In the
internal combustion engine, combustion takes place inside the cylinder and is directly
responsible for forcing the piston to move down. With an external combustion engine, such
as a steam engine, combustion takes place outside the engine. The external combustion
engine requires a boiler to which heat is applied. This combustion causes water to boil to
produce steam. The steam passes into the cylinder under pressure and forces the piston to
move downward.
The transformation of heat energy to mechanical energy by the
engine is based on the fundamental law of physics which states that gas expands when
heated. The law also states that when gas is compressed, the temperature of the gas
increases. if the gas is confined with no outlet for expansion, then the pressure of the
gas increases when heat is applied. In the internal combustion engine, the burning of fuel
within an enclosed cylinder results in an expansion of gases. This expansion creates
pressure on top of the piston, causing it to move downward. In an internal combustion
engine, the piston moves up and down within the cylinder. The relationship between volume,
pressure, and temperature within a cylinder of the engine is explained in the chart below
and shown in Figure 2-2. Note the changes
within the cylinder while the temperature outside remains a constant 70°F.
This
up-and-down motion is known as reciprocating motion.This motion
(straight-line motion) mustbe changed into rotary motion(turning motion)
to turn the wheels of a vehicle. A crankshaft and a connecting rod change their
reciprocating motion to rotary motion.
All internal
combustion engines, whether gasoline or diesel, are basically the same. We can best
demonstrate this by saying they all rely on three things—fuel, air, and ignition.
Fuel
contains potential energy for operating the engine; air contains the oxygen necessary for
combustion; and ignition starts combustion. Each one is fundamental, and an engine cannot
operate without them. Any discussion of engines must be based on these three factors and
the steps and mechanisms involved in delivering them to the combustion chamber at the
proper time.
Published
by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy
