Indicated vs. Brake Work: IMEP & BMEP

An engine's indicated power is the work the combustion gas does on the piston, found by integrating cylinder pressure over volume (the indicator diagram). The brake power is what actually comes out at the crankshaft — measured by a dynamometer — and it is always less, because the engine spends some of its indicated work overcoming its own internal friction (pistons, bearings, valve train) and pumping losses (drawing air in and pushing exhaust out). That difference is the friction power. So brake power = indicated power − friction power, and the ratio brake/indicated is the mechanical efficiency — typically 80–90% at full load, falling at light load where friction is a bigger slice of a smaller pie.

Raw torque or power depends on engine size, so they're poor for comparison. Mean effective pressure (MEP) is the trick: it's the constant pressure that, acting over one power stroke, would produce the cycle's work. Because MEP is a pressure, it normalises away the displacement — a 2-litre engine and a 5-litre engine with the same BMEP are equally efficient at turning displacement into work. IMEP uses indicated work; BMEP uses brake work; the difference is the FMEP (friction MEP). Naturally-aspirated petrol engines run ~8–13 bar BMEP at full load; turbos push higher; diesels sit a little higher still. A high BMEP at a given speed means the engine is working its displacement hard.