USES
In internal combustion engines with pistons, the camshaft is used to
operate poppet valves. It consists of a cylindrical rod
running the length of the cylinder bank with a number of oblong lobes protruding from it, one for each valve. The cam lobes
force the valves open by pressing on the valve, or on some intermediate
mechanism, as they rotate.
AUTOMOTIVE: MATERIALS
Camshafts can be made out of several types of material.
These include:
Chilled iron castings: Commonly used in high volume
production, chilled iron camshafts have good wear resistance since the chilling
process hardens them. Other elements are added to the iron before casting to
make the material more suitable for its application.
Billet Steel: When a high quality camshaft or
low volume production is required, engine builders and camshaft manufacturers
choose steel billet. This is a much more time consuming process, and is
generally more expensive than other methods. However, the finished product is
far superior. CNC lathes, CNC milling machines, and CNC camshaft grinders will be used during production.
Different types of steel bar can be used, one example being EN40b. When
manufacturing a camshaft from EN40b, the camshaft will also be heat treated via gas nitriding, which changes the micro-structure of the material. It
gives a surface hardness of 55-60 HRC. These types of camshafts can be used in high-performance
engines.
TIMING
A Steel Billet racing camshaft with noticeably broad
lobes(very long duration)
The
relationship between the rotation of the camshaft and the rotation of the crankshaft is of critical importance. Since the valves control
the flow of the air/fuel mixture intake and exhaust gases, they must be opened
and closed at the appropriate time during the stroke of the piston. For this
reason, the camshaft is connected to the crankshaft either directly, via a gear mechanism, or indirectly via a
belt or chain called a timing belt or timing chain. Direct drive using gears is
unusual because of the cost. The frequently reversing torque caused by the
slope of the cams tends to cause gear rattle which for an all-metal gear train
requires further expense of a cam damper. Rolls-Royce V8 (1954) used gear drive
as, unlike chain, it could be made silent and to last the life of the engine. Where
gears are used in cheaper cars, they tend to be made from resilient fibre
rather than metal, except in racing engines that have a high maintenance
routine. Fibre gears have a short life span and must be replaced regularly,
much like a timing belt. In some designs the camshaft also drives the distributor and the oil and fuel pumps. Some vehicles may have the power steering pump driven by
the camshaft. With some early fuel injection systems, cams on the camshaft would operate the fuel
injectors. Honda redesigned the VF750 motorcycle from chain drive to the gear
drive VFR750 due to insurmountable problems with the VF750 Hi-Vo inverted chain
drive.
An
alternative used in the early days of OHC engines was to drive the camshaft(s)
via a vertical shaft with bevel gears at each end. This system was, for
example, used on the pre-World War I Peugeot and Mercedes Grand Prix cars. Another
option was to use a triple eccentric with connecting rods; these were used on
certain W.O. Bentley-designed engines and also on
the Leyland Eight.
In a two-stroke engine that uses a camshaft, each valve is opened once for
every rotation of the crankshaft; in these engines, the camshaft rotates at the
same speed as the crankshaft. In a four-stroke engine, the valves are opened only half as
often; thus, two full rotations of the crankshaft occur for each rotation of
the camshaft.
The timing of
the camshaft can be advanced to produce better low RPM torque, or retarded for
better high RPM power. Changing cam timing moves the overall power produced by
the engine down or up the RPM scale. The amount of change is very little
(usually < 5 deg), and affects valve to piston clearances.
DURATION
Duration is
the number of crankshaft degrees of engine rotation during which the valve is
off the seat. In general, greater duration results in more horsepower. The RPM
at which peak horsepower occurs is typically increased as duration increases at
the expense of lower rpm efficiency (torque).
Duration
specifications can often be misleading because manufacturers may select any
lift point from which to advertise a camshaft's duration and sometimes will
manipulate these numbers. The power and idle characteristics of a camshaft
rated at a .006" lift point will be much different from one with the same
rating at a .002" lift point.
Many
performance engine builders gauge a race profile's aggressiveness by looking at
the duration at .020", .050" and .200". The .020" number
determines how responsive the motor will be and how much low end torque the motor will make. The .050" number is used to
estimate where peak power will occur, and the .200" number gives an
estimate of the power potential.
A secondary
effect of increased duration can be increased overlap, which is the number of crankshaft degrees during which
both intake and exhaust valves are off their seats. It is overlap which most
affects idle quality, inasmuch as the "blow-through" of the intake
charge immediately back out thru the exhaust valve which occurs during overlap
reduces engine efficiency, and is greatest during low RPM operation. In
general, increasing a camshaft's duration typically increases the overlap,
unless the intake and exhaust lobe centers are moved apart to compensate.
LIFT
The camshaft
"lift" is the resultant net rise of the valve from its seat. The
farther the valve rises from its seat the more airflow can be provided, which
is generally more beneficial. Greater lift has some limitations. Firstly, lift
is limited by the increased proximity of the valve head to the piston crown and
secondly, greater effort is required to move the valve springs to a higher
state of compression. Increased lift can also be limited by lobe clearance in
the cylinder head casting. Higher valve lift can have the same effect as
increased duration where valve overlap is less desirable.
