Page 198 - Handbook of Energy Engineering Calculations

e = 2545/(sfc)(HHV), where sfc = specific fuel consumption, lb/(bhp · h);
b
HHV = higher heating value of fuel, Btu/lb. For this engine, e = 2545/[(0.42)
b
(18,920)] = 0.32, or 32.0 percent.

Related Calculations. Use the same procedure for gas and gasoline engines.
Obtain the higher heating value of the fuel from the supplier, a tabulation of
fuel properties, or by test.

I-C ENGINE HORSEPOWER AND MEAN EFFECTIVE
PRESSURE

A 500-hp (373-kW) internal-combustion engine has a brake mean effective
2
pressure of 80 lb/in (551.5 kPa) at full load. What are the indicated mean
effective pressure and friction mean effective pressure if the mechanical
efficiency of the engine is 85 percent? What are the indicated horsepower and
friction horsepower of the engine?

Calculation Procedure:

1. Determine the indicated mean effective pressure
2
Indicated mean effective pressure imep lb/in for an internal-combustion
engine is found from imep = bmep/e , where bmep = brake mean effective
m
2
pressure, lb/in ; e = mechanical efficiency, percent, expressed as a decimal.
m
2
For this engine, imep = 80/0.85 = 94.1 lb/in (659.3 kPa).

2. Compute the friction mean effective pressure
For an internal-combustion engine, the friction mean effective pressure fmep
2
lb/in is found from fmep = imep − bmep, or fmep = 94.1 − 80 = 14.1 lb/in 2
(97.3 kPa).

3. Compute the indicated horsepower of the engine
For an internal-combustion engine, the mechanical efficiency e = bhp/ihp,
m
where ihp = indicated horsepower. Thus, ihp = bhp/e , or ihp = 500/0.85 =
m
588 ihp (438.6 kW).