Layout and Working Progress of the Valve Train

1. Working process

When the working cycle of the cylinder requires the valve to be opened for ventilation, the crankshaft drives the camshaft to rotate through the timing gear, so that the cam on the camshaft pushes the rocker arm to swing through the tappet, push rod, and adjustment screw, and the other end of the rocker arm is Push down on the valve stem to open the valve while allowing further compression of the valve spring. As the cam rotates, the valve lift gradually increases.

When the cam rises to the top of the peach, the valve opens and the cam continues to rotate. At this time, the apex of the convex part of the cam turns over the tappet, and the thrust to the tappet gradually decreases. Under the action of its spring tension, the opening of the valve gradually decreases until it is completely closed, that is, during the compression and power stroke of an intake or exhaust process, the valve is tightly closed under the action of the spring tension, so that the cylinder is airtight. Since the four-stroke engine completes each working cycle, the crankshaft rotates twice, and each cylinder only takes in and exhausts once, that is, the camshaft only needs to rotate once, so the transmission ratio of the crankshaft to the camshaft is 2:1.

The characteristics of the cargo overhead valve arrangement are: the valve stroke is large, although the structure is relatively complex, but its combustion chamber is compact, which is conducive to combustion and heat dissipation, and is conducive to improving the compression ratio and improving the power of the engine.

2. Camshaft layout

The arrangement of the camshaft can be divided into three types: lower, middle and upper. The camshaft of the valve-overhead valve mechanism can be placed on the top, the middle or the bottom, and the camshaft of the side-mounted valve valve mechanism can only be placed on the bottom.

1) Lower camshaft type valve train:

The camshaft arranged in the crankcase is called the lower camshaft type. This kind of valve mechanism is the most widely used. Its characteristic is that the valve and the camshaft are far away, so the valve transmits the movement and force through the tappet, push rod and rocker arm. , the whole system will produce elastic deformation, affecting the valve movement law and the accuracy of opening and closing, so it is not suitable for high-speed vehicle engines, but due to the close distance between the crankshaft and the camshaft, the transmission device between the two can be simplified. Conducive to the layout of the whole machine.

2) Center-mounted camshaft valve train:

when the engine speed is high, in order to reduce the reciprocating quality of the valve transmission mechanism, the camshaft position can be moved to the upper part of the cylinder block, and the camshaft can be directly driven by the tappet The push rod is omitted according to the arm, and this structure is called a central camshaft valve train.

3) Upper camshaft valve train:

The camshaft of the upper cam valve train is arranged on the cylinder head. The figure shows a single camshaft engine with an overhead valve. One camshaft drives the intake and exhaust rocker arms, the intake arm drives the intake valve to open and close, and the exhaust rocker arm drives the exhaust valve to open and close.

For the upper camshaft valve mechanism, there are usually two forms of valve driving, one is that the camshaft directly drives the valve, and the other is to drive the valve through the rocker arm.

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