FAA Powerplant Written Practice Test

Why is it important for the spark to occur before the piston reaches TDC in a compression stroke?

1. To ensure maximum fuel efficiency

2. To achieve effective combustion

3. To cool the combustion chamber

4. To increase fuel flow

The correct answer is: To achieve effective combustion

The timing of the spark in relation to the piston reaching Top Dead Center (TDC) during the compression stroke is critical for achieving effective combustion within the engine cylinder. Igniting the air-fuel mixture before the piston reaches TDC allows the flame front to propagate throughout the combustion chamber while the piston is still moving upward. This ensures that the maximum pressure is achieved as the piston begins to move downward on the power stroke. If the spark were to occur too late—specifically after the piston reaches TDC—the combustion would not have sufficient time to develop and could result in incomplete combustion. This would lead to a reduction in the engine's efficiency and power output, as the energy release would not be timed to coincide with the downward movement of the piston. This meticulous timing is essential for optimizing the pressure and temperature conditions within the cylinder, facilitating the best possible combustion process, and maximizing the engine's overall performance.