Brake Mean Effective Pressure (BMEP) Calculator

Background.

Torque figures flatter big engines. A 5-litre V8 making 500 N·m sounds stronger than a 2-litre four making 300 N·m, yet the small engine is extracting half as much again from every litre it sweeps. Brake mean effective pressure puts engines of any size on one scale: take the work the crankshaft delivers over one complete engine cycle and divide it by the volume the pistons displace. Work divided by volume is a pressure, quoted in bar, kilopascals or psi.

No gauge in the cylinder ever reads that pressure. Real cylinder pressure spikes just after ignition and falls almost to nothing on the exhaust stroke. BMEP is the steady pressure that, pushing on the pistons through one stroke per cycle, would deliver the same work the crankshaft actually delivered. The word brake means the torque was measured at the output shaft, on a dynamometer, after friction and pumping losses have taken their share.

The cycle matters as much as the numbers. A four-stroke engine needs two crankshaft revolutions to complete a cycle, a two-stroke only one, so the same torque from the same displacement gives a two-stroke exactly half the BMEP. Compare four-strokes with four-strokes.

What is brake mean effective pressure (bmep) calculator?

Brake mean effective pressure is brake work per engine cycle divided by total swept volume. For a four-stroke engine it equals 4π × torque ÷ displacement; for a two-stroke, 2π × torque ÷ displacement. With torque in newton-metres and displacement in cubic metres the result is in pascals. Dividing out engine size leaves a measure of how hard each litre is working.

How to use this calculator.

1. 01Enter brake torque in N·m, measured at the crankshaft at the operating point you care about, usually peak torque. A figure in lb·ft multiplies by 1.3558 to give N·m.
2. 02Enter total engine displacement in litres: cc divided by 1,000, or cubic inches multiplied by 0.016387.
3. 03Choose four-stroke for car, truck and most motorcycle engines, two-stroke for engines that fire every revolution, such as many small outboards and chainsaws.
4. 04Read BMEP in whichever unit the figures you are comparing against use. Brake work per cycle is the joules delivered in one complete cycle.

The formula.

Torque multiplied by the angle turned, in radians, gives work. One crankshaft revolution is 2π radians, so the work per cycle is W = 2π × T × n, where T is brake torque and n is the number of revolutions per cycle: 2 for a four-stroke, 1 for a two-stroke. Dividing by displacement V gives BMEP = W ÷ V. Litres become cubic metres by dividing by 1,000, pascals become bar by dividing by 100,000, and psi comes from dividing pascals by 6,894.757. Collapsing the constants gives two useful shortcuts for four-strokes: BMEP in bar ≈ 0.1257 × N·m per litre, and BMEP in psi ≈ 150.8 × lb·ft ÷ cubic inches. The result describes one operating point only. It says nothing about peak cylinder pressure, combustion quality or how close the engine is to its mechanical limits.

A worked example.

Frequently asked questions.

References& sources.

1. [1]U.S. EPA. Fuel Consumption Modeling in PERE, Equation 12; publishes the four-stroke torque/displacement BMEP relationship and describes normalized engine torque.