How does a tandem pump work?

The vacuum side of the pump is a large oil lubricated vane pump. Air in and air out in yellow, out at the lowest point allowing oil to drain:

IMG_0642.jpg


Interestingly the oil supply appears to bypass the pump (purple arrow). I need to work out where that goes to.

There is what looks like a reed valve on the return of air and oil to the cylinder head. Need to check it out.

There may also be a valve in the vac connection.

Also I've found the reason this pump leaked. Diesel has destroyed the O ring where diesel under pressure passes from the outer to inner casing (red arrow). That seal cost me for a set of hoses and a Bosch pump.
 
Vac pump lubrication, the oil enters the pump under the vanes then flows into the vanes via a notch in the central shaft (purple arrows)

IMG_0644.jpg


@RAB the seal between the diesel and air sides of the pump does not, as I speculated, prevent diesel passing to the air side. It prevents lubricating oil passing to the diesel side. It looks like the air pump runs at crankcase pressure max (reed valve to be investigated) and oil pressure is higher than crankcase pressure. Conclusion, there is no possible failure within the pump that will allow air or crankcase vapors to enter the fuel supply.
.
 
The air pump is a vacuum pump and will have the lowest pressure of all three. The diesel pump is just a feed pump and will have a low positive pressure; the high pressure pumping is done by the injectors. The oil will just lubricate the vacuum pump; diesel does that for the diesel side.

RAB
 
There is a reed valve on the return of air and oil to the cylinder head. This will prevent crankcase vapors entering the pump. Once the pump has created vacuum it will be held and the pump will not work constantly to keep crankcase vapors out.

IMG_0646.jpg


If anybody else has a scrap pump could you check please that the shaft seal is fitted correctly, thank you.

IMG_0645.jpg
 
Guest 4123 said:
The air pump is a vacuum pump and will have the lowest pressure of all three. The diesel pump is just a feed pump and will have a low positive pressure; the high pressure pumping is done by the injectors.
Click to expand...
Did pick up a 2005 BHC TDI today. I would like to test both the diesel pressure produced by the pump and the vacuum.
Could anyone please advise what acceptable values would be?
Are test requirements for Luk and Bosck pumps the same?
 
Are test requirements for all diesel pumps the same? (AMF, BHC and ATL)
Any idea of what the test values should be ?
 
Brilliant @philward this is a new area to me so I am totally lost.... We (family of five) have now passed 1.200.00km with our A2s and I have until now not had any issue with tandem pumps... as I know about.

I look forward to follow next chapter of your interesting thread 👍
 
I have finally found time to check out the return connection check valve:

Tandem pump.jpg


Described as a pressure regulating valve for return line. It's there and I suspect its purpose is to stop the return side of the injectors draining down to negative pressure when the car is not in use. Also to maintain a small back pressure on the return side of the injectors when the engine is running.

Thoughts?
 
Is what's called a strainer, what we think is actually an air eliminator, and the air to return path is via the restrictor? Flow across the restrictor port will create a slight pressure drop, "sucking" the air out. Strainer seems an odd name, as, other than air, there shouldn't be anything to strain at that point!
I'd think the pressure regulating valve is actually a bypass valve that open if pressure exceeds it's spring setting.
But it will retain a positive pressure, engine off, too.
Mac.
Edit: It'd be interesting to know what the original German name is for the "strainer"
 
The pressure regulating valve will also act as a thermal relief valve, in the, (unlikely in UK), event of car getting very hot, in the sun.
Mac
 
PlasticMac said:
Is what's called a strainer, what we think is actually an air eliminator, and the air to return path is via the restrictor? Flow across the restrictor port will create a slight pressure drop, "sucking" the air out. Strainer seems an odd name, as, other than air, there shouldn't be anything to strain at that point!
I'd think the pressure regulating valve is actually a bypass valve that open if pressure exceeds it's spring setting.
But it will retain a positive pressure, engine off, too.
Mac.
Edit: It'd be interesting to know what the original German name is for the "strainer"
Click to expand...
It is an air eliminator not a strainer the fit is rubbish so muck would just bypass it. As you say there should be no rubbish at that stage. Agreed interesting know the German translation.
The pressure regulating valve is best described in English as an inline non return valve it sits in the only return line. Again perhaps a translation from German issue?
The restrictor connects to the main return flow on the outside of a 90 degree turn of the return flow. I guess the increased velocity on the outside of the of the turn provides the Venturi effect To draw the air through the restrictor.
 
I've realised that my memory was a bit wooly. The term for the weir arrangement, to remove air, was actually called an air separator, (not air eliminator). Separator and strainer are close aren't they. Probably closer in the original German.
The restrictor is sized so that air will pass easily, but diesel will struggle, I think.
Did your picture come from an Audi SSP? If so, Google the German version perhaps.
Mac.
 
PlasticMac said:
I've realised that my memory was a bit wooly. The term for the weir arrangement, to remove air, was actually called an air separator, (not air eliminator). Separator and strainer are close aren't they. Probably closer in the original German.
The restrictor is sized so that air will pass easily, but diesel will struggle, I think.
Did your picture come from an Audi SSP? If so, Google the German version perhaps.
Mac.
Click to expand...
The picture came from something called SSP, @Catnip64 posted it in this thread.

Yes the restrictor is very small, too small for me to measure. Diesel would pass but through such a small hole it would be very slow. As the name suggests it would be restricted.
 
Joga said:
The part which is named strainer in english is in german named Sieb.
Click to expand...
Sieb is 'sieve' in English - a strainer or a filter - so the translation is accurate.

Whether people think it’s an accurate descriptor of the function being performed is another matter.

Cheers,

Tom
 
Last edited:
Joga said:
SSPs can be downloaded from here btw: Link
Confirming the theories of this thread I think? :
View attachment 121856
Click to expand...
It does confirm my thinking. The only thing I’m struggling with is why dump the excess feed pressure straight back into the supply line? It will just get hotter.

Edit: because it only happens when you are stupid enough to pull max revs in a diesel. Now I think I get it 👍
 
Little Dog said:
It does confirm my thinking. The only thing I’m struggling with is why dump the excess feed pressure straight back into the supply line? It will just get hotter.

Edit: because it only happens when you are stupid enough to pull max revs in a diesel. Now I think I get it 👍
Click to expand...
Bit late, just re-reading this thread.
The pressure relief valve will drop the pressure that is bled off to the return, so it won't be full pump pressure. The maximum pressure that can be present in the return line is whatever the check valve in the return is set at.
Mac.
 
You must log in or register to reply here.
Back
Top