Oct 032012

Otto CycleOtto cycle is a gas power cycle that is used in spark-ignition internal combustion engines (modern petrol engines). This cycle was introduced by Dr. Nikolaus August Otto, a German Engineer.

An Otto cycle consists of four processes:

  1. Two isentropic (reversible adiabatic) processes
  2. Two isochoric (constant volume) processes

These processes can be easily understood if we understand p-V (Pressure-Volume) and T-s (Temperature-Entropy) diagrams of Otto cycle.

p-V and T-s Diagrams of Otto Cycle:

p-V Diagram T-s Diagram
Otto Cycle p-V Diagram Otto Cycle T-s Diagram


In the above diagrams,

p → Pressure

V → Volume

T → Temperature

s → Entropy

Vc → Clearance Volume

Vs → Stroke Volume

Processes in Otto Cycle:

As stated earlier, Otto cycle consists of four processes. They are as follows:

Process 1-2: Isentropic compression

In this process, the piston moves from bottom dead centre (BDC) to top dead centre (TDC) position. Air undergoes reversible adiabatic (isentropic) compression. We know that compression is a process in which volume decreases and pressure increases. Hence, in this process, volume of air decreases from V1 to V2 and pressure increases from p1 to p2. Temperature increases from T1 to T2. As this an isentropic process, entropy remains constant (i.e., s1=s2). Refer p-V and T-s diagrams for better understanding.

Process 2-3: Constant Volume Heat Addition:

Process 2-3 is isochoric (constant volume) heat addition process. Here, piston remains at top dead centre for a moment. Heat is added at constant volume (V2 = V3) from an external heat source. Temperature increases from T2 to T3, pressure increases from p2 to p3 and entropy increases from s2 to s3. (See p-V and T-s diagrams above).

In this process,

Heat Supplied = mCv(T3 – T2)


m → Mass

Cv → Specific heat at constant volume

Process 3-4: Isentropic expansion

In this process, air undergoes isentropic (reversible adiabatic) expansion. The piston is pushed from top dead centre (TDC) to bottom dead centre (BDC) position. Here, pressure decreases fro p3 to p4, volume rises from v3 to v4, temperature falls from T3 to T4 and entropy remains constant (s3=s4). (Refer p-V and T-s diagrams above).

Process 4-1: Constant Volume Heat Rejection

The piston rests at BDC for a moment and heat is rejected at constant volume (V4=V1). In this process, pressure falls from p4 to p1, temperature decreases from T4 to T1 and entropy falls from s4 to s1. (See diagram above).

In process 4-1,

Heat Rejected = mCv(T4 – T1)

Thermal efficiency (air-standard efficiency) of Otto Cycle,

{eta _th}~=~{Heat~Supplied~-~Heat~Rejected}/{Heat~Supplied}

eta _th~=~{mC_v(T_3~-~T_2)~-~mC_v(T_4~-~T_1)}/{mC_v(T_3~-~T_2)}


The complete derivation of air-standard efficiency of Otto cycle can be found here and the complete derivation of mean effective pressure (m.e.p) of Otto Cycle can be found here.

How petrol engine works using Otto Cycle – Animation video

Introductory image source

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I am a mechanical engineer with a passion for technical stuff. I am the founder and former editor-in-chief of Mechteacher.com.

  • POi Magno

    What is blowdown?

  • greatodu

    You forgot to explain the WORK in the cycle. The engine is of no worth if it can’t deliver work. Also, explain the resulting temperatures at the end of the isentropic expansions. Thanks, great job ‘though.

  • Aktham Adel

    great topic really u have no idea how much it helped me and my friends >>>>>thanks alot

  • StYyx Ns

    you my sir are saving livesss !!!!!!! thankk you so muchhhhhhhhhhhhhhhhhhhhhh

  • Fahd

    WELL DONE Sir Surjeet
    keep it up
    your a life saver of many students out there

  • Dusan Kacurak

    In the process of combustion you mentioned that the air is compressed… Should it be the mixture of air/fuel together ? since we are dealing with petrol engines ?

    thank you

    • Nikko Nicholas

      same question here,, so i think the mass used is a mixture of air and fuel..correct me if i am wrong

  • vivek kumar

    Does heat rejection process actually occur at constant volume , if yes how ?
    I will be grateful to you sir if you clarify me regarding this question.

  • Abdul Rahman
  • abu adil khan

    sir can we say constant volume heat rejection as isochoric heat rejection process in otto cycle