Brake Mean Effective Pressure

Brake Mean Effective Pressure is the average effective cylinder pressure that does useful work calculated from the brake horse power. Abbreviated BMEP, this is a fundamental parameter associated with an engine design.

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.

BMEP equation

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:

BMEP equation in litres

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. The table below gives a BMEP range for different engine types.

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
Example values of BMEP for different engine types

Indicated Mean Effective Pressure

During the cycle of an engine useful work is only done on the power stroke. By measuring the cylinder pressure through a cycle it is possible to calculate the average pressure that is useful. This is the Indicated Mean Effective Pressure.

Bouland Motors 1/3rd scale cosworth DFV

A 1/3rd Scale Cosworth DFV model engine by Bouland Motors

It was December 2015 and Marcel Bouland, scale model engine enthusiast and collector decided to develop a fully working 1:3 scale Cosworth DFV engine. Many people were asking if he wanted to sell one of his scale model engines, the answer was no. So it was time to make a small series from a very attractive scale model engine and yes, that had to be the COSWORTH DFV. Not only looking great, but with 155 Grand Prix victories between 1967 and 1983, the most legendary F1 engine ever built!

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