The thermodynamic cycleThermodynamic Cycles used in internal combustion gasoline powered engines. The first such
engine was built by the German engineer Otto in 1861. The Otto cycle consists of four strokes of the piston for each
explosion.
- 1. Intake stroke. A mixture of air and gasoline is drawn into the cylinder.
- 2. Compression stroke. The fuel-air mixture compressed, causing a rise in temperature.
- 3. Explosion. The mixture is ignited by spark plugs with the piston staying nearly fixed, leading to a
rise in pressure and temperature.
- 4. Power Stroke. The hot gas is then allowed to do mechanical work by pushing the piston back.
- 5. Valve exhaust. An outlet valve is opened and some exhaust escapes with the piston remaining
fixed.
- 6. Exhaust stroke. The piston forces the remaining exhaust to escape. The outlet valve is then closed,
the inlet valve opened, and the cycle repeats.
The maximum efficiency of this cycle is
where
is the engine's compression ratio and
is the heat capacity ratio (Zemansky 1968). For a car with r = 10, the theoretical expression gives , but the practical efficiency is more like 0.3 (Mandl 1989). A table of thermal efficiencies and peak cylinder
pressure and combustion temperature is given by Anderson (1989, p. 183).
Diesel Cycle
References
Anderson, H. L. (Ed.-in-Chief). A Physicist's Desk Reference. New York: American Institute of Physics, 1989.
Mandl, F. Statistical Physics, 2nd ed. Chichester, England: Wiley, pp. 121-123, 1989.
Zemansky, M. W. Heat and Thermodynamics: An Intermediate Textbook, 5th ed. New York: McGraw-Hill, pp. 173-177, 1968.
© 1996-2007 Eric W. Weisstein
