• Overlap of the opening of the starting air valve is provided so that when one cylinder air starting valve is closing , other one is simultaneously opening. • This is to ensure a positive start position of the engine at all positions of the crank shaft. • If one cylinder air start valve of the unit is closed before the other unit valve has opened, than the engine at that position of the crank will not start. • Usual overlap period is 15 to 20 degrees. • CONDITIONS FOR ADMITTING STARTING AIR IN THE CYLINDER. • Starting air is admitted on working stroke. Period of opening is governed by a) Firing Interval of the Engine. No. of degrees in Engine Cycle i.e. 360 for 2 stroke engine./ No. of Cylinders. E.g. For 6 cylinder 2 stroke engine = 360/6 = 60 degrees. b) Starting air valve must close before Exhaust valve opens. c) Valve should open after firing dead centre to give a positive turning moment in correct direction.
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