Air Motor Starting System for Auxiliary Engines on Ships
Auxiliary Engine Automation System
The sensors and indicators are installed on engine properly and connected to the power system panel for control and monitoring. The engine responds to the control signals via pneumatic and electronic mechanism of the engine.
Therefore, electrical power of DC24V and compressed air of about 30 bar should be supplied consistently during engine operation. The compressed air supplied from the air reservoir is lowered to a proper pressure through reducing valve around staring air motor, which is used for starting and stopping the engine.
The basic functions of the engine automation system are as follows;
- Engine Starting System.
- Engine Stop System.
- Engine Speed control System.
- Engine Safety system
Auxiliary Engine Starting System
In air motor starting system, the engine is started by a starting air motor which is operated by compressed air. Figure below shows the compressed air system for starting, stopping and fuel limiting for auxiliary engines on ships.
Refer to the figure above. Compressed air reaches the auxiliary engine at 30 bar pressure. The air pressure is reduced to 6 bar with a reduction valve. A safety valve is also fitted in the line after reducing valve to protect the air starting system components. Air then enters air starting valve (5) and wait there. When ‘START’ button on the control panel is activated, starting solenoid valve (5) is opened to supply compressed air into the starting air motor (1). Then, the pinion of the air starting motor is engaged with the gear rim of the engine flywheel. As the pinion moves, relay valve (2) is supplied with air and it allows air to the starting air motor turbine wheel. Now air motor turns crankshaft of the engine. When the engine rotating speed reaches predetermined speed, fuel oil is injected into the combustion chamber. Then, starting is completed and the pinion of the air starting motor is disengaged from the gear rim at predetermined speed.
Purpose of Fuel Rack Limiter
During starting period, the turbocharger is out of normal operation and therefore diesel engine is always in the incomplete combustion due to lack of air, which results in heavy smoke. The fuel rack limiter (9) is used to avoid excessive fuel injected into cylinder during starting period to avoid heavy smoke. During starting period, the engine automation system activate starting solenoid valve to supply compressed air to push the fuel rack limiter (9) piston. A fuel rack limiter valve (8) supplies air to a pneumatic cylinder or fuel rack limiter (9). The limiting position is set to about 50% load normally. The limiting position can be adjusted by guide when loosening locking screw.
On-off valve (3) is for stopping the engine when engine shut down is necessary or over speed trip is activated. This valve provides air to each stop cylinder (6), connected to each fuel pumps and pulls the rack to cut off fuel to the engine.
Auxiliary Engine Stop System
The engine is stopped when pressing ‘STOP’ button or ‘EMERGENCY STOP’ button on control panel intentionally, or by ‘AUTO STOP’ signal. Engine automation system generate ‘AUTO STOP’ signal when abnormal condition of the engine is detected.
However, the engine is stopped fundamentally when the fuel injection into the combustion chamber is stopped. This means that the rack of each fuel injection pump is moved to stop position by stop signal. Every fuel rack is connected to common control shaft mechanically and also connected to common compressed air line pneumatically.
Therefore, there are two ways of moving fuel racks to stop position (Zero index) as shown in figure below.
The one is by the mechanical stop, which pull the racks to stop position by the governor or the manual control lever. ‘STOP’ button activates the governor to be ‘STOP’ position.
The other is by the pneumatic stop by compressed air (as discussed above with on-off valve 3), which pushes the rack to stop position regardless of the governor control. ‘EMERGENCY STOP’ button or ‘AUTO STOP’ signals activates the stop solenoid valve to supply the compressed air for all fuel injection pumps. This ‘EMERGENCY STOP’ signal also activates governor’s stop simultaneously.
However, these two ways are mechanically independent each other and the spring-loaded levers provide mechanical flexibility between them.
Reference:
Himsen Instruction Book by HYUNDAI HEAVY INDUSTRIES CO., LTD. Engine & Machinery Division