Page 197 - Handbook of Energy Engineering Calculations

A 12 × 18 in (30.5 × 44.8 cm) four-cylinder four-stroke single-acting diesel

engine is rated at 200 bhp (149.2 kW) at 260 r/min. Fuel consumption at
rated load is 0.42 lb/(bhp · h) (0.25 kg/kWh). The higher heating value of the
fuel is 18,920 Btu/lb (44,008 kJ/kg). What are the brake mean effective
3
pressure, engine displacement in ft /(min · bhp), and brake thermal
efficiency?

Calculation Procedure:

1. Compute the brake mean effective pressure
Compute the brake mean effective pressure (bmep) for an internal-

combustion engine from bmep = 33,000 bhp /LAn, where bmep = brake mean
n
2
effective pressure, lb/in ; bhp = brake horsepower output delivered per
n
2
cylinder, hp; L = piston stroke length, ft; A = piston area, in ; n = cycles per
minute per cylinder = crankshaft rpm for a two-stroke cycle engine, and 0.5

the crankshaft rpm for a four-stroke cycle engine.
For this engine at its rated hbp, the output per cylinder is 200 bhp/4
2
cylinders = 50 bhp (37.3 kW). Then bmep = 33,000(50)/[(18/12)(12) (π/4)
2
(260/2)] = 74.8 lb/in (516.1 kPa). (The factor 12 in the denominator converts
the stroke length from inches to feet.)

2. Compute the engine displacement
3
The total engine displacement V ft is given by V = LAnN, where A = piston
d
d
2
area, ft ; N = number of cylinders in the engine; other symbols as before. For
3
2
this engine, V = (18/12)(12/12) (π/4)(260/2) (4) = 614 ft /min (17.4
d
3
m /min). The displacement is in cubic feet per minute because the crankshaft
speed is in r/min. The factor of 12 in the denominators converts the stroke
2
and area to ft and ft , respectively. The displacement per bhp = (total
3
3
displacement, ft /min)/bhp output of engine = 614/200 = 3.07 ft /(min · bhp)
3
(0.12 m /kW).

3. Compute the brake thermal efficiency

The brake thermal efficiency e of an internal-combustion engine is given by
b