Combustion road vehicles
Cars, trucks and buses burning petrol, diesel, CNG or LPG.
Conversion and use · Transport systems · No. 111
- Conversion and use
- Typical efficiency: About 25-30% (petrol engine)
A petrol car wastes about three litres of every four as heat, yet road engines burn a fifth of world energy.
Chart: Illustration: Combustion road vehicles
Where it comes from
Chart: Map of where it comes from
Burning fuel in a sealed cylinder pushes a piston, and the crankshaft turns that push into rotation. The more the charge is squeezed first, the more heat becomes work, which is why diesels beat petrol engines.
Three things to know
- An ideal petrol engine at a compression ratio of 13 could reach 64%, but Toyota's best production engine peaks at 40%.
- In city driving, idling alone wastes about 6% of a car's fuel, which stop-start systems and hybrids remove.
- Electric trucks made up 9% of global truck sales in 2025, so most freight still runs on diesel.
The journey
- 1 Fuel tankPetrol, diesel, CNG or LPG on board
- 2 CombustionCompressed charge burns into hot gas
- 3 PistonsExpanding gas turns the crankshaft
- 4 DrivelineGearbox sends power to the wheels
- 5 ExhaustCatalysts clean gases; most heat escapes