Internal combustion engine
An engine that burns fuel inside its cylinders and uses the pressure to push pistons. Petrol and diesel engines are both of this kind.
A steam engine burns its fuel in a separate boiler; an internal combustion engine burns it in the very place where the power is produced. In 1876 Nicolaus Otto made a practical engine that repeats four strokes: draw in the mixture, compress it, burn it, expel it[1]. Compressing before burning raises both power and efficiency considerably.
Since it turns heat into work, it cannot escape the efficiency ceiling of Carnot's theorem. Today's petrol engines reach a thermal efficiency of about 40% at best; the rest is thrown away as heat. To cut exhaust emissions and carbon dioxide, hybrids and electric vehicles are replacing it, though ways of keeping it by burning hydrogen or synthetic fuel are also being explored.
Related terms
- Carnot's theoremThe law that any engine producing work from heat has an efficiency ceiling set only by the temperatures of its hot and cold sides.
- KnockingAbnormal combustion in an engine in which the fuel-air mixture explodes before the spark ignites it, with a metallic knocking sound.
- Diesel engineAn internal combustion engine that compresses air alone until it is hot, then injects fuel, which ignites by itself. It is more efficient than a spark-ignited petrol engine.