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The mean effective pressure is a quantity relating to the operation of a reciprocating engine and is a valuable measure of an engine's capacity to do work that is independent of engine displacement.^{[1]} When quoted as an indicated mean effective pressure or IMEP (defined below), it may be thought of as the average pressure acting on a piston during a different portions of its cycle.
Let,
The power produced by the engine is equal to the work done per operating cycle times the number of operating cycles per second. If N is the number of revolutions per second, and n_c is the number of revolutions per cycle, the number of cycles per second is just their ratio. We can write
By definition:
so that
Since the torque T is related to the angular speed (which is just N 2 π) and power produced by
Then the equation for mep in terms of torque becomes,
Notice that speed has dropped out of the equation and the only variables are the torque and displacement volume. Since the range of maximum brake mean effective pressures for good engine designs is well established, we now have an engine displacement independent measure of the torque producing capacity of an engine design (a specific torque of sorts). This is useful for comparing engines of different displacements. Mean effective pressure is also useful for initial design calculations; that is, given a torque, we can use standard mep values to estimate the required engine displacement. However, it is important to remember that mean effective pressure does not reflect the actual pressures inside an individual combustion chamber—although the two are certainly related—and serves only as a convenient measure of performance.
Brake Mean Effective Pressure or bmep is calculated from measured dynamometer torque. Net indicated mean effective pressure or imep_{n} is calculated using the indicated power; i.e., the pressure volume integral in the work per cycle equation. Sometimes the term fmep (friction mean effective pressure) is used as an indicator of the mean effective pressure lost to friction (or friction torque) and is just the difference between imep_{n} and bmep
Mean effective pressure (MEP) is defined by the location measurement and method of calculation, some commonly used MEPs are given here.
For example, a four-stroke motor producing 160 N·m from 2 litres of displacement has a bmep of (4π)(160 N·m)/(0.002 m³) = 1,005,000 N/m^{2} =1,005 kPa (10.05 bar). If the same engine produces 76 kW at 5400 rpm (90 Hz), its torque is 134 N·m and its bmep is 8.42 bar (842 kPa). As piston engines always have their maximum torque at a lower rotating speed than the maximum output, the BMEP is lower at full power.
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