30.1 POWER

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À  p

We can estimate the power generated in the combustion cycle using the gas pressure and change in volume.

Basically in an engine the volume of gas is constant, except during ignition, where the volume of gas is almost instantly increased. This results in a corresponding increase in pressure, that drives the displacement of the piston.

For a typical engine the cylinder pressure curves will look something like those below from Shigley and Uicker for a 401 cubic inch V6 truck engine.

We can also estimate actual horsepower and torque of the motor by braking the engine, and calculating powers over a range of speeds. [Shigley and Uicker]

In an engine typical component massed are, [Shigley and Uicker]

We can typically relate engine power to displacement volume, with some variance considered for engine type,

The ideal pressure volume diagram for an engine is shown below. It does not have the rounding seen on the practical graph before (the rounding is the result of slow valve openings, and the finite time for combustion).

We can find the work done by the engine by looking at the change in pressure and volume on the expansion stroke.