How does a tandem pump work?

Little Dog

Little Dog

A2OC Donor
European-Union
This is a genuine question, I don't know. What I do know is that there are two sides to the pump; hence tandem. One side supplies fuel to the injectors, the other side draws air and creates the vacuum for the brake servo and I guess also the vacuum operated valves around the engine? The side of the pump that delivers fuel I know is a positive displacement vane pump.

So I'm going to take apart a Luk pump and see if I can figure it out:

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Contributions please, I'd like to get this right.
 
Under the top cover of the pump is the fuel delivery pump. It sits in what is most likely to be a reservoir of pressurised diesel. I'm assuming this because two holes in the cover plate (green arrow) provide lubrication to the vanes. It is a two vane pump. This means that the other hole in the cover (red arrow) is an outlet for diesel under pressure.
The other obvious reason to assume the diesel under the cover is under pressure is because this is the cover that leaks and damages cooling hoses.

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Let's have a look under the cover:

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This doesn't tell me much more other than this is a two vane positive displacement diesel pump. It is positive displacement pump and, will without exception, deliver more diesel than the engine requires. I'm expecting to find a pressure relief valve that directs excess fuel back to tank. At this point I suspect it is the plug on the cover plate next to the outlet.
This turns out not to be the case and that plug is actually something very useful.
 
If there was a leak between the two parts it would be from the fuel side to the vacuum side, not vice versa, given that the former is under positive pressure and the latter under vacuum.

RAB
 
RAB said:
If there was a leak between the two parts it would be from the fuel side to the vacuum side, not vice versa, given that the former is under positive pressure and the latter under vacuum.

RAB
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Yes I think there is a seal between the two, I only got the diesel side off today.
Vac side tomorrow and I'll pull the shaft out and see.

Edit: only just woken up to the importance of your comment, doh! Most likely the air can’t be forced into the diesel but I’ve not yet worked out how the shaft is lubricated on either side nor where the air pump pushes the air to. The shape of the shaft seal will tell a story, the side it is preventing flow from.
 
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An interesting topic, thanks for taking the time to investigate.
I'd have thought that the red channel is the fuel inlet to the pump. Once you remove the shaft you will see the outlet behind this going through the main body of the pump and into the fuel rail within the head.
 
Remove the pump and all is revealed, there are two low pressure inputs (blue arrows) and the second high pressure output (red arrow). The pump does indeed sit in a case of pressurised diesel:

IMG_0635.jpg
 
The diesel under pressure flows out of the reservoir around the pump and into a chamber with a metal gauze filter in it. From there it flows into the head via a small hole in the underside of the pump (Red arrows). A second small hole in the underside of the pump allows return of fuel from the head (blue arrows).

IMG_0635-1.jpg


The chamber is something of a mystery, I assumed at first it was to stop and particles from the pump reaching the injectors but then I noticed the gauze was badly fitting. My best guess is that the chamber and gauze are there to promote laminar flow before the diesel enters the head?

More interesting is the bung (purple circle), I'm reasonably confident it is a priming drain for the pump. Cracking that open would allow the pump to draw fuel from the tank in seconds. How the underside looks:

IMG_0636.jpg
 
In summary, diesel flows through a tandem pump (red arrows diesel under pressure, blue arrows feed or return, negative or low pressure).

Fuel in and out:

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Fuel out of pump, lower chamber and in to lubricate pump vanes:

IMG_0633-1.jpg



Second pump chamber pressure outlet and fuel drawn in:

IMG_0635-2.jpg


Fuel delivery via a chamber to the cylinder head and return from the cylinder head:

IMG_0635-1.jpg



No automatic bleed valves, no non return valves and no pressure relief valves. The pump delivers diesel to the head at a rate primerily determined by its rotational speed. I think I have created more questions for myself than I have answered :rolleyes:.
 
