The internal combustion engine is at the heart of modern transport, providing the power for most cars and many motorcycles. The energy is provided when a spark ignites a mixture of air and petrol vapour, making the mixture expand and push a piston which in turn turns a set of wheels.
The process has four steps, shown below.
A. The induction stroke. The inlet valve is opened and the pistons moves down. A mixture of air and petrol vapour is sucked in to the chamber.
B. The compression stroke. The valves close and the piston moves up to compress the fuel air mixture to around 12% of its original volume. The mixture heats up.
C. Power stroke. An electric spark from the spark plug ignites the mixture which then expands rapidly and forces the piston down.
D. The exhaust stroke. The exhaust valve opens. The piston moves up to expel the burnt gases.
The cycle repeats. The internal combustion engine is about 25% efficient. This might not sound very much, but this is a much higher efficiency than used to be the case.