Mean effective pressure | |
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Common symbols | p |
SI unit | Pascal (Pa) |
In SI base units | 1 kg⋅m^{−1}⋅s^{−2} |
Derivations from other quantities | p = W·V^{−1} |
Dimension |
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 the 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 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 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 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.
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 :
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:
Mean effective pressure (MEP) is defined by the location measurement and method of calculation, some commonly used MEPs are given here.
Engine type | Typical max. BMEP |
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Motorbike engine | 1.2 MPa (174.0 lbf/in^{2}) |
Race car engine (Formula 1) | 1.6 MPa (232.1 lbf/in^{2}) |
Passenger car engine (naturally aspirated Otto) | 1.3 MPa (188.5 lbf/in^{2}) |
Passenger car engine (turbocharged Otto) | 2.2 MPa (319.1 lbf/in^{2}) |
Passenger car engine (turbocharged Diesel) | 2.0 MPa (290.1 lbf/in^{2}) |
Lorry engine (turbocharged Diesel) | 2.4 MPa (348.1 lbf/in^{2}) |
High-speed industrial Diesel engine | 2.8 MPa (406.1 lbf/in^{2}) |
Medium-speed industrial Diesel engine | 2.5 MPa (362.6 lbf/in^{2}) |
Low-speed two-stroke Diesel engine | 1.5 MPa (217.6 lbf/in^{2}) |