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This extract from the SSP indicates that there are a couple of valves within the unit, perhaps these are only fitted for some versions of the pump or are manufacturer dependent, but it would be good if you are able to confirm whether they are present in the pump that you are working on.

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Catnip64 said:
This extract from the SSP indicates that there are a couple of valves within the unit, perhaps these are only fitted for some versions of the pump or are manufacturer dependent, but it would be good if you are able to confirm whether they are present in the pump that you are working on.

View attachment 105158
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Thats a great contribution thank you. They must be built in and non serviceable. I’ll check them out later with compressed air.
That does change things a lot and explain some of the reservations I had about what I was seeing.

Edit, I’ve found one of the features and it will stop air getting to the injectors. @PlasticMac you asked about this feature.
 
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It is impossible to say if the inlets and outlets have valves as in the cross section illustration that @Catnip64 posted:

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There is something in both pipes but it does not behave as the diagram suggests it should. I will have to cut up the pump to find out. Whatever is there is non serviceable, you just can't get to it. Ill come back to this when I have investigated the vacuum side.

There is a capillary between the pressure side and return side of the pump. Labeled restrictor above, it is very tiny and very little diesel would pass through it even with a pressure differential. Air however will be trapped in the chamber and will pass through it. Providing air in the fuel does not overwhelm that tiny little hole then no air will get to the injectors. The hole is marked with a red arrow, it is the tiny black dot in the center of the silver circle. Its so small I can't measure it. The blue line illustrates a theoretical level of fuel with air above leaking out.

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The strainer is as my first thought a strainer, the fit is bad though and it would easily allow debris past.
 
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I like this kind of investigation, actually physically seeing how stuff works. I can add a little to the thread. The 'bung' in the purple circle is to attach a pressure tester to so you can see the fuel pressure being provided by the pump. I'm pretty certain of that one as I've used it several times.

I think the gauze is to help keep air in the upper chamber for bleeding out of the tiny hole as described. You might think that air would pass through the gauze easier that fuel but you would be wrong. It's quite a well known technique I've come across in other areas. When air arrives at the gauze the surface tension of the fuel blocks the gauze and stops the air passing through. The effect is used on multiple fuel pick ups on bikes and chainsaws and stuff that doesn't stay upright in order to shut off a fuel pick up when air is present (ie the thing is upside down). This will be why, provided the engine starts, the pump is capable of bleeding quite a lot of air and keep the engine running.
 
Lubrication of the pump lobes (inside), drive and shaft. Took some working out, diesel enters under pressure via a groove in the underside of the pump top plate (red arrow):

IMG_0639.jpg


The lubricating diesel is drawn out at the underside of the pump via a groove to the diesel input which we know is low or negative pressure (blue arrow).

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I'm not yet able to see a seal between the air and diesel sides of the pump. Therefore it is very likely that the seal protects the air side from contamination from the diesel side. I am not seeing the potential for air or gas leaking into fuel within the pump.
 
Before I take the vac pump off I've taken a look at the pump to head gasket:

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There are four sealed areas which I believe are (I may have to edit this):

Yellow, open to the cylinder head / crank case pressures, the vac pump must pump into the cylinder head.
Green, I assume this is an oil supply for the vac pump. Oil return would be via the yellow zone.
Red, fuel at pressure to cylinder head
Blue, fuel return from cylinder head.

Again no potential for gasses to transfer to the fuel.

In my experience those annoying tandem pump oil leaks originate from the yellow rejoin. There is also potential for oil leaks from the green rejoin but all of my pump oil leaks have been on the top of the pump.
 
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Looking at it and sadly having fitted a few in wonder why the design has so little fixing bolts, sure one more would help the centre section seal!
 
CrispyEdd said:
Looking at it and sadly having fitted a few in wonder why the design has so little fixing bolts, sure one more would help the centre section seal!
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Exactly my thoughts too, they tend to leak either side of the input shaft where the blobs of blue silicone are. You can see I was taking no chances with this one and applied extra engine silicone, white smudges.
 
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