The mean effective pressure (MEP) is the average pressure exerted on the piston during each power stroke, and is determined from a formula or by means of a planimeter. There are two kinds of mep: indicated mean effective pressure (imep), which is developed in the cylinder and can be measured; and brake mean effective pressure (bmep), which is computed from the brake horsepower (bhp) delivered by the engine.

Length of Stroke

The distance a piston travels between top and bottom dead centers (TDC, BDC) is known as the length of stroke. This distance is one of the factors that determines the piston speed. In some modern diesel engines, piston speeds may reach about 1600 feet per minute (fpm).

Cylinder Bore

Bore is used to identify the diameter of the cylinder. The cylinder bore must be known in order to compute the area of the piston crown upon which the pressure acts to create the driving force. This pressure is calculated and ex-pressed for an area of one square inch as pounds per square inch (psi). The ratio of length of stroke to cylinder bore is fixed in engine design; in most slow speed engines, the stroke is greater than the bore.