The Actual Engine Cycles - EngineKnowHow

An Actual Otto Engine Cycle

The plot below represents the relationship between the cylinder pressure and volume of an actual spark ignition engine operating under part load conditions.  The beauty of the actual Pressure-Volume loop is it gives a good overview of what’s occurring in the engine in terms of work required and outputted as the area contained within the curves are equal to energy / work.

 

An Actual Diesel Engine Cycle

The plot below represents the relationship between the cylinder pressure and volume of an actual compression ignition engine operating under part load conditions.

 

Differences between Theoretical and Actual Engine Cycles

In reality there a several differences between the ideal Otto and Diesel thermodynamic cycles and their actual engine cycles.  These differences account for why the ideal thermal efficiency can never be achieved in real life:

 

  • Not all of the fuel is combusted. Combustion Efficiency, ηc < 100%.
    • Lower maximum temperature and pressure
  • 100% of the combustants will not be exhausted from the cylinder, there will be residuals.
    • Lower maximum temperature and pressure
  • Combustion takes time and won’t occur at a constant volume in a spark ignition engine nor at constant pressure in a compression ignition engine.
    • Lower maximum temperature, pressure and efficiency
  • During compression and expansion heat will be transferred to the piston and cylinder.
    • Not adiabatic / isentropic
  • Valve timing does not occur at TDC and BDC.
  • Flow through the valves changes with the valve lift.
  • There is work required to pump the fresh air into the engine and the combustants out of the engine.
    • Requires additional work from the engine
  • There will be blow-by between the piston and the cylinder walls.
    • Lower maximum pressure
  • The specific heat coefficients, cp and cv increase with temperature.
    • Lower maximum temperature and pressure
  • The composition of the working fluid changes from air to air / fuel to combustants.
  • As the engine draws air from the atmosphere and exhausts the combustants to the atmosphere it is considered an open system and is therefore an engine cycle (open loop) and not a thermodynamic cycle (closed loop).

 

Despite these differences between the ideal and actual cycles the ideal cycles are good approximations and aid in the development of engines by showing the effects of, for example combustion temperature, pressure, compression ratio and cutoff ratio on engine efficiency and performance.

 

Due to these approximations, for a spark ignition engine the actual thermal efficiency relates to the ideal thermal efficiency by approximately: