Improving Energy Efficiency
- Improving vehicle efficiency means less fuel is consumed during a set journey
- The advantages of this are:
- The journey costs less
- Fewer greenhouse gases are emitted into the environment
- The efficiency of a vehicle can be improved in the following ways:
- Reducing air resistance to limit aerodynamic losses
- Reducing friction between the tyres and the road limits rolling resistance
- Reducing fuel consumption limits idling losses from the engine
- Reducing the fuel used during acceleration limits inertial loses
Reducing Aerodynamic Losses
- Air resistance causes a frictional force between the moving object and the air that opposes its motion
- This results in unwanted thermal energy transfers to the object and the surroundings
- Air resistance can be reduced by streamlining the shapes of moving objects
- For example, a racing car is designed with a low profile and small surface area at the front to reduce air resistance
The Air Resistance of a Truck vs A Racing Car
A racing car has a lower profile with less surface area facing forward meaning the air resistance it creates is much less than a truck with a large forward-facing surface area
- A vehicle that is lower to the ground with more enclosed wheel arches also has improved aerodynamics, which reduces the air resistance it creates
Reducing Rolling Losses
- Friction occurs between two surfaces
- When one or both surfaces move then friction acts opposing the surface's direction of motion
- This results in unwanted thermal energy transfers to the objects and the surroundings
- Tyre friction can be reduced by correctly inflating the tyres to the correct pressure
- This is monitored by tyre pressure sensors warning the driver when the pressure is not correct
- Tyres are designed to ensure they grip the road safely but also need to minimise the rolling resistance between the tyre and the ground
- The rolling resistance is the friction created between the tyre and the road as the tyres rotate
A Fully Inflated vs a Partially Inflated Tyre
A fully inflated tyre uses less fuel because less force is needed to move it along the road compared to the partially inflated tyre
Reducing Idling Losses
- When a car is stationary and the engine is still running (e.g. when waiting in traffic at a red traffic light or a junction), fuel is consumed
- This is called idling
- Some cars have automatic start-stop systems to turn the engine off completely to reduce the amount of fuel consumed when a car is stationary
Reducing Inertial Losses
- Inertia is the resistance of any object to a change in its state of motion or rest
- The inertia of an object is related to its mass:
- The greater the mass then the greater the inertia
- So the greater the resultant force required to change its motion
- Reducing the mass of a car reduces the amount of force needed for it to accelerate
- As a result, this reduces the amount of fuel consumed during the acceleration
- This is done using lighter materials in vehicle construction
- For example, carbon fibre instead of heavier metals for certain components
- Installing a regenerative braking system converts the kinetic energy from a car's motion into electricity for the brakes when the car needs to slow down
Worked example
Energy can be lost from cars in a variety of ways. These include losses with rolling resistance, inertia and idling.
State the design features manufacturers can use to reduce each of these energy losses.
Answer:
Rolling resistance losses can be reduced by:
- Installing a tyre pressure warning system
- OR
- Using materials in the tyre / narrower tyres that do not gain heat easily from the rotation of the wheel against the ground
Inertia losses can be reduced by:
- Constructing a car using lighter / lower mass materials
OR - Installing a regenerative braking system
Idling losses can be reduced by:
- Installing an automatic stop-start system
Examiner Tip
Make sure you can describe the four main energy losses from a vehicle motion, as well as how car manufacturers and drivers can reduce these. Make sure you read questions on this topic carefully. This worked example asks how car manufacturers can reduce these energy losses. So you will get no marks by saying how drivers can also reduce them.
