Saturday, May 14, 2011

Valve-trains OHC engines

Valve-trains OHC engines
Overhead-camshaft engines have the camshaft mounted on the top of the Cylinder-Head. This provides a short valve train, although a long timing chain or belt is needed between the crankshaft and the camshaft or the camshafts.
Having the camshafts overhead also eliminates the reciprocating parts that are used in the valve trains of overhead-valve engines. This is an advantage with smaller high speed engines we reciprocating parts could produce liberation.
The valve is in overhead camshafts are operated by rocker arms or, more directly, by bucket type tappets. There are a number of different arrangements.

Rocker arms
Figure 2.19 shows a section through a Cylinder-Head with a single overhead camshaft and rocker arms to operate the valves. This is the basic arraignment for OHC engines. It has a hemispherical combustion chamber with the valves set at an angle in the head. It has one camshaft, but two sets of rocker arms-one set for the intake valves and the other for the exhaust-valves.
Rocker arms are a form of a lever, with one end against the cam of the camshaft and the other end against the stem of a valve. As the camshaft rotates, cam movement is transferred by the rocker arm to open and close the valve.
There has to be some clearance in the valve-train to allow for changes in temperature, otherwise the valve could be prevented from seating properly. This would not only allow as a gas leak, but would cause the valve to burn.
The rocker arms in the illustration have an adjusting screw on the end of the rocker arm to adjust the valve clearance.
The components of fig. 2.19 are labeled below:
1.Rocker cover, 2. Spring retainer, 3. Spring, 4. Seal, 5.Exhaust valve, 6. Rocker arm, 7.Rocker shaft, 
8. Camshaft, 9. Cylinder head, 10. Intake valve
Bucket tappets
Fig 2.20
Figure 2.20 shows the arrangement for bucket tappets and a double overhead camshaft. The camshafts are mounted directly above the valves. Only two valves are shown, but the cylinder actually has four valves two for intake and two for exhaust. The bucket tappets are located between the cams of the camshaft and the ends of the valve stems; cam movement is transferred directly through the tappet to the valve.
The bucket tappet is cup-shaped So that it fits over the end of the valve and spring.  It operates in a guide which protects the valve against side thrust that it would receive if the cam operated to directly against the valve. There is a small clearance between the cam and the tappet and this is adjusted by altering the thickness of shims or spacers that are in the tappet.

§    Bucket tappets can also be identified in figure 2.24 and in figure 2.43, which also has two camshafts.

Hydraulic lash adjusters for OHC engines
Fig. 2.21
Some overhead camshaft engines are fitted with hydraulic lash adjusters that are used to remove the lash from the valve-train. These operate in a similar way to the hydraulic valve-lifters for OHV engines discussed previously.  With hydraulic lash adjusters, there is no clearance and the Valve-stem and this eliminates the need for adjustment. There are three general locations for hydraulic lash adjusters in OHC engines:
  1. In the valve end of the rocker arm.
  2. In the Cylinder-Head at the end of the rocker arm.
  3. In bucket type tappets.

Adjusters in the rocker arm
Figure 2.21 shows hydraulic lash adjusters fitted in the ends of the rocker arms which operate against the Valve-stems. They have a body with a plunger which is held against the Valve-stem by a spring. Oil supplied to the adjuster keeps the plunger in contact with the valve and eliminates lash from the valve-train.
Fig. 2.22
The details of a hydraulic lash adjuster can be seen in figure 2.22. When the lash adjuster is under load, the plunger is held outwards against the valve tip by the plunger spring. Oil is trapped in the high pressure chamber by the check ball and this eliminates any lash in the valve train. Lash adjuster acts like a solid lifter.
There is a reservoir of oil in the adjuster that is supplied by the engine’s lubricating system. Any loss of oil from the high pressure chamber will allow the plunger to move back into the body. But, when the load is released, the spring will move the plunger outward and allow oil from the reservoir to enter the high pressure chamber will always be full and the plunger will always be in contact with the tip of the valve.
Fig. 2.23

Lash adjuster in the cylinder-head
The cylinder-head assembly in figure 2.23 has a double cam shaft and pivot type hydraulic lash adjusters mounted in the cylinder-head.  Rocker arms are a roller type, with a roller bearing operating against cam to reduce friction and wear.
The lash adjusters operate in the same way as those in a rocker arm, except that they are stationary and provide a ball pivot for one end of the rocker arm.  The ball is on the end of the lash adjuster plunger and this holds the roller up against that cam.

Fig. 2.24
Lash adjuster in bucket tappet
The arrangement for a bucket tappet is shown in figure 2.24, where the lash adjuster is installed inside the bucket tappet.
In this design, the hydraulic action of the plunger tends to spread that tappet.  This holds the bucket body against the cam on the camshaft and the plunger against the tip of the Valve-stem.  This gives zero clearance.
The parts of fig.2.24 are labeled below
1. Bucket body, 2.Ball seat, 3. Check ball, 4. Plunger spring, 5. Check ball spring, 6. Oil passage, 7. Chamber A, 8. plunger, 9. Sleeve, 10. Chamber B

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