7  Car

Adapted from (Cullen, Allwood, and Borgstein 2011)

In a car, the engine/motor and drive system convert electrical or chemical energy into rotational energy driving the wheels. The car passive system then provides the service of transporting people and goods.

7.1 Characterisation

The thrust force provided by the car’s drive train is resisted by mechanical rolling resistance and aerodynamic drag. In addition, for practical driving, additional thrust is required to accelerate the car.

7.2 Key issues that affect efficiency

  • Lower rolling resistance in the tyres leads to reduced mechanical drag. Rolling resistance is mostly due to dissipation of energy in the tyre material as it deforms, which is primarily affected by the vehicle mass.

  • The aerodynamic drag on the car is determined by the car velocity, its projected frontal area and the drag coefficient based on its shape.

  • Lower vehicle mass means less energy must be used to drive up gradients, and to accelerate the car. The energy used for acceleration dominates during urban driving.

7.3 Efficiency limits

Cullen, Allwood, and Borgstein (2011) estimated the current efficiency and efficiency limits for European average cars are:

Current 530 kJ/km
Practical limit 50 kJ/km

This estimate considers:

  • Large reduction in mass from 1300 kg to 300 kg (including passengers)
  • Rolling resistance coefficient of friction reduced to match 45-75R16 radial ply tubeless Michelin tyre
  • Reduced drag coefficient, and some reduction in frontal area of the car.

This does not consider:

  • energy recovery in the braking system
  • turning off the engine during idling
  • reducing energy use by accessories (e.g. air conditioning)
  • reducing maximum speed and acceleration
  • putting more passengers in the same car

Since the service delivered is measured in “passenger-kilometers”, doubling the occupancy of the car would halve the utilisation intensity of the car measured in “kg per passenger kilometer”.

See section S.4.1 of the Supplementary Information from Cullen, Allwood, and Borgstein (2011) for more details.

7.4 Final Energy used for cars

This table shows the quantity of final energy \(F\) used in the UK in one year, for all road transport, both cars and trucks:

      Energy [PJ/year]
Fuel Device EndUse  
Electricity Electric Motor Mechanical 13.3
Liquid fuel Diesel Engine Mechanical 977.9
Spark Ignition Engine Mechanical 572.6

You will need to use this figure to estimate how much of the energy in the table above (which is for all road transport) is relevant to cars. The fractions for each fuel type are [ref ECUK, 2013 values]:

  • Petrol: 97% is used for cars
  • Diesel: 45% is used for cars
  • Electricity: assume 100% is used for cars