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In order to operate the valves properly, the camshaft rotation must be timed to open and close the valves at the proper time in the engine cy-cle. The opening and closing of the valves must coincide with the proper position of the piston in each cylinder. For this reason, the cam-shaft is always driven by the crankcam-shaft, either directly or indirectly.

The camshaft must be designed to work hand in hand with the crank-shaft in order to time the valve operation correctly.

Remember that an engine usually contains one cam lobe for each valve. Each valve opens and closes once in each engine cycle, and each cycle requires two complete crankshaft rotations. Since each valve opens only once in each cycle, the camshaft needs to turn only one time for every two crankshaft rotations. In other words, the crankshaft needs to turn twice as fast as the camshaft in order for the valves to work properly. Therefore, the drive assembly that connects the cam-shaft to the crankcam-shaft is designed to turn the camcam-shaft half as fast as the crankshaft.

The camshaft can be connected to, and driven by, the crankshaft in one of several ways. The method that’s used depends on the engine design and on the camshaft placement. The first method that you’ll learn about is called a direct drive system. In this method, a gear is attached to the end of the crankshaft. This gear, in turn, drives a gear that’s placed on the end of the camshaft (Figure 62A). The teeth of the two gears mesh together, so as the crankshaft rotates, the camshaft does also.

Small marks called timing marks are usually stamped on each of the gears. In order to set the proper timing of the two shafts, these timing marks must be aligned with each other when the gears are installed.

Also notice in Figure 62A that the camshaft gear is twice as large as the crankshaft gear. This difference in size allows the camshaft to turn at one-half the speed of the crankshaft.

In the example shown in Figure 62A, only two gears are present—one on the crankshaft, and one on the camshaft. This system works well if the camshaft is mounted in the engine block near the crankshaft. How-ever, if the camshaft is mounted far away from the crankshaft (for ex-ample, in the cylinder head), then several intermediate gears would be needed to connect the two timing gears together. This method would be somewhat complicated and inconvenient.

For this reason, when the camshaft and the crankshaft aren’t close to-gether, the camshaft is usually driven indirectly by a metal chain called a timing chain. This type of system is called an indirect drive system. In this system, a sprocket is placed on the end of the crankshaft and on the end of the camshaft. A timing chain is then placed around the sprockets to drive the camshaft. Figure 62B shows an example of this type of drive system. Notice how the camshaft sprocket is still twice the size of the crankshaft sprocket. Therefore, just like the direct drive system we looked at earlier, the camshaft will turn at one-half of the crankshaft speed.

In the engine shown in Figure 62B, the length of the chain is short.

However, if the camshaft is mounted far away from the crankshaft (in the cylinder head, for example) the chain would be quite long. In such a situation, it’s common for the system to use an additional spring-loaded sprocket or guide to keep the chain tension tight at all times.

This device is called a chain tensioner. The chain tensioner compensates for any stretching that the chain experiences as it operates. If a tension-ing device isn’t used, the chain could become so loose that it would simply slip on the sprockets and not turn the camshaft. One common chain tensioning device is illustrated in Figure 63.

You can also use a flexible rubber belt called a timing belt to drive a camshaft. A timing belt is usually made of rubber and fiber, is about

FIGURE 62—In the direct camshaft drive method shown in Figure 62A, a gear is attached to the end of the crankshaft. This gear then drives another gear on the end of the camshaft. Note the position of the timing marks on the gears. These marks must be aligned in order for the valve timing to be correct. In the indirect drive method shown in Figure 62B, a sprocket is placed on the end of the crankshaft and on the end of the camshaft. Then, a chain is placed around the sprockets and is used to drive the camshaft.

one inch wide, and has a series of teeth molded into it. A typical timing belt is illustrated in Figure 64. The teeth on the belt are called cogs. The cogs fit into special grooves in the pulleys that attach to the crankshaft and camshaft. The belt is placed around the pulleys in the same way that a timing chain is placed around sprockets.

A timing belt is usually used in engines in which the camshaft is mounted in the cylinder-head area (Figure 65). As you can see in the figure, the timing belt is quite long. Therefore, an additional adjustable

FIGURE 63—This illustra-tion shows one method used to keep the chain tension tight in an indi-rect drive camshaft.

FIGURE 64—A typical timing belt is shown here. (Courtesy of Goodyear Replacement Products)

sprocket is used to keep the proper tension on the belt at all times. Also notice that the camshaft pulley is once again twice the size of the crankshaft pulley, thus allowing the camshaft to turn at one-half the crankshaft speed.

As you can see, no matter what drive method is used, the camshaft will always turn at one-half the crankshaft speed. Also note that in each drive method, timing marks are made on the crankshaft and cam-shaft to ensure that the two components are correctly aligned. The proper relationship of the camshaft and crankshaft speeds is called valve timing. Valve timing is very important to the operation of an en-gine. If the opening and closing of the valves and the up-and-down motion of the pistons aren’t matched properly, the engine won’t run properly and will produce only a small amount of power. In some cases, the engine may not run at all. In addition, if a valve is open when a piston rises to the top of a cylinder, the piston may actually strike the valve head and cause serious engine damage.

FIGURE 65—This illustration shows how a timing belt is used to drive the cam-shaft in an overhead camcam-shaft engine.