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Valve System Operation
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| Valve System Operation |
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| Updated |
Jul 7, 2004 20:29:29 |
| Rating |
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Description:
Below is information on the valvetrain and
camshaft
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. |
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