Marine diesel engines are normally started by compressed air which is admitted at a pressure of about 15 to 30 bar. The air is admitted into the cylinder by the Starting Air Valve fitted on each cylinder head, but the sequence of airflow to the starting air valve is controlled by the Starting air distributor.
Once the speed is picked up, the fuel is injected, and starting air is stopped. The compression pressure is then sufficient to ignite the high-pressure atomized fuel.
Before admitting the air, the control system should make sure that the turning gear is not engaged, and make sure the starting air will start the engine in the correct and desired direction for engines with a reversible option. Admit fuel when the engine reaches the firing speed and cut off the starting air etc.
Diesel engines intended for the propulsion of ships fitted with neither a controllable pitch propeller nor a reversing gearbox are made in the direct-reversing form.
Astern running involves carrying out the events of the cycle in the reverse order, i.e. altering the timing of valves and fuel pumps to cause them to start the engine in the opposite direction and then continue its operating cycle in this direction. Reversing is normally required during manoeuvring of the ship, while berthing, passing through canals, lock gates, anchoring, etc.