The Physics of a Four-Stroke Internal
Combustion Engine

The most commonly used type of car engine today, is based on the Otto cycle, named after its creator Nikolaus Otto. The term, four-stroke refers to the four distinguished motions that the piston goes through during the conversion of chemical energy into rotational energy that can be harnessed for a practical use, in this case, to propel a car.

This picture is used as a reference of the parts of the engine which are mentioned on this page of the website.They are described in greater detail on the PARTS page.


1.Intake Stroke

The first stroke of the cycle is described as the intake cycle, where the piston that starts at the top of the cylinder chamber, begins to move downward. At the same time that the piston begins it path downward, the intake valve opens and allows air to be drawn into the cylinder chamber by the downward moving piston. Also during this time a small quantity of gasoline is squirted into the chamber through a fuel injector and mixes with the air. The gasoline must be mixed with the air because liquid gasoline will not burn so it must be vaporized by the injector and mixed with air.The perfect air to gas ratio is 14 parts air to one part fuel.This ratio is electronically controlled by a computer connected to the fuel pump and injectors that supply the amount of fuel in relation to the amount of air that the engine can draw into the cylinder.


2. Compression Stroke

The second stroke, also known as the compression stroke, begins with the closing of the intake valve. As the intake valve closes a sealed chamber is created between the piston and the top of the cylinder where the valves are located. The piston then begins its upward path, the gasoline and air mixture is compressed at a ratio of roughly 10:1. This ratio comes from the differences in volume between the volume of the cylinder chamber at the top of the pistons stroke compared to the volume of the cylinder chamber when the piston is at the bottom of its path.The greater that this ratio can be made the more power an engine can produce.When cars a given 454 in3 or 5.0 liters, this is the total volume ofall cylinders at their intake stroke.Therefore the 454 in3 engine with 8 cylinders can hold 56.75 in3 per cylinder and therefore with a compression ratio of 10:1 can compress this to 5.67 in3. This compression creates a lot of pressure in the cylinder chamber.


3. Combustion Stroke

The third stroke of the cycle, the power stroke refers to the combustion itself. Now that the cylinder chamber is full of highly compressed air and gasoline, a spark from a spark plug initiates an explosion in the chamber that causes a rapid expansion of the compressed mixture, resulting in the piston being forced downward very quickly. The expansion of gas, caused by the combustion is the single most important stage of the cycle. It is also very important that there are no leaks in the system or the pressure will be lost and result in a power loss.


4. Exhaust Stroke 

The fourth and final stroke, is known as the exhaust stroke. Once the piston reaches the bottom of its path after the explosion, all that remains in the cylinder chamber is waste. Once the piston begins its movement upward in the cylinder the exhaust valve opens and the piston forces the exhaust out of the chamber and away from the engine. Following this exhaust removal the intake valve opens allowing air to enter the chamber and continue the cycle.


Pressure vs. Volume relationship


NOTE· The timing of the both the piston-stroke and the valves is crucial to the operation of the engine. To synchronize the timing the crankshaft, which all pistons are attached to and the camshaft, which controls the opening and closing of the valves, are linked using a timing belt. If the timing is off between the motions of the pistons and valves then combustion cannot occur properly, or may just not produce enough force to keep the cycle moving.
A good simulation of the pressure vs volume ratio during the otto cycle:
A visual representation of the Otto Cycle:
A brief history of the Otto Cycle and the car engine: