Tores mailbox VIIC and VIIC/41 operation and technical details

[Don Prince]

Hello Mr. Tore,


When the 1st stage piston moves backward it uncovers the housing for the 4th stage piston.  Therefore the bouncing chamber is exposed to the compressor incoming air. and this air when compressed provides the bouncing effect.  It this correct?


Regards,
Don_

[Don Prince]

Hello Mr. Tore,


Looking at the Ju stage 2 and 3... 


1. They don't show a connection between the input valve and the lower chamber for stage 2?
2. There looks to be no connection between the 2nd stage compressed air the 3rd stage?
3. How does stage 2 compressed air get into the 3rd stage cylinder so it can be compressed and output to the 4th stage input?
4. There looks to be a empty valve seat at the bottom of the 2nd stage piston cylinder, why?
5. There looks like there is a port at the bottom of the 3rd stage piston cylinder.  That can't be otherwise the 3rd stage piston would be useless?
6. eventually we get 2nd stage compressed air into the 3rd stage chamber, but that chamber also houses the 2nd stage input valve....


Is this compressor a 2 cycle or a 4 cycle compressor?  Really confused....


Kind regards,
Don_

[tore]

Don.
There is no connection between the first stage and the 4 stage cylinder., the only sources for filling the scavenging- combustionchamber are as I mention, the under side of stage 1 piston via valves B and the first stage cylinder port valves C. The 2nd 3rd and fourth stages do not take part in this process.
Tore

[tore]

Don.
From the compressor stage 1 after the piston passes the scavenging portvalves at 1/3 of the compression stroke the air, which from now on do not take part in the combustion process, goes via stage 1 end cover outlet valves into a pipe to the first stage aircooler under the compressor and then to the other side of the compressor where the pipe enters the inlet for stage 2, from stage 2 outlet it goes via pipe to 2 nd stage aircooler and then enters stage 3 inlet by pipe, not clearly shown,  from stage 3 outlet the air goes by pipe to stage 3 aircooler and then by another pipe to the other side of the compressor and stage 4 inlet. From stage 4 outlet the air goes by pipe via stage 4 cooler to the airbanks. Stage 2, 3 and 4 do not take part in the air supply for the diesel section.
The reason for having stage 1 and 4 at one side and 2 and 3 at the other is balancing of the free piston. Each stage has to be cooled before it is passed on to the next stage as there is substantial heat generated by the high pressures the compressor is working. All the 4 tubecoolers are placed underneat the compressor.
The supply pipes to the coolers and between the various stages are not shown on the crossection drawing only the inlet and outlet connections.
Tore

[tore]

Mark.
First of all, the ligths were not that bright as on to days museums U 995 as we did not have fluorescent lights. In order not to destroy the night vision we used when needed in the control room and tower, red lights. During nights we usually reduced the lights in the living compartments.
The general lights throughout the boat did not vary during the various running condition.
Tore

[tore]

Don.
The inlet pipe  for stage 2 is not shown on the sketch, only the inlet chamber in the endcover.
As stated before it is a two stroke ("cycle") uniflow scavenged four stage opposed free piston dieselcompressor.
Could you indicate on the crossection drawing the items what you are assuming under your questions 4 and 5?
I see I am returning to the bad habit of  including the bouncing chamber in the scavenging, combustion air chamber. As I explained in my reply 2456, page 164 January 18th 2015 it is not really a bouncing chamber, as the major bouncing effect is maintained in the compressor cylinders by the pressure maintenance valve in the HP pressureline. I guess the correct name should be scavenging air chamber which means it is the airsupply chamber both for scavenging and filling of combustion air.
Tore

[tore]

Hello Mr. Tore,


When the 1st stage piston moves backward it uncovers the housing for the 4th stage piston.  Therefore the bouncing chamber is exposed to the compressor incoming air. and this air when compressed provides the bouncing effect.  It this correct?


Don.
I don`t think so. Down below I have tried to make an image of the piston in the inward dead end, may be this gives you an idea how it works.
Tore

[VIC20]

First of all, the ligths were not that bright as on to days museums U 995 …
The general lights throughout the boat did not vary during the various running condition.

