FAA Powerplant Written Practice Test

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

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

2. The distributor turns at 1/2 crankshaft speed

3. The distributor turns at twice the crankshaft speed

4. The distributor turns at one-fourth crankshaft speed

The correct answer is: The distributor turns at 1/2 crankshaft speed

In an aircraft reciprocating engine, the distributor is typically connected to the ignition system, and its primary function is to sequentially distribute the high voltage from the ignition coil to the correct spark plug at the right time during the engine's operation. The relationship between the distributor's speed and the crankshaft speed is crucial for proper engine timing and performance. The correct understanding is that the distributor turns at half the speed of the crankshaft. This relationship is a result of the engine's design, where the ignition system operates in a manner that requires the distributor to synchronize with the two revolutions of the crankshaft necessary to complete one full cycle of ignition for each cylinder. Since each cylinder in a reciprocating engine completes one power stroke for every two revolutions of the crankshaft, the ignition system (and consequently the distributor) only needs to rotate once for every two rotations of the crankshaft. This means that, as the crankshaft rotates and moves through its cycles, the distributor must keep up with the timing requirements, leading to its operation at half the speed of the crankshaft. Understanding this relationship is essential for mechanics and engineers who work with reciprocating engines, as it impacts ignition timing, engine efficiency, and overall engine performance.