Drives for DOHC and Variable valve timing

Fig. 2.31

Drives for DOHC

Drive for double overhead-camshaft can be arranged in a number of different ways. For example, the upper chain in figure 2.31 drives boats camshafts.

In other engines, the chain our belt is used between the crankshaft and the exhaust camshafts. A drive chain is then used between a sprocket on the exhaust camshaft and a sprocket on the intake camshaft (figure 2.36).

The exhaust camshaft is time to the crankshaft, but the intake camshaft has to be timed to the exhaust camshaft. For this purpose, there are timing marks on the chain and on the sprockets.

Another method of driving double camshaft is by gears. Gears between the camshaft are sometimes referred to as scissor gears (figure 2.37). These are provided with timing marks.

Fig. 2.36

To reduce lash between the gears, intake camshaft has a thin sub gear that is held against its drive gear. This creates side friction between the two gears.

Both gears are in mesh with the exhaust camshaft gear, but that teeth of the sub gear mesh differently because the profile of the sub gear is different to the drive gear.

The result of this arrangement is that lash between the two drive gears is dampened out.

Tensioners and dampers

Various methods are used to dampen out the lash and to provide tension on timing belts and chains.

Fig 2.37

When operating under a constant load, a chain or belt will have one slack side and one tight side. At engine idle, the slack will vary due to the power impulses of the engine and the chain or belt with tend to vibrate.  Also, each time the engine speeds up or slows down, tension will alter and the slack will move from one side to the other. To prevent this and to reduce noise, some form of damper or tension is used.

Chain devices

In general, there are three devices that are used with chains:

Dampers: that act on the back of the chain to prevent it from vibrating. 

Fig. 2.33

Guides: that directs the chain in relation to the sprockets. 

Tensioners: provide a load against the chain to keep a tight. 

Devices can be seen in figure 2.30 and 2.31 in Camshafts post.

Dampers, guides and Tensioners are used for slightly different purposes; but they all act in a similar manner against the back of the chain. In shorter chains, the action is mainly one of tensioning but, with longer chains they also act as guides. 

A damper consists of a synthetic rubber pad that may be either fixed or held against the back of the chain by the spring blade. A guide is usually a fixed taped against which the back of the chain operates.

A tensioner voice command voice command is either spring loaded or provided that the hydraulic plunger to hold the rubber pad against the back of the chain.  Hydraulic Tensioners are self-adjusting to compensate for wear of the pad and stretching of the chain.

Fig. 2.34

Hydraulic Tensioners are operated by engine oil pressure.  The rubber pad that bears against the chain is attached to a plunger that operates in a small cylinder. When the engine stopped and no engine oil pressure, a spring holds the plunger outwards with the rubber pad against the chain. With the engine running, oil pressure acts against the plunger to force the pad against the chain. 

Belt devices

Fig. 2.35

Timing belts use by idlers and tensioners in the form of rollers that run on the back of the belt.  The idlers guide the belt so that it to wraps around the crankshaft and the camshaft pulleys. This increases the length of the belt that is in contact with the pulleys.

There are many arrangements of idlers and tensioners.  The belt in figure 2.33 has a simple spring loaded tensioner that is manually adjustable. Figure 2.34 has idlers and the hydraulic tensioner. Figure 2.35 also has an idler pulley and a hydraulic tensioner.

Fig. 2.38

Variable valve timing

A variable camshaft timing unit for a single overhead-camshaft is shown in figure 2.38.  This is bolted to the camshaft sprocket.  Camshafts sprocket is driven in the normal way by the timing chain and this rotates the camshaft timing unit as well as the camshaft.  However, while the camshaft is rotating, the timing unit is able to move the camshaft in relation to the sprocket to advance or retard the valve timing. 

As well as the units shown, there is a housing bolted to the front of the engine that contains oil passages and an oil control valve that is operated by a solenoid. The timing unit is operated hydraulically by engine oil pressure and the oil control valve controls the supply of engine oil to and from the timing unit.

Fig.2.39

A partly sectioned camshaft timing unit is shown in the figure 2.39.  Its main parts are:

1.      Camshaft Sprocket

 This is driven by the timing chain and has the timing unit attached.

2. Inner sleeve

This is bolted to the front of the camshaft.  It has helical gear which meshes with the ring gear.

3. Ring gear

This ring gear has external teeth as well as internal teeth. Its internal teeth mesh with the teeth on the sleeve and its external teeth mesh with teeth inside the housing. The ring gear is attached to the ring piston which is operated by engine oil pressure.

4.  Housing

This encloses the parts. It has internal teeth that mesh with the external teeth of the ring gear. It also provided some cylinder in which the ring piston operates.

5.  Ring piston and return spring

This ring piston is operated by engine oil pressure to advance the camshaft. This spring is used to return the ring piston to retard the camshaft.

Continued 

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