What is Starting air overlap & Why it is provided ?

• 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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Marine Engineering

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