Starting air overlap (or air-overlap) refers to the crank angle range over which starting air is admitted to a cylinder during the starting of a two-stroke engine. In a two-stroke engine starting air must be available to each cylinder during the upper part of its expansion stroke, i.e. shortly after TDC and while the piston is moving down, so that the air pressure acting on the piston crown helps to turn the engine. The overlap is the period when the starting air valve is open. In practice, for a cylinder to be started at any crankshaft position, the starting air valve is arranged to open a small angle before TDC and to close a certain angle after TDC, giving a range (overlap) of admission. To ensure smooth starting at any position, the valve timing is set so the air is admitted to a cylinder shortly after the piston passes TDC (for a two-stroke the air should act on the down-going piston) and the distributor/starting sequence provides air to each cylinder in turn. The overlap prevents air being admitted continuously and is set to give a compact starting torque. The starting air supply must be admitted once per revolution to the cylinders in firing order during the starting stroke.

For a two-stroke engine the starting air valve for each cylinder is opened by the air distributor shortly after top dead centre of the cylinder (typically opening about 5 to 15 degrees after TDC, depending on design) and closed before the exhaust ports are passed or before the firing stroke, i.e. the valve is closed well before the piston reaches bottom dead centre and the exhaust blows down. The duration is set so the air push contributes to rotation without wasting excessive air. In practice the injection of starting air into a particular cylinder is timed to begin after the top dead centre position of the same cylinder and before the commencement of firing/ignition; some designs open a little before TDC to build up pressure. The exact opening/closing angles are set by the cam or by the electronic control of the distributor.

Slow turning is a feature by which the engine is first turned over very slowly (on turning gear or by a special low-speed air supply) for a short period before normal starting, without firing. The purpose is to:

• ensure the engine can be turned over freely before the actual start, detecting any seizure, hydraulic lock (e.g. water in cylinders), or foreign objects;
• expel any liquid (condensate, water, accumulated oil) from the cylinders and combustion space to avoid hydrostatic damage;
• lubricate the cylinder and running gear before the engine starts;
• check the engine is safe to start, particularly after standing idle or after maintenance;
• on engines reversing, to check the direction control.

The slow turning is achieved either by the turning gear (slow mechanical rotation) or by giving a very short burst of starting air to turn the engine a fraction of a revolution (air-driven slow turn), after which the control system checks that the crankshaft is turning and then proceeds to the normal start. It also verifies the starting air system is operational.