marine diesel engine starting air overlap period

 Starting air overlap required:
Overlap is a period when two or more cylinder 
unit are receiving starting air when one unit is phasing "out" while the other one is phasing in the start air period .
This is essential to ensure that the positive starting 
of the engine in any crank position . It depends on the start air period exhaust  timing & noumber of the cylinder 
For example 3 cylinder engine (two stroke)
Firing interval=360deg/3unit=120)