Chapter 5 Principles of Engine Operation, Two- and Four-Stroke Engines
91
Valve Timing
Valve timing is measured in degrees of crank-
shaft rotation. The point at which the valves open
or close before or after the piston is at top dead
center (TDC) or bottom dead center (BDC) varies
with different engines.
Figure 5-12 shows one complete operating cycle
of a four-stroke engine. Beginning at point A, the
intake valve opens 10° before TDC and stays open
through 235°. The exhaust valve closes 30° after TDC.
Valve overlap occurs when both valves are open
at the same time.
During the compression stroke, the intake valve
closes and ignition occurs 30° before TDC. The
power stroke continues through 120° past TDC.
The exhaust valve opens 60° before BDC and stays
open through 270°. During the last 40°, the intake
valve is also open and the second cycle has begun.
Note
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between the electrodes of the spark plug. This
ignites the air-fuel mixture, and the force of com-
bustion (rapid expansion of burning gases) pushes
the piston downward.
Actually, the full charge does not burn at once.
The fl ame progresses outward from the spark plug,
spreading combustion and providing even pressure
over the piston face throughout the power stroke.
The entire fuel charge must ignite and expand
in an incredibly short period of time. Most engines
have the spark timed to ignite the fuel slightly
before the piston reaches top dead center (TDC) of
the compression stroke. This provides a little more
time for the mixture to burn and accumulate its
expanding force.
Basically, the amount of power produced by
the power stroke depends on the volume of the air-
fuel mixture in the cylinder and the compression
ratio of the engine. The compression ratio is the
proportionate difference between the volume of
cylinder and combustion chamber at bottom dead
center and the volume of cylinder and combustion
chamber at top dead center. If the compression ratio
is too high, the fuel may be heated to its fl ash point
during the compression stroke and ignite too early.
Exhaust Stroke
After the piston has completed the power stroke,
the burned gases must be removed from the cylin-
der before introducing a fresh charge. This takes
place during the exhaust stroke. The exhaust valve
opens and the rising piston pushes the exhaust
gases from the cylinder. See Figure 5-10D.
The exhaust valve has to function much like
the intake valve. When closed, the valve must seal.
When open, it must allow a streamlined fl ow of
exhaust gases out through the port. The removal
of gases from the cylinder is called scavenging.
The passageway that carries away exhaust gases
is referred to as the exhaust manifold or exhaust
port. The exhaust manifold must be designed for
the smooth fl ow of gases.
The heat absorbed by the exhaust valve must
be controlled or the valve will deteriorate rapidly.
Some valve heat is carried away by conduction
through the valve stem to the guide. However, the
hottest part of the valve, the valve head, transfers
heat through the valve seat to the cylinder block.
See Figure 5-11.
Goodheart-Willcox Publisher
Figure 5-11.
The exhaust valve must cool during an incredibly short
period (1/50 sec. at 3600 rpm). Heat is conducted from
the valve through the seat to the cylinder block. Some
heat travels down the stem and to the valve guide.
Closed approx.
1/50 sec.
Cylinder block
Valve stem
Valve guide
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