In the intricate world of automotive
engineering, the camshaft plays a
pivotal role in the functionality
and efficiency of an engine. The
question of "What does a camshaft
do?" reveals the complexity and
innovation behind internal
combustion engines. This component
is essential for controlling the
engine's intake and exhaust valves,
orchestrating the precise timing
necessary for optimal performance.
By converting rotational motion into
linear motion, the camshaft ensures
that fuel intake and exhaust
expulsion occur at just the right
moments. Through understanding the
function of the camshaft, one gains
insight into the remarkable
engineering that powers vehicles
around the globe.-By
NewCams
At the heart of every
combustion engine lies the camshaft, a
component whose role is crucial yet
often understated. It's essential to
recognize that this piece of engineering
genius directly influences an engine's
performance and efficiency.
The camshaft's primary
function is to regulate the opening and
closing of the engine's intake and
exhaust valves. It does so through a
synchronized dance of rotation and
timing. As the camshaft rotates, each
camshaft lobe-a meticulously designed
protrusion¡ªinteracts with valve lifters
or pushrods to precisely control the
timing and duration that valves stay
open. This harmonious operation ensures
that fuel can enter the combustion
chamber and exhaust can exit at the
optimum moments, directly impacting the
engine's power output and fuel economy.
The role of the
camshaft in a combustion engine cannot
be overstated. By dictating the timing
of the valve openings, it plays a
pivotal part in the engine's breathing
process. The efficiency of this process
is what allows a vehicle to glide
effortlessly on the road or roar to life
with power.
The innovation behind
each camshaft lobe's design and the
precision with which the camshaft
rotates underscore the complexity of
modern automotive engineering. These
components work tirelessly and unseen,
yet they are fundamental to the engine's
capability to harness energy from fuel.
Now that you have an understanding of
the camshaft's function, it's important
to know how they work. Let's start with
the basics.-By
NewCams
Camshaft 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. But what happens if you
increase the RPM? Let's find out.
When you increase the RPM, the 10 to 20
RPM 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 above 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.
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.-By
NewCams
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
In
the next section, we'll look at
each of these configurations.-By
NewCams
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).-By
NewCams
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.
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¡¤ CamShaft
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
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¡¤ CamShaft
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.
The final configuration we'll go into in
this article is the pushrod engine.-By
NewCams
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.
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.-By
NewCams
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.
A big thing in designing camshaft
systems is varying the timing of each
valve. We'll look into valve timing in
the next section.-By
NewCams
There are a couple of novel ways by
which carmakers vary the valve timing.
One system used on some Honda engines
is called VTEC.-By
NewCams
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.
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¡¤ CamShaft
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.
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...-By
NewCams
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