A four-cycle engine runs at 3600 rpm's. The number of sparks per minute required at the spark plug would be:

In a four-cycle engine, each complete cycle consists of four distinct strokes: intake, compression, power, and exhaust. This means that for every two revolutions of the crankshaft, one power stroke occurs and, consequently, one spark is needed to ignite the fuel-air mixture. Therefore, for an engine running at 3600 revolutions per minute (rpm), the number of cycles per minute can be calculated as half that number.

Thus, if the engine is operating at 3600 rpm, you divide that by 2 to find the number of power strokes (or sparks required at the spark plug). This calculation results in 1800 sparks per minute.

Understanding the cycle of a four-stroke engine is crucial for determining how frequently the spark plug fires to optimize engine operation. Each spark occurs only when a power stroke is happening, which is why the number of sparks is half the number of revolutions per minute. This mechanical configuration is what distinguishes a four-cycle engine from its two-cycle counterpart, where a spark occurs with every revolution.