Process 0-1 :
Also known as the intake stroke
Mass of air at constant pressure is fed into the cylinder/piston
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.
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.