Basic Concept of Starting:
High initial torque is necessary to accelerate the engine, so that sufficient compression pressure is produced to create high temp in the cylinder for fuel to ignite ( 540OC ) .
Initial torque has to overcome:
Ü Forces of adhesion of bearing surfaces due to presence of cold oil
Ü Load coupled to the engine
Ü Compression force
Ü Effect of cold combustion chamber walls, which lowers the temp of
compression
compression
Principles of Air Starting System:
Ü Medium & high powered engines are started with compressed air to a sufficient speed for operation on fuel
Ü To achieve this, engines are fitted with Starting air valves controlled by a Distributor
Ü Distributor is to ensure:
air is introduced into relevant cylinder in correct time starting of engine in a desired direction staring of engine from any position of rest
Ü Starting sequence is same as the firing order of the engine
Ü Air is usually introduced slightly after TDC during the power stroke
Ü Air admission is stopped before opening of exhaust valve
Ü Overlapping is provided to ensure positive starting in correct direction
Starting Air System Precautions
Great care is to be exercised in the operation and maintenance of starting air systems. The hazard of compressed air and lubricating oil forming an explosive mixture must be avoided. Oil from any source must be excluded from the starting air system.
Air compressor, starting air reservoir blow down drains are to be operated at regular intervals, and if automatic, their function verified.
Periodic inspection of air starting system pipelines is to be carried out to ensure that no build up of oil is occurring. Highly flammable cleaning fluids must never be used in any part of the starting air system. Any residue of liquid or vapors could result in an explosion.
Routine duties must include the manual checking of the main air starting valve pipes for any increase in temperature, which would indicate leakage of combustion gasses into the system. It is particularly relevant during maneuvering when the main air starting reservoirs are open to the system despite the existence of non-return valves and other devices.
It is absolutely essential that if an air starting valve is in any way suspect that immediate action is taken i.e. shutting the fuel off the unit in accordance with the manufacturer's instructions, "gagging" the valve shut, and replacing the valve at the first opportunity.
Only in exceptional circumstances and according to the Master's authority is the main engine to be operated with leaking air starting system valves.
Air starting systems usually incorporate two or more air reservoirs. According to experience with the main engine plant, the Chief Engineer is to issue standing instructions regarding the air reservoirs which are to be online during maneuvering.
Following maneuvering and during full away passage conditions and dependent on the particular system i.e. connections with auxiliary engine air starting system etc., as many air reservoirs as possible are to be taken off line and their isolating and stop valves shut. The pressure in these reservoirs is to be maintained at the operating maximum.
International Association of Classification Society rules state:
In order to protect starting air mains against explosion arising from improper functioning of starting valves, the following devices must be fitted:
i. An isolation non-return valve or equivalent at the starting air supply connection to each engine.
ii. A bursting disc or flame arrester in way of the starting valve of each cylinder for direct reversing engines having a main starting manifold. OR
ii. At the supply inlet to the starting air manifold for non-reversing engines
Devices under (ii) above may be omitted for engines having a bore not exceeding 230 mm.
The system may also be provided with a relief valve.
Safety Features
Ü Turning gear interlock
Ü Telegraph interlock
Ü Start blocking
Ü Running direction interlock
Ü Over speed cut-out
Ü Low LO press cut out
Ü Low piston cooling press cut out
Ü Low JCW press cut out
Ü Air spring pressure
Interlocks & Blocking Devices
ÜProtects engine by preventing it from starting or reversing unless certain conditions are fulfilled
ÜVaries considerably with different designs of engines
ÜStarting interlock prevents engine being put on to fuel before all sequences of starting system has been completed
ÜReversing interlock prevents engine from starting in the wrong direction on fuel
ÜBlocking devices can be mechanical, pneumatic, electrical or hydraulic type to make the engine operation safer
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