Brake Mean Effective Pressure

This is the average effective cylinder pressure that does useful work calculated from the brake horse power.

The work accomplished during one engine cycle divided by the engine swept volume. It is essentially the engine torque normalized by the engine displacement. The word ′brake′ denotes the actual torque/power available at the engine flywheel as measured on a dynamometer. Thus, BMEP is a measure of the useful power output of the engine.

Where

BMEP = Brake Mean Effective Pressure [Nm-2 = Pa]
T = Torque [Nm]
nc = number of revolutions per cycle [for a 4-stroke engine nc = 2 and for a 2-stroke engine nc = 1]
Vd = displacement volume [m3]

This is easily re-written so that it works for engine displacements in litres:

Where

Vlitres = displacement volume [litres]

From the above equations you can see that the peak BMEP is achieved at peak torque. In most engines peak torque is achieved below peak power.

Examples
Naturally aspirated 4-stroke diesels700 to 900 kPa
Naturally aspirated spark-ignition engines850 to 1050 kPa
Turbocharged automotive spark ignition engines1250 to 1700 kPa
Turbocharged automotive 4-stroke diesels1400 to 1900 kPa
Large 2-Stroke Diesel1900 kPa
High performance ultra-boosted 4-stroke2800 kPa
Top fuel dragster8000 to 10000 kPa

See also: Brake Specific Fuel Consumption, Combustion Pressure, Indicated Mean Effective Pressure, Mean Effective pressure.

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Subjects: Automotive Engines