FAA Powerplant Written Practice Test

What is the relationship between the distributor and crankshaft speed in aircraft reciprocating engines?

1. The distributor turns at the same speed as the crankshaft

2. The distributor turns at twice the crankshaft speed

3. The distributor turns at half the crankshaft speed

4. The distributor turns at a quarter of the crankshaft speed

The correct answer is: The distributor turns at half the crankshaft speed

In aircraft reciprocating engines, the distributor's speed is closely linked to the crankshaft's speed due to the engine's design and function. The distributor is responsible for directing the high-voltage electrical current to each spark plug at the appropriate time to ensure efficient combustion. In a typical four-stroke engine, the crankshaft completes two revolutions for every one cycle of firing, which means that the camshaft, which often drives the distributor, rotates at half the speed of the crankshaft. Since the distributor is usually geared to the camshaft, which turns at half the crankshaft's speed, the distributor also rotates at half that speed. This relationship is crucial because it ensures that spark timing aligns with the opening and closing of intake and exhaust valves, which is fundamental for the engine's performance and efficiency. Understanding this connection is vital for anyone working with aircraft engines, as it informs maintenance, troubleshooting, and overall engine operation.