Engine Performance Parameters Of an IC Engine

The role of an internal combustion engine is to convert heat energy into mechanical energy. The performance of an IC engine means how it will convert heat energy into mechanical energy. This performance can be measured in terms of certain parameters & these parameters are called performance parameters.

The following are some important parameter which is measured in order to calculate the performance of the engine:

1. Indicated Power
2. Brake Power
3. Frictional Power 
4. Volumetric efficiency 
5. Mechanical efficiency
6. Thermal efficiency
7. Indicated Mean Effective Pressure 
8. Brake  Mean Effective Pressure 
9. Air-Fuel ratio
10. Relative Air-Fuel ratio
11. Brake Specific fuel consumption
12. Specific output

Indicated Power (IP)

• The power which is developed during the combustion of fuel inside the combustion chamber(engine cylinder)  is called Indicated Power (IP). 
• It can be calculated as: 

 I.P=PmALn60000in kW

 were,

• Pm=Indicated mean effective pressure in bar
• n=Number of power strokes per minute
• n=N, for a two-stroke engine 
• n=N2, for a four-stroke engine 
• N=Speed of the engine in rpm
• A=Cross sectional area of the piston in m2
• Length of stroke in m

Brake Power (BP)

• The power which is available at an engine output shaft (crankshaft)  is called brake power.
• It is the power that is used to drive the vehicle 
• It can be calculated as:

 B.P=2NT60000in kW

 where,

• T=Torque at output shaft(crankshaft) in N-m.
• N=Speed of the engine in rpm

Frictional Power (FP)

• It is the power that is lost due to friction.
• It is the difference between Indicated power and Brake Power.
• It can be calculated as:

F.P=I.P-B.P in kW

Mechanical efficiency(m)

• Mechanical efficiency basically measures the effectiveness of a  mechanical system.
• It is the ratio of brake power to the indicated power of an IC Engine.
• It can be calculated as:

  m=B.PI.P

Volumetric efficiency(v)

• It is defined as the ratio of actual volume of air drawn inside the engine cylinder during the suction stroke to the swept volume of the piston.
• Swept volume is the volume between  TDC to BDC.
• Volumetric efficiency is an important performance parameter, which directly affects the power output of the engine. 
• More the volumetric efficiency, the more power will be produced.
• It can be calculated as

 v=VaVs

 were,

• Va=Actual volume of air drawn inside the engine cylinder during suction stroke in m3
• Vs=swept volume of the piston in m3

Thermal efficiency

• It is the ratio of work done or power developed by an engine to the rate of heat supplied by burning of fuel in the engine. 
• In an IC engine, thermal efficiency is either based on indicated power or brake power and accordingly, we have two types of thermal efficiencies
• Indicated thermal efficiency

        i=I.

• Brake or overall thermal efficiency

        b=

 were,

• I.P=Indicated Power in kW
• B.P=Brake Power in kW
• mass of fuel consumed in kg/sec
• C.V =Calorific value in kJ/kg

Indicated means effective pressure

• It is a theoretical mean effective pressure that, if imposed on the pistons uniformly throughout the power stroke, would produce work that will be equal to mechanical work generated during the actual cycle.
• It is usually determined with the help of a P-V diagram taken with the help of an indicator mechanism. The area of the PV diagram drawn by the indicator mechanism can be calculated by any measurement method.
• • ma=mass of air in kg 
  • mf=mass of fuel in kg 
    • Then, the indicated mean effective pressure can be determined as

    Pm=Area of indicator diagram Spring stiffnessLength of the indicator diagram

    Brake mean effective pressure

    • It is a theoretical mean effective pressure which, if imposed on the pistons uniformly throughout the power stroke, would produce work that will be equal to the measured brake power.
    • It can be calculated as:

    BMEP=Work done per cycleStroke volume

    Air-Fuel ratio

    • Air-fuel ratio (A: F)is the ratio between the mass of air ma and mass fuel mf, used by the engine when running
    • It can be calculated as

    A: F=mamf

     were,

  Relative Air-Fuel ratio

  • It is defined as the ratio of actual fuel-air ratio to that of stoichiometric air-fuel ratio or chemically correct fuel-air ratio.
  • The stoichiometric fuel-air ratio is the ratio in which complete combustion of fuel takes place and no excess air is left.
  • In a given Air-Fuel mixture, if the relative Air-Fuel ratio is less than 1 it is a rich mixture and if it is more than 1, it is a lean mixture.
  • It can be calculated as

    Relative Air fuel ratio=Actual Air fuel ratioStoichiometric Air fuel ratio

  Brake Specific fuel consumption

  • Brake-specific fuel consumption (BSFC) is a parameter that is used to measure the fuel efficiency of an engine that burns fuel and produces rotational power. 
  • In automotive applications, BSFC is used to evaluate the efficiency of the internal combustion engines (ICE).
  • Brake-specific fuel consumption is defined as the amount of fuel required to be supplied to an engine to develop 1kW power per hour.
  • It can be calculated as

                 BSFC=mfB.Pin kg/hour per kW 

   were,

  • mf=mass of fuel consumed in kg/hr
  • B.P=Brake power in kW

  Specific output

  • It is defined as the brake power output per unit of piston displacement. 
  • It is a parameter that relates the power of an engine with its size so it is an important performance parameter.
  • It can be calculated as

                 Specific output=B.PAL=PmbN in kW/m3 

   were,

  • Pmb=Brake mean effective pressure in bar
  • B.P=Brake power in kW
  • N=Speed of the engine in rpm
  • A=Cross sectional area of the piston in m2
  • L=length of stroke in m
  • From the expression of specific output, we can conclude that for the same piston displacement and brake mean effective pressure, an engine running at a higher speed will give more power.