Mazda Training manual - part 243

 

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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 

 

58

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. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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