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Mechanical Efficiency (η_m)

Ratio of brake to indicated power. A falling η_m at fixed IMEP is the earliest sign of rising friction (piston rings, bearings, seals).

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Formula, assumptions, and limits

ηm=PbPi,Pf=PiPb\eta_m = \frac{P_b}{P_i}, \quad P_f = P_i - P_b
SymbolMeaningUnit
PbP_bBrake powerkW
PiP_iIndicated powerkW
ηm\eta_mMechanical efficiency
PfP_fFriction power losskW

Source: Pounder's Marine Diesel Engines

In short

Ratio of brake to indicated power.

Learn the theory Engine Performance Monitoring: PMI Systems