Otto Cycle

Otto Cycle is the theoretical thermodynamic cycle which describes the working of a spark ignition engine. This type of spark ignition engines is the most common type of engines used in automobiles. Today we will attempt to study it and understand what comprises of this Otto Cycle.

The Otto cycle is the study of what happens to a mass of gas when it is subjected to changes in pressure, temperature, volume, adding heat and removing heat. The system is the term given to the mass of gas that is subjected to these changes. Otto Cycle also studies the effect of this system on the environment. The effect in question here is the net output or the work generated by the Otto Cycle to move the automobile in which the engine is installed.

otto cycle

The name Otto Cycle comes from the name of the person who has put forward the theory of this system.His name was Dr Nicolas August Otto.
The Otto Cycle comprises a top and bottom of the loop process which is called isentropic process. This process is frictionless and adiabatic. And an isochoric process which happens at left and right side of the loop and has the constant volume.

The reason we consider the Otto cycle as a theory is because of its premise that it operates in a completely efficient system where no energy is lost. However, we know that in reality, it is still not possible.

Also Read: Diesel Cycle vs Otto Cycle

The isentropic process implies that during compression cycle there will be no loss of mechanical energy and considers that no heat will either enter the system or leave it.
In theory, heat flows through the left pressurizing process and some of it passes back through the right depressurizing process. The difference of heat here gives the net mechanical work generated.

The Following Are The Four Processes In Otto Cycle-

Process 0-1 :

Also known as the intake stroke

Mass of air at constant pressure is fed into the cylinder/piston

Process 1-2:

Also known as Compression stroke.

In this process, the isentropic compression of the charge happens. This happens due to movement of the cylinder from bottom dead center to top dead center.This is the time when air-fuel mixture is compressed.

Process 2-3 :

This is also known as Ignition phase.

Here the piston for a moment of time rests at the top dead center. There is a little air-fuel mixture present at the top during this process. Heat is then introduced into the system which ignites the air-fuel mixture. Due to this, the volume remains constant whereas the pressure rises.

Process 3-4 :

Also known as Expansion stroke.

Also Read: Diesel Cycle vs Otto Cycle

The rise in pressure due to ignition causes the piston to move to bottom dead centre.Gases are expanded isentropically and hence the system works on the piston.In simpler terms, the expansion of gases leads to movement of piston here.

Process 4-1 :

Also known as Heat Rejection phase.

The piston comes to rest at bottom dead center for a while. This drops the gas pressure immediately as the heat is removed using a heat sink at the cylinder head. The gas returns to its original state as was in step 1.

Process 1-0 :

Also known as the Exhaust stroke.

The exhaust valve opens in this process as the piston moves from Bottom dead center to top dead center. The remainder gas is expelled and the process again starts from 0-1.

So here we have in theory how an Otto process works.

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