Figure 2-8.Comparison of two-stroke and four-stroke cycles.
the diesel engine, many parts are somewhat heavier and
stronger, so they can withstand higher temperatures and
pressures that the engine generates. The engines differ
also in the type of fuel used and how the air-fuel mixture
is ignited. In a gasoline engine, the air and fuel are
mixed together in a carburetor or fuel injection system.
After this mixture is compressed in the cylinders, it is
ignited by an electrical spark from the spark plugs.
A diesel engine has no carburetor. Air alone enters
the cylinder where ii is compressed and reaches a high
temperature due to compression.
The heat of
compression ignites the fuel injected into the cylinder
and causes the air-fuel mixture to burn. A diesel engine
requires no spark plugs; the contact of diesel fuel with
hot air in the cylinders causes ignition. In a gasoline
engine, the heat from compression is not enough to
ignite the air-fuel mixture, so spark plugs are required.
The discussion so far has been on a single cylinder
engine. A single cylinder provides one power impulse
every two crankshaft revolutions in a four-stroke-cycle
engine and is delivering power only one fourth of the
time. To provide for a more continuous flow of power,
modem engines use four, six, eight. or more cylinders.
The same series of cycles discussed previously take
place in each cylinder.
In a four-stroke cycle, six-cylinder engine, for
example, the throws on the crankshaft are set 120
degrees apart, the throws for cylinders 1 and 6, 2 and 5, 3
and 4 being in line with each other (fig. 2-9). The
cylinders fire or deliver power strokes in the following
order: l-5-3-6-2-4. The power strokes follow each
other so closely that there is a fairly continuous and even
delivery of power to the crankshaft.
Even so, additional leveling off of the power
impulses is desirable, so the engine runs more smoothly.
A flywheel (fig. 2-9) is used to achieve this result.
To understand how the flywheel functions, lets
consider a single cylinder engine. It is delivering power
only one fourth of the time during the power stroke.
Figure 2-9.Crankshaft for a sixcylinder engine.