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The mean effective pressure (MEP) is a quantity relating to the operation of a
reciprocating engine A reciprocating engine, more often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common features of al ...
and is a measure of an engine's capacity to do work that is independent of
engine displacement Engine displacement is the measure of the cylinder volume swept by all of the pistons of a piston engine, excluding the combustion chambers. It is commonly used as an expression of an engine's size, and by extension as an indicator of the ...
. Despite having the dimension of pressure, MEP cannot be measured. When quoted as an ''indicated mean effective pressure'' (''IMEP''), it may be thought of as the average
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
acting on a
piston A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder (engine), cylinder a ...
during the different portions of its cycle. When friction losses are subtracted from the IMEP, the result is the brake mean effective pressure (BMEP).


Derivation

Let: :P = power output in watt; :p_\text = mean effective pressure in megapascal; :V_\text = displacement volume in cubic centimetre; :i = number of cycles per revolution (for a 4-stroke engine, i =0.5, for a 2-stroke engine, i =1);Wankel engines are four-stroke engines, so i =0.5; the displacement V_\text is derived from the chamber volume V_\text by multiplying it with the number of rotary pistons i and 2: V_\text = 2 V_\text i (see Wolf-Dieter Bensinger: ''Rotationskolben-Verbrennungsmotoren'', Springer, Berlin/Heidelberg/New York 1973, , p. 66) :n = number of revolutions per second; :\omega = angular velocity, i.e. \omega = 2 \pi n; :M = torque in newton-metre. Then, BMEP may be used to determine an engine's power output as follows: : P=i \cdot n \cdot V_\text \cdot p_\text Since we know that power is: :P = \omega \cdot M = 2 \pi \cdot n \cdot M We now see that, BMEP is a measure of expressing torque per displacement: :P = i \cdot n \cdot V_\text \cdot p_\text = 2 \pi \cdot n \cdot M And thus, the equation for BMEP in terms of torque is: : p_\text = . 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 a 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, standard MEP values can be used to estimate the required engine displacement. However, 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 (BMEP) is calculated from measured
dynamometer A dynamometer or "dyno" is a device for simultaneously measuring the torque and rotational speed ( RPM) of an engine, motor or other rotating prime mover so that its instantaneous power may be calculated, and usually displayed by the dyna ...
torque. Net indicated mean effective pressure (IMEP) 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 and BMEP.


Examples

;MEP from torque and displacement A four-stroke engine produces 159 N·m of torque, and displaces 2000 cm3 *i=0.5 *M=159 \, \text \text *V_\text=2000 \, \text^3 :p_\text = \cdot = \cdot \approx 100 \, N \cdot cm^ = 1 \, \text ;Power from MEP and crankshaft speed If we know the crankshaft speed, we can also determine the engine's power output from the MEP figure: P=i \cdot n \cdot V_\text \cdot p_\text
In our example, the engine puts out 159 N·m of torque at 3600 min−1 (=60 s−1): *i=0.5 *n= 60 \, \text^ *V_\text=2000 \, \text^3 *p_\text= 1 \, \text Thus: :P

60,000 \, N \cdot m \cdot s^ = 60,000 \, \text = 60 \, \text
As piston engines usually have their maximum torque at a lower rotating speed than the maximum power output, the BMEP is lower at full power (at higher rotating speed). If the same engine is rated 72 kW at 5400 min−1 = 90 s−1, and its BMEP is 0.80 MPa, we get the following equation: *i=0.5 *n= 90 \, \text^ *V_\text=2000 \, \text^3 *p_\text= 0.80 \, \text Then: :P

72,000 \, N \cdot m \cdot s^ = 72 \, \text


Types of mean effective pressures

Mean effective pressure (MEP) is defined by the location measurement and method of calculation, some commonly used MEPs are given here: * Brake mean effective pressure (BMEP, p_) - Mean effective pressure calculated from measured brake torque. * Indicated mean effective pressure (IMEP, p_) - Mean effective pressure calculated from in-cylinder pressure over the complete engine cycle (720° in a four-stroke, 360° in a two-stroke). IMEP may be determined by planimetering the area in an engine's pV-diagram. Since naturally aspirated four-stroke engines must perform pumping work to suck the charge into the cylinder, and to remove the exhaust from the cylinder, IMEP may be split into the high-pressure, gross mean effective pressure (GMEP, p_) and the pumping mean effective pressure (PMEP, p_). In naturally aspirated engines, PMEP is negative, and in super- or turbocharged engines, it is usually positive. IMEP may be derived from PMEP and GMEP: p_ = p_-p_.Ulrich Spicher: ''Kapitel 3 · Kenngrößen – table 3.16: Effektiver Mitteldruck heutiger Motoren'', in Richard van Basshuysen, Fred Schäfer (eds.): ''Handbuch Verbrennungsmotor – Grundlagen · Komponenten · Systeme · Perspektiven'', 8th edition, Springer, Wiesbaden 2017, ISBN 978-3-658-10901-1, DOI 10.1007/978-3-658-10902-8_3, p. 24 * Friction mean effective pressure (FMEP, p_) - Theoretical mean effective pressure required to overcome engine friction, can be thought of as mean effective pressure lost due to friction: p_=p_-p_. FMEP rises with an increase in engine speed.Ulrich Spicher: ''Kapitel 3 · Kenngrößen – table 3.16: Effektiver Mitteldruck heutiger Motoren'', in Richard van Basshuysen, Fred Schäfer (eds.): ''Handbuch Verbrennungsmotor – Grundlagen · Komponenten · Systeme · Perspektiven'', 8th edition, Springer, Wiesbaden 2017, ISBN 978-3-658-10901-1, DOI 10.1007/978-3-658-10902-8_3, p. 26, formula 3.48


BMEP typical values


See also

*
Compression ratio The compression ratio is the ratio between the maximum and minimum volume during the compression stage of the power cycle in a piston or Wankel engine. A fundamental specification for such engines, it can be measured in two different ways. Th ...


Notes and references


Notes


References


External links


Brake Mean Effective Pressure (bmep), Power and Torque
Factory Pipe
All About Mean Effective Pressure
Harleyc.com

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