Actual Valve Timing Of Four-Stroke Petrol/ Spark Ignition Engine.
What is Valve timing Diagram?
Valve timing is the regulation of the points in the cycle at which the valves are set to open and close.
In the ideal cycle inlet and exhaust valves open and close at dead centres, but in actual cycles they open or close before or after dead centres as explained below.
There are two factors, one mechanical and other dynamic, for the actual valve timing to be different from the theoretical valve timing.
(a) Mechanical factor.
The poppet valves of the reciprocating engines are opened and closed by cam mechanisms. The clearance between cam, tappet and valve must be slowly taken up and valve slowly lifted, at first, if noise and wear is to be avoided. For the same reasons the valve cannot be closed abruptly, else it will ‘bounce’ on its seat. (Also the cam contours should be so designed as to produce gradual and smooth changes in directional acceleration). Thus the valve opening and closing periods are spread over a considerable number of crankshaft degrees. As a result, the opening of the valve must commence ahead of the time at which it is fully opened (i.e., before dead centres).
b) Dynamic factor;
Besides mechanical factor of opening and closing of valves, the actual valve timing is set taking into consideration the dynamic effects of gas flow.
Actual Valve Timing Of Four-Stroke Petrol /Spark Ignition Engine.
IV opens 20 deg before TDC.
IV closes 35 deg after BDC to take the advantage of momentum of rapidly moving gases (Ram Effect).
Ignition occurs 35 deg before TDC; this is to allow the time delay between the spark and commencement of combustion.
EV opens at 35 deg before BDC; else pressure will rise enormously, and the work required to expel the gas will increase.
EV closes at 10 deg after TDC; this is to increase the volumetric efficiency.
Valve Overlap: The time during which both the valves (inlet and exhaust) remain open at the same instant.
The values quoted in the actual valve timing diagrams are the typical values, and may vary from engine to engine.For HIGH SPEED engines, higher values of angles are desirable to take into account the short time interval.
How Valves Timing Diagram works?
The IV opens before TDC and closes after BDC. This is arranged so as to get maximum charge inside the cylinder. When the IV opens, the charge outside the valve has to be accelerated upto the inlet velocity, and this takes time. In order that maximum inlet velocity occurs at the earliest possible moment during the intake stroke, the IV is opened early. The kinetic energy of the moving charge is used at the end of the intake stroke to produce a ramming effect by closing the IV slightly after BDC. The ramming effect thus increases the volumetric efficiency.
The EV opens before BDC so as to exhaust the combustion products efficiently. Thus, by virtue of its excess pressure above atmosphere, some exhaust gas leaves the cylinder. This makes the exhaust gas to flow freely from the cylinder by the time piston commences the exhaust stroke. Again, by closing the EV after TDC, the kinetic energy of the exhaust gas can be utilized to assist in maximum exhausting of the gas. In the process, the IV begins to open before the EV closes – and this is called valve overlap.
Actual Valve timing Diagram For two stroke Petrol/ Spark Ignition Engine: