BMEP Calculator

What Is BMEP?

Brake mean effective pressure (BMEP) is the average pressure acting on the piston during the power stroke. It is a useful way to compare engine efficiency independent of displacement and speed.

BMEP Formula

$$BMEP = \frac{2\pi \cdot T}{V_d \cdot n_r}$$

$T$ is torque (N·m), $V_d$ is displacement in m³, and $n_r$ is revolutions per power cycle: 2 for four-stroke engines, 1 for two-stroke engines.

Example: 200 N·m torque, 2 L (0.002 m³) displacement, four-stroke ($n_r = 2$) at 6000 RPM gives BMEP ≈ 314 kPa and power ≈ 228 hp.

Engine Power

$$Power_{hp} = \frac{T_{lb\text{-}ft} \times RPM}{5252}$$

In SI units, power in kW is $P = T \times RPM \times 2\pi / 60 / 1000$. BMEP and power together characterize how hard an engine works per unit displacement.

Frequently Asked Questions

Why use BMEP instead of just horsepower?

BMEP normalizes output by displacement, making it easier to compare engines of different sizes. A high BMEP means efficient use of cylinder volume.

What is n_r for a four-stroke engine?

n_r = 2 because a four-stroke engine needs two crankshaft revolutions per power stroke. Two-stroke engines use n_r = 1.

What is a typical BMEP value?

Naturally aspirated gasoline engines often reach 800–1200 kPa peak BMEP. Turbocharged engines can exceed 2000 kPa.

Where does the 5252 constant come from?

It converts torque in lb·ft and RPM to horsepower: $HP = (2\pi / 33{,}000) \times T_{lb\text{-}ft} \times RPM \approx T \times RPM / 5252$.

Does BMEP include frictional losses?

BMEP is derived from brake (output) torque, so it already reflects mechanical losses. It represents useful work per unit displacement.

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What Is BMEP?

BMEP Formula

Engine Power

Frequently Asked Questions

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