Diverse methods are employed for starting the marine diesel engines on board ships depending on the type and kind of the engine. Some of the most common forms of methods used on board ships are manual, electrical and mechanical systems. In ship’s main propulsion or in auxiliary engines, considerable torque is needed to overcome the inertia of large reciprocating masses. For this purpose, the energy stored in the compressed air is used.
Cited below are important points that marine engineers must be consider while operating starting air system of marine engines.
- The required range of starting air pressure
The starting air pressure should be such that it provides enough speed to the piston during its compression stroke for quickly compressing the charge air and reaching the required temperature to initiate combustion of the injected fuel. The starting air pressure is generally of the same range for both the main propulsion engines and the auxiliary engines i.e. between 25 and 42 bars. If the air pressure goes higher than this, then the components of the engine should be sturdy and robust to cater for the same. The regulation says that the starting air reservoirs should be able to provide 12 consecutive starts without replenishment. For non-reversible engines, 6 consecutive starts are sufficient.
- The time period for the induction of starting air
It is in the expansion stroke that the starting air valves are opened to provide a positive torque to the engine. For 2 Stroke engines, the starting air valves are opened when the piston just passes the top dead centre and closed when the exhaust valves are about to open in uniflow scavenged engines and exhaust ports in closed and loop scavenged engines. In 4 Stroke engines, the exhaust valves are open for a similar phase when the piston passes the top dead centre and closes before the exhaust valve opens in the expansion stroke.
For 2 stroke engines the starting air valve is open approximately 10 degrees before TDC (this is actually provided for the valve to open fully when the piston passes the TDC) and around 5 degrees before the exhaust valve opens. In a pulse turbocharged 2 stroke engine, the maximum starting air angle is of 115 Degrees.
For 4 stroke engines, the starting air valve begins to open 4 degrees before TDC and begins to close 130 degrees after TDC.
- The overlap period
Overlap is the simultaneous opening of two starting air valves during the starting air sequence. It is necessary to start the engine in any crank position and thus this ensures that at least one valve will open when the starting air is inducted in. If there is no overlap provided, then the engine could stop in any position with all the starting air valves closed, when the starting air was given.
There should be a minimum overlap of 15 degrees provided and the ideal condition should be between 20 degrees and 90 degrees.
For a 4 cylinder 2 stroke engine the firing interval is 90 degrees (360/4) and if the starting air period is 115 degrees then the total overlap period would be the difference between the two, i.e. 115-90=25 degrees.
- Indications of leaking starting air valves and the cause of their leakage
The leakage of starting air valves is indicated by the overheating of line between the starting air valve and the starting air manifold, when the engine is in operation. The heating generally occurs due to the passage of hot gases from the engine cylinder to the starting airline. Thus during maneuvering, each starting air line should be felt for temperature close to the starting air valves. The common causes of leakage includes foreign particles deposited between the valve and the valve seat from the starting air supply system, preventing the valve to close fully or valve operating sluggishly because of incorrect clearance between the operating parts.
To determine the leakage in the starting air valve if the engine is standstill, the automatic starting air valve is kept in open position and the air to the distributor is kept shut. Indicator cocks for all the units to be kept opened. The air is now opened from the starting air receiver. Engage the turning gear and bring each unit’s piston to TDC. The leakage of air can be checked from the indicator cocks of the corresponding unit. This will indicate the starting air valve which is leaking for a particular unit.
- Running of engine with the leaking starting air valve
If overheating of a particular line is felt and the starting air valve leakage is detected, then the starting air branch on the starting air manifold will have to be blanked off. If two or more starting air valves are removed from the engine, then there could be a possibility of engine failing to start in a particular crank position during maneuvering. Thus, the reversing control can be operated and the engine can be given a small starting air in the reverse direction to obtain a different crank position or the turning gear could be engaged and one of the pistons shall be moved in position just after top dead centre to get the positive torque to turn the engine.
- Slow turning valve
If during maneuvering, the starting air is not inducted for 30 minutes, while the engine is on wheel house control, then there’s an automatic activation of the slow turning mode in which the engine is turned very slowly for at 8-10 rpm and the air is restricted by a slow turning valve. This is done as a precautionary measure to prevent damage to the engine while starting, if there were an oil or water leakage.
- Running direction interlock
Interlocks are the blocking devices which ensure that the engine is started or reversed only when some conditions are fulfilled or satisfied. Running direction interlock is an essential trait that prevents the injection of fuel to the engine when the telegraph doesn’t synchronise with the running direction of the engine. It is an important application in the crash maneuvering when the starting air is used to apply brakes on the engine by reversing the operation.
- Turning gear interlock
Turning gear interlock is another important thing that prevents the admission of starting air to the engine cylinders when the turning gear is engaged. If the starting air is admitted with the turning gear engaged, then the turning gear along with the motor will fly off puncturing the bulkhead. Thus the interlock is necessary to prevent such accidents. These are some of the most important points marine engineers must know about the air starting system on ships.