Power Plant Engineering

DIESEL POWER PLANT 259

Solution. Brake thermal efficiency, ηb = 30%

Air-fuel ratio by weight = 20

Calorific value of fuel used, C = 41800 kJ/kg

Brake mean effective pressure, pmb =?

Brake thermal efficiency = work produced/heat supplied

0.3 = work produced/41800

Work produced per kg of fuel = 0.3 × 41800 = 12540 kJ

Mass of air used per kg of fuel = 20 kg

S.T.P. conditions refer to 1.0132 bar and 15°C

Volume of air used =

RT

P

m

= 5

(20 287) (273 15)

1.0132 10

××+

×

= 16.31 m^3

Brake mean effective pressure, Pmb =

work done

cylinder volume

= 5

(12540 1000)

16.31 10

×

×

= 7.69 bar.

Example 6. A 2-cylinder C.I. engine with a compression ratio 13:1 and cylinder dimensions of
200mm × 250mm works on two stroke cycle and consumes 14kg/h of fuel while running at 300 r.p.m.
The relative and mechanical efficiencies of engine are 65% and 76% respectively. The fuel injection is
effected upto 5% of stroke. If the calorific value of the fuel used is given as 41800 kJ/kg, calculate the
mean effective pressure developed.

Solution. Refer Fig. 8.18.

Diameter of cylinder, D = 200 mm = 0.2 m

Stroke length, L = 250 = 0.25 m

Number of cylinders, n = 2

Compression ratio, r = 14

Fuel consumption = 14 kg/h

Engine speed, N = 300 r.p.m.

Relative efficiency, ηrelative = 65%

Mechanical efficiency, ηmech = 76%

Cut-off = 5%n of stroke

Calorific value of fuel, C = 41800 kJ/kg

k = 1 for two-stroke cycle engine V 3

Cut-off ratio, p =^3

2

ν

ν