  Yes I doubt you used such lamps. But would you say the light in "Das Boot" comes close?

[tore]

Mark.
I dont think the lights in "Das Boot" is too bad.
Tore

[Don Prince]

Hello Mr. Tore,
 
I still have a problem understand how you get air into the bouncing chambers… I propose that when the diesel pistons are at full compression that both bouncing chambers have uncovered the internal piston casings and created an opening for air to enter the chambers.
 
If you examine the 1^st stage piston and the 2^nd stage piston, both have an undercut area on the piston face that rides over the internal piston casing. This undercut provides the space to assure the bouncing chambers are open to air from the external air supply in the case of the 1^st stage bouncing chamber, and the 1^st stage compressed air supply in the case of the 2^nd stage bouncing chamber.
 
If this opening/condition did not exist, then there would be no way physical to get air into these chambers because they both would be sealed.  There are two other possibilities:
 
1. The seals between the 1^st stage piston and the 2^nd stage housing leaked and allowed air to enter the chamber, but that would most likely negate the bouncing effect as well!
 
2.  There would have to be a valve on the bouncing chamber which would allow air to enter when the 1^st stage and 2^nd stage pistons were drawing air into the 1^st stage and 2^nd stage piston chambers.  However, these valves do not exist for either bouncing chamber.
 
I have drawn what I proposed with measurements from my computer screen, and the same could be done with printed diagrams.  This looks to be a working solution to the bouncing chamber air issue.  What do you think?
 
I have attached my measured drawing to go along with my proposal…
 
Kind regards,
Don_

[Don Prince]

Hello Mr. Tore,


I believe I have stages 2 and 3 figured out based on a photo of a compressor and the drawing from u-historia.com.  what do you think?

I believe those different stages of pressurized air pipes run through a water cooling system...

Regards,
Don_

[tore]

Don.
The U Historia image gives a better understanding of the 2nd and 3rd stage air connections, and each stage outlet goes through a tube water cooler before it enters the next stage. I agree you can use these sketches. After the air passes the four stages tubecoolers there is a pressure maintainace valve. This valve is shut until the pressure is reaching a preset level which I have forgotten, but it is quite high, before the compressor starts to deliver air to the airbanks. This pressure is, as I previously ( Jan. 18th) and repeated in my post 2826 of yesterday mention, creating the real bouncing effect.
 I shall revert to your question on the air supply to the scavenging chamber as soon as I manage to get access to my paint programme.
Tore

[tore]

Don.
I have tried to illustrate the first stage inlet stroke as follows( sketch below). When the piston is at the outward dead end and start to move inwards, air is drawn into the cylinder at a suction      ( under)- pressure via the air inlet chamber and valves D, this air is coloured yellow, piston position 1. As the underpressure is created, the first stage outlet valves E shuts.The inwards side of the piston is now creating an overpressure ( blue) shutting valves A and opening the valves B and C, supplying air to the scavenging chamber, for C via the dotted blue hatched duct.
 As the piston moves to position 2 the outward part of the piston side has still an underpressure and shuts now valve C as the piston passed the scavenging air ports, thus shutting the  blue dotted hatched connection to the scavenging chamber. The inward side of the piston is having an overpressure, valves B are still open and  in connection with the inward side (overpressure) of the piston, supplying air to the scavenging chamber.
When the piston is at the inwards dead end, position 3, I agree to your sketch showing a possibility of a leakage  from the first stage "piston pocket" however at the dead inward end the pocket is at its max. expansion and there is hardly any pressure difference between the " pocket " and the outwards side of the piston, thus no significant leakage. Moreover  a  possible leakage to the 1 st. stage piston underpressure side (yellow) would be contained due to the sealing  of the first stage pistonrings, the air can only enter the scavenging chamber when the outward pistonchamber pressure open valves C.
The fixing nut for the 4 stage piston rod is sealing the "pocket" towards the scavenging chamber.

Tore                           

[tore]

Don.
I guess it has been cut, see image below, may be you are confused by the exhaust water trap.
Tore

[Don Prince]

Hello Mr. Tore,


It was an optical illusion with the point of view...  I sent another photo showing it was cut...


Regards,
Don_