The camshaft uses lobes, called 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 way the shape of the cam lobes and the way the engine performs in different speed (revolutions per minute or RPM) ranges.
To understand why this is the case, imagine that we are running an engine extremely slowly -- at just 10 or 20 RPM -- so that it takes the piston a couple of seconds to complete a cycle. It would be impossible to actually run a normal engine this slowly, but let's imagine that we could. At this slow speed, we would want cam lobes shpaed so that:
Just as the piston starts moving downward in the intake stroke (called top dead center, or TDC), the intake valve would open. The intake valve would close right as the piston bottoms out.
Then the exhaust valve would open right as the piston bottoms out (called bottom dead center, or BDC) at the end of the combustion stroke, and would close as the piston completes the exhaust stroke.
This set-up would work really well for the engine as long as it ran at this very slow speed.
When you increase the RPM, however, this configuration for the camshaft does not work well. 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/fuel mixture rushing into the cylinder is moving very quickly as well.
When the intake valve opens and the piston starts its intake stroke, 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 is moving at a pretty high speed. If we were to slam the intake valve shut, all of that air/fuel would come to a stop and not enter the cylinder. By leaving the intake valve open a little longer, the momentum of the fast-moving air/fuel continues to force air/fuel into the cylinder as the piston starts its compression stroke. So the faster the engine goes, the faster the air/fuel moves, and the longer we want the intake valve to stay open. We also want the valve to open wider at higher speeds This parameter, called valve lift, is governed by the cam lobe profile.
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. This is why car makers have developed schemes to vary the cam profile as the engine speed changes.
There are several different arrangements of camshafts on engines; we'll talk about some of the most common ones. You've probably heard the terminology:
single overhead cam (SOHC)
double overhead cam (DOHC)
Single Overhead Cams
This arrangement denotes an engine with a single cam per head. So if it is an inline 4-cylinder or inline 6-cylinder engine, it will have one cam; if it is a V-6 or V-8, it will have two cams (one for each head).
The cam actuates rocker arms that press down on the valves, opening them. Springs return the valves to their closed position. These springs have to be very strong because at high engine speeds, the valves are pushed down very quickly, and it is the springs that keep the valves in contact with the rocker arms. If the springs were not strong enough, the valves might come away from the rocker arms and snap back. This is an undesirable situation that would result in extra wear on the cams and rocker arms.
On single and double overhead cam engines, the cams are driven by the crankshaft, via either a belt or chain called the timing belt or timing chain. These belts and chains need to be replaced or adjusted at regular intervals. If a timing belt breaks, the cam will stop spinning and the piston could hit the open valves.
Double Overhead Cam
A double overhead cam engine has two cams per head. So inline engines have two cams, and V engines have four. Usually, double overhead cams are used on engines with four (or more) valves per cylinder -- a single camshaft simply cannot fit enough cam lobes to actuate all of those valves.
The main reason to use double overhead cams is to allow for more intake and exhaust valves. More valves means that intake and exhaust gases can flow more freely because there are more openings for them to flow through. This increases the power of the engine.
Like SOHC and DOHC engines, the valves in a pushrod engine are located in the head, above the cylinder. The key difference is that the camshaft on a pushrod engine is inside the engine block, rather than in the head
The cam actuates long rods that go up through the block and into the head to move the rockers. These long rods add mass to the system, which increases the load on the valve springs. This can limit the speed of pushrod engines; the overhead camshaft, which eliminates the pushrod from the system, is one of the engine technologies that made higher engine speeds possible.
The camshaft in a pushrod engine is often driven by gears or a short chain. Gear-drives are generally less prone to breakage than belt drives, which are often found in overhead cam engines.
Here is a brief explanation on what lifter are and what they do:
Also called "followers" or "tappets," they are the components that ride on the cam lobes and help "lift" the valves open. There are two basic types: solid and hydraulic. Hydraulic lifters are hollow and fill up with oil to take up slack in the upper valve train. Low oil pressure, loss of pressure from the lifters or plugged oil holes in the lifters can result in a "clattering" sound that's referred to as "noisy lifters." Hydraulic lifters do not require periodic adjustment but solid lifters do to maintain the correct amount of valve lash.
This conclude this lesson.
(Edited by bryan at 3:28 pm on July 28, 2001)
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