The power developed by an engine depends upon the type of engine as well as the speed of the engine. A cylinder of a single-acting, 4-stroke cycle engine will produce one power stroke for every two crankshaft revolutions, while a single-acting, 2-stroke cycle engine produces one power stroke for each revolution.

Indicated Horsepower

The power developed within a cylinder can be calculated by measuring the imep and the engine speed. (The rpm of the engine is converted to the number of power strokes per minute.) With the bore and stroke known (available in engine manufacturers technical manuals), the horsepower (hp) can be computed. This power is called indicated horsepower (ihp) because it is obtained from the pressure measured with an engine indicator. Power loss due to friction is not considered in computing ihp.

Using the factors which influence the engines capacity to develop power, the general or standard formula for calculating ihp is as follows:

To illustrate the use of this formula, assume that a 12-cylinder, 2-stroke cycle, single-acting engine has a bore of 8.5 inches and a stroke of 10 inches. Its rated speed is 744 rpm. With the engine running at full load and speed, the imep is measured and found to be 105 psi. What is the ihp developed by the engine?

In this case

Substituting these amounts in the formula, you have

This amount represents the horsepower developed in only one cylinder; since there are 12 cylinders in this engine, total horsepower for the engine will equal 12 times 111.9, or approximately 1343.