Diesel Cycle vs Otto Cycle

Diesel cycle is also called as constant pressure cycle. Diesel engine operates on this cycle. This cycle also contains four processes,out of which two processes are adiabatic,third one is constant pressure process and forth process is constant volume process.

Petrol or spark ignition engines use otto cycle while diesel or compression ignition engines use diesel cycle. Today, we are going to learn about the differences between these two cycles i.e., Otto cycle and Diesel cycle. The main and basic difference between Otto-cycle and diesel cycle is that of the way the way they supply heat to the engine to initiate combustion.

Otto Cycle –

otto cycle, diesel cycle

German scientist Dr. Nikolaus August Otto devised the compressed charge internal combustion engine. Due to this reason, the working process behind the petrol engine got known as Otto cycle. The modern petrol engines work on this cycle.

Also ReadWhat Is Diesel Cycle? What Are The Four Processes Of Diesel Cycle

  • In the Otto cycle, heat addition takes place at constant volume
  • At the time of heat addition, the piston is at the Top Dead Centre (TDC).
  • The Otto cycle has compression ratio from 7:1 to 10:1 which is lesser than that of a diesel cycle.
  • It has comparatively has higher thermal efficiency.
  • It uses spark plugs to ignite the charge (fuel+air).
  • The charge is drawn in by the cylinders during the intake stroke.
  • Overall efficiency is lesser than the diesel cycle.
  • During the backward stroke of the piston, complete adiabatic expansion takes place.
  • As it is pretty evident by now that otto cycles are used in petrol or spark engines. So otto cycles are working in all four stroke and two stroke petrol engines running on the roads.

Diesel Cycle-

diesel cycle

  • Rudolph Diesel invented the diesel engine that is why the working principle is known as diesel cycle.
  • In the diesel cycle, heat addition takes place at constant pressure.
  • In the diesel cycle, heat addition takes place when the piston is retreating.
  • The diesel cycle has comparatively higher compression ratio. It ranges from 11:1 to 22:1.
  • It has lower thermal efficiency in comparison to Otto cycle.
  • Due to high compression ratio, the fuel ignites on its own.
  • After drawing air during the intake stroke, fuel is injected by an injector.
  • Overall efficiency is higher than the Otto cycle
  • In the diesel cycle, the adiabatic expansion takes place when the heat addition is cut-off.
  • A diesel engine is used in large trucks, where high efficiency, low-stress cycle results in better and longer engine life at lower operational cost. Due to this factor, diesel engines are ideal for heavy hauling works.

Also Read: What Is Diesel Cycle? What Are The Four Processes Of Diesel Cycle

This is all about Otto cycles and Diesel cycles. If you find something missing or want something new to be discussed, leave your thoughts in the comment box.


  1. Shanmukha datta March 5, 2018
  2. Shanmukha datta March 5, 2018

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