Tores mailbox VIIC and VIIC/41 operation and technical details

[Don Prince]

Hello Mr. Tore,

4. Where is the starting air valve located relative to each cylinder head?
5. Do you have a photo of a starting air valve?
6. Is the shifting cam shaft one long continuous shaft to all cylinders?  yes/no
7. There is only one shifting cam shaft with cams for both the push rods driving the intake and exhaust valves, and cams which drive the starting air valves?  yes/no
8. Does the shifting cam shaft rotate one full revolution per two (2) revolutions of the crank shaft?  yes/no

Regards,
Don_

[tore]

Don.
4.Down below is Simons image of the cylindercover where the starting valve for each cylinder is shown fitted.
5. I don`t have a photo of a separate starting airvalve.
6. The camshaft is the same as the original camshaft along the engine.
7. Hence the originalcamshaft is driven from the camshaftdrive as normal.


Tore
 

[Don Prince]

Hello Mr. Tore,


9. How does the starting air get to the middle connection on the inserted starting air valve which is placed into the engine valve head? I see the top connection pass air through the shut-off valve to the upper section of the starting air valve, and then it forces the roller to engage the cams...


Regards,
Don_

[Don Prince]

Hello Mr. Tore,


Thanks for the last few posts...  I think that I'm beginning to understand how the starting air system works.


I added some ID tags on one of your drawings and I hope that I got it right...


10. when is the Shut-off valve used on a cylinder? Why?
11. Was that spring an added modification by crewmen (Starting valve spring.jpg attached)?

Regards,
Don

[tore]

Don.
Below is one of Simons drawings which shows the starting air supply pipe to the starting airvalves of each cylinder. As you see the startingair valve casing is protruding up over the cylinder covertop allowing space for an airchamber flangeconnection to the starting air supplypipe going along the main engine at the level of the starting airvalve connection. The shut off valve to the engaging piston on the top of the valve is clearly seen, I guess the shut off valve could be used if for some reason you would disconnect the rockerarm from the camshaft. The spring for retaining the rocker arm from the camshaft is a typical device of the prewar technology and is an intergral part of the system, not somebodys funny idea. If f.i. a starting valve for some reason stays open or start leaking, the combustion pressure in the relevant cylinder forces hot exhaust gases into the starting air system overheating the pipes. It happened to me ones upon a time and the painting on the startingpipes became so hot that the painting started to boil.


Tore

[Don Prince]

Hello Mr. Tore,

During WWII, was there ever maintenance at sea on a diesel engine where a head valve, piston, or any other major component was replaced where the lifting bars were utilized? I don't believe in your situation at sea where you could go back to port that these types of repairs were ever done...

Regards,
Don_

[tore]

Don
I have no records of any maintenance at sea by the germans during WWII, but based on my experience from the VIICs I guess it could be quite a few. You are right in your assumtion that we carried out major repairs at shipyards along our coast, being about 30.000 km long. Repairs like changing big end bearings and exhaust valves, even a seized supercharger was done by our own crew, otherwise planned maintenance was carried out a our yards.
Tore

[Don Prince]

Hello Mr. Tore,

Would you carry spares like: fuel pumps, oil pumps, air starting valves, etc? Intake and exhaust valves would require pulling a head casting and all those connections; would your crew do that at sea? It seems like replacing an end bearing would be more difficult at the clutch (aft) end than at the forward end of the engine. Were you limited to the spare parts because of shortages of parts from German shipyards that were destroyed during the war?

Regards,
Don_

[tore]

Don.
Spares like oilpumps, and starting air valves were not stored onboard. Consumeables like fuel nozzles, fuelpump plungers were stored. Exhaust and inletvalves with cages was a routine job. Big end bearings were not a problem. The germans left behind a considerable amount of spares in the large submarine pens which were constructed for a larger submarine fleet than ours.
Tore

[Don Prince]

Hello Mr. Tore,

Then if your spares included Intake and Exhaust valves, then your crew used the lifting bar to pull the head casting to replace the valves. That seems like a very time consuming Job to me, along with replacing the crankshaft bearings - Wow! It's a darn fortunate thing that you were not being hassled by Allied aircraft and British Destroyers when attempting to repair an ailing diesel engine...

Regards,
Don_

[Don Prince]

Hello Mr. Tore,


Simon and I were having a discussion about the lines going to the Starting Valves, and I think that the Germans installed a pressure relief valve at the end of the air pressure lines feeding the starting valves lower chamber. If there was a water stroke, then the air pressure would build up and pop the relief valve and possibly avoid any engine damage? See the image and what do you think?

Question - How do you blow out the cylinders with HP air? Open the cylinder drain cocks and what else is done?

Bear with me for a moment... What is a water stroke. Near TDC the exhaust and the intake valves overlap by about 20 to 60 degrees. So a piston could be near the top of the cylinder with both valves open. If there was a water intrusion; then water could go from the exhaust valve chamber, to the cylinder chamber, and then to the intake valve chamber. Now, the first time the piston does an intake stroke, the water from the intake valve chamber is drawn into the cylinder, and then on the compression stroke we get a "Water Stroke" because the piston cannot cycle to the top of the cylinder??? Comments?

