What is Starting Air Overlap? Why is it provided?

Close view of the crude oil tanker's engine room.

Overlap is a period when two (or more) cylinders are receiving the starting Air simultaneously where one cylinder is “phasing out” and the other is “phasing into” the starting air peroid.

Overlap is provided because it is necessary to start the engine in any crank position and thus this ensures that at least one valve will open when the starting air is inducted in.

If there is no overlap provided, then the engine could stop in any position with all the starting air valves closed, when the starting air was given.

Overlap ensures that at every crank angle there is sufficient air turning moment to enable positive starting.

Overlap is reduced if there are more numbers of cylinder units.

It depends on the start air period, exhaust timing and the number of cylinders.

Minimum No. of cylinder units for overlap:

For 2 stroke Engine, a Minimum of 4 cylinders is required to get the overlap.

Firing interval = (360 degree/ 4 unit) = 90 degrees.

if the starting air peroid is 115 degrees,

Then the total overlap period = starting air period – firing interval


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