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Computer drawing of the Wright 1903 aircraft engine operation
 during the exhaust stroke

For the forty years following the first flight of the Wright brothers, airplanes used internal combustion engines to turn propellers to generate thrust. Today, most general aviation or private airplanes are still powered by propellers and internal combustion engines, much like your automobile engine. On this page we will discuss the fundamentals of the internal combustion engine using the Wright brothers' 1903 engine, shown in the figure, as an example.

The brothers' design is very simple by today's standards, so it is a good engine for students to study to learn the fundamentals of engine operation. This type of internal combustion engine is called a four-stroke engine because there are four movements (strokes) of the piston before the entire engine firing sequence is repeated. In the figure, we have colored the fuel/air intake system red, the electrical system green, and the exhaust system blue. We also represent the fuel/air mixture and the exhaust gases by small colored balls to show how these gases move through the engine. Since we will be referring to the movement of various engine parts, here is a figure showing the names of the parts:

Computer drawing of the Wright 1903 aircraft engine showing the
 labeled parts in a single cylinder.

Mechanical Operation

At the end of the heat rejection process, the piston is at the far left and exhaust gas has been cooled to nearly atmospheric conditions. The exhaust valve is then opened to begin the exhaust stroke. The purpose of the exhaust stroke is to clear the cylinder of the spent exhaust in preparation for another ignition cycle. The exhaust stroke begins at Stage 6 as the piston is pushed towards the combustion chamber (to the right in the figure). The intake valve is closed, the electrical contact is open, and the exhaust valve is opened by the cam pushing on the rocker arm. The exhaust gas is pushed past the valve and exits the engine. At the end of the exhaust stroke, Stage 1, the piston is located at the far right and is ready to begin another intake stroke after the exhaust valve is closed and the intake valve is opened.


The exhaust stroke takes place at a nearly constant atmospheric pressure because the exhaust valve is open to the atmosphere throughout the stroke. There is (theoretically) no work done on the exhaust during this process. The random motion of the gas causes to exit the combustion chamber and cylinder as the volume is decreased by the piston motion. The pressure and temperature ratios are both 1.0 during the exhaust stroke.


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Editor: Nancy Hall
NASA Official: Nancy Hall
Last Updated: May 13 2021

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