Camshafts

Camshafts

While the basic purpose of the camshaft is to open and close the valves, cams perform a far more detailed function, because the entire action of the valve depends on the shape of the cam.

The shape of the cam is referred to as the cam profile or cam contour.  This will determine when the valve commences to open, and when it closes.  All of these are part of the design of a particular engine.

Fig.2.25

The three basic camshaft arrangements are: single overhead camshaft (SOHC), double overhead camshaft (DOHC) and overhead valve (OHV) camshafts.

Single overhead camshafts (OHC)

A single camshaft is located on the top of the Cylinder-Head and driven by the toothed belt and toothed pulley, or by a chain and sprocket.  I camshaft in figure 2.25 is driven by toothed belt.  It has five journals which support it on top of the Cylinder-Head and cams to operate the valves.

Fig 2.26

§         Toothed pulleys are also referred to as sprockets, although this term is usually reserved for gears that are used with the chains.

Double overhead camshafts (DOHC)

Two camshafts (DOHC) are used in many engines.  Each cylinder of a DOHC engine has four valves –two intake and two exhaust valves.  One camshaft is used for the intake valves and the other for exhaust valves.  Cylinder-head with two camshafts is shown in figure 2.26.  These have toothed pulleys that are driven by the toothed belt.

Witt v-type engines, two camshafts are used for each bank of cylinders, so the engine has four camshafts.  This arrangement is sometimes referred to as quad-cam.

Fig 2.16

§         DOHC arrangements can also be seen in the number of other figures in this chapter.

Camshafts for overhead valves

In overhead-valve engines, a single camshaft is mounted in the cylinder block and valve lifters and push-rods on top of the cylinder-head.  The arraignment for an in-line engine was shown princely in figure 2.16.

With a v-type engine the camshaft is situated in the cylinder block directly above the crankshaft, as shown in figure 2.27.  Camshaft is fitted with the sprocket that is driven by a short timing chain from a sprocket on the end of the crankshaft.  In the assembly shown, the oil pump is driven directly by the crankshaft.

Fig 2.27

Camshaft drives and timing

Camshafts are driven at half the crankshaft speed by means of timing gears, timing chain or timing belts that are, connected between the camshaft and the crankshaft. The camshaft is designed so that the valves opened and closed correctly in relation to each other and in the correct firing order for the engine.  However, camshaft must also be timed to the crankshaft so that the valves open and close at the correct times in relation to piston movement. This is a function of the camshaft drive.

§         The drive must be correctly timed and timing marks are provided for this purpose.

Fig 2.28

Timing gears

Where timing gears are used, the camshaft gear is twice the size of the crankshaft gear (figure 2.28).  The crankshaft gear has marks on to adjacent teeth, while camshaft gear has marks on only one tooth. For correct valve timing, the single marked tooth on the camshaft gear is Meshed between the two marked teeth on the camshaft gear as shown.

This is a simple arrangement that was used for OHV engines in passenger vehicles, but which now has a limited application.

Timing chains

Figure 2.29 illustrates the chain arrangement for a v-type OHV engine. This is a double roller chain with a sprocket on the camshaft and another on the crankshaft. The sprockets carry timing marks. For correct timing of the arrangement shown, the timing mark on each sprocket and the center of each after should be in line when No.1 piston is on top dead center (TDC).

Fig 2.30

Fig 2.29

Overhead camshaft chains

Over head camshafts can be driven by chains in a number of different ways. In figure 2.30, a single chain drives the camshafts sprocket and also an auxiliary shaft sprocket. The auxiliary shaft drives the oil pump. The chain has a damper, our guide, on one side and a hydraulic tensioner on the other.

There are two timing chain in figure 2.31, which has two overhead camshafts. One chain drives an auxiliary shaft for the oil pump and other drives two sprockets for the two overhead-camshafts. This arrangement avoids what would otherwise have to be a long chain drive.

Timing belts

Fig 2.31

Timing toothed belts are used with many overhead-camshafts. There are quieter in operation that chains and require no lubrication. Belts are also able to be wrapped further around the pulleys or sprockets than chains. That’s ten shooting police and idler police are relatively simple as they are able to run on the smooth back of the belt to guide the belt and keep it tight.

The construction of the timing belt is shown in figure 2.32. It contains of a core of glass fiber that has a high tensile strength to resist stretching, a canvas reinforced tooth section to resist wear, and a rubber backing that has good heat and wear resistant properties.

Timing belt arrangements

The timing belt for a single overhead-camshaft is shown in figure 2.33. This has timing marks that are identified by arrowheads. The crankshaft pulley is positioned with the key to the top, and the camshaft pulley is positioned in line with its timing mark as shown.

Fig 2.32

Figure 2.34 shows the timing belt for a horizontally opposed engine. This is a long belt drives a water pump as well as the two camshafts. There are two idler pulleys on the back of the belt that guides the belt and keep it wrapped around the pulleys. And automatic tensioner pulley keeps the belt tight.

The arrangement of the timing belt and timing marks for an overhead camshaft v-type engine is shown in figure 2.35. This is a long timing belt that drives the camshaft pulleys on each bank and also the coolant pump. An idler on the back of the belt acts as a guide and hydraulic tensioner keeps the belt tight.

Continued

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