Pistons are key components of many engines, notably those in cars, and they utilize basic scientific principles to generate force and energy. Below is a simplified schematic of a diesel engine cycle:

The piston engine is composed of a cylindrical container (presented in 2D as a rectangle), an intake valve (left) and an exhaust valve (right), as well as the piston (top), which is a cylindrical plate attached to a movable rod allowing it to go up and down along the walls of the container. The first step in the cycle is for fuel to enter the chamber (A). Once the fuel is inside, the intake valve closes and the piston will compress the fuel (B). This adiabatic compression (defined as no loss of heat to the environment) causes the fuel to combust and ignite. The resultant explosion is an isobaric expansion – at constant pressure – that pushes the piston back up and generates force (C). Finally, the piston goes down again and expels the remaining exhaust by the exhaust valve and the cycle resumes (D).

Ideally, fuel will ignite at the maximal compression of the piston in order to generate maximum force. However, if the fuel ignites prior to the piston reaching its maximum compression length, the piston is still compressing while the fuel is expanding, and this results in opposite forces that decrease the efficiency of the piston engine. This is called “”knocking””. For many years, tetra-ethyl lead (TEL) was added to fuel because it helped prevent premature auto-ignition.

TEL combusts by the following reaction:

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