8.8 PERFORMANCE OF DIESEL ENGINE The performance of the diesel engine means the power and efficiency. The engine develops as
the various parameters of the engine, e.g. piston speed, air-fuel ratio, compression ratio, inlet air-pres-
sure and temperature are varied.
The two usual conditions under which I.C. engines are operated are: (1) constant speed with
variable load, and (2) variable speed with variable load. The first situation is found in a.c. generator
drives and the second one in automobiles, railway engines and tractors etc. A series of tests are carried
out on the engine to determine its performance characteristics, such as: indicated power (I.P.), Brake
power (B.P.), Frictional Power (F.P.), Mechanical efficiency (
η
m ), thermal efficiency, fuel consumption
and also specific fuel consumption etc. Below, we shall discussed how these quantities are measured:
8.8.1 INDICATED MEAN EFFECTIVE PRESSURE (IMEP) In order to determine the power developed by the engine, the indicator diagram of engine should
be available. From the area of indicator diagram it is possible to find an average gas pressure that while
acting on piston throughout one stroke would account for the network done. This pressure is called
indicated mean effective pressure (I.M.E.P.).
8.8.2 INDICATED HORSE POWER (IHP) The indicated horse power (I.H.P.) of the engine can be calculated as follows:
I.H.P. =
(P . L.A.N. )
(4500
)
×
m n k
240
POWER PLANT ENGINEERING
where
P
m = LM.E.P. in kg/cm
2
L = Length of stroke in metres
A = Piston areas in cm
2
N = Speed in R.P.M.
n = Number of cylinders
k = 1 for two stroke engine
= 2 for four stroke engine.
