The air start distributor is a device in the starting system of a large two-stroke diesel engine that provides timed compressed air to each cylinder's starting air valve (the large pneumatically-operated valve in the cylinder head) in the correct sequence. It consists of a rotary valve whose rotor is driven by the engine crankshaft. As the engine turns, the distributor admits starting air to each cylinder only during that cylinder's "starting stroke" (when the piston is moving down into the cylinder to receive the air push), in the correct firing order. The effect is that the compressed air, admitted into the cylinder, pushes the piston down and turns the engine until the first fuel injection/combustion takes over and the engine is self-sustaining. It is critical because: without correctly-timed starting air no engine can start - the air has to reach each cylinder at the right crank angle to produce a continuing turning force and to stop once the engine is firing (to avoid air passing out and wasting the receivers and overheating the piston). Hence it governs both starting and the load on the starting air system.

Compressed starting air from the air receivers is supplied to the distributor inlet. Inside, a rotor (driven from the crankshaft at engine speed) carries ports/holes. As the rotor sweeps over the stationary body's outlet ports - one to each cylinder - it directs air at high pressure down to the corresponding cylinder's starting valve, opening it and admitting air into the cylinder at the correct crank angle. The sequence is laid out so that the rotor presents outlet pressures in the same order as the firing order of the cylinders (for a direct-reversing engine the rotor is driven in either direction by a reversing drive, and the outlet sequence is arranged for both directions). The distributor thus ensures cylinder 1 is given air as piston 1 descends, then cylinder 2 as piston 2 descends, etc. In the two-stroke, because each cylinder fires once per revolution, all cylinders are given a start every revolution during the starting phase. Once the engine fires and runs, the starting air is shut off (by the pilot valve/direction interlock) so no more air is admitted.

The distributor body's outlet ports are positioned around the arc corresponding to the firing/phase relationship between cylinders (e.g. on a six-cylinder two-stroke the cylinders fire every 60 deg). The rotor carries one or more radial ports that line up with the individual cylinder outlets as the shaft rotates. The profile (the positions and widths of the ports) is such that each outlet is uncovered and covered at the same crank angle as that cylinder's piston is near top dead centre / just after, giving a short admission to the descending piston. The inlet air is thus presented to each cylinder in the correct order and for the correct duration. Because the rotor is keyed to the crankshaft, the timing relative to each piston's TDC is fixed geometrically. On reversal, the rotor is driven in the opposite sense (or a double-acting distributor is used) so that the port that opens for each cylinder in the astern direction is presented - i.e. the profile allows admission in both directions - ensuring the correct firing order and timing when the engine is to run astern.