Why won't the main engine start on air?

<p>A main engine that will not start on air is nearly always held up by the starting air system, the fuel system, or an interlock, and those three should be checked in that order.</p>When a large two-stroke main engine will not start, the fault almost always sits in one of three systems, the starting air system, the fuel system, or the interlocks and safety controls, and working through them in a fixed order finds the cause quickly rather than guessing. It helps to separate the symptom first: does the engine fail to turn on air at all, or does it turn but fail to fire.

If the engine will not turn at all, suspect the starting air side first. Check that the main air receivers actually hold adequate pressure, typically maintained around 25 to 30 bar and topped up automatically by the air compressors, since a low receiver pressure alarm that was ignored earlier is a common root cause; check that the automatic and manual starting air stop valves are open, since these are deliberately closed for maintenance and sometimes left that way; confirm the turning gear is fully disengaged, since its interlock is designed to physically block starting air admission while it is in mesh and this is one of the most common causes of a no start; and check that control air supply to the starting air distributor and pilot valves is present, since a loss of control air can prevent the sequence starting even with plenty of main air available.

If the engine turns on air but will not fire, the problem is almost always fuel related. Check that the fuel booster and supply pumps are actually running and producing pressure, that no air is locked in the fuel piping, that heavy fuel oil has been heated to the correct viscosity and temperature for good atomization, since cold, viscous fuel will not inject properly, that fuel filters are not choked, and that the fuel rack or governor is not sitting at a zero fuel or stop position, which on electronically controlled engines can also mean checking the engine control system has actually released a fuel command.

Control and safety systems commonly block starting even when air and fuel are both fine: an unreset shutdown from a previous trip such as low lubricating oil pressure, overspeed or high jacket water temperature will latch the engine out until it is deliberately acknowledged and reset at the control system, and starting will also be blocked if bridge and engine room control has not been properly transferred to the station attempting to start.

The systematic sequence, then, is to confirm the turning gear is out and its interlock clear, verify starting air receiver pressure and that isolating valves are open, check control air pressure and that no safety shutdown remains unreset, confirm fuel is circulating hot with pumps running and the fuel rack free, and only then attempt to start again, since repeated attempts without fixing the actual cause simply drain the air receivers and can leave too little air for a real attempt once the fault is found.