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Untitled Document
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The Engine Top
End

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General:
An engine's top end is designed to
combine air and fuel in the correct ratio and deliver the mixture
to the combustion chambers. The top end also provides a means for
combustion gases to exit the engine. The top end consists of four
major components: intake manifold (1), cylinder head (2), valve
train (3), and exhaust manifold (4).
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Intake
Manifold:
The intake manifold (1) distributes
air to each cylinder. Intake runners (2) connect each intake port
to the manifold. Piston action creates a vacuum in the manifold,
which draws outside air through the throttle body and into the
manifold. The vacuum will increase when the throttle valve closes
(air flow is restricted) and will decrease when the throttle valve
opens (air flow is less restricted.) Diesel engines do not have
a throttle valve, therefore the vacuum in the manifold is weaker
than a gasoline engine.
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Vacuum:
A vacuum is pressure that is below
atmospheric pressure. Systems such as power brakes, emission control,
fuel, and cabin ventilation use manifold vacuum to operate components.
Some examples include the power brake booster, EGR valve, and fuel
pressure regulator. Since Diesel engines produce a weak vacuum,
an engine-driven vacuum pump is provided to generate the necessary
vacuum for systems requiring it.
Manifold
Pressure:
Manifold pressure is a good indicator
of engine load. Pressure sensing, electronic multi-point fuel injection
systems have a sensor that provides manifold pressure feedback to
the engine's computer. A high pressure signal indicates a high load
requiring a rich mixture. A low manifold pressure signal indicates
reduced load requiring a leaner mixture. The computer will lengthen
fuel injection pulse time to richen the mixture. For a leaner mixture,
the computer will shorten pulse time.
The
information on this page is provided from the "Road Machines"
learning CD. Buy it Today and continue
your learning !
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