If you read the article How Car Engines Work, you know about the valves that allow air / fuel into the engine and exhaust on the engine. The camshaft (called party uses cam lobes) which push against the valves open as the camshaft rotates; springs on the valves return them to their closed position. This is a critical work, and can have a big impact on engine performance at different speeds. On the next page of this article, you can see the animation, we have built to really show you the difference between a camshaft and a performance standard.
Start with the basics.
The key elements of any camshaft are the lobes. As the camshaft turns, the lobes open and close the intake valves and exhaust valves in time with the movement of the piston. It appears that there is a direct relationship between the shape of the cam lobes and how the engine produces in different speed ranges.
To understand why this is the case, imagine that we are running an engine extremely slowly – only 10 or 20 revolutions per minute (RPM) – so it takes the piston seconds to complete a cycle. It would be impossible to actually run a normal engine this slowly, but imagine that we could. At this slow speed, we want cam lobes shaped so that:
As the piston starts moving downward in the intake stroke (called top dead center, or TDC), the intake valve open. The intake valve would close right as the piston funds.
The exhaust valve would open right as the piston bottom (called bottom dead center based, or BDC) at the end of the power stroke, and close when the piston makes the exhaust stroke.
This configuration could work very well for the engine as it ran at that speed very slow. But what happens if you increase the speed? Let’s see.
When you increase the speed of rotation, the configuration of 10 to 20 RPM for the camshaft does not work well. If the motor runs at 4000 RPM, the valves open and close 2,000 times per minute, or 33 times per second. At these speeds, the piston moves very quickly, so that the air / fuel mixture rushing into the cylinder moves very quickly too.
When the intake valve opens and the piston begins its induction stroke, the air / fuel mixture in the inlet channel starts to accelerate in the cylinder. When the piston reaches the bottom of its intake stroke, air / fuel moves at a fairly high speed. If we were to close the intake valve closes, all the air / fuel would come to a stop and not enter the cylinder. Leaving the intake valve open a little more time, the dynamics of fast moving air / fuel continues to force air / fuel into the cylinder when the piston begins its compression stroke. So more the engine will, over the movements of the air / fuel, and we want the intake valve remains open. We also want the valve to open wider at higher speeds – this parameter, called valve lift, is governed by the cam lobe profile.
The animation below shows how a regular cam and a cam performance have different timing. Note that the cycles of the exhaust (red circle) and intake (blue circle) overlap much performance on the cam. For this reason, the cars with this type of cam tend to run very rough at idle.
Two different cam profiles: Click the button below the play button to switch between the cams. The circles indicate how long the valves stay open, blue for the home, red exhaust. The valve overlap (when the two intake valves and exhaust are open at the same time) is shown at the beginning of each animation.
Any given camshaft will be perfect only one engine speed. At each engine speed on the other hand, the engine will not run to its full potential. A fixed camshaft is, therefore, always a compromise. That’s why carmakers have developed schemes to vary the cam profile as engine speed changes.
There are several different arrangements of camshafts on engines. We’ll talk about some of the most common. You’ve probably heard of the terminology:
Single overhead camshaft (SOHC)
The dual overhead camshaft (DOHC)
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