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Valve
System Operation
Valve
Lifter
(Tappet)
The
valve lifter is the unit that makes contact with the valve stem and
the camshaft. It rides on the camshaft. When the cam lobes push it
upwards, it opens the valve.
The
engine oil comes into the lifter body under pressure. It passes
through a little opening at the bottom of an inner piston to a cavity
underneath the piston. The oil forces the piston upward until it
contacts the push rod. When the cam raises the valve lifter, the
pressure is placed on the inner piston which tries to push the oil
back through the little opening. It can't do this, because the
opening is sealed by a small check valve.
When
the cam goes upward, the lifter solidifies and lifts the valve.
Then, when the cam goes down, the lifter is pushed down by the push
rod. It adjusts automatically to remove clearances.
Lifter
Body
The
valve lifter body houses the valve lifter mechanism. The valve
lifter is the unit that makes contact with the valve stem and the
camshaft. It rides on the camshaft. When the cam lobes push it
upwards, it opens the valve.
Valve
Cover
The
valve cover covers the valve train. The valve train consists of
rocker arms, valve springs, push rods, lifters and cam (in an
overhead cam engine). The valve cover can be removed to adjust
the valves. Oil is pumped up through the pushrods and dispersed
underneath the valve cover, which keeps the rocker arms lubricated.
Holes are located in various places in the engine head so that the
oil recirculates back down to the oil pan. For this reason, the valve
cover must be oil-tight; it is often the source of oil leaks.
The
valve cover is often distorted on older cars, because at some point
the valve cover screws were over-tightened, bending the valve cover.
This happens because the valve cover is made of very thin sheet metal
and cannot withstand the force of an over-tightened bolt.
One
way to determine if your valve cover is bent is to remove the gasket
and put the valve cover back on to the cylinder head. When the valve
cover and cylinder head come into contact, the cover should sit flat.
If it rocks, it is bent. Cast aluminum valve covers cannot be
straightened, they need to be replaced. Sheet metal valve covers can
be straightened.
A
symptom of a bent or leaking valve cover is a pinching of the valve
cover gasket. This means that the gasket is sealing one area and not
sealing another area. This condition produces a leak; oil could be
leaking down the side of the engine. Some valve covers are hard to
access, because they are covered with other engine parts.
Chronic
valve cover leakage can sometimes be fixed by using two gaskets
glued together instead of using just one.
Valve
Ports
Valve
ports are openings in the cylinder head. Intake ports let the fuel
mixture into the cylinder head, and exhaust ports let the exhaust out.
Valves
The
valve's job is to open and close the valve ports. If the ports were
always open, the fuel exploded in the combustion chamber would leave
through the ports. The explosion has to be kept in the combustion
chamber to push the piston down. The valves are set up to open and
close at exactly the right moment. One lets the fuel mixture in and
closes. After the fuel explodes and pushes the piston down, the other
valve lets the exhaust out.
Valve
Guides
The
valves are usually held in an upright position by the valve stem.
The valve stem is the long straight side of the valve, like the stem
of a flower. Holes are bored in the cylinder head for the valve
stems. Worn valve guides allow oil to enter the combustion chamber
and cause blue smoke in the exhaust.
Valve
Springs
The
valve springs keep the valves closed tightly against their seats
until the valve is opened by the cam. After the cam turns (releasing
pressure), the valve springs close the valves.
Valve
Seals
The
valve seal is a unit that goes over the end of the valve stem. It
keeps excess oil from getting between the valve guide and the valve stem.
Camshaft
The
camshaft is a round shaft with "lobes" (specially
formed bumps) which is driven by the timing belt or timing chain.
It, directly or through "lifters" and "pushrods"
opens and closes the fuel and exhaust valves. The camshaft turns at
one-half of the crankshaft speed. It is supported by bearings located
in the front and rear of the crankcase.
Rocker
arms
Rocker
arms are used to transmit force from cam to valve. Riding on a cam
on the camshaft, rocker arms direct the upward motion of the lobe of
the cam into an opening motion of the valve stem.
Push
Rods
Push
Rods attach the valve lifter to the rocker arm. Through their
centers, oil is pumped to lubricate the valves and rocker arms. |
