FAA Powerplant Written Practice Test

During a 4-stroke cycle, what event occurs at about 60 degrees after BTC?

1. The exhaust valve opens

2. The intake valve opens

3. The intake valve closes

4. The power stroke begins

The correct answer is: The intake valve closes

In a 4-stroke engine, the cycle consists of four distinct events: intake, compression, power, and exhaust. The piston moves through these strokes to facilitate the engine's operation. At about 60 degrees after Bottom Dead Center (BDC) during the compression stroke, the intake valve is typically in a closed position. The reason why the intake valve closes at this point relates to the engine's operation cycle. After the intake stroke, when the piston is at BDC, the intake valve is closed, and the piston moves up towards Top Dead Center (TDC) to compress the air-fuel mixture. The closure of the intake valve at this time is crucial to ensure that the air-fuel mixture is adequately compressed before the ignition event occurs. Therefore, the correct event occurring at about 60 degrees after BDC is the closure of the intake valve, as this marks the end of the intake phase and the beginning of the compression phase in the 4-stroke cycle. The closing of the intake valve is essential for maintaining the engine's efficiency and proper functioning, as it prevents the escape of the mixture that will be compressed and ignited in the next stages of the cycle.