Sketch: The air starting valve is fitted in the cylinder cover (head) of each cylinder. It consists of:

• A valve body (housing) screwed/ bolted into the cylinder head, with a central bore.
• A valve spindle (stem) with a valve head (disc) at its lower end that seats on a valve seat in the body, sealing the cylinder.
• A piston (servo/ pilot piston) on the upper part of the spindle, working in a cylinder in the body.
• A spring (closing spring) that holds the valve closed.
• A pilot (control) air connection to the top of the servo piston, and the main starting air connection to the underside of the valve head.
• A small pilot valve/ distributor connection from the starting air distributor.

Working principle: Main starting air (from the air receiver, ~30 bar) is supplied to the space below the valve head. The valve is held closed by the spring and by cylinder pressure. To start the engine, the starting air distributor (a cam-driven or electronic distributor) sends a pilot air signal to the top of the servo piston at the correct time for each cylinder (in firing order). The pilot air pressure acts on the servo piston, overcoming the spring and the cylinder pressure, and lifts the valve head off its seat. Main starting air then flows into the cylinder, driving the piston down. When the distributor cuts off the pilot air, the servo piston is vented, the spring closes the valve, and starting air is shut off. The valve thus admits starting air to each cylinder in the correct sequence to turn the engine over and start it.

Common faults:

• Sticking/ seized valve spindle (from carbon, dirt, or corrosion): the valve fails to open or close properly.
• Leaking valve seat (worn/ damaged seat or valve head): starting air leaks into the cylinder continuously, causing the engine to creep/ turn when starting air is on, or air leakage into the cylinder during running.
• Broken/ weak closing spring: the valve does not close fully, allowing air leakage.
• Worn/ damaged servo piston or O-rings: loss of pilot air pressure, valve fails to open.
• Blocked/ leaking pilot air line or distributor fault: valve opens at the wrong time or not at all.
• Carbon build-up on the valve head/ seat: poor sealing and sticking.

Consequences:

• A leaking starting valve can cause the engine to turn slowly/ creep when starting air is applied, or can admit air into a cylinder during normal running, causing abnormal combustion/ back-pressure and possible damage.
• A valve that fails to open prevents that cylinder from contributing to starting, making starting difficult or impossible.
• A valve that sticks open can cause the engine to run away/ overspeed on starting air or cause a cylinder to fire on air, risking damage.

Maintenance/ safety measures:

• Regular inspection and cleaning of the valve, seat, spindle and servo piston; remove carbon.
• Check and renew the spring, O-rings and seals at overhaul.
• Check the valve seat and head for wear/ damage and re-grind or renew.
• Check the pilot air line and distributor for correct operation and timing.
• Test the valve for correct opening/ closing and for leakage (air test).
• Ensure the starting air system is isolated and drained before maintenance; follow the maker's overhaul procedure and use correct tooling.
• Keep the starting air system clean and dry (drain the air receivers) to prevent moisture/ dirt entering the valves.