FAA Powerplant Written Practice Test

During which stroke does the piston experience the highest temperature in a four-stroke engine?

1. Intake stroke

2. Compression stroke

3. Power stroke

4. Exhaust stroke

The correct answer is: Power stroke

In a four-stroke engine, the piston experiences the highest temperature during the power stroke. This is because the power stroke is when the combustion of the air-fuel mixture occurs. After the mixture has been compressed during the compression stroke, it is ignited, typically by a spark in gasoline engines. This ignition leads to a rapid rise in temperature and pressure as the gases expand, driving the piston down and generating power. The heat generated during combustion significantly raises the temperature of the gases, which in turn raises the temperature of the piston and surrounding components. The power produced during this stroke is crucial for the engine's operation, as it is the moment where energy is converted into mechanical work. Consequently, this phase of the engine cycle is characterized by the highest thermal energy due to combustion, hence the highest temperature. In contrast, the intake stroke involves drawing in the air-fuel mixture, which is at a much lower temperature. The compression stroke raises the temperature of the mixture due to compression but not to the extreme levels reached during combustion. The exhaust stroke expels the burnt gases from the combustion process and also does not reach the high temperatures experienced during the power stroke.