Theory of Operation

The function of all internal combustion engines is to convert fuel (chemical energy) to power. This is accomplished by burning a fuel in a closed chamber and using the increase in temperature within the closed chamber to cause a rise in pressure. The pressure produces a force on the head of the piston causing it to move. The linear movement of the piston is converted to rotary motion (at the crankshaft), Figure 5.1. Rotary motion is more useful than linear movement.

All internal combustion engines have eight requirements for operation:

1) Air (oxygen) is drawn into the engine cylinder.

2) A quantity of fuel is introduced into the engine.

3) The air and the fuel are mixed.

4) The fuel-air mixture is compressed.

5) The fuel-air mixture is ignited by the spark plug in gasoline engines or by the heat of compression in diesel engines.

6) The burning of the fuel-air mixture causes a rapid pressure increase in the cylinder, which acts against the piston, producing a force on the piston.

7) The use of a connecting rod and a crankshaft converts the linear movement of the piston to rotary motion. The force on the piston is converted to torque on the crankshaft.

8) The products of combustion are expelled from the engine.

The next sections illustrate two ways in which these events are arranged in— four-stroke cycle and two-stroke cycle engines.

FIGURE 5.1. Parts of an internal combustion gasoline engine.
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