Engine Performance Parameters

Engine performance parameters

Engine performance is an indication of the degree of success of the engine performs its assigned task, i.e. the conversion of the chemical energy contained in the fuel into the useful mechanical work. The engine performance is indicated by the term efficiency, η. Five important engine efficiencies and other related engine performance parameters are:

  1. Power
  2. Indicated Thermal Efficiency (ηith)
  3. Brake Thermal Efficiency (ηbth)
  4. Mechanical Efficiency (ηm)
  5. Volumetric Efficiency (ηv)
  6. Relative Efficiency or Efficiency Ratio (ηrel)
  7. Mean Effective Pressure (Pm)
  8. Specific Fuel Consumption (sfc)
  9. Fuel-Air or Air-Fuel Ratio (F/A or A/F)
  10. Calorific Value (CV)
Figure- Engine performance parameters


    Power is the most important engine performance parameters. The main purpose of running an engine is to obtain mechanical power. Power is defined as the rate of doing work and is equal to the product of force and linear velocity or the product of torque and angular velocity.

Power (P)= Torque (T) x Angular velocity (ω)

P=T ω


Power can also be written in the form of workdone per unit time i.e.

Power= Workdone/Time

In I.C. Engine, power is classified into three categories i.e.

  1. Brake Power or Brake Horse Power (bhp or bp)
  2. Indicated power or indicated horse power (ihp or ip)
  3. Frictional Power (F.P.)

Brake Power (B.P.)

The power developed by an Engine at the output shaft is called the brake power.

Brake Power= Brake Workdone/Time


Indicated power (I.P.)

The total power developed by Combustion of fuel in the combustion chamber is called indicated power.

Indicated Power= Indicated Workdone/Time


Frictional Power (F.P.)

The difference between I.P. and B.P. is called frictional power (f.p.).

FP = IP – BP

Mechanical Efficiency (ηm)

It is the ratio of brake power to the indicated power i.e.

ηm =BP/IP

Thermal Efficiency (ηth)

This is also one of the most important engine performance parameters. Thermal efficiency is the ratio of Power to energy supplied by the fuel.

ηth= Power/ Energy

In I.C. Engine, thermal efficiency can be classified into two categories i.e.

Indicated Thermal Efficiency (ηith)

Indicated thermal efficiency is the ratio of indicated power to the heat supplied or added.

ηith= IP/Qs

Brake Thermal Efficiency (ηith)

Brake Thermal Efficiency is the ratio of brake power to the heat supplied or added.  

ηbth= BP/Qs

Volumetric Efficiency v)

This is one of the most important parameters which decide the performance of four-stroke engines. Four stoke engines have distinct suction stoke, volumetric efficiency indicates the breathing ability of the engine.

Volumetric efficiency is defined as the ratio of actual flow rate of air into the intake system to rate at which the volume is displaced by the system.

ηv= (m ̇”a/Ρa )/(Vdispaced x N/2)

“Ρa”= Inlet density is taken atmospheric air density

N= Number of cylinder in use

Relative Efficiency or Efficiency Ratio(ηrel)

   Relative efficiency is the ratio of  thermal efficiency of an actual cycle to that of the ideal cycle. The efficiency ratio is a very useful criterion which indicates the degree of development of the engine.

ηrel = (Actual thermal Efficiency)/(Air-Standard Efficiency)

Mean Effective Pressure (Pmean)

Mean effective pressure is the average pressure inside the cylinders of an internal combustion engine based on the calculated or measured power output. It can be expressed as the ratio of Workdone to the Swept volume.

Pmean= Workdone/Swept volume(Vs)

Pmean= W.D./V1-V2

Also read: I.C. Engine

Specific Fuel Consumption (sfc)

The Fuel Consumption characteristics of an engine are generally expressed in terms of specific fuel consumption in kilograms of fuel per kilowatt-hour.

It is an important parameter that reflects how good the engine performance is. It is inversely proportional to the thermal efficiency of the engine

Sfc= (Fuel consumption per unit time)/Power

Brake Specific fuel consumption (bsfc)

bsfc= (Fuel consumption per unit time)/(Brake Power)

Indicated Specific fuel consumption (isfc)

isfc= (Fuel consumption per unit time)/IndicatedPower (IP)

Calorific Value (CV)

  Calorific value of a fuel is the thermal energy released per unit quantity of the fuel when the fuel is burned completely and the products of combustion are cooled back to the initial temperature of the combustible mixture.

Heat added per kg of fuel = Calorific Value (CV)

HA/sec = C.V. x mf/sec

HA/sec= (HA/kg of fuel) x mf/sec

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