Steam Engine Pressure-Volume Diagram    
Steam Engine Pressure-Volume Diagram
Blue = theoretical maximum diagram
Red = typical indicator card (PV) plot
Mean effective pressure, pm:
pm (lb/in2) =  area of PV indicator card (lb/in2)(in3)
swept volume of cylinder (in3)

Indicated Horsepower, ihp:

ihp =  pm L a n
33,000
      Where:
    pm = mep, lb/in2
    L = stroke, ft
    a = piston area, in2
    n = number of cycles per minute
    33000 = ft-lb/min / hp (definition)
Brake horsepower, bhp:

bhp = ihp x mechanical efficiency   (mech. eff. typically 80% - 94%)

Friction horsepower, fhp:

fhp = ihp - bhp

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