Page 196 - Handbook of Energy Engineering Calculations

deducted. Since the mechanical efficiency of the engine is 82 percent, the fuel
required to produce the indicated horsepower is 82 percent of that required
for the brake horsepower, or (0.82)(0.485) = 0.398 lb/kWh (0.179 kg/kWh).
The indicated thermal efficiency of an internal-combustion engine driving
a generator is e = 3413/f (HHV), where e = indicated thermal efficiency,
i
i
i
expressed as a decimal; f = indicated fuel consumption, lb/kWh; HHV =
i
higher heating value of the fuel, Btu/lb.
Compute the HHV for a diesel fuel from HHV = 17,680 + 60 × °API. For
this fuel, HHV = 17,680 + 60(25) = 19,180 Btu/lb (44,612.7 kJ/kg).
With the HHV known, compute the indicated thermal efficiency from e =
i
3,413/[(0.398)(19,180)] = 0.447 or 44.7 percent.

4. Compute the overall thermal efficiency *
The overall thermal efficiency e is computed from e = 3413/f (HHV),
o
a
a
where f = overall fuel consumption, Btu/kWh; other symbols as before.
o
Using the engine-generator fuel rate from step 2, which represents the overall
fuel consumption, e = 3413/[(0.527)(19,180)] = 0.347, or 34.7 percent.
a

5. Compute the brake thermal efficiency
The engine fuel rate, step 1, corresponds to the brake fuel rate f . Compute
b
the brake thermal efficiency from e = 3413/f (HHV), where f = brake fuel
b
b
b
rate, Btu/kWh; other symbols as before. For this engine-generator set, e =
b
3413/[(0.485)(19,180)] = 0.367, or 36.7 percent.

Related Calculations. Where the fuel consumption is given or computed in
terms of lb/(hp · h), substitute the value of 2545 Btu/(hp · h) (1.0 kW/kWh)
in place of the value 3413 Btu/kWh (3600.7 kJ/kWh) in the numerator of the

e , e , and e equations. Compute the indicated, overall, and brake thermal
i
b
o
efficiencies as before. Use the same procedure for gas and gasoline engines,
except that the higher heating value of the gas or gasoline should be obtained
from the supplier or by test.

DIESEL ENGINE ENERGY EFFICIENCY AND

CHARACTERISTICS