The automatic valve is only open whilst an air start is taking place. It incorporates a non return valve to prevent any explosion in the air start system getting back to the air receivers. A slow turn valve is incorporated in the smaller bore pipework to the side of the valve. This is used to turn the engine slowly before starting, to prevent damage which could be caused if liquid had found it's way into the cylinder.

The valve shown in the photos and the diagram opposite is from an MAN B&W slow speed two stroke. The valve itself is a simple ball valve which is turned through 90° by pneumatic actuator. The actuator consists of a central spindle with gear teeth machined onto it. This is rotated by two racks which are driven by pistons.

Two guide rods which maintain the alignment of the pistons and racks are bored to  allow air to either side of the pistons. (only one guide rod is shown on the diagram)

When a start signal is given, the space behind the pistons is pressurised and they move together, rotating the spindle and opening the ball valve.

At the end of the start sequence air is admitted through the second guide rod (not shown) pressurising the space between the pistons, moving them apart and closing the valve.