Power is the amount of work done per unit time or the rate of doing work. For a diesel
engine, power is rated in units of horsepower. Indicated horsepower is the power
transmitted to the pistons by the gas in the cylinders and is mathematically calculated.
Brake horsepower refers to the amount of usable power delivered by the engine to the
crankshaft. Indicated horsepower can be as much as 15% higher than brake horsepower.
The difference is due to internal engine friction, combustion inefficiencies, and parasitic
losses, for example, oil pump, blower, water pump, etc.
The ratio of an engine's brake horsepower and its indicated horsepower is called the
mechanical efficiency of the engine. The mechanical efficiency of a four-cycle diesel is
about 82 to 90 percent. This is slightly lower than the efficiency of the two-cycle diesel
engine. The lower mechanical efficiency is due to the additional friction losses and power
needed to drive the piston through the extra 2 strokes.
Engines are rated not only in horsepower but also by the torque they produce. Torque
is a measure of the engine's ability to apply the power it is generating. Torque is
commonly given in units of lb-ft.