Valvetrain
Valvetrain Design
The valvetrain on an engine includes the camshaft, the valves,
the valve configuration, and the followers.
Lets break down the various aspects and function of the camshaft to begin. The camshaft is a shaft with
off center lobes machined onto it whose function it is to open and close the intake and exhaust valves.
The camshaft is directly coupled to the crankshaft through gears, a chain, or a belt, and spins at one half
of the engine or crankshaft speed. The design of the camshaft depends on the valve configuration of the motor,
with pushrod Overhead Valve motors utilizing each camshaft lobe to open valves on the opposite side of the
motor. See the diagram below.
For an overhead cam motor, there is typically one lobe for each valve to be opened. In either case, there
is typically some type of follower that takes the rotating motion of the camshaft and converts it into linear
motion that the valve requires to open. Overhead valve motors require rocker arms to reverse the direction of
this linear motion to push the valve in the correct direction. Rocker arms also typically multiply the movement
of the pushrod by way of their center of rotation being offset as shown. This minimizes the required movement
of the lifter and pushrod. On an overhead cam motor, on the other hand, typically the follower pushes directly
on the valve as the cam is generally lined up right over the top of the valve stem as shown. Most Single overhead
cam motors have 2 valves per cylinder, but it is possible to have more valves opened by a single camshaft and some
sort of an arm that distributes the motion over more than one valve. Typically, however, a motor with 4 valves
per cylinder utilizes Dual overhead camshafts, thereby again utilizing one cam lobe for every valve on the engine.
A 2v per cylinder SOHC motor has the advantage that the valve spring does not have to reverse the motion of the
rocker arm and pushrod every cycle, which helps to increase the high rpm capability of the engine. A 4v per
cylinder DOHC motor has the advantage that 4 valves per cylinder allows for smaller, lighter valves, a higher
surface area for a given cylinder bore size for the intake and exhaust valve openings, and further improved high
rpm capability.
Some of the drawbacks of an overhead cam motor are the increased cost, size, complexity, and weight due to the
additional camshafts and drive mechanisms required. A 2V SOHC motor does not provide any real advantage in
airflow characteristics over a 2V pushrod motor that is smaller, lighter, and cheaper to manufacture. With
modern technology such as in the GM© LS series engines and the Chrysler© Hemi series engines, the advancements
in 2V pushrod design such as lightweight hollow valves, roller camshafts, and advanced materials, these engines
are still able to compete in today’s market of increasingly OHC motors. The smaller overall size of a pushrod
motor typically allows for a larger displacement engine than would be possible in the same space for a SOHC or
DOHC motor. On the other hand, 4V DOHC motors continue their popularity due to their flexibility, high power
output, and despite their increased cost.
There are two basic types of camshafts, the roller cam and the flat tappet cam. Most modern OHV motors use a
roller cam design, which has a small roller cam follower attached to the follower body. This results in much
lower forces between the cam and follower, and allows much more aggressive cam profiles without undue wear on
the materials. A flat tappet camshaft has a slightly different lobe design that simply pushes on the follower
that results in much higher contact forces and typically results in higher wear rates. OHC motors typically use
a flat tappet design for simplicity. Roller followers are not needed due to lower valve train mass and lighter
springs used on OHC motors. Be sure to check out the camshaft article
for more details on camshaft and rocker arm design.
The issue of valve float is the final topic on valve train design. Valve float typically occurs on pushrod motors,
when the rpm capability of the valve springs for a given amount of lift or camshaft ramp rate is too high.
Under normal operation the valve spring closes the valve and maintains pressure on the rocker arm, which maintains
pressure on the pushrod, which maintains pressure on the follower, which maintains pressure on the camshaft.
If the valve spring cannot close the valve fast enough, the valve will “hang” open, and when it does close will
do so in an uncontrolled manner. Then, when the slack is taken back up in the valvetrain, the resulting impact
forces can cause breakage or excessive wear. In the worst case, the valve can actually hang open and be contacted
by the piston, usually resulting in a bent valve or worse. Valve float is generally the result of over revving, or
installing inadequate springs or worn out springs on an engine.