Brake Specific Fuel Consumption - EngineKnowHow

During engine testing the fuel consumption of the engine is the mass flow rate of the fuel.  However, to easily compare different engines to one another that have different displacements, ignition systems etc.  a comparable parameter is the brake specific fuel consumption, or bsfc.  The specific fuel consumption is a measure of how efficiently the fuel is being used to generate power at the crankshaft and is calculated by:

 

 

 

 

 

 

 

 

When looking at BSFC it’s common to review it as a function of BMEP, as both parameters are independent of engine characteristics and allow for easy comparison between engines.

 

Examples of Engine BSFC Maps

Below is a BSFC map for a 1.8L, Inline-4, turbocharged petrol engine:

From the plot of BSFC over the entire engine operating range the maximum fuel efficiency occurs in an area at approximately half of the maximum engine speed and at 80% of the maximum load.  This area of maximum fuel efficiency is in a similar location for nearly all spark ignition engines, regardless of natural aspiration or boosting.

 

From the area of maximum fuel efficiency, if the load is increased whilst the engine speed remains constant, the fuel efficiency decreases due to:

  • The spark being retarded as the engine becomes knock limited.
  • Overfuelling for turbine protection.

 

From the area of maximum fuel efficiency, if the engine speed is increased whilst the load remains constant, the fuel efficiency is dominated by the:

  • Increasing engine friction.

 

From the area of maximum fuel efficiency, if the load is decreased whilst the engine speed remains constant, the fuel efficiency is dominated by the:

  • Increasing pumping work.

 

From the area of maximum fuel efficiency, if the engine speed is decreased whilst the load remains constant, the fuel efficiency is dominated by the:

  • Increasing heat losses.

 

Below is a BSFC map for a 2.0, Inline-4, turbocharged diesel engine:

Comparing the BSFC map of the petrol engine to the diesel engine, the diesel engine offers lower fuel consumption across its (more speed limited) engine map.  This reasons being:

  • The diesel engine operates without a throttle (unthrottled) which reduces pumping losses, especially in the lower load regions.
  • The diesel engine operates lean (excessive air to fuel) increasing the likelihood that the fuel is combusted.
  • The diesel engine has a higher compression ratio.