BMEP Calculator (Brake Mean Effective Pressure Calculator)

Created by Krishna Nelaturu
Last updated: Nov 16, 2022

Our BMEP calculator will help you determine an IC engine's brake mean effective pressure (BMEP) from the engine displacement volume and its maximum torque output. Learning what BMEP is in the following article will help you recognize its significance. You will also learn how to calculate the BMEP of any engine using the BMEP formula.

Our torque calculator will help you with the fundamentals of torque if you need a refresher before diving in.

What is BMEP?

The brake mean effective pressure (BMEP) is the average pressure exerted on the engine's piston by an internal combustion engine's air/fuel mixture combustion. Specifically, this pressure translates into the torque at the driveshaft. It is one of the metrics used to gauge an engine's performance.

During the power stroke of an IC engine, the pressure exerted on the piston varies as the piston reciprocates (and the gas expands). Mean effective pressure (MEP) is the average value of this pressure and is responsible for the engine's torque and the internal frictional forces the piston must overcome during its reciprocating motion. BMEP is the part of MEP that is translated to the engine's torque - it contains no information about the frictional losses. Naturally, the higher the BMEP value, the more "efficient" or "tuned" the engine performance.

BMEP formula

The BMEP formula is given by:

BMEP=2πnTD\rm{BMEP} = \frac{2 \pi n T}{D}

where:

  • BMEP\rm{BMEP} - The BMEP of the engine;

  • n\rm{n} - Number of revolutions per cycle (or per power stroke). n=1n=1 for two-stroke and n=2n=2 for four-stroke engines;

  • T\rm{T} - Torque generated at the driveshaft; and

  • D\rm{D} - Engine displacement volume.

BMEP has the same units as pressure. In SI, it is common to use kilo- or megapascals, while bar is the common imperial unit.

For instance, the Ferrari 458 Italia's 4497 cc4497 \text{ cc} engine produces a maximum torque of 540 Nm540 \text{ } \rm{N \cdot m} at 6000 rpm. Let's using the BMEP equation above to calculate its BMEP. Since the engine is four-stroke, n=2n = 2.

BMEP=2πnTD=2π2540 Nm4497×106 m3=1508970 PaBMEP=1509 kPa or 15.09 bar\begin{align*} \rm{BMEP} &= \frac{2 \pi n T}{D}\\[1em] &= \frac{2 \pi \cdot 2 \cdot 540 \text{ } \rm{N\cdot m}}{4497 \times10^{-6} \text{ } \rm{m^3}}\\[1em] &= 1508970 \text{ Pa}\\[1em] \rm{BMEP} &= 1509 \text{ kPa or } 15.09 \text{ bar} \end{align*}

Using this BMEP calculator

This BMEP calculator is straightforward to use:

  • Select whether the engine is two-stroke or four-stroke. Based on the selection, the BMEP calculator will automatically choose the value for nn. Alternatively, you can enter a custom value for nn by clicking on the advanced mode button.

  • Enter the value of the engine displacement volume.

  • Provide the maximum torque generated by the engine. This tool will then employ the BMEP equation to determine the BMEP of the engine.

⚠️ BMEP is a valuable metric to compare engine efficiency with other engines. However, comparing the BMEPs of entirely different kinds of engines isn't very sensible! For example, you can compare the BMEPs of different-sized petrol engines, but comparing them against a diesel engine's BMEP will not tell you much.

Enjoying our BMEP calculator? We have other such tools that can interest an automotive geek, such as yourself - piston speed calculator and the piston force calculator. Explore them to your heart's content!

Krishna Nelaturu
Engine type
Four Stroke Engine
Engine displacement (Volume)
cu in
Torque
ft-lb
Brake mean effective pressure
psi
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