Question

In a four-stroke spark ignition engine, calculate the engine speed in RPM if the spark plug fires 16 times per second.

Answer 1

A Four Stroke SI Engine consists of 4 strokes namely

1) Suction

2) Compression

3) Expansion/Explosion/Power Stroke

4) Exhaust

The crankshaft completes one revolution after Suction and Compression strokes and further completes second revolution after Power and Exhaust Stroke, The Spark occurs after completion of Compression Stroke where in spark is ignited and there is explosion which causes Expansion stroke followed by Exhaust stroke.

So to conclude in Four Stroke SI Engine there is 1 Spark after 2 revolutions of Crankshaft.

Mathematically speaking it would be 0.5 spark in 1 revolution.

Now assuming that the given 4 stroke SI engine is a 4 cylinder engine, in order to determine how many times this engine fires per revolution the expression to find that would be (0.5 * the number of cylinders)

The Number of Cylinders = 4,

therefore , 0.5 * 4 = 2 sparks per revolution. Refer the Following two images for answer.