Horsepower:

Power is the amount of work complete per unit time or the rate of doing work. To a diesel engine, power is rated in units of horsepower. Denoted horsepower is the power transmitted to the pistons through the gas in the cylinders and is mathematically calculated.

Brake horsepower submit to the amount of usable power delivered through the engine to the crankshaft. Indicated horsepower could be as much as 15 percent higher than brake horsepower. The difference is because of internal engine friction, combustion inefficiencies and parasitic losses, for instance, blower, water pump, oil pump, etc.

The ratio of an engine's brake horsepower and its indicated horsepower is known as the mechanical effectiveness of the engine. A mechanical efficiency of a four-cycle diesel is about 82 to 90 %. This is slightly lower than the efficiency of the two-cycle diesel engine. The lower mechanical efficiency is because of the additional friction losses and power required to drive the piston through the extra 2 strokes.

Engines are rated not just in horsepower but also through the torque they generates. Torque is a measure of the engine's ability to apply the power it is producing. Torque is generally provided in units of lb-ft.