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Valve timing is crucial to the efficient operation of the 4-stroke petrol or diesel engine. The opening and closing

of the inlet and exhaust valves at the correct time in relation to piston position must be precisely controlled,

either at a fixed compromise position, or variable within finely controlled limits.

The valves are opened and closed by the camshaft(s) which are driven by chain, toothed belt or gear train from

the crankshaft.

Intake Stroke

As the piston rises, and at a point just before it reaches TDC, the intake valve opens, and a mixture of fuel and air begin to enter the combustion chamber.

As the piston travels downwards the mixture of fuel and air continues to enter the combustion chamber and at a point just after the piston reaches BDC, the intake valve closes.

Compression Stroke

The air-fuel mixture is compressed as the piston travels upwards to a point just before it reaches TDC, at which time the spark plug fires and combustion takes place forcing the piston downwards.

Power Stroke

As the piston travels downwards, and at a point just before it reaches BDC, the exhaust valve opens and the exhaust gases begin to leave the combustion chamber.

Exhaust Stroke

As the piston travels upwards the exhaust gases continue to leave the combustion chamber and at a point just after the piston reaches TDC, the exhaust valve closes.

Valve timing

It is crucial that the timing marks on the flywheel and camshaft are correctly aligned before removing and replacing the cambelt (timing belt). Check using a maintenance manual (eg: Ha*nes).

Before commencing the engine must be cold.

Remove the sparkplugs to release compression. This will make it easier to turn the crankshaft by hand.

Align the timing mark on the flywheel by turning the crankshaft slowly. On most FWD cars the cranskshaft pulley can be accessed by removing the plastic wheel liner

Before removing the old cambelt the camshaft sprocket wheel must be locked up to prevent rotation. If the camshaft sprocket wheel rotates even slightly when you are fitting the new cambelt a valve timing error is introduced then when the car is started up an increase in tappet noise is heard.

An valve timing error of a just few degrees introduced when replacing the camshaft belt can cause the tappets to sound noisy and this can happen if the cambelt position is 'out' by just 2-3 teeth on the camshaft sprocket wheel. The tendency then is to stop the engine and immediately adjust all the valve clearances to reduce the tappet noise. Do not adjust the tappets because the engine will not fire up when started.

Leave the engine to cool down. The new cambelt must then be removed from the camshaft sprocket wheel and replaced into the correct position ensuring that the timing marks on the flywheel and the camshaft sprocket wheel are lined up together. This ensures that the spark in the cylinder occurs at the correct timing in relation to the position of the piston in the cylinder.

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โˆ™ 2010-09-27 16:25:56
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Q: Valve timing diagram of petrol engine?
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