If you have read the article How Car Engines
Work, you know about the valves that let the air/fuel
mixture into the engine and the exhaust out of the engine. 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. On the next page of this
article you can see the animation we built to really show you
the difference between a performance camshaft and a standard
one.
The camshaft (click on image to see
animation)
In this edition of HowStuffWorks,
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 Basics 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.
Two different cam profiles: Click the button under
the play button to toggle between cams. The circles show how
long the valves stay open, blue for
intake, red for exhaust. The
valve overlap (when both the intake and exhaust valves are
open at the same time) is highlighted at the beginning of each
animation.
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 carmakers have developed
schemes to vary the cam profile as the engine speed changes.
Camshaft Arrangements 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)
Pushrod
Single Overhead Cams This
arrangement denotes an engine with one 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.
A single overhead cam
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.
Damage from a piston
striking a
valve
The picture above shows what can happen when a piston hits
an open valve.
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.
Pushrod Engines 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.
A pushrod
engine
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.
A pushrod engine
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.
Variable Valve Timing There are a couple of
novel ways by which carmakers vary the valve timing. One
system used on some Honda engines is called VTEC.
VTEC (Variable Valve Timing and Lift Electronic Control) is
an electronic and mechanical system in some Honda engines that
allows the engine to have multiple camshafts. VTEC engines
have an extra intake cam with its own rocker, which
follows this cam. The profile on this cam keeps the intake
valve open longer than the other cam profile. At low engine
speeds, this rocker is not connected to any valves. At high
engine speeds, a piston locks the extra rocker to the two
rockers that control the two intake valves.
Some cars use a device that can advance the valve
timing. This does not keep the valves open longer;
instead, it opens them later and closes them later. This is
done by rotating the camshaft ahead a few degrees. If the
intake valves normally open at 10 degrees before top dead
center (TDC) and close at 190 degrees after TDC, the total
duration is 200 degrees. The opening and closing times can be
shifted using a mechanism that rotates the cam ahead a little
as it spins. So the valve might open at 10 degrees after TDC
and close at 210 degrees after TDC. Closing the valve 20
degrees later is good, but it would be better to be able to
increase the duration that the intake valve is open.
Ferrari has a really neat way of doing this. The
camshafts on some Ferrari engines are cut with a
three-dimensional profile that varies along the length
of the cam lobe. At one end of the cam lobe is the least
aggressive cam profile, and at the other end is the most
aggressive. The shape of the cam smoothly blends these two
profiles together. A mechanism can slide the whole camshaft
laterally so that the valve engages different parts of the
cam. The shaft still spins just like a regular camshaft -- but
by gradually sliding the camshaft laterally as the engine
speed and load increase, the valve timing can be optimized.
The variable cam system used on some
Ferraris
Several engine manufacturers are experimenting with systems
that would allow infinite variability in valve timing. For
example, imagine that each valve had a solenoid on it that
could open and close the valve using computer control rather
than relying on a camshaft. With this type of system, you
would get maximum engine performance at every RPM. Something
to look forward to in the future...
