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Automobile Engineering

Specific Output, Volumetric Efficiency And Fuel-air Ratio (F/a)

Introduction:

Specific output of an engine is defined as the brake power (output) per unit of piston displacement

 

Specific Output

 

  • The specific output consists of two elements – the bmep (force) available to work and the speed with which it is working.
  • Therefore, for the same piston displacement and bmep an engine operating at higher speed will give more output.
  • It is clear that the output of an engine can be increased by increasing either speed or bmep. Increasing speed involves increase in the mechanical stress of various engine parts whereas increasing bmep requires better heat release and more load on engine cylinder.

 

Volumetric Efficiency

 

  • Volumetric efficiency of an engine is an indication of the measure of the degree to which the engine fills its swept volume.
  • It is defined as the ratio of the mass of air inducted into the engine cylinder during the suction stroke to the mass of the air corresponding to the swept volume of the engine at atmospheric pressure and temperature.
  • Alternatively, it can be defined as the ratio of the actual volume inhaled during suction stroke measured at intake conditions to the swept volume of the piston.

 

 

 

The amount of air taken inside the cylinder is dependent on the volumetric efficiency of an engine and hence puts a limit on the amount of fuel which can be efficiently burned and the power output. For supercharged engine the volumetric efficiency has no meaning as it comes out to be more than unity.

 

Fuel-Air Ratio (F/A)

  • Fuel-air ratio (F/A) is the ratio of the mass of fuel to the mass of air in the fuel-air mixture. Air-fuel ratio (A/F) is reciprocal of fuel-air ratio.
  • Fuel-air ratio of the mixture affects the combustion phenomenon in that it determines the flame propagation velocity, the heat release in the combustion chamber, the maximum temperature and the completeness of combustion.
  • Relative fuel-air ratio is defined as the ratio of the actual fuel-air ratio to that of the stoichiometric fuel-air ratio required to burn the fuel supplied.
  • Stoichiometric fuel-air ratio is the ratio of fuel to air is one in which case fuel is completely burned due to minimum quantity of air supplied.