80 Auto Engine Performance and Driveability
number of engines have some form of variable valve timing.
Variable valve timing allows the timing of the valve opening
and closing to be varied based on driving conditions. Two
general types of variable valve timing are used on engines:
p
Camshaft timing adjustment. These adjusters are
mounted on the end of the camshaft where it is fas-
tened to the driven sprocket or gear.
p
Camshaft lift and duration adjustment. These adjusters
are mounted at the camshaft lobes and rocker arms.
The vehicle computer operates the adjusters based on sen-
sor inputs. The types of variable valve timing devices are
discussed in more detail in Chapter 10.
Vibration Damper
When the engine cylinders fire, force is transmitted
to the crankshaft. When it receives this force, part of the
crankshaft tends to rotate before the rest of the crankshaft.
This causes a twisting of the crankshaft. When the force is
removed, the partially twisted shaft unwinds. This unwind-
ing action, although minute, causes what is known as tor-
sional vibration. To stop the vibration, a vibration damper,
sometimes called a harmonic balancer, is attached to the
front of the crankshaft. It consists of two heavy rings con-
nected by rubber plugs, spring-loaded friction discs, or a
combination of the two.
When a cylinder fires and the crankshaft speeds up,
the outer ring of the damper has a tendency not to rotate. As
a result, the rubber connecting the two rings of the damper
flexes. As the crankshaft tries to unwind after the cylinder
has fired, the outer ring of the damper again tends not to
rotate in the opposite direction and the rubber flexes. The
unwinding force of the crankshaft is cancelled out by the
damper. On some newer engines, the crankshaft pulley is
an integral part of the balancer.
The engine flywheel also absorbs vibration. The
flywheel used with manual transmissions is heavy and
absorbs vibration. Automatic transmission flywheels are
lightweight steel stampings. The torque converter absorbs
most of the vibration.
Balance Shafts
In some engines, one or more balance shafts are
added to counterbalance vertical and torsional vibrations.
A balance shaft has offset weights that rotate in the opposite
direction of the crankshaft. These shafts are either turned
by the camshaft through direct gearing or by the crankshaft
through a belt or chain. Balance shafts help to provide a
smoother idle and less vibration from the engine.
Engine-Related Systems That Can
Affect Driveability
The following sections cover systems related to the
engine that can affect driveability. These systems are all
part of the overall basic engine and affect engine operation
if they are not properly operating.
Cooling System
The cooling system is a set of components that remove
unwanted engine heat and regulate engine temperature.
A cooling system is needed because not all of the heat
of combustion creates pressure to move the pistons. The
excess heat must be removed to prevent engine damage.
Even a slightly overheating engine experiences excess wear
Figure 5-13. Belt camshaft drive mechanisms are used on
overhead camshaft engines. Some dual overhead camshaft
engines use a combination belt and chain system to drive the
camshafts.
Camshaft
sprocket
Stationary
mark
Timing mark
Stationary mark
Timing mark
Crankshaft
sprocket
Belt
Tensioner