are first mixed together in the carburetor. After this
CLASSIFICATION OF ENGINES
mixture is compressed in the cylinders, it is ignited by
LEARNING OBJECTIVE: Identify various
an electrical spark from the spark plugs.
ways engines are classified. Identify the major
The diesel engine has no carburetor. Air alone
differences between gasoline and diesel en-
enters its cylinders, where it is compressed and reaches
high temperature due to compression. The heat of
Engines for aviation support equipment may be
compression ignites the fuel injected into the cylinder
classified in a number of ways: type of fuel used, type
and causes the fuel-air mixture to burn. The diesel
of cooling employed, or valve and cylinder
engine needs no spark plugs; the very contact of the
arrangement. They all operate on the internal
diesel fuel with the hot air in the cylinders causes
combustion principle, and the application of basic
ignition. In the gasoline engine, the heat from
principles of construction to particular needs or
compression is not enough to ignite the fuel-air
systems of manufacture has caused certain designs to
mixture; therefore, spark plugs are necessary.
be recognized as conventional. The most common
method of classification is by the type of fuel used; that
is, whether the engine burns gasoline or diesel fuel.
Figure 3-5 illustrates the strokes of the 4-stroke
GASOLINE ENGINES VERSUS DIESEL
cycle gasoline engine, and figure 3-8 illustrates the
strokes of the 4-stroke cycle diesel engine. In each
cylinder of either engine, four strokes of the piston
Mechanically and in overall appearance, gasoline
produce two revolutions of the crankshaft. In the
and diesel engines resemble one another. However, in
2-stroke cycle engine shown in figure 3-6, two piston
the diesel engine, many parts are somewhat heavier and
strokes are required to accomplish these four events.
stronger, so that they can withstand the higher
This engine produces less energy per power stroke. But
temperatures and pressures the engine generates. The
since the power stroke occurs twice as often, it is as
engines differ also in the fuel used, in the method of
efficient, or more so, than a 4-stroke cycle engine of the
introducing it into the cylinders, and in how the fuel-air
same size and operating speed.
mixture is ignited. In the gasoline engine, air and fuel
Figure 3-8.--Four-stroke cycle diesel engine.