FAA Powerplant Written Practice Test

In a four-stroke cycle aircraft engine, when does the ignition event occur during the compression stroke?

1. At bottom dead center (BDC)

2. After the piston passes TDC

3. Before the piston reaches TDC

4. Immediately at TDC

The correct answer is: Before the piston reaches TDC

In a four-stroke cycle aircraft engine, the ignition event occurs shortly before the piston reaches the top dead center (TDC) during the compression stroke. This timing is critical because it allows for optimal combustion of the fuel-air mixture, ensuring the maximum expansion of gases in the following power stroke. Igniting the mixture just before TDC initiates combustion at the right moment, allowing it to build pressure as the piston begins its downward motion. This timing also helps to prevent engine knock and improves efficiency. If the ignition were to occur at TDC or after, the engine would lose power and efficiency due to less effective combustion at the beginning of the power stroke. Understanding this timing is essential for maintaining engine performance and efficiency in four-stroke engines, where precise control over ignition timing can significantly affect overall engine operation.