Higher lift
allows greater airflow; although even by allowing a larger volume of air to
pass thru the larger opening, the brevity of the typical duration with a higher
lift cam results in less airflow than with a cam with lower lift but more
duration, all else being equal. On forced induction motors this higher lift
could yield better results than longer duration, particularly on the intake side.
Notably though, higher lift has more potential problems than increased
duration, in particular as valve train rpm rises which can result in less
efficient running or loss of torque.
Cams that
have excessive valve lift, running at high rpm, can cause what is called
"valve float", where the valve spring tension is insufficient to keep
the valve following the cam at its apex. This could also be a result of a very
steep rise of the lobe, where the valve is effectively shot off the end of the
cam rather than following the cams’ profile. This is typically what happens
when a motor over revs. This is where the engine rpm exceeds the maximum design
rpm. The valve train is typically the limiting factor in determining the
maximum rpm the engine can maintain either for a prolonged period or
temporarily. Sometimes an over rev can cause engine failure when the valves
become bent as a result of colliding with the piston crowns.
POSITION
Depending on
the location of the camshaft, the cam operates the valves either directly or
through a linkage of pushrods and rockers. Direct operation involves a simpler
mechanism and leads to fewer failures, but requires the camshaft to be
positioned at the top of the cylinders. In the past when engines were not as
reliable as today this was seen as too much trouble, but in modern gasoline
engines the overhead cam
system, where the camshaft is on top of the cylinder head, is quite common.
NUMBER OF CAMSHAFTS
While today
some engines rely on a single camshaft per cylinder bank, which is known as
a single overhead camshaft (SOHC),
most modern engines are driven by a two camshafts per cylinder bank arrangement (one camshaft for
the intake valves and another for the exhaust valves); such camshaft
arrangement is known as a double or dual overhead cam (DOHC), thus,
a V engine, which has two separate cylinder banks, may have four
camshafts (colloquially known as a quad-cam
engine).
More unusual
is the modern W engine (also known as a 'VV' engine to distinguish itself
from the pre-war W engines) that has four cylinder banks arranged in
a "W" pattern with two pairs narrowly arranged with a 15-degree
separation. Even when there are four cylinder banks (that would normally
require a total of eight individual camshafts), the narrow-angle design allows
the use of just four camshafts in total. For the Bugatti Veyron, which has a 16-cylinder W engine
configuration, the four camshafts are driving a total of 64 valves.
The overhead
camshaft design adds more valve train components that ultimately result in more
complexity and higher manufacturing costs, but this is easily offset by many
advantages over the older design: multi-valve design, higher RPM limit, and design freedom to
better place valves, spark plugs (Spark-ignition
engine), and
intake/exhaust ports.
MAINTENANCE
The rockers
or cam followers sometimes incorporate a
mechanism to adjust the valve lash through manual adjustment, but
most modern auto engines have hydraulic lifters, eliminating the need to adjust the valve lash at regular
intervals as the valve train wears, in particular the valves and valve seats in the combustion chamber.
Sliding friction between the surface of the cam and the cam follower
which rides upon it can be considerable. In order to reduce wear at this point,
the cam and follower are both surface hardened, and modern lubricant motor oils contain additives specifically to reduce sliding
friction. The lobes of the camshaft are usually slightly tapered and the faces
of the valve lifters slightly domed, causing the lifters to rotate to
distribute wear on the parts. The surfaces of the cam and follower are designed
to "wear in" together, and therefore each follower should stay with
its original cam lobe and never be moved to a different lobe. You can put new
lifters on an old cam but never old lifters on a new cam. In some engines the
followers have rollers which eliminate the sliding friction and wear but add
mass to the valve train.
Camshaft
bearings are similar to crankshaft main bearings, being pressure-fed with oil.
However, overhead camshaft bearings do not always have replaceable bearing
shells, meaning
that a new cylinder head is required if the bearings suffer wear due to
insufficient or dirty oil.
ALTERNATIVES
In addition
to mechanical friction, considerable force is required to compress the valve
springs used to close the engine's valves. This can amount to an estimated 25%
of an engine's total output at idle, reducing overall efficiency. Some
approaches to reclaiming this "wasted" energy include:
·
Camless valvetrains using solenoids or magnetic systems have long been investigated
by BMW and Fiat, and are currently being prototyped
by Valeo and Ricardo
·
The Wankel engine, a rotary engine which uses neither
pistons nor valves, best known for being used by Mazda in the RX-7 and RX-8 sports cars.
·
Koenigsegg has developed an electric valve actuator as a more fuel efficient and space saving alternative
to the traditional camshaft.
IGNITION SYSTEMS
In mechanically timed ignition
systems, a separate cam in the distributor is geared to the engine and operates
a set of breaker points that trigger a spark at the correct time in the
combustion cycle.
ELECTRICAL
Before the
advent of solid state
electronics, camshaft controllers were used
to control the speed of electric motors. A camshaft, driven by an electric motor or a pneumatic motor, was used to operate contactors in sequence. By this means, resistors or tap changers were switched in or out of the circuit to vary the
speed of the main motor. This system was mainly used in electric multiple
units and electric locomotives.
WORKING