Regards,
Don_

[tore]

Don, I have rewritten your two starting air pages of the Skizzenbuch but have trouble in mailing the correct images, at my age (92) I am not very familiar with the E-mail and shall get some help from my grandson next week. My pages explains the starting air system bit further incooperating the maneuvrehandles with manifold. The relief valve on the end of the starting air common rail is adjusted to 75kg/cm2 and with a lifting hight of 12mm too small as a waterstroke cylinderprotection, remember water is acting like a solid rather than a gas. The starting system allows for manual control of the starting air independently of the fuelinjection, hence you can run the engines on air only. As a safetyprecaution you always run the engines by air and open indicatiorcocks, one for each cylinder, open. We called that "blowing through the engine". By massive intrusion of water we sometime cranked the engine by hand with open indicatorcocks.

Tore

[Don Prince]

Hello Mr. Tore,

Thanks for the update...  The U-Boats HP air is capable of 205 Kg/cm square and that's 2,915.79 psi which is huge. Then the pressure relief valve for the starting valve line to the lower chamber was limited to 75 Kg/cm square, or 1,066.75 psi, and it looks like this is the limit of air pressure that may be used to start the diesel engine without popping the relief valve.

Question #1.  Exactly, what was the function of the relief valve? Could you over pressurize the diesel engine with the starting air valve since the bottle source could be 205 Kg/cm square?

A water stroke would happen during the very first 360 degree turning of the crank shaft when you attempt to start the diesel engine with air. The crank shaft speed would be slow and only the air pressure is driving the piston. When you turn the handle on the starting air valve, the air pressure would go from low-to-high. I would think that a water stroke would just stop the crank shaft from turning and not damage the piston, piston connecting rod, the crank shaft journals, or the crank shaft???

However, if I do the math - The piston diameter is 400 mm = 15.745 inches, so the piston surface area is 194.6 sq inches. Now, if we apply 1,066 Psi (75 Kg/cm square) of air pressure, then the total pressure could be 207,449.40 pounds on the piston top... Yep! I guess things could break...

Question #2.  Mr. Tore, what usually broke?

Question #3.  Was there an air pressure gauge that displayed the Starting Air pressure that was applied to start the diesel engine?

Regards,
Don_

[tore]

Don.
It seems I have overlooked a basic element in your understanding of a four stroke engine, the compression and ingnition.
 If you have water intrusion in the engine it usually enters via the exhaust system. The combustion air enters via the engine room as the induction air ducts ends near the outboard bilge. On the exhaust side the usual reason for the water intrusion is the carbonization of the valve seating of the main hull exhaust valve. Allthough you having the pneumatic grindingdevice for the valveseating, leakages occur for several reasons. Time is a major factor, grinding of the main hullexhaustvalve starts as soon as the engines are stopped and boat is ready for diving e.g. totally shut pressure hull and on the COs order. An important element in the grinding is seawater backpressure on the flapvalve disc, hence you want grinding the valvedisc as soon as you leave the surface. Two elements works against this, the pneumatic grinding is noisy and consumes a lot of HP air building up the pressure inside the sub. A third element reduce the grinding time, as you reach about 5-6 meter the flapvalve backpressure becomes so great that the pneumatic motor can`t overcome the grinding friction and stop. All these elements counts in the risk of water intrusion. An ideal dive from the exhaustvalve grinding point of view is 4- 5 meter for some 3-5 minutes, but that is wishful thinking.
 If you are unlucky and have to keep the periscope depth after a crashdive you might have a massive waterintrusion, the 2nd hull exhaustvalve has a special high drain capacity to the bilge, but in the worst case not large enough and might overflow and flood the exhaust manifold along the engine. In spite of drain to the bilge the manifold can be flooded  and water might enter the cylinders. If you start an engine with water in the cylinder equal to the compression volume (dead volume)you might have a waterstroke in the cylinders reaching the compression (ingniton) stroke as the water is trapped solid in the compressionvolume. The collateral damage by the rotating masses can be substatial.
Tore

[Don Prince]

Hello Mr. Tore,

After reviewing all the material we post last week... This is my understanding about a water stroke:

1. A diesel engine is shutdown with one cylinder/piston in the exhaust stroke position
2. The piston is low in the cylinder and the exhaust valve is open (creating an entrance and space for sea water)
3. Another cylinder/piston is in the power stroke position
4. The exhaust system failed and there is a massive sea water intrusion
5. The cylinder/piston in the exhaust stroke fills up with sea water (also the valve head area and the manifold)
6. Starting air is enabled for all cylinders
7. Starting air is applied to the cylinder/piston in the power stroke position
8. The crank shaft attempts to rotate
9. The piston with the sea water filled cylinder attempts to move upward
10. The sea water pushing through the open exhaust valve port creates a huge back pressure (pressure shock)
11. The exhaust cylinder piston barely moves as the pressure increases on the power cylinder piston
12. The starting air valve continues to supply HP air to the power stroke cylinder (increase pressure)
13. The exhaust cylinder piston has a "dead head" (no motion)
14. This is a water stroke

When a diesel engine is normally running; in the exhaust stroke, while the piston is moving upward the burned fuel gasses are compressed as they are forced around the exhaust valve and out the exhaust head port. However, with a water filled cylinder there is nothing to compress which stops all crank shaft rotation, and applies excessive back pressure on the driving piston in another cylinder chamber.

Did the pressure relief valve pop/release when you had a water stroke? Is it just a signaling device, or did the pressure relief have some effect on the starting air system (Shut-off the starting air system)?

Regards,
Don_