Ideal Thermal Efficiency - EngineKnowHow

For an internal combustion engine it is possible to determine the maximum theoretical efficiency of the engine, or how much of the heat generated from the combustion of fuel could under perfect conditions be converted into useful work.  This maximum theoretical efficiency is known as the Ideal Thermal Efficiency, or the Indicated Fuel Conversion Efficiency of the engine and is calculated from the ideal air-standard thermodynamic cycles.


For both spark and compression ignition engines perfect, or ideal conditions do not exist which would allow them to reach their ideal thermal efficiency.  Losses occur due to:

  • Incomplete combustion of the fuel;
  • Engine knock and pre-ignition of the fuel;
  • The pumping work required to pump the air and exhaust though the intake and exhaust systems;
  • The mechanical friction of the engine (parts sliding against one another);
  • Heat losses to the exhaust and cooling system;
  • The exhaust components will never be completing removed from the cylinder and therefore allow a 100% fresh charge to enter;
  • Incomplete filling of the cylinder with air due to the valve timing and pressure pulsations in the intake system;
  • Combustion gases can escape between the piston and the cylinder walls; and,
  • The engine being designed to reduce noise and optimise emissions.


Spark Ignition

The ideal thermal efficiency for a spark ignition engine is calculated by:





In an engine, the specific heat ratio is the ratio between the heat capacity of the fuel air mixture under constant pressure conditions and under constant volume conditions, where the heat capacity is the amount of energy which must be added to change the temperature.   So for example, for air in a cylinder with a constant volume, the heat capacity is how much energy must be delivered to the air to result in 1°C temperature change.


When calculating the ideal thermal efficiency of an engine it is common to use the specific heat ratio for just air as those for fuel are very similar and the cylinder is predominantly filled with air, therefore:



The plot below shows how the ideal thermal efficiency of a spark ignition engine increases with the compression ratio.

Testing has shown that the maximum efficiency for spark ignition engines is at a compression ratio of approximately 17.  Above this compression ratio, real world efficiency begins to drop due to cylinder pressures forcing fuel into crevices in the cylinder, eg. between the piston and the cylinders walls prior to combustion and increasing heat losses.


Compression Ignition

For a compression ignition engine, the ideal thermal efficiency is calculated by:






TThe cutoff ratio is the ratio between the cylinder volume at the start of combustion and at the end of combustion.  The plot below compares the ideal thermal efficiency of a spark ignition engine against a compression ignition engine over compression ratio.

Comparing the ideal thermal efficiency of the spark ignition engine to the compression ignition engine, the spark ignition engine is capable of higher thermal efficiencies.  In reality these thermal efficiencies are limited by knock and pre-ignition of the fuel, limiting the compression ratio.  The diesel engine also runs unthrottled, reducing pumping losses and lean, improving the combustion efficiency.