A diagram of a an internal combustion engine, where fuel is burned inside the engine.

• This is the second main type of heat engine.

The diagram shows the sequence of events that takes place in one cylinder of a four stroke engine.

• In the intake stroke (Figure A), a mixture of air and gasoline vapor enters the cylinder.
• Next, in the compression stroke (Figure B), the piston compresses the gas mixture. At the end of compression, the spark plug ignites the mixture, which heats the gas under pressure.
• In the power stroke (Figure C), the hot gas expands and drives the piston down.
• During the exhaust stroke (Figure D), gas leaves the cylinder, and the cycle repeats.