4.5.5 Valve timing of a four-stroke engine
The moments of opening and closing of valves in four-stroke engines do not coincide with the positions of the pistons at TDC and BDC for better filling the cylinders with a fresh charge and the most complete cleaning of them from exhaust gases. But they occur with a certain advance or delay. In other words, the inlet valve can close once the piston passes BDC, and the exhaust valve - once the piston passes TDC.
The moments of opening and closing of valves that are expressed in degrees, corresponding to the angles of rotation of the crank of the crankshaft relative to the dead points, are called valve timing. The valve timing can be plotted on a radius chart, which is called the valve timing diagram, as shown in Figure 4.11.
Perhaps it will be easier to show this with an example. Thus, if one says that the valve opens for 5 ° before TDC, then it started opening at a time when the crankshaft crank, to which the piston connecting rod is attached, was 5 ° before the top dead center.
Figure 4.11 Four-stroke engine gas distribution diagram
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