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Computer drawing of IC engine.

To move an airplane through the air, thrust is generated by some kind of propulsion system. Beginning with the Wright brothers' first flight, many airplanes have used internal combustion engines to turn propellers to generate thrust. Today, most general aviation or private airplanes are powered by internal combustion (IC) engines, much like the engine in your family automobile. When discussing engines, we must consider both the mechanical operation of the machine and the thermodynamic processes that enable the machine to produce useful work. On this page we consider the thermodynamics of a four-stroke IC engine.

On the figure we show an internal view of the Wright brothers' 1903 engine at six times, or stages, during a thermodynamic cycle. The Wright engine has been chosen because of its simplicity, but the same six stages occur in all four-stroke IC engines. The stages proceed from the upper left to the bottom left, then from the bottom right to the upper right in a continuous cycle. We label the stages for the same reasons that we labeled the stations of a gas turbine engine; to better organize our analysis of the performance of the engine. The thermodynamic cycle for the four-stroke IC engine was developed by Dr. N. A. Otto, in 1876. The cycle proceeds as follows:

  • The cycle begins when the intake valve opens and a mixture of fuel and air is drawn into the cylinder from the intake manifold. The piston is pulled towards the crankshaft, to the left in the figure, at constant pressure because the valve is open. The motion of the piston is called a stroke. Stage 1 is the beginning of the intake stroke.
  • At the end of the intake stroke, the intake valve is closed and the piston is moved back towards the combustion chamber. Since the valves are closed, the pressure and temperature are increased by the adiabatic compression. Stage 2 is the beginning of the compression stroke.
  • At the end of the compression stroke, the pressure in the combustion chamber is a maximum. The spark plug in a modern engine, or the contact switch of the Wright engine, then generates an electric spark which ignites the fuel-air mixture. Stage 3 is the beginning of the combustion process.
  • Combustion occurs very quickly in an IC engine and occurs at constant volume in the combustion chamber. The high pressure forces the piston back towards the crankshaft. Stage 4 is the beginning of the power stroke.
  • At the end of the power stroke, heat is rejected to the surroundings as required by the second law of thermodynamics. Stage 5 is the beginning of the heat rejection.
  • Following heat rejection, the exhaust valve is opened and the residual gas is forced out into the surroundings to prepare for the next intake stroke. Stage 6 is the beginning of the exhaust stroke.

At the end of the exhaust stroke the conditions have returned to Stage 1 conditions and the cycle repeats itself. The variation of pressure, and cylinder volume can be displayed on a p-V diagram for the Otto Cycle. The area enclosed by the plot is equal to the useful work generated by one cylinder of the engine.



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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: Jun 12 2014

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