Figure 12-26.-Operation of a hydraulic valve lifter.
Figure 12-26 shows the operation of one type of
other a mark on only one tooth. Timing the valves
hydraulic valve tappet mechanism. Oil under pressure
is forced into the tappet when the valve is closed. This
pressure extends the plunger in the tappet so that all
valve clearance, or lash, is eliminated. When the cam
lobe moves around under the tappet and starts to raise
it, you hear no tappet noise. The movement of the tappet
forces the oil upward in the lower chamber of the tappet.
This action closes the ball check valve so that oil cannot
escape. Then the tappet acts as though it were a simple,
one-piece tappet and the valve is opened. When the lobe
moves out from under the tappet and the valve closes,
the pressure in the lower chamber of the tappet is
relieved. Any slight loss of oil from the lower chamber
is replaced by the oil pressure from the engine
lubricating system. This oil pressure causes the plunger
to move up snugly against the push rod so that any
clearance is eliminated.
Timing Gears (Gear Trains)
Timing gears keep the crankshaft and camshaft
turning in proper relation to one another so that the
valves open and close at the proper time. Some engines
use sprockets and chains.
The gears or sprockets, as the case may be, of the
camshaft and crankshaft are keyed into position so that
they cannot slip. Since they are keyed to their
respective shafts, they can be replaced if they become
worn or noisy.
With directly driven timing gears (fig. 12-27), one
gear usually has a mark on two adjacent teeth and the
properly requires that the gears mesh so that the two
marked teeth of one gear straddle the single marked
tooth of the other.
We have discussed the main parts of the engine
proper; but other parts, both moving and stationary, are
essential to engine operation. They are not built into the
engine itself, but usually are attached to the engine block
or cylinder head.
The fuel system includes a fuel pump and carburetor
mounted on the engine. In diesel engines the fuel
injection mechanism replaces the carburetor. An
Figure 12-27.-Timing gears and their markings.