cylinder engine
2.9 VALVE OPERATING MECHANISMS
2.9.1 Cams and camshafts
In section 2.8, it was highlighted that valves are used to open and close the intake and exhaust ports; also that most four-stroke engines rely on a cam system to open the valves (section 2.8.9). Cams and camshaft based systems have been used since the very earliest internal-combustion engines and have only really changed significantly in the method used to transfer the lift of the cam to the valve, and the way in which the camshaft is driven.
A cam is a component, which is shaped so that when it rotates on the shaft it causes another component (which is in contact with it) to move in a different manner. There are very many types of cams in use in all manner of machines but with regard to inlet and exhaust valves, the rotating cam causes the valve to move in a linear way, which enables the valve to open and close the ports.
The type of cam used to operate the valves of a motor-vehicle engine is a relatively simple one such as that illustrated in Figure 2.83.
As previously described in (section 2.8.9), the cam does not normally act directly on the end or stem of the valve; a cam follower (sometimes referred to as a tappet) follows the profile of the cam as it rotates (Figure 2.84). The follower then transmits the
movement to the valve (either directly or via a pushrod or rocker mechanism).
The cam follower remains stationary during that part of the cam’s rotation when it rests upon the base circle, and its ‘lift’ begins at the point where the opening flank joins the base circle. At the peak of the cam (highest point of the cam lobe) the lift is maximum, and the follower then falls as the closing flank passes beneath it. When the follower is again in contact with the base circle it will have returned to its original position.
Figure 2.83 Details of cam
In some engines there may be fairly large variations of clearance under different running temperatures. To ensure quiet operation, even when the clearances are large, the flanks are joined to the base circle by ramps, which extend over about 30° of cam rotation. When checking valve clearances it is most important that the cam follower should be resting on the base circle of the cam (refer to Figure 2.83). One way of ensuring this is to turn the engine to the position where the applicable valve is at full lift, and then turn the crankshaft one complete revolution.
On simple engines, all of the cams required to open the valves for all of the cylinders are formed on a single shaft called the camshaft. It is, however, normal practice on modern engines to use two camshafts; one for the inlet valves and one for the exhaust valves.
Camshaft materials
Camshafts are usually made of steel, either by forging or casting, with subsequent machining. Forged camshafts usually have the cams case-hardened, while the cams of cast shafts are generally hardened by chilling during casting.
Camshaft bearings
The camshafts are usually carried in plain bearings, but in some cases ball or roller bearings may be used. In many engines the shaft is moved into position from the front end of the engine (either into the cylinder block or cylinder head) and must therefore pass through the bearing holes. To enable this to be done the bearings must be larger than the cams.
2.9.2 Camshaft location
Side-valve engines
Earlier designs of engine located the camshaft in the cylinder block, often just to one side and slightly above the crankshaft. This location was very convenient on
older engines that had side valves; each of the cams was effectively located just beneath the valve stem and therefore only required a cam follower to transmit the cam movement to the valve stem (Figure 2.85).
The location of the camshaft, which was close to the crankshaft, allowed the use of a short chain and sprocket system to drive the camshaft, although some engines used gears to transmit drive.
Due to the inefficiency of side-valve engines, vehicle manufacturers no longer use this layout.
Figure 2.84 Cam acting onto a cam follower
Figure 2.85 Side-valve layout showing location of the camshaft
Overhead-valve (OHV) engines with pushrods To improve engine efficiency and allow greater power outputs, engine designs progressively made use of overhead-valve layouts, where the valves were located in the cylinder head. However, many overhead-valve engines were based on the original side-valve designs and therefore retained the camshaft in the cylinder block. It was therefore necessary to use a pushrod and rocker shaft system to transfer the movement of the cam to the valve (Figure 2.86).
Although the pushrod and rocker system added some complication to the valve-operating mechanism, the camshaft remained conveniently located in the cylinder block, which allowed the simple drive mechanism of the side-valve engines to be retained and allowed engine height to be kept to a minimum.
Overhead camshafts (OHC)
Further developments in engine design resulted in a trend towards overhead camshafts. The advantages include more accurate control of valve operation, especially at higher engine speeds, which could not be reliably achieved with the pushrod system.
Many overhead camshaft designs retain the use of a rocker to transmit the cam movement to the valve, the cam usually acting directly onto the rocker, which in turn moves the valve. Many variations exist for overhead camshaft designs; Figure 2.87 shows a commonly used simple, overhead, camshaft system using a rocker.
follower then transmits the movement to the valve, directly or via pushrods and/or rockers. In fact, a rocker can function as a cam follower as shown in Figure 2.87.
By using a cam follower, the stem of the valve is only forced to follow a linear (up and down) motion. If, however, the stem of the valve is in direct contact with the cam it is also forced sideways by the cam, which can result in the valve binding on the valve guide and accelerated wear of the valve stem and guide.
