Mazda Training manual — part 243

4 – VALVE TRAIN

57

Piston Engine Fundamentals

TC010-05-01S

Chain Adjuster

Timing chain tension changes with engine temperature and wear. A chain
adjuster mounted on the cylinder block uses both engine oil pressure and
spring tension to automatically maintain timing chain tension.

When the engine is started, oil pressure builds in the adjuster. As shown in
Figure 40, oil pressure and spring tension push the chain adjuster sleeve
outward if the chain is loose. The ratchet pawl skips to the next tooth,
pushing the chain lever against the chain, taking out excess slack. When
the engine is stopped, the oil pressure drops, but the ratchet pawl prevents
the adjuster sleeve from being pushed back into the chain adjuster.

FIGURE 40. The
chain adjuster
uses oil
pressure and
spring tension to
automatically
maintain timing
chain tension.

Ratchet
spring

Ratchet pawl

Chain aduster
sleeve

Drain

Plunger
spring

Chain lever

Timing chain

4 – VALVE TRAIN

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Piston Engine Fundamentals

TC010-05-01S

FIGURE 41. A
pulley and a belt
or chain transfers
rotation from the
crankshaft to the
camshaft.

Camshaft

pulleys

Tensioner

pulley

Timing belt

Timing belt

pulley

Idler pulley

OHC Drive

To drive the camshaft(s), a pulley on the end of the crankshaft drives a
timing belt or timing chain that turns the camshaft(s).

Figure 41 shows how this works on an OHC engine (in this case, an
engine with dual overhead cams). The timing belt is driven by the
crankshaft pulley. The camshaft pulleys connected to this belt then turn
each camshaft.

As Figure 41 shows, the timing belt pulley has half as many teeth as the
camshaft pulleys. This means that the camshafts turn once for every two
turns of the crankshaft. OHC drives also include a tensioner pulley and
tensioner spring or hydraulic auto tensioner, which maintain timing belt
tension and valve timing.

4 – VALVE TRAIN

59

Piston Engine Fundamentals

TC010-05-01S

Timing Belt Auto Tensioner

OHC drives use a timing belt auto tensioner, which works similar to the
chain adjuster, except that the auto tensioner uses a rod and piston
instead of a ratchet. As shown in Figure 42, when the engine is cold, there
is little tension on the belt. The auto tensioner rod pushes the tensioner
pulley up, taking the slack out of the belt.

When the engine is warm, expansion causes increased belt tension, which
pushes the rod into the tensioner.

FIGURE 42. The
maintenance-
free timing belt
auto tensioner
automatically
adjusts the belt
tension
according to
engine
temperature.

Tensioner
pulley

Auto
tensioner

4 – VALVE TRAIN

60

Piston Engine Fundamentals

TC010-05-01S

FIGURE 43. The
combination
DOHC drive uses
a timing belt for
the intake
camshaft and a
timing chain for
the exhaust
camshaft.

Camshaft
pulley

Camshaft
chain

Chain
adjuster

Intake
camshaft

Exhaust
camshaft

Timing belt
pulley

Timing belt

Tensioner
pulley

Belt and Chain Drive

Another type of DOHC drive is the combination belt and chain drive. In
this design, shown in Figure 43, a timing belt drives the intake camshaft,
and a timing chain drives the exhaust camshaft. The major advantage of
this design is that it allows the valves to be placed at a more vertical
angle. This angle produces enhanced combustion efficiency, better fuel
economy, and lower emissions.