Engines

Petrol engines
With petrol engines, fuel enters the cylinders and is ignited by spark plugs. The fuel then burns very quickly and the engine runs at higher revs to produce more power than a similar sized diesel engine.

Diesel engine
With a diesel engine, pistons rapidly compress the air inside the cylinders creating heat. Diesel fuel is then injected into the cylinders and ignites spontaneously. Road diesel is designed to lubricate the injection system and burn cleanly. Even so, diesel engines produce more 'particulates' than petrol engines because the fuel is less refined.

Revs
Revs is short for 'revolutions per minute' (or rpm). When an engine's revs are high, the engine is turning quickly - when low, it is turning more slowly. A change of gear affects the revs of an engine, with the higher gears creating lower revs.

Torque
Torque is the 'turning force' an engine produces. The higher the torque, the more 'pulling power' is produced. Diesel engines generally deliver a higher torque at lower engine speeds when compared to petrol engines. (Torque and engine speed multiplied together give the engine's power.)

Power
For vehicles, power is the amount of mechanical energy delivered to propel the vehicle per second. Its units are either horsepower (hp) or kilowatts (kW).