Chapter 5 Fundamentals of Engine Construction and Operation 79
On overhead cam engines, the camshaft is installed
on top of the cylinder head. It opens the valves from above.
In some overhead cam engines, the lifter is placed between
the cam lobe and the rocker arm. In many cases, the rocker
arm is directly operated by the camshaft. A hydraulic lash
adjuster, similar in operation to a hydraulic lifter, maintains
the proper valve clearance. See Figure 5-10.
Camshaft Drive Mechanisms
To maintain the relationship between the valves and
pistons, one of three types of camshaft drive mechanisms
is used. In all cases, the gear or sprocket on the crankshaft
has exactly half the number of teeth on the camshaft gear
or sprocket. This causes the camshaft to turn at exactly half
of the crankshaft speed.
A few vehicles use a gear drive, Figure 5-11. This type
of drive has two meshing gears. The crankshaft gear rotates
to drive the camshaft gear.
The majority of overhead valve, cam-in-block engines
use a chain drive, Figure 5-12. The crankshaft gear drives
the chain. The chain then drives the camshaft gear.
A few overhead camshaft engines use a chain drive.
However, most overhead camshaft engines use a belt drive,
Figure 5-13. In this design, the belt is driven by a crankshaft
sprocket. The belt then drives the camshaft sprocket. On
these designs, the belt often drives the water pump and oil
pump. Some overhead camshaft engines use a system that
is a combination of chain and belt drives.
Variable Valve Timing Devices
In the past, the valve timing set by the relationship
of the drive and driven sprockets or gears could not be
changed once the engine was assembled. An increasing
Figure 5-10. Lash adjusters are used in the valve train for an
overhead camshaft engine. They use the same principle as
the hydraulic lifter, but are installed on the rocker arm or on the
opposite side of the valve stem. (Ford)
Hydraulic
lash adjuster
Roller
follower
Valve
stem
Cam
Figure 5-11. A few spark-ignition engines use gears as the cam-
shaft drive mechanism. Gear drive is more common in diesel
engines.
Camshaft
gear
Crankshaft
gear
Timing marks
Figure 5-12. Chain camshaft drive mechanisms are most com-
mon on cam-in-block engines. However, some overhead cam-
shaft engines also have chain camshaft drives. Note the timing
marks on both the camshaft and crankshaft timing gears.
Camshaft
gear
Crankshaft
gear
Timing
marks
Chain