 Brake Mean Effective Pressure

Brake Mean Effective Pressure

Any model engine enthusiasts who are familiar with the excellent series of model engine tests conducted by the late Peter Chinn over many years will have noticed the appearance of a vertical scale on Chinn’s performance graphs designated BMEP. This stands for Brake Mean Effective Pressure. The significance of this figure is not widely understood, making it appear advisable to present a brief summary of the parameter and its significance.

BMEP is a measurable quantity relating to the operation of a reciprocating engine. It is a valuable measure of an engine's capacity to do work that is independent of engine displacement. It may best be thought of as an approximation of the average pressure in the combustion chamber of the engine over a working cycle.

Since the goal of any model engine designer is to maximize the pressure achieved in the cylinder through combustion of the fuel, it follows that a high BMEP implies that the engine in question is operating efficiently. More importantly, this is true quite independently of the engine’s displacement.  Although geometric considerations mean that the horsepower and torque developed by two engines of different displacements at the same BMEP figures will differ due to the different piston areas and strokes involved, the fact remains that those two engines will be working at or near comparable levels of overall efficiency.

It was for this reason that Peter Chinn always stated quite openly that in his view BMEP was the most relevant measure of an engine’s performance. Consequently, he always included such figures in his tests, often comparing different models on that basis, regardless of actual displacement.

For those interested, I have summarized the relevant calculations below. It's my hope that these calculations may help the reader towards a better understanding of this very useful parameter. I’ve done the necessary unit conversions throughout – it’s the principles that matter here. We’ll start with the basics:

Horsepower (BHP) =     RPM x Torque (oz-in)
1,008,000

where 1,008,000 is the required unit conversion factor.

Rearranging:

Torque (oz-in) = BHP x 1,008,000
RPM

Now, for a two-stroke engine, the standard engineering formula is:

BMEP (psi) = 75.4 x Torque  (lb-ft) / Displacement (cuin.)

Converting to model units:

Torque (oz-in) x 192 = Torque (ft-lb)

Displ. (cc) x 16.39 = Displ. (cuin.)

Therefore, substituting the relevant conversion factors:

BMEP (psi) = 75.4 x Torque (oz in)  x  16.39     =    Torque (oz in)   x  6.436

Displ. (cc)         192                Displ. (cc)

BMEP (psi) =   BHP x 1,008,000    x 6.436
RPM x Displ. (cc)

BMEP (psi) =   BHP  x  6487488
RPM x Displ. (cc)

The latter formula is the one used by Chinn to calculate BMEP. Since BHP appears on the top of the fraction while displacement appears on the bottom, it should be obvious that, as we would expect, the BHP of a smaller engine will be proportionally lower than that of a larger engine at the same BMEP and RPM figures. It should also be apparent that the achievement of high BHP at a given speed requires the development of a high BMEP figure at that speed.

In summary, the BMEP developed by a given engine is easily calculated from that engine’s horsepower curve. Since the parameter takes an engine’s displacement into account, it is a very useful parameter for comparing the operating efficiencies of engines having different displacements.

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