Camshaft
The camshaft is an apparatus often used in piston engines of cars to operate
poppet valves. It consists of a cylindrical rod running the length of
the cylinder bank with a number of oblong lobes or cams protruding from
it, one for each valve. The cams force the valves open by pressing on
the valve, or on some intermediate mechanism, as they rotate.
Timing
The rotation of the camshaft and the rotation
of the crankshaft is of critical importance. Since the valves control
the flow of 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 some 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. Also on early fuel
injection systems, cams on the camshaft would operate the fuel
injectors.
In a other four-stroke engine case, the valves are opened only half as
often; two full rotations of the crankshaft occur for each rotation of
the camshaft.
In a two-stroke engine that uses a camshaft as
car parts, each valve
is opened once for each rotation of the crankshaft; in these engines,
the camshaft rotates at the same rate as the crankshaft.
Camshaft position
Depending on the different location of the camshaft, the cams operate
the valves either directly or through a linkage of pushrods and rockers.
Direct operation involves a simpler mechanism and leads to fewer
failures in car parts, 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 bother, but in modern gasoline
engines with modern car parts the overhead cam system, where the
camshaft is on top of the cylinder head, is quite common. Some engines
use two camshafts each for the intake and exhaust valves; such an
arrangement is known as a double or dual overhead cam (DOHC), thus, a V
engine may have four camshafts.
Duration
Duration can often be confusing point because manufacturers may select
any lift point to advertise a camshaft's duration and sometimes will
manipulate these numbers. The power and idle characteristics of a
camshaft rated at .006" will much different than one rated the same at
.002". Whenever duration is quoted, be sure to note the lift at which it
is given.
Many performance engine builders have learned to 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.
In Conclusion, duration determines how many crankshaft degrees a
camshaft maintains more than a given tappet lifts.
Maintenance
The rockers or cam followers sometimes incorporate a mechanism to adjust
and set the valve play 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, and in particular
the valves and valve seats in the combustion chamber.
The surfaces of the cam and follower are designed to "wear in" together,
and therefore when either is replaced, the other should be as well to
prevent excessive rapid wear.
Sliding friction between the surface of the cam and the cam follower
which rides upon it is 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. In some engines, the flat contact
surfaces are replaced with rollers, which eliminate the sliding friction
and wear but add mass to the valvetrain.
