The performance of the diesel engine means power and efficiency. The diesel engine develops as the various parameters of the engine such as piston speed, air-fuel ratio, compression ratio, inlet air pressure and temperature are varied.
The two usual conditions under which Internal combustion engines are operated are:
- Constant speed with variable load, and
- Variable speed with variable load.
The first situation is found in alternating current 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 horsepower (I.H.P), Brake horsepower (B.H.P), Frictional horsepower (F.H.P), Mechanical efficiency (ηm), indicated thermal efficiency (ηi), brake thermal efficiency (ηb), fuel consumption and also specific fuel consumption etc. Below, how these quantities are measured is discussed briefly.
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).
The indicated horsepower (I.H.P.) of the engine can be calculated as follows:
where, Pm = LM.E.P. in kg/cm2
L = Length of stroke in metres,
A = Piston areas in cm2,
N = Speed in R.P.M,
n = Number of cylinders,
k = 1 for two-stroke engine and
k = 2 for four-stroke engine.
Brake horse power is defined as the net power available at the crankshaft. It is calculated by measuring the output torque with a dynamometer.
where T = Torque in kg.m.
N = Speed in R.P.NT.
The difference between the Indicated horsepower (I.H.P) and Brake horsepower (B.H.P) is called frictional horsepower (F.H.P). Frictional horsepower is utilized in overcoming the frictional resistance of rotating and sliding parts of the engine.
Indicated thermal efficiency (ηi) is defined as the ratio of indicated work to thermal input.
where, W = Weight of fuel supplied in kg/min,
Cv = Calorific value of fuel oil in k.cal/kg,
J = Joules equivalent = 427.
Brake thermal efficiency (ηb) is defined as the ratio of brake output to thermal input.