36.1. POWER OF STEAM ENGINE
36.1.1. Indicated Power (ip)
The power developed inside the engine cylinder is called the indicated power (ip). For a reciprocating steam engine, indicated power can be measured by estimating the mean effective pressure of the indicated diagram.
For a single acting engine
Power is developed on one side of the piston.
From equation (35.2a), the mean effective pressure, pm (bars) is given by
Pm = 
or pm = 
Therefore, work done/cycle = 100 pm . Vs = 100 pm . A . L, kJ/cycle
where, A = area of cross-section of piston, m2 ; L = stroke of engine, m
Actual work done/min = (work done/cycle) x (No of cycles per min)
Actual work done/min = (100 pm . A . L) x (n), kJ/min
where ‘n’ is No of power cycles per min
In steam engine number of power cycle per min ‘n’ is equal to number of crankshaft revolutions per min (rpm) ‘N’,
Therefore, actual work done/min = (100 pm . A . L) x (N), kJ/min
Indicated Power Output of engine = 
, kW ......................……….. (36.1)
or = 
, hp
For a double acting engine
Power is developed on both side of the piston.
Let Pm,1, A1 and Pm,2, A2 be the mean effective pressures and effective piston areas of head end and crank end sides of the piston, respectively, then from equation (36.1)
Indicated Power Output of engine on the head end side = 
, kW
Indicated Power Output of engine on the crank end side = 
, kW
Total indicated power output of the engine
........................…..(36.2)
or 
,hp
where, A1 = 
and
A2 = 
where, ‘D’ and ‘d’ is diameter of piston and piston rod, respectively.
If the power developed on both side of the piston is same and area of piston rod is neglected
then A1 = A2 = A and Pm1 = Pm2 = Pm
From equation (36.2)
Total indicated power output of the engine
= 
kW
Therefore, Indicated Power Output of engine = 
, kW
or = 
, hp
36.1.2. Brake Power
The actual power produced at the crank shaft may be termed shaft power (sp) or Brake power (bp). It is equal to the indicated power minus the power spent to overcome the mechanical friction in the engine and to drive the auxiliary mechanisms attached to the engine.
The power absorbed due to mechanical friction in the engine and to drive the auxiliary mechanisms called friction power (fp).
So bp = ip – fp
The brake power (bp) is measured by dynamometer with the following relation:
Brake power (bp) = 
(kW or kJ/s)
For detail of brake power refer section “Performance Criteria for IC Engines” in lesson 46 of module IX.
36.2. EFFICIENCY OF STERAM ENGINE
There are several efficiencies which are to be considered for the performance of a steam engine.
36.2.1. Mechanical efficiency:
It is the ratio of brake power (bp) to indicated power (ip) ;
i.e. ηm = 
(normally = 80 - 94%)
As it depends primarily on engine mechanism, so it may be called the efficiency of the engine mechanism.
36.2.2. Thermal efficiency
It is the ratio of output energy as work to the heat energy supplied by the steam in the cylinder. If the output is indicated work, it will be indicated thermal efficiency (ηith) and if it be brake work it will be brake thermal efficiency (ηbth).
Indicated Thermal Efficiency (ηith) = 
(normally = 17 - 24%)
Brake thermal efficiency (ηbth) = 
; (normally = 14 - 21%)
where M is mass of steam admitted in per min into the engine
hg is enthalpy of dry steam at the inlet of the engine (from steam table)
hf is enthalpy of water at saturation temperature of exhaust pressure (from steam table).
36.2.3. Overall efficiency
The overall efficiency (ηo) of a steam engine power plant is defined as the ratio of work available at the crank shaft of steam engine to the heat energy supplied by the fuel in boiler.
Overall efficiency (ηo) = 
where is mass of fuel supplied per min
CV is calorific value of fuel.
36.2.4. Relative efficiency
The relative efficiency of a steam engine is the ratio between the indicated thermal efficiency and Rankine efficiency
Relative efficiency = 
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