78 Auto Engine Performance and Driveability
until the camshaft lobe allows the valve spring to reseat
the valve. On overhead camshaft engines, the cam lobes
usually push directly on the valve rocker arm, Figure 5-8.
There are no push rods.
Valve Timing
The valves must open and close in proper relation to
the movement of the piston or the engine will not run. This
relationship is called valve timing, not to be confused with
ignition timing. Valve timing is determined by the relative
positions of the crankshaft and camshaft. The intake valve
must also open wide enough and long enough to allow the
air-fuel mixture to get into the cylinder. The exhaust valve
must do the same to allow the exhaust gases to get out of
the cylinder.
Lift is how wide the valve opens. Duration is the
amount of time that the valve stays open. Overlap is the
amount of time that both intake and exhaust valves are
open. Lift and duration are determined by the shape of the
camshaft lobes. Valve timing, lift, and duration have a big
effect on engine driveability.
The crankshaft always turns two complete revolutions
for every one revolution of the camshaft. This is because any
cylinder in a four-stroke cycle engine, whether gasoline or
diesel, requires two complete revolutions of the crankshaft
to complete all four cycles. However, each valve in the
engine opens only once during all four strokes. To accom-
plish this, the driving gear on the crankshaft always has half
the number of teeth as the driven gear on the camshaft.
Valve Lifters
Valve lifters transmit the motion of the camshaft lobes
to the push rods or rocker arms. Lifters can be mechanical
or hydraulic. Mechanical, or solid lifters, must be periodi-
cally adjusted. Hydraulic lifters are self adjusting.
A typical hydraulic lifter is shown in Figure 5-9. The
outer lifter body contacts the camshaft lobe. The inner
piston (plunger) contacts the push rod or rocker arm. The
space between the lifter body and the plunger is filled with
engine oil. This oil is supplied by the lubrication system
through a small passage.
When the camshaft lobe pushes the lifter body upward,
the oil passage into the lifter is sealed off. Since the oil can-
not escape or compress, the hydraulic lifter acts as a solid
unit and opens the valve. When the cam lobe allows the
lifter to move down, the lifter oil passage is again open and
oil can flow into the lifter. Engine oil pressure pushes the
plunger upward to remove any valve train clearance, but
does not have enough force to open the valve.
The friction between the lobes and lifters is the highest
friction in the engine and can cause the camshaft and lifters
to rapidly wear out. To reduce friction, the camshaft lobes
are tapered and the lifters offset. This causes the lifters to
rotate as they are pushed up by the lobe. This rolling action
between the lobe and lifter helps the lobes to evenly wear.
To further reduce friction, many late model engines use
roller lifters. A roller is installed on the bottom of the lifter
and turns with the lobe as the camshaft rotates.
Push Rods and Rocker Arms
Push rods are used only on cam-in-block engines.
They transmit the lifter motion to the rocker arm. Many
push rods are hollow. Oil from the lifter flows through them
to lubricate the rest of the valve train. Rocker arms are piv-
oting levers that convert the upward movement of the push
rod or lifter into downward movement of the valve.
Figure 5-8. The overhead camshaft valve train allows quicker
valve response with less play and friction between the com-
ponents. Overhead camshafts are either belt or chain driven.
(Subaru)
Spring
Oil hole
Plunger
Plunger
spring
Check ball
retainer
Check
ball
Body
Valve
rocker
Lash
adjuster
Figure 5-9. Hydraulic lifters use the engine oil pressure to auto-
matically eliminate play from the valve train. Hydraulic lifters are
used on almost all cam-in-block engines. (Ford)
Push rod cup
Check valve
Body
Plunger
Lock
ring
Check valve
retainer
Check valve spring
Plunger spring
Metering valve disk