Performance of the Diesel Engine

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The performance of the diesel engine means the power and efficiency. 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:

  1. Constant speed with variable load, and
  2. 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 horse power (I.H.P), Brake horse power (B.H.P), Frictional horse power (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 discussed briefly.

Diesel Engine performance

Diesel engine. Image source:

Indicated Mean Effective Pressure (I.M.E.P):

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).


Indicated Horse Power (I.H.P):

The indicated horse power (I.H.P.) of the engine can be calculated as follows:
Indicated horse power formula for diesel enginewhere, 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 (B.H.P):

Brake horse power is defined as the net power available at the crankshaft. It is calculated by measuring the output torque with a dynamometer.
Brake horse power formula for diesel enginewhere T = Torque in kg.m.
N = Speed in R.P.NT.


Frictional Horse Power (F.H.P):

The difference between the Indicated horse power (I.H.P) and Brake horse power (B.H.P) is called frictional horse power (F.H.P). Frictional horse power is utilized in overcoming frictional resistance of rotating and sliding parts of the engine.

Frictional Horse power (F.H.P) = Indicated horse power (I.H.P) – Brake Horse Power (B.H.P)


Indicated Thermal Efficiency:

Indicated thermal efficiency (ηi) is defined as the ratio of indicated work to thermal input.
Indicated thermal efficiency formula for diesel engine
where, W = Weight of fuel supplied in kg/min,
Cv = Calorific value of fuel oil in,
J = Joules equivalent = 427.


Brake Thermal efficiency (Overall efficiency):

Brake thermal efficiency (ηb) is defined as the ratio of brake output to thermal input.

Brake thermal efficiency for diesel engine


Mechanical Efficiency :

Mechanical efficiency (ηm) is defined as the ratio of brake horse power (B.H.P) to indicated horse power(I.H.P). Therefore,
Mechanical efficiency formula for diesel engine

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