Camshafts and Valve Train Basics

July 25, 2004 | By: Abdul Rehman (UK)

If cylinder heads are the heart of an engine, then the camshaft and valve train have to be the brains of the operation. Timing the opening, closing, lift, and duration of each valve event is central to increasing power and torque.

A valve train consists of valves and a mechanism which opens and closes them. The opening and closing system is called a camshaft. The camshaft uses lobes (cams) that push against the valves to open them as the camshaft rotates; springs on the valves return them to their closed position. This is a critical job, and can have a great impact on an engine's performance at different speeds.

The key parts of any camshaft are the lobes. As the camshaft spins, the lobes open and close the intake and exhaust valves in time with the motion of the piston. It turns out that there is a direct relationship between the shape of the cam lobes and the way the engine performs in different speed ranges.

To understand why this is the case, imagine running an engine extremely slowly at just 10 or 20 revolutions per minute (RPM) so that it takes the piston a couple of seconds to complete a cycle. To actually run an engine this slow would be impossible, but let's imagine that we could. At this slow speed, we would want cam lobes shaped so that just as the piston starts moving downward in the intake stroke, the intake valve would open. The intake valve would then close right as the piston reaches the bottom and the exhaust valve would then open right at the end of the combustion stroke and would close as the piston completes the exhaust stroke.

This kind of setup would work really well for the engine as long as it ran at this very slow speed. Now what happens if you increase the RPM?

When you increase the RPM, the 10 to 20 RPM configuration for the camshaft will not work very well. Just say if the engine is running at 4,000 RPM, the valves are opening and closing 2,000 times every minute, or 33 times every second. At these speeds, the piston is moving very quickly, so the air and fuel mixture rushing into the cylinder will also be moving at a very quick rate.

When the piston starts its intake stroke and the intake valve opens, the air/fuel mixture in the intake runner starts to accelerate into the cylinder. By the time the piston reaches the bottom of its intake stroke, the air/fuel mixture is moving at a pretty high speed. If you were to slam the intake valve shut, all of that air/fuel mixture would come to a halt and will not enter the cylinder. By leaving the intake valve open a little longer, the momentum of the fast-moving air/fuel mixture continues to force air and fuel into the cylinder as the compression stroke is started by the piston. In theory the faster the engine goes, the faster the air/fuel mixture flows and the longer we would want the intake valve to stay open. We would also want the valve to open wider at higher speeds. Also affecting the cams performance is lift, the duration, overlap and timing.

Lift

Lift refers to maximum valve lift. This is how much the valve is "lifted" off its seat at the cam lobe's highest point. The intake and exhaust valves need to be open to let air/fuel in and exhaust out of the cylinders. Generally, opening the valves quicker and further will increase engine output. Increasing valve lift, without increasing duration, can yield more power without much change to the nature of the power curve. However, an increase in valve lift almost always is accompanied by an increase in duration. This is because ramps are limited in their shape which is directly related to the type of lifters being used, such as flat or roller.

Duration

Duration is the angle in crankshaft degrees that the valve stays off its seat during the lifting cycle of the cam lobe. Increasing duration keeps the valve open longer, and can increase high-rpm power. Doing so increases the RPM range that the engine produces power. By increasing duration without a change in lobe separation angle will result in increased valve overlap.

Overlap

Overlap is the angle in crankshaft degrees that both the intake and exhaust valves are open. This occurs at the end of the exhaust stroke and the beginning of the intake stroke. Increasing lift duration and/or decreasing lobe separation increases overlap. At high engine speeds, overlap allows the rush of exhaust gasses out the exhaust valve to help pull the fresh air/fuel mixture into the cylinder through the intake valve. Increased engine speed enhances the effect. Therefore increasing overlap, increases top-end power and reduces low-speed power and idle quality.

Timing

For timing take this as an example 22-62, 62-22.

The first 22 shows how many degrees the inlet valve opens before top dead center,
The first 62 shows how many degrees the inlet valve closes after bottom dead center,
The second 62 shows how many degrees the exhaust valve opens before bottom dead center,
The second 22 shows how many degrees the exhaust valve closes after top dead center.

The duration and overlap is calculated by using these values.

To conclude, any given camshaft will be perfect only at one engine speed. At every other engine speed, the engine won't perform to its full potential. A fixed camshaft is, therefore, always a compromise. In the next article we will look at the different camshaft arrangements and how these help the cams perform better.

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