OTTO CYCLE         Combustion Internal Combustion Engines         Emissions internal combustion engine         Testing Internal combustion engines          highlights internal combustion engines          Numerical internal combustion engines          Q. Answers internal Combustion engines         Trouble shooting internal combustion engines          Fuels internal combustion engines          Terminology internal combustion engines



Otto cycle is a gas power cycle. Otto cycle is used in petrol engines. It has two isentropic and two constant volume processes. It is a  4 stroke cycle. The first engine based on Otto cycle was built by the German engineer Otto in 1861.

Four Strokes Of Otto Cycle 

Suction Stroke

Piston moves outwards and Intake valve opens but the exhaust valve remains closed.   A homogeneous mixture of air and fuel at low pressure (near to atmosphere) is sucked into the cylinder. Thus piston moves to top dead center (T D C) and the suction stroke is completed.

Compression Stroke

Now piston starts moving towards bottom dead center (B D C),  intake valve closes. Thus both the valves are closed. On further piston movement, the fuel-air mixture is compressed  to high pressure and high temperature .

Power Stroke

This high pressure and high temperature mixture is ignited by a spark plug. Piston remaining at BDC. There is a sudden  rise in pressure and temperature. These gases push the piston back.

Exhaust Stroke

Due to high pressure,  exhaust valve is opened and some exhaust gases go out . The piston forces out the remaining gases. Then the exhaust valve is closed. On further movement of piston, the intake valve opens and the cycle repeated.


  1. Air standard cycle is assumed.

  2. Closed cycle is assumed.

  3. A fixed amount of air is used time and again.

  4. Combustion process is not considered.

  5. Intake process is not considered.

  6. Exhaust process is not considered

  7. Engine friction losses are neglected.

  8. Heat losses are neglected.

  9. Specific heats are considered constant i.e. specific heats do not vary with temperature.

  10. All the processes are reversible.

 However In nature, no process is reversible. INTAKE AND EXHAUST ARE NOT THERE BECAUSE OF THE CLOSED CYCLE ASSUMPTION. It is only to make the analysis simpler. But in any case, we are going far away from reality.

Thus the four REVERSIBLE PROCESSES of the Air Standard Otto cycle are:

Isen-tropic compression (ideal) AB

Heat addition at constant volume BC

Expansion as an Isen-tropic (ideal) CD

Constant volume heat rejection DA