Valve Timing - Briefing

“Keep Going

Your hardest times often lead to the greatest moments of your life. Keep going. Tough situations build strong people in the end.”
― Roy T. Bennett

 

  

In general, an internal combustion engine functions befittingly with the assistance of every single component working inside. Each one depends on each. One of the momentous parameters used here for getting perfection in running the engine to harvest better output is called Valve Timing. Of course, this valve timing plays a vital role in improving the efficiency of an IC engine by opening and closing the valves with specialized adjustments, which are slightly abstruse.

Before entering, you need to gain some input about valves, piston movements, and the strokes of an engine. The valves consist of an inlet valve, which allows fresh charge into the cylinder, and an exhaust valve, which permits burned gases to send out from the cylinder. The piston reciprocates between the top dead center(TDC), which is the topmost position to which the piston can travel inside the cylinder, and the bottom dead center(BDC), which is the bottom-most position to which the piston can travel inside the cylinder. Consider it as a 4-stroke engine, which consists of
1)suction - sucks the fresh charge inside the combustion chamber,
2)compression - compress the air-fuel in SI engine and air alone in CI engine,
3)combustion or power - the mixture gets combusted and produces indicated power,  
4)exhaust - the burned charges get out of the combustion chamber.

Theoretically, the inlet valve opens when the piston is at TDC then closes when the piston is at BDC. Likewise, the exhaust valve opens when the piston is at BDC then closes when the piston is at TDC. It is a ceaseless cycle.

 

While coming to reality, valves can't function scrupulously because each mass has its inertial effects. So consequently, more time has to be granted for intake and exhaust openings.

For the SI engine, the inlet valve opens about 15-20 degrees before the piston reaches TDC, then closes around 30-35 degrees after the piston reaches BDC for sucking the air-fuel mixture inside. While compression, a spark is produced about 35 degrees before TDC, then power stroke takes place, and the exhaust valve opens about 35 degrees before BDC, then closes after 10 degrees from TDC.

For the CI engine,  the inlet valve opens about 15-25 degrees before the piston reaches TDC, then closes around 30 degrees after the piston reaches BDC for sucking the fresh charge inside. While compression, fuel injection starts around 5 degrees before TDC, then ends about 25 degrees after TDC. Combustion begins during the process of injection and produces power, then about 45 degrees before the BDC exhaust valve gets opened, and closes at 15 degrees after TDC.

Generally, if the valve opens earlier before reaching TDC or BDC, then it is called valve lead. If the valve closes later after crossing TDC or BDC, then it is called valve lag. The condition when both inlet and exhaust valves are open simultaneously during certain degrees of crankshaft rotation is called valve overlap. These parameters help manufacturers to achieve performance precisely.

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