The mean effective pressure (MEP) is a quantity relating to the operation of a
reciprocating engine
A reciprocating engine, also often known as a piston engine, is typically 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 featu ...
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 a loose indicator of the ...
.
[Heywood, J. B., "Internal Combustion Engine Fundamentals", McGraw-Hill Inc., 1988, p. 50] 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 e ...
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 and is made gas-tig ...
during the different portions of its
cycle.
Derivation
Let:
:
= work per cycle in joule;
:
= power output in watt;
:
= mean effective pressure in pascal;
:
= displacement volume in cubic metre;
:
= number of revolutions per power stroke (for a 4-stroke engine,
);
[Wankel engines are four-stroke engines, so ; the displacement is derived from the chamber volume by multiplying it with the number of rotary pistons and 2: (see Wolf-Dieter Bensinger: ''Rotationskolben-Verbrennungsmotoren'', Springer, Berlin/Heidelberg/New York 1973, , p. 66)]
:
= number of revolutions per second;
:
= torque in newton-metre.
The
power
Power most often refers to:
* Power (physics), meaning "rate of doing work"
** Engine power, the power put out by an engine
** Electric power
* Power (social and political), the ability to influence people or events
** Abusive power
Power may a ...
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
is the number of revolutions per power stroke, the number of power strokes per second is just their ratio. We can write:
:
Reordering to put work on the left:
:
By definition:
:
so that
:
Since the
torque
In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
''T'' is related to the angular speed (which is just ''N''·2π) and power produced,
:
then the equation for MEP in terms of torque is:
:
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" for short, 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 ...
torque. Net indicated mean effective pressure (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.
Examples
;MEP from torque and displacement
A four-stroke engine produces 160 N·m of torque, and displaces 2000 cm
3=2 dm
3=0.002 m
3:
*
*
*
:
We also get the megapascal figure if we use cubic centimetres for
:
*
*
*
:
;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:
In our example, the engine puts out 160 N·m of torque at 3600 min
−1:
*
*
*
*
:
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 76 kW at 5400 min
−1 = 90 s
−1, and its BMEP is 0.844 MPa, we get the following equation:
*
*
*
*
:
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) - Mean effective pressure calculated from measured brake torque.
* Gross indicated mean effective pressure (IMEP
g) - Mean effective pressure calculated from in-cylinder pressure over compression and expansion portion of engine cycle (360° in a
four-stroke
A four-stroke (also four-cycle) engine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either directio ...
, 180° in a
two-stroke
A two-stroke (or two-stroke cycle) engine is a type of internal combustion engine that completes a Thermodynamic power cycle, power cycle with two strokes (up and down movements) of the piston during one power cycle, this power cycle being comple ...
). Direct measurement requires cylinder pressure sensing equipment.
* Net indicated mean effective pressure (IMEP
n) - Mean effective pressure calculated from in-cylinder pressure over the complete engine cycle (720° in a four-stroke, 360° in a two-stroke). Direct measurement requires cylinder pressure sensing equipment.
* Pumping mean effective pressure (PMEP) - Mean effective pressure from work moving air in and out of the cylinder, across the intake and exhaust valves. Calculated from in-cylinder pressure over intake and exhaust portions of engine cycle (360° in a four-stroke, 0° in a two-stroke). Direct measurement requires cylinder pressure sensing equipment. PMEP = IMEP
g - IMEP
n.
* Friction mean effective pressure (FMEP) - Theoretical mean effective pressure required to overcome engine friction, can be thought of as mean effective pressure lost due to friction. Friction mean effective pressure calculation requires accurate measurement of cylinder pressure and dynamometer brake torque. FMEP = IMEP
n - BMEP.
BMEP typical values
See also
*
Compression ratio
The compression ratio is the ratio between the volume of the cylinder and combustion chamber in an internal combustion engine at their maximum and minimum values.
A fundamental specification for such engines, it is measured two ways: the stati ...
Notes and references
Notes
References
External links
Brake Mean Effective Pressure (bmep), Power and Torque Factory Pipe
All About Mean Effective Pressure Harleyc.com
{{Aircraft piston engine components
Piston engines
Engine technology