In this section you will learn how
the camshaft affects engine performance. We've got some great
animations that show you how different engine layouts, like single
overhead cam (SOHC) and double overhead cam
(DOHC), really work. And then we'll go
over a few of the neat ways that some cars adjust the camshaft so
that it can handle different engine speeds more efficiently!
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 that we are 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. 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 shaped 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.
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 setup 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.
The animation below shows how a
regular cam and a performance cam have different valve timing.
Notice that the exhaust (red circle) and intake
(blue circle) cycles
overlap a lot more on the performance cam. Because of this, cars
with this type of cam tend to run very roughly at idle.
