The Ideal Diesel Cycle - EngineKnowHow

Using the ideal air-standard cycle it is possible to determine the thermodynamic cycle for a compression ignition, or a diesel engine.  This thermodynamic cycle is known as the Diesel Cycle, named for one of the originators of the compression ignition engine.   The plot below shows the Diesel cycle over cylinder pressure and volume under full load conditions and is another representation of the four stroke engine cycle.

 

The Diesel cycle differs from the Otto cycle in that combustion occurs at constant pressure as the fuel must be injected into the cylinder following the compression of the air.  As there is a delay in ignition combustion will occur during the initial part of the expansion stroke.

Stage Process Assumptions
1 → 2 Compression

Work completed by the piston compressing the air

  • No heat transfer to surroundings (adiabatic)
  • Process reversible (therefore isentropic)
  • Air is an ideal gas, therefore the specific heat ratio is constant
2 → 3 Combustion

Heat is added to the air by combustion

  • No heat transfer to surroundings (adiabatic)
  • Combustion efficiency,  = 100%
  • Combustion starts at TDC and occurs at a constant pressure
3 → 4 Expansion

Work is extracted from the air by expansion against the piston

  • No heat transfer to surroundings (adiabatic)
  • Process reversible (therefore isentropic)
  • Air is an ideal gas, therefore the specific heat ratio is constant
4 → 5 Heat Rejection

Heat is removed from the air by exhausting from the cylinder

  • No heat transfer to surroundings (adiabatic)
  • Heat rejected at a constant volume
4 → 5 → 6  

Exhaust Stroke

The air leaves the engine

 

 

 

  • No heat transfer to surroundings (adiabatic)
  • Valve events occur at TDC and BDC
  • Inlet and exhaust pressures constant (at atmospheric pressure)
  • The velocity effects are negligible
  • Reversible and no work required
6 → 1  

Intake Stroke

The air enters the engine

From these processes and assumptions, it is possible to determine several operating points on the Diesel cycle:

 

Compression

 

 

 

 

 

 

 

 

 

Combustion

 

 

 

Cut Off Ratio

 

 

 

Expansion

 

 

 

 

 

 

 

 

 

 

Heat Rejection

 

 

 

 

Ideal Thermal Efficiency

For a thermodynamic cycle, the thermal efficiency is calculated by:

 

 

 

 

 

Substituting the equations above, the ideal thermal efficiency for compression ignition engines is: