The mean effective pressure is an abstraction of the pressure exerted into the combustion chamber of an internal combustion engine. Mean effective pressure is abbreviated MEP and often BMEP for "Brake" MEP for denoting dynamometer testing. Pressure (symbol: p) is the force per unit area acting on a surface in a direction perpendicular to that surface. ... A colorized automobile engine The internal combustion engine is a heat engine in which the burning of a fuel occurs in a confined space called a combustion chamber. ...

It is calculated by taking the torque exerted by the engine over a revolution for a two-stroke engine and two revolutions for a four-stroke, and dividing it by its displacement. In SI units, for MEP in pascals In physics, torque can be thought of informally as rotational force. Torque is commonly measured in units of newton metres; although, centiNewton Meters (cNm), Foot Pounds (Lb-Ft), Inch Pounds (Lb-In) and Inch Ounces (Oz-In) are also frequently used expressions of torque. ... The two-stroke cycle of an internal combustion engine differs from the more common four-stroke cycle by having only two strokes (linear movements of the piston) instead of four, although the same four operations (intake, compression, power, exhaust) still occur. ... The four-stroke cycle of an internal combustion engine is the cycle most commonly used for automotive and industrial purposes today (cars and trucks, generators, etc). ... Engine displacement is defined as the total volume of air/fuel mixture an engine can draw in during one complete engine cycle; it is normally stated in cubic inches, cubic centimeters, or litres. ... Cover of brochure The International System of Units. ... The pascal (symbol: Pa) is the SI unit of pressure. ...

MEP = 2 πT/V for a two-stroke,

MEP = 4 πT/V for a four-stroke.

where Lower-case π (the lower case letter is usually used for the constant) The mathematical constant π ≈ 3. ...

T is torque (in newton metres)

V is volume (in cubic metres)

For example, a four-stroke motor producing 160 N·m (the rotating speed is irrelevant, but the torque is the power divided by it) from 2 litres of displacement is going to have (4π)(160 N·m)/(0.002 m³) = 1005000 N/m² = 1005000 Pa = 1.005 MPa of MEP (or 10.05 bars). Newton metre is the unit of moment in the SI system. ... The cubic meter (symbol m³) is the SI derived unit of volume. ... In physics, power (symbol: P) is the amount of work done per unit of time. ... The bar (symbol bar) and the millibar (symbol mbar, also mb) are units of pressure. ...

If the same engine produces 76 kW at 5400 rpm (90Hz), its torque is 76000 N·m·s^-1 / 2π / 90 s^-1 = 134 N·m; its MEP will be (4π)(134 N·m)/(0.002 m³) = 0.842 MPa (8.42 bars). As piston engines mostly have their maximum torque at a lower rotating speed than the maximum output, the MEP is lower at full power, it's because the efficiency is lowered by more friction losses, air viscosisty, below optimum combustion, etc.

To gain a step, the formulas for coming directly from power are, because P/(2πf) = T

MEP = P/(f·V) for a two-stroke,

MEP = 2P/(f·V) for a four-stroke

where

P is power (in watts)

V is volume (in cubic metres)

f is rotating speed (in hertz)

MEP is a useful comparison tool between different engines, and is a good indicator of the level of performance achieved by its conceptor and manufacturer. It measures the efficiency of the conversion from the indicated mean effective pressure in the cylinder to the output shaft and the level of pressure attained in an engine. It’s important to remember that the values produced by the formula are for theoretical analysis only, and do not reflect the actual pressures inside an individual combustion chamber. The watt (symbol: W) is the SI derived unit of power. ... The cubic meter (symbol m³) is the SI derived unit of volume. ... The hertz (symbol: Hz) is the SI unit of frequency. ...

External links

• Brake Mean Effective Pressure (BMEP), Power and Torque, Factory Pipe

• All About Mean Effective Pressure, Harleyc.com