How does a exhaust valve of auxiliary engine rotate ? How does the main enigne exahast valve rotate?

Rotocap Working....!!!

ROTOCAP

PURPOSE OF ROTOCAP.
The rotocap is a mechanical means to rotate the valve unlike the main engine exhaust valves which has got vanes in it...

The rotocaps are usually fitted tot the exhaust valves, this ensures that the valve rotates and inturn ensures that the valve is kept at an even temperature so it does not distort and helps in even distribution of carbon deposits.

 CONSTRUCTION AND WORKING.

                              There is a collet or cotter which is on the valve cover fitted around the exhaust valve. The valve cover has number of inclined tracks machined into it. Each contains a ball and a spring. The balls runs on the race. Now when the valve is closed, the belville washer locks the valve cover and the spring carrier together preventing rotation, and at the same time acts on the ball race, pushing it hard up against the balls, locking them half way up the inclined track and compressing the springs.

                               As the valve starts to open, the increasing valve spring force cause the belville washer to flatten. The ball race no longer locks the balls in the inclined tracks and therefor the valve cover is free to  move. The valve cover now moves over the ball bearings until the balls are at the end of inclined tracks, as the cover rotates, so does the collet and the valve spindle.

                               

                                 As the valve shuts, the belville washer locks the valve cover to the spring carrier and applies pressure to the ball races as mentioned above and forces balls back up the inclined tracks and locking them up preventing rotation.

FACTS 

1.  Rotocaps stop operating below about 25 % of engine load.
2. Contaminants in the lube oil which lubricates the valve gear may cause deposits in the inclined tracks and makes the rotocap ineffective..
3. It should be noted that rotocaps rotate the valve as it opens and not when it closes as it is in the main engine exhaust valve.

MAIN ENGINE EXHAUST VALVE WORKING.

The main engine exhaust valve has vanes fitted to them just above the head of the valve, which help in rotation of the exhaust valve.

The high velocity exhaust gases acts on these vanes and then the valve rotates while closing.

Exhaust valve fitted with VANES. 

Auxiliary Engine Crankcase Inspection.

Before starting with auxiliary engine crankcase inspection, the following must be done,

The stand by generator should be switched on.
The generator to be inspected should be cooled down.
Air starting valve closed.
Indicator cocks open.
Engage turning gear.
Generator should be put to manual and tagged "MEN AT WORK".
Tool box talk and Permit to work made and filed.
All tools kept ready.




After doing all the tasks above,

Open the crankcase door of all units.
Check the gaskets of all the door, check for any wear.
Check the colour of the lube oil.
Touch the lube oil and feel for the sludge content in it.
Check the underside of the liner with a flash light, for all units. The liner is checked for any scoring marks, or any cracks.
Check the play of the connecting rod with a bar.
Check the tightness of the connecting rod bolts.
Check the connecting rod bearing clearance.
Check the main bearing clearance.

Maneuvering Diagram with explanation. (MAN B&W)

Before going to the Maneuvering diagram, just understand how the fuel pump and air distributor is reversed,

CLICK http://www.marineengineeringdoubts.blogspot.in/2014/04/fuel-pump-and-air-distributor.html

 to understand fuel pump and air distributor reversing arrangement,,,,


MAN B&W (Starting of the engine in Ahead position.)

MAN B&W ENGINE STARTING SYSTEM (SIMPLIFIED)

The system shown shows the valves in position whist an ahead start is in progress. There are two control levers (not shown); a stop-start-run lever and an ahead/astern lever.

Valve 1, the stop signal solenoid valve is activated throughout the air start sequence. This operates valve 5, which in turn operates valve 8 which puts an air signal on the fuel pump puncture valves, preventing high pressure fuel being pumped to the injectors. It also sends an air signal to operate valve 13 which will allow 30 bar control air as far as valves 12 and 14 when valve 16 is operated.

The direction lever in the control room is set to AHEAD. This activates solenoid valve 2 which sends a signal to operate valve 14 which will allow 30 bar control air to the distributor when valve 16 is operated. It also operates valve 11 which allows air to the fuel pump cam follower servo motor.

