PRIN. OF INTERNAL COMBUSTION ENGINES - OD1619 LESSON 1/TASK 1
(5) Valve Train. It is very important to operate the valves in a timed sequence.
Therefore, the engine is fitted with a valve train, which operates the valves. If
the exhaust valve were to open in the middle of the intake stroke, the piston would
draw burnt gases into the combustion chamber with a fresh mixture of fuel and air.
As the piston continued to the power stroke, there would be nothing in the
combustion chamber that would burn.
A simplified valve train is illustrated in view A of figure 8 on page 10. A
camshaft is made to rotate with the crankshaft through the timing gears. The
raised piece on the camshaft is called a cam lobe. As illustrated in view B of
figure 8, the valve spring is designed to hold the valve closed.
The cam lobe contacts the bottom of the lifter as it rotates with the camshaft, as
shown in view C of figure 8. As the cam lobe pushes up on the lifter it will, in
turn, push the valve open against the pressure of the spring. In view D of figure
8, the cam lobe has passed the center of the lifter bottom. As it rotates away
from the lifter, the valve spring pulls the valve closed.
By proper position of the cam lobes on the camshaft, a sequence can be established
for the intake and exhaust valves. It is described, in subparagraphs 2a(4) (a)
through 2a(4) (d) on page 7, how the intake valve and the exhaust valve must each
open once for every operating cycle. As explained in subparagraph 2a(4), the
crankshaft must make two complete revolutions to complete one operating cycle.
Using these two facts, a camshaft speed must be exactly onehalf the speed of the
crankshaft. To accomplish this, the timing gears are made so that the crankshaft
gear has exactly onehalf as many teeth as the camshaft gear, as shown in view A of
figure 9 on page 11. The timing marks indicated are used to put the camshaft and
the crankshaft in the proper position relative to each other.
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