Currently, there are two types of designs used in the automobile engine for the
placement of the valves and camshaft. They are the overhead valve (OHV) and the
overhead cam (OHC). Both designs have their valves mounted above the cylinders
in the cylinder head. It is the location of the camshaft, which operates the
valves, that distinguishes the two designs.
If the camshaft is located in the cylinder head, the engine is called an
overhead cam design. If the camshaft is located in the engine block, the engine
is called an overhead valve design. A dual overhead cam (DOHC) engine has two
camshafts on each cylinder head, one camshaft to operate the intake valves,
while the other one operates the exhaust valves.
On an overhead valve engine (OHV), the camshaft is located in the engine block.
The camshaft uses lifters, push rods, and rocker arms to activate the valves.
Its operation is fairly simple. As the camshaft rotates, each off-center
(eccentric) cam lobe pushes against a lifter. The upward motion of the lifter is
transferred through the push rod and onto the rocker arm.
The upward motion is changed to downward motion as the rocker arm pivots. The
downward motion is then used to open the valve. As the camshaft continues to
rotate, the lobe passes by the lifter and allows the valve to close. A spring is
attached to the valve which returns the valve to its seated position.
Every cylinder in the internal combustion engine contains at least two valves.
One intake valve and one exhaust valve. Both valves are used to open and close
internal passages in the cylinder head.
The intake valve is the larger of the two valves. It controls the flow of the
fuel mixture (gasoline) or air (diesel) into the combustion chamber. The exhaust
valve controls the flow of exhaust gases out of the cylinder.
The valves consist of a round head, a stem and a groove at the top of the valve.
The head of the valve is the larger end that opens and closes the passageway to
and from the combustion chamber. The stem guides the valve up and down and
supports the valve spring. The groove at the top of the valve stem holds the
valve spring in place with a retainer lock.
The valves must open and close in order for the air/fuel mixture to enter and
exit the combustion chamber. The opening and closing of the valves must be
properly timed in order for the engine to run smoothly. Valve operation
(sequence and timing) is controlled by the camshaft.
CamshaftThe camshaft consists of several camshaft journals and a set of cam lobes. The camshaft journals, like the crankshaft journals, are used to hold the camshaft in place. A bearing is placed around each journal, allowing it to rotate freely. The cam lobes are used to open the intake and exhaust valves in each cylinder. The number of cam lobes equals the number of valves in an engine.
The angular positioning of the cam lobes on the shaft determines the sequence of
the valves opening and closing. The design of the lobes determines how high the
valves will open (lift) and how long they will remain open (duration). These
designs vary from one engine to another.