When the lever is moved into the start position, it activates solenoid valve 4. This operates valve 6 which in turn operates valve 9. This allows air to operate the automatic valve. Should solenoid valve 15 be activated, then only the slow turning valve will open. Valve 16 is also activated allowing 30 bar air to the start air distributor.

When the lever is moved to the run position, solenoid valves 1 and 4 are deactivated and vented. This causes valve 8 to vent, shutting the fuel pump puncture valve. Valve 13 also vents, as does valve 16, venting the air start distributor. Valve 16 moves across allowing control air to shut the automatic and slow turning valves.

 Points to Note:

·     Valve 6 has a 1 second delay timer fitted. This allows the main starting valve to remain open, so as to supply air to those cylinders that are in the start position.

·     There is a manual shut off valve fitted between the air start manifold and valve 16 for maintenance purposes.

·     The fuel pump reversing servo motor is fitted with relief valves and restrictors to damp operation.

·     Valve 7 is vented by engaging the turning gear. This prevents a start sequence taking place.

Fuel Pump and Air Distributor arrangement for Reversing.

The fuel pump cam on the MAN B&W MC series engine is designed to raise the plunger on the injection stroke and then keep the plunger at the top of its stroke while the follower stays on the peak of the cam until just before the next delivery stroke when the follower returns to the base circle of the cam, and the fuel pump plunger moves down on its suction stroke.
The diagram on the top shows the cam follower just beginning to move up the slope of the cam with the camshaft rotating in anticlockwise direction. (i.e. start of injection)
If the engine direction is reversed at this point, then air will enter the pneumatic cylinder as shown and will move the piston to the right. The cam follower will be moved across and would finish in the position shown which would be at the correct fuel pump timing for running astern.

It should be noted that the reversal of the follower only takes place while the engine is rotating. If the engine had been stopped from running ahead, and then started astern, the fuel pump followers would move across as the engine starts to rotate, and before the fuel is admitted by venting the fuel pump puncture valves.

A micro switch shown on the left hand side detects whether the follower has moved across. If not, an indicator light is lit in the control room, however the engine will still start if a follower fails to move, perhaps due to corrosion in the servo cylinder a high exhaust temperature deviation alarm would operate within a short time. Allowing the engine to start in this situation could be useful during maneuvering in confined waters

AIR DISTRIBUTOR WORKING

The larger MC engines use a radial air start distributor operated by two cams fixed on the end of the camshaft. The pilot valves, arranged radially in a housing, are either pushed by the pilot air onto the inner cam profile when starting ahead, or onto the outer profile when starting astern. When not in use, a spring holds the pilot valve follower clear of both profiles, thus reducing wear.

Photo down below shows distributor mounted on end of camshaft. This is for a 12 cylinder engine, there is another distributor at the other end of the camshaft for the other 6 cylinders. No cover was removed to take this picture; There is actually a perspex window, so you can see the distributor!! 

There is one pilot valve for each air start valve. Each pilot valve has 3 inlet ports and one outlet port. When the engine is required to start, Pilot air at 30 bar from the main air start manifold is led to the middle inlet port.The air cannot pass through the pilot valve at this point because the port is shut off by the operating piston on the valve slide.

If the engine is required to start ahead then control air at 30 bar from the air start manifold is led to the top port. This acts on the top of the operating piston of the valve slide and forces the valve slide down onto the inner cam profile. As the slide moves downwards to admit pilot air to the air start valve, the top port is closed off and the main pilot air now acts on top of the actuating piston. As it does this, the pilot air passes through the exit port to actuate the air start valve in the cylinder.

As the cam profile moves the valve slide upwards, the piston on the operating slide closes off the pilot air and air to the air start valve is vented through the center of the valve slide.For astern operation the action is similar, but this time the control air enters through the lower port.

What is the difference between Supercharghing and Turbocharging?

Supercharging is the process of providing air to the engine at higher pressure than atmospheric pressure.

Turbocharging is one of the type of supercharging were exhaust gas is use to drive the turbine.

The main function is that if more air is induced then more fuel can be burnt.

The air is sent to a cooler, were the mass of air is increased when cooled, i.e density of the air increases.