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What is Valve Timing Diagram ? Why do We Need Valve Timing diagram?


What is Valve Timing Diagram :-


A valve timing diagram is a representation of the positions of the crank when the various operations as inlet valve opening, closing, exhaust valve opening and closing and also the beginning and end of various strokes. The extreme position of the bottom of the cylinder is called “Bottom Dead Centre” [BDC]. The position of the piston at the top of the cylinder is called “Top Dead Centre” [TDC].


Theoretically it may be assumed that the valves open and close and the spark occurs at the engine dead centers. However, in actual operation, the valves do not operate at dead center positions but operate some degree on either side of the dead centers. The opening occurs earlier and the exhaust continues even at later crank angles. The ignition is also timed to occur in advance of the completion of compression stroke. The timing of these events, referred in terms of crank angles from dead center positions, is represented on a valve timing diagram. The correct timings are of fundamental importance for the efficient and successful running of the I.C. engine.


Why do We Need Valve Timing diagram? 


The normal engine completes around 100000 cycles per minute, as we know there are number of processes involve in a single cycle (from the intake of the air-fuel mixture to the exhaust of the combustion residual) of a internal which makes it necessary to be equipped with an effective system that can enable
  • Synchronisation between the steps of a cycle of the engine from intake of air-fuel ratio to the exhaust of combustion residual.
  • Complete seizure of the combustion chamber at the instant at which the combustion of air-fuel mixture takes place as the leakage can cause damage to the engine and can be hazardous.
  • Provide engine with a mixed air and fuel or air in case of diesel engine when required ( at the time of suction) which is the necessity of the engine.
  • Provide the exit for the combustion residual so that the next cycle of the engine can take place.
  • Ideal timing for the opening and closing of the inlet and outlet valve which in turn protect the engine from defects like knocking or detonation.
  • A high compression ratio required to combust the fuel especially in case of diesel engine by overlapping the closing of the valve.
  • The cleaning of engine cylinder which in turn maintain the quality of combustion and decreases wear and tear inside the cylinder.
  • The study of the details of the combustion that is required for the modification of the power of the engine.
So due to these reasons a engine weather it is 2-stroke or 4-stroke is designed according to the valve timing diagram, so that the movement of piston from TDC to BDC is provided with the ideal timing of opening and closing of the intake and exhaust valves respectively.



Valve Timing Diagram For A Four Stroke Cycle Petrol Engine :-
The petrol engine are also know as Spark Ignition Engines.The valve timing diagram for a four stroke cycle engine as shown in the below figure :-


(a) The inlet valve open (IVO) at 10°-20° before top dead center(TDC) and closes 30°-40° after bottom dead center (BDC).
(b) The compression of charge starts at 30°-40° after BDC and ends at 20°-30° before TDC.
(c) The ignition (IGN) of charge takes places at 20°-30° before TDC.
(d) The expansion starts at 20°-30° before TDC and ends at 30°-50° before BDC.
(e) The exhaust valve opens (EVO) at 30°-50° before BDC and closed at 10°-15° after TDC.

Note :-

  • The inlet valve of a four stroke IC engine remains open for 230°.
  • The charge is compressed when both the valves (i.e, inlet valve and exhaust valve) are closed .
  • The charge is ignited with the help of a spark plug.
  • The pressure inside the engine cylinder is above the atmospheric pressure during the exhaust stroke.



Valve Timing Diagram For A Four Stroke Cycle Diesel Engine :-
The diesel engines are also know as compression ignition engines.The valve timing diagram for a four stroke cycle diesel engine engine is show in the below figure :-

The following particular are important for a four stroke cycle diesel engine regarding valve timing diagram :-




(a) The inlet valve opens at 10° -20° before TDC and closes at 25°-40° after BDC.
(b) The fuel valve opens at 10°-15° before TDC and closes at 15°-20° after TDC.
(c) The compression starts at 25°-40° after BDC and ends at 10°-15° before TDC.
(d) The expansion  starts at 10°-15° after  TDC and ends at 10°-15° before BDC.
(e) The exhaust valve opens at 30°-50° before BDC and closes at 10°-15° after TDC.

Note :-
In diesel engines, the fuel is injected in the form of very fine spray into the engine cylinder, which gets ignited due to high temperature of the compressed air.

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