Overhead and side-valve tappets/cam followers Figure 2.88a shows a type of tappet typically made of cast iron. The bottom surface that rubs against the cam would be chilled in the casting process to provide a hardened surface. These tappets, which sit in a machined bore, are hollow and may have ‘windows’
which help to reduce weight and assist lubrication. This type of tappet operates directly in the cylinder block and would be used in side-valve engines. The valve stem sits in the top of the adjuster, which is used to set the working clearance. Figure 2.88c shows a slightly different type of side-valve tappet.
Figure 2.88b illustrates a tappet/cam follower that is used in an OHV pushrod-type engine. The pushrod sits in the top of the follower and an adjuster forms part of the rocker. The location and construction are the same as for the side-valve type tappet.
Figure 2.88d shows a tappet or cam follower, which has a roller that makes contact with the cam lobe. This type of cam follower is no longer in common use but was in the past effective on lower speed engines.
Figure 2.87 Simple overhead camshaft and rocker layout
2.9.3 Cam followers
Cam followers and tappets
The term ‘tappets’ has long been associated with the component that immediately contacts the cam, i.e.
where the cam lobe taps the component to move it. On older engine designs (e.g. a side-valve engine) the term tappet was universally applied to these components, which often contained some form of adjuster to set the working clearance of the valve. In general, however, these components are now referred to as ‘cam followers’
on all engines, but reference is made to tappets within this book with regard to those components that were originally referred to as tappets.
The purpose of cam followers
As mentioned previously, cam followers are used to follow the movement of the cam as it rotates. The
Figure 2.88 Types of tappet
Most tappets/cam followers used on pushrod or side-valve engines have a flat base, and are used with cams that have convex flanks. Some cams have straight flanks, and these operate on followers with curved ends.
Flat-based tappets are usually allowed to rotate in their guides; in fact the design promotes rotation because this reduces the rubbing speed between cam and tappet and spreads the wear over the whole of the tappet base. Rotation of the tappet is encouraged by offsetting the tappet from the centre-line of the cam or by grinding the cams with a very slight taper (about 1°) in which case the foot of the tappet is ground very slightly convex, forming part of a sphere of large radius.
Figure 2.86 Simple overhead valve system using pushrods and rockers. The camshaft is located in the cylinder block
Overhead camshaft type cam followers
For overhead camshaft engines where the cam is directly in line with (or above) the valve stem, the cam follower is located in a machined bore and sits between the cam and the valve stem. The followers are usually made of steel and arranged so that they sit over the valve and valve spring (Figure 2.89).
Traditionally, a pre-sized shim or spacer is inserted between the top of the valve stem and the internal face of the follower. The thickness of the shim selected during manufacture dictates the working gap. When wear occurs, which results in too large or too small a working gap, different thickness shims need to be inserted to obtain the correct working clearance. Note that some cam followers of this type included a tapered adjusting screw that screwed into the follower from the side and effectively replaced the shim. By adjusting the tapered screw, the working clearance could be altered.
Other types of overhead camshaft systems make use of rockers that act as cam followers. Figure 2.87 shows a typical example.
Figure 2.89 Overhead camshaft with cam follower
Figure 2.90 A hydraulic cam follower
earlier designs were not favoured for high-speed engines, modern high revving sports and even some racing engines are now fitted with hydraulic followers.
A typical hydraulic cam follower is shown in Figure 2.90.
The hydraulic cam follower body (1) contains a plunger (2), which is formed into two chambers: a feed chamber (3) and a pressure chamber (4). Oil from the main engine lubrication system is passed to the feed chamber and then to the pressure chamber via a one-way ball valve (5). The oil flow from the pressure chamber is controlled by the amount of clearance between the follower body and the plunger. By accurately setting this clearance, a given amount of oil is allowed to escape up the side of the plunger each time the tappet is operated. A spring (6) tends to force the plunger out of the follower body and therefore eliminate any clearance that may exist, but although this spring reduces the valve clearance to zero, it does not have the strength to operate the engine valve.
Figure 2.91 shows the operation of the hydraulic follower.
When the cam rotates and begins to open the valve, (Figure 2.91a) oil in the pressure chamber is trapped, since the ball valve will not allow it to return to the feed chamber. As a result, the upward movement of the tappet body pressurizes the oil and causes the plunger to be moved a similar amount. During this stage the hydraulic follower behaves like a solid cam follower.
2.9.4 Hydraulic cam followers
It has been previously identified (section 2.8.9) that a working clearance is required to ensure that the valve can fully close and is not held slightly open by the cam.
On the valve-operating systems examined so far, a means of manual adjustment is normally provided to enable the correct clearance to be obtained.
An automatic means of adjusting the clearance is achieved using ‘hydraulic cam followers’. In fact this type of cam follower provides what is effectively a zero working clearance at all times and at all temperatures.
The hydraulic follower allows the valve to fully close but does not then allow any additional clearance.
Hydraulic cam followers have been used for many years and have been fitted to pushrod operating systems and to overhead camshaft systems. Although
hydraulic follower designs and modern oils have now virtually eliminated this problem, wear in the follower or incorrect oil can cause rattle. Engine oil that has not been changed at the specified intervals can also affect hydraulic follower operation.