braking	A system that converts some kinetic energy into electrical energy when a vehicle slows down.

Section 7 of 7Exam technique

1

Describing what drivers can do when the question asks about design features chosen by manufacturers.

2

Saying energy is 'used up' or 'lost'. It is transferred to the surroundings, mainly as thermal energy, and is no longer useful.

3

Confusing inertia with weight. Inertia depends on mass and is about resisting changes in motion, not about the pull of gravity.

4

Stating that wider or softer tyres always save fuel. Reducing rolling resistance means less friction heating, but the tyre must still grip the road safely.