FAA Powerplant Written Practice Test

When does ignition occur in a four-stroke cycle aircraft engine?

1. After the piston reaches BDC on the exhaust stroke

2. At the bottom dead center of the intake stroke

3. Before the piston reaches TDC on the compression stroke

4. After the piston reaches TDC on the power stroke

The correct answer is: Before the piston reaches TDC on the compression stroke

In a four-stroke cycle aircraft engine, ignition occurs just before the piston reaches the top dead center (TDC) during the compression stroke. This timing is crucial as it allows for the air-fuel mixture to be adequately compressed, which aids in efficient combustion. The ignition spark ignites the mixture just prior to the piston reaching TDC, creating an explosion that pushes the piston down during the power stroke. This ignition timing maximizes the pressure and temperature within the combustion chamber, leading to a more complete and effective combustion process. This is why the proper timing of the spark is so important for optimizing performance and efficiency in an aircraft engine.