The Ideal Otto Cycle - EngineKnowHow

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

 

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

  • Combustion occurs at a constant volume at TDC
  • No heat transfer to surroundings (adiabatic)
  • Combustion efficiency,  = 100%
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

  • Heat rejected at a constant volume
  • No heat transfer to surroundings (adiabatic)
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 Otto cycle:

Compression:

 

 

 

 

 

 

 

 

 

Combustion

 

 

 

Expansion

 

 

 

 

 

 

 

 

Heat Rejection

 

 

 

The Ideal Thermal Efficiency

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

 

 

 

 

 

Substituting the equations above, the ideal thermal efficiency for spark ignition engines, or Otto engine is: