Figure 12-26.-Operation of a hydraulic valve lifter.Figure 12-26 shows the operation of one type ofother a mark on only one tooth. Timing the valveshydraulic valve tappet mechanism. Oil under pressureis forced into the tappet when the valve is closed. Thispressure extends the plunger in the tappet so that allvalve clearance, or lash, is eliminated. When the camlobe moves around under the tappet and starts to raiseit, you hear no tappet noise. The movement of the tappetforces the oil upward in the lower chamber of the tappet.This action closes the ball check valve so that oil cannotescape. Then the tappet acts as though it were a simple,one-piece tappet and the valve is opened. When the lobemoves out from under the tappet and the valve closes,the pressure in the lower chamber of the tappet isrelieved. Any slight loss of oil from the lower chamberis replaced by the oil pressure from the enginelubricating system. This oil pressure causes the plungerto move up snugly against the push rod so that anyclearance is eliminated.Timing Gears (Gear Trains)Timing gears keep the crankshaft and camshaftturning in proper relation to one another so that thevalves open and close at the proper time. Some enginesuse sprockets and chains.The gears or sprockets, as the case may be, of thecamshaft and crankshaft are keyed into position so thatthey cannot slip. Since they are keyed to theirrespective shafts, they can be replaced if they becomeworn or noisy.With directly driven timing gears (fig. 12-27), onegear usually has a mark on two adjacent teeth and theproperly requires that the gears mesh so that the twomarked teeth of one gear straddle the single markedtooth of the other.AUXILIARY ASSEMBLIESWe have discussed the main parts of the engineproper; but other parts, both moving and stationary, areessential to engine operation. They are not built into theengine itself, but usually are attached to the engine blockor cylinder head.The fuel system includes a fuel pump and carburetormounted on the engine. In diesel engines the fuelinjection mechanism replaces the carburetor. AnFigure 12-27.-Timing gears and their markings.12-22
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