PV diagrams and why we love them.

http://www.grc.nasa.gov/WWW/K-12/airplane/otto.html

The four-stroke engine and all internal combustion engines are based on the Otto cycle and have the same steps.

    PV diagrams show the comparison of the state variables, Pressure vs Volume, and are used to describe the changes of state. These diagrams give us an idea of what is happening during this thermodynamic procces.