Large two stroke marine diesel engines are started by starting air at 30 bar pressure. The starting air is inserted in...............
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REVERSING SYSTEM :
A marine diesel engine directly coupled to the propeller shaft must be reversible. The reversing system must be capable of turning the engine in opposite direction by compressed air.
The condition which must be satisfied are :
1. change in the sequence of starting air admission
2. set the fuel pump timing to reverse running direction
3. air inlet valves (if fitted) and exhaust valve timing as per the reverse direction.
In four stroke engines to obtain all these changes on the same camshaft, a separate set of astern cams is fitted. Each astern cam is fitted to camshaft adjacent of ahead cam.
To engage the correct cams for ahead and astern running the camshaft slides axially. The incline between the ahead and astern cams allow the cam rollers to transfer from one cam to the other.
The axial movement is controlled by camshaft reversing gear, which is usually a piston operating in hydraulic cylinder fitted to the camshaft. Locking devices and safety cutouts ensure that camshaft has carried out its full axial movement before the engine can be restarted. Hydraulic system can be fed from the main engine lubricating system.
In some engines, the axial movement of the camshaft is carried out by air pressure. Pneumatic servomotors are usually supplied with air from starting air system.
The two stroke cycle can also be illustrated on a timing diagram.
|1 -2 Compression||1. approx 110º BTDC|
|2 - 3 Fuel Injection||2. approx 10º BTDC|
|3 - 4 Power||3. approx 12º ATDC|
|4 - 5 Exhaust Blow-down||4. approx 110º ATDC|
|5 - 6 Scavenging||5. approx 140º ATDC|
|6 - 1 Post Scavenging||6. approx 140º BTDC|
Large two stroke engines have scavenge ports which control scavenge timings. This will be symmetrical and hence will be unchanged when reversed. Engines operating with constant pressure turbo-charging system have almost symmetrical exhaust valve timing, so no change in timing is necessary for exhaust cams.
Fuel pump timings need to be re-adjusted when the engine is required to run in reverse direction. Two such methods are explained below:
- By moving fuel pump cam follower positions (B&W)
When reversing is carried out, air enters the pneumatic reversing cylinder and the piston is moved to the other end of the cylinder. The cam follower moves across and attains a position which changes the fuel pump timing in the new direction.
The link is self locking in either position and a limit switch is fitted to each pump to indicate the position of cam follower.
- by moving the fuel pumps cam (Sulzer)
Fuel pumps and their cams are grouped in pairs along the camshaft and a servomotor is fitted for each pair of adjacent cams.
Servo motor uses a rotating vane, which when oil is supplied under pressure through the drain, will rotate through the lost-motion angular distance to change the fuel timing for astern operation
“Lost motion” is the term used to indicate that the timing has been retarded, through a given angle with respect to new direction of rotation.
Since same cam is used for both ahead and astern movements, the camshaft is rotated by hydraulic servomotor through a definite angular distance. The angle through which the cams are turned is known as “Lost Motion Angle”.
The servomotor consists of pair of vanes, fitted on the camshaft which moves between another pair of vanes fitted within gearwheel rim. By putting lubricating oil under pressure between opposite pair of vanes, the camshaft is moved related to the gearwheel and engine crankshaft. The relative movement changes the fuel pump timing for ahead and astern operation and change the firing order.The rotation of Camshaft is independent of crankshaft and hence it is called as “Lost Motion”.