BMEP Calculator (Brake Mean Effective Pressure from Torque and Displacement)
The average pressure an engine would need to hold on its pistons throughout the cycle to produce its torque, plus power and output per litre.
Results
The dashed line marks the value you entered.
What this tool does
Torque on its own flatters big engines, so engineers divide it by the displacement and the number of strokes per cycle to get BMEP: the equivalent steady pressure on the pistons, in bar. It measures how hard an engine works its cylinders regardless of size. As a rough guide, naturally aspirated petrol engines sit around 9–13 bar, while turbocharged petrol and diesel engines run well above that; very high figures point to high boost and heavy loads on the engine. A four-stroke completes one cycle in two revolutions, a two-stroke in one.
Formula
BMEP = 2π × n_r × T ÷ V_d (n_r = 2 for a four-stroke, 1 for a two-stroke; T in N·m, V_d in m³) · P = T × 2π × rpm ÷ 60
Variables
| Symbol | Meaning | Unit |
|---|---|---|
tq | Engine torque | N·m |
vd | Engine displacement | L |
st | Engine cycle | — |
rp | Engine speed at that torque | rpm |
BM | Brake mean effective pressure (BMEP) | bar |
BK | BMEP in kPa | kPa |
BP | BMEP in psi | psi |
PW | Power at that speed | kW |
TL | Torque per litre | N·m/L |
PL | Power per litre | kW/L |
Worked example
- Engine torque250 N·m
- Engine displacement2 L
- Engine cycle2
- Engine speed at that torque5000 rpm
- Brake mean effective pressure (BMEP)15.71 bar
- BMEP in kPa1571 kPa
- BMEP in psi228 psi
- Power at that speed130.9 kW
- Torque per litre125.0 N·m/L
- Power per litre65.4 kW/L
Limitations
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
- Mixing units is the most common source of error. Convert every input to the units shown next to each field before calculating.