Page 195 - Handbook of Energy Engineering Calculations
P. 195
of 25° API fuel for a 900-kWh output; mechanical efficiency, 82.0 percent;
generator efficiency, 92.0 percent. Compute engine fuel rate, engine-
generator fuel rate, indicated thermal efficiency, overall thermal efficiency,
brake thermal efficiency.
Calculation Procedure:
1. Compute the engine fuel rate
The fuel rate of an engine driving a generator is the weight of fuel, lb, used to
generate 1 kWh at the generator input shaft. Since this engine burns 1.5 bbl
(238.5 L) of fuel for 900 kW at the generator terminals, the total fuel
consumption is (1.5 bbl)(42 gal/bbl) = 63 gal (238.5 L), at a generator
efficiency of 92.0 percent.
To determine the weight of this oil, compute its specific gravity s from s =
141.5/(131.5 + °API), where°API = API gravity of the fuel. Hence, s =
141.5(131.5 + 25) = 0.904. Since 1 gal (3.8 L) of water weighs 8.33 lb (3.8
kg) at 60°F (15.6°C), 1 gal (3.8 L) of this oil weighs (0.904)(8.33) = 7.529 lb
(3.39 kg). The total weight of fuel used when burning 63 gal is (63 gal)(7.529
lb/gal) = 474.5 lb (213.5 kg).
The generator is 92 percent efficient. Hence, the engine actually delivers
enough power to generate 900/0.92 = 977 kWh at the generator terminals.
Thus, the engine fuel rate = 474.5-lb fuel/977 kWh = 0.485 lb/kWh (0.218
kg/kWh).
2. Compute the engine-generator fuel rate
The engine-generator fuel rate takes these two units into consideration and is
the weight of fuel required to generate 1 kWh at the generator terminals.
Using the fuel-consumption data from step 1 and the given output of 900 kW,
we see that engine-generator fuel rate = 474.5-lb fuel/900 kWh output =
0.527 lb/kWh (0.237 kg/kWh).
3. Compute the indicated thermal efficiency
Indicated thermal efficiency is the thermal efficiency based on the indicated
horsepower of the engine. This is the horsepower developed in the engine
cylinder. The engine fuel rate, computed in step 1, is the fuel consumed to
produce the brake or shaft horsepower output, after friction losses are