[Sketch notes: the air start valve in the cylinder head is a large, spring-closed poppet valve (pilot-operated). A small pilot valve feeds pilot air to the top of the main starting air valve, which is held closed by a spring. The pilot air from the air distributor opens the large valve pneumatically, admitting starting air through the valve into the cylinder.]

In the air start system of a large two-stroke engine, each cylinder has a large starting air valve (an inlet-valve type) in the cylinder head. It is operated indirectly by pilot air:

1. The main starting air (from the air receivers at 20-30 bar) is supplied to the starting air valve's underside/connection.
2. A pilot (starting) air distributor supplies low-flow pilot air at the correct time to the top (pilot) connection of the main valve.
3. When the pilot air admits to the valve's pilot chamber above the piston, the small pilot pressure acting on the large-area pilot piston generates a force that overcomes the spring closing the main valve (which has a larger area but is held closed by a spring plus the cylinder pressure), so the large valve opens.
4. Starting air then flows into the cylinder through the open main valve, pushing the piston down.
5. When the distributor cuts off the pilot air, the spring closes the main valve, sealing the cylinder for combustion or the exhaust blowdown.

The advantages of indirect (pilot) operation:

the large valve is opened by a small pilot signal so timing is precise; only small pilot air is wasted; the main valve is a robust, spring-closed valve which automatically closes if the pilot air is lost.

A large two-stroke direct-reversing engine reverses by reversing the direction of rotation - the engine can run in the opposite direction by changing the timing of the fuel injection and the starting air relative to the crank. The reversal mechanism:

1. The engine is stopped, the fuel and the starting air are cut off, and the direction selector (ahead/astern control) is selected on the control system/camshaft mechanism.
2. The camshaft (or the reversing gear) is axially moved so that the fuel pump cams (and exhaust/starting control) now have the ahead profile in the position for the astern rotation - the cams are double-profile (ahead and astern faces) and axial movement selects the correct one.
3. When starting in astern, the air distributor (driven by the crankshaft through a reversing gearing) admits starting air to each cylinder in the correct order for the astern rotation - the distributor is driven in either direction so ports align for the reverse direction.
4. The fuel injection is timed so that injection occurs when the piston is moving down for the reversed rotation; the governor tells the system the new direction.
5. With fuel now injected on the reversed strokes, the engine accelerates astern. Electronic camshaftless engines reverse by the ECU simply switching the injection/valve timing and firing order, since there is no physical camshaft to shift, making the reversal fast.

Key:

the reversal interlock ensures fuel is cut off while the direction changes, preventing the engine firing in the wrong direction and preventing damage.