There are two types of valves—intake and exhaust.
The intake valve is the larger valve and it allows a fuel
charge to flow into the cylinder. The exhaust valve is the
smaller valve and it opens to let burned gasses (exhaust)
out of the cylinder. Figure 1-15 shows how the air-fuel
mixture flows through the intake port, past the valve, and
into the combustion chamber when the valve is open.
Four-Stroke Cycle
The four-stroke cycle needs four up or down piston
movements, or strokes, to produce one complete cycle.
Every two up and two down strokes of the piston results
in one power-producing cycle. Two complete revolutions
of the crankshaft are needed to complete one four-stroke
cycle. Automotive engines, both gas and diesel, are
four-stroke-cycle engines.
The four strokes are intake, compression, power,
and exhaust. With the engine operating, these strokes
happen over and over very rapidly. At idle, an engine
might be running at 800 revolutions per minute (rpm),
which means the crankshaft rotates 800 times in one
minute. Since it takes two complete revolutions of the
crankshaft to complete a four-stroke cycle, an engine
completes 400 four-stroke cycles per minute at idle. In
other words, the piston must slide up 800 times and
down 800 times per minute. You can imagine how fast
these events are happening at highway speeds!
Intake Stroke
A gasoline engine’s intake stroke draws air and fuel
into the combustion chamber. Figure 1-16A shows the
basic action during the intake stroke. Study the position of
the valves and movement of the piston. The piston slides
down to form a vacuum (low pressure area). The intake
valve is open and the exhaust valve is closed. Atmospheric
pressure (outside air pressure) pushes the air-fuel charge
into the vacuum in the cylinder. This fills the cylinder with
a burnable mixture of fuel and air.
Compression Stroke
The compression stroke squeezes the air-fuel mixture
to make it more combustible. See Figure 1-16B. Both the
intake and exhaust valves are closed. The piston slides up
and compresses the mixture into the small area in the com-
bustion chamber.
Chapter 1 Review of Engine Operation 15
Intake
valve
Intake
port
Exhaust
valve
Exhaust
port
Valve springs
Figure 1-13. Valves fit into guides in the cylinder head. Valve
springs hold the valves closed. The valves seal against valve
seats in the head to close off ports from the combustion chamber.
Cylinder
head
Valve guide
Valve
seat
Valve
face
Valve
head
Margin
Port
Figure 1-14. Study the valve action. When the valve slides
open, the valve face is lifted off of the valve seat.This opens the
port to the combustion chamber and gasses are free to enter or
exit the cylinder.
Air-fuel
mixture
Port in
cylinder head
Intake
valve open
Figure 1-15. Note the action as the intake valve opens.
Downward movement of the piston forms a vacuum in the
cylinder. Atmospheric pressure pushes the air-fuel charge into
the cylinder. (Ford)
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