Mean effective pressure

The mean effective pressure is a quantity related to the operation of an internal combustion 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 over a cycle in the combustion chamber of the engine.
Let,

W = Work per cycle in Joules

P = Power in Watts

p_mep = mean effective pressure in Pa

V_d = displacement volume in m³

n_c = number of revolutions / cycle (for a 4-stroke engine n_c = 2)

N = number of revolutions per second

T = Torque in Nm

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:

W = p_mep * V_d

so that

Since the torque T is related to the angular speed (which is just N 2 π) and power produced by

P = TN2π

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, as usual, calculated from measured dynamometer torque. Indicated mean effective pressure or 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.

BMEP Typical values

• Naturally aspirated spark-ignition engines : maximum values are in the range 8.5 to 10.5 bar ( ~ 125 - 150 lb/in²), at the engine speed where maximum torque is obtained. At rated power, bmep values are typically 10 to 15% lower.
• Turbocharged automotive spark ignition engines : the maximum bmep is in the 12.5 to 17 bar range (180 to 250 lb/in²).
• Naturally aspirated four-stroke diesels : the maximum bmep is in the 7 to 9 bar range (100 to 130 lb/in²).
• Turbocharged automotive four-stroke diesels : the maximum bmep is in the 14 - 18 bar range.
• Two-stroke diesels have comparable values, but very large low speed diesels like the Wärtsilä-Sulzer RTA96-C can have bmep up to 19 bar.

For example, a four-stroke motor producing 160 N·m from 2 litres of displacement is going to have a bmep of (4π) (160 N·m) / (0.002 m³) = 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. As piston engines always have their maximum torque at a lower rotating speed than the maximum output, the BMEP is lower at full power.

See also

• Compression ratio

Notes and External links

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

• All About Mean Effective Pressure, Harleyc.com

• The paragraph on typical values for bmep has been taken from Heywood and the Wikipedia article on the Wärtsilä-Sulzer RTA96-C.

1. ^ Heywood, J. B., "Internal Combustion Engine Fundamentals", McGraw-Hill Inc., 1988

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