How does a tandem pump work?

@PlasticMac I’ve not managed bottom this one. I have a pump to cut up to determine if there is a capillary bleed at the return check valve. Also I need to follow the pipes to check for further check valves and check how return and feed flows interact at the fuel filter. It is going to be some time before I fully understand how a tandem pump works.
 
I’ve now taken the pump apart as far as I can. The return connector does have a check valve in it and there is also a capillary bypass of the check valve.

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PlasticMac said:
What do you mean by a capillary bypass Sir?
Do you mean a restrictor, like in air separator vent, that'll pass air, but not fuel?
Mac.
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Yes that is what I mean. It is there on the right. A slight flat on valve seat. I sealed off the base of the valve and blew air through that gap.
I don’t think it is there though to separate air. I think it is to allow pressure to equalise throughout the return when not running. The valve sets the back pressure on the injectors when running.

Edit. SSP thinks it is an air bypass, I’m not convinced. Need to do some thinking.

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Little Dog said:
Yes that is what I mean. It is there on the right. A slight flat on valve seat. I sealed off the base of the valve and blew air through that gap.
I don’t think it is there though to separate air. I think it is to allow pressure to equalise throughout the return when not running. The valve sets the back pressure on the injectors when running.
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Pressure equalisation/balance, perhaps, but seems odd to me to have a NRV with a deliberate, albeit, calibrated, leak. Why have a valve there at all, in that case, I wonder.
And, is the valve, especially the "leak" identical on a new pump, as an original one?
Maybe there's a check valve at the sender end, to keep the line from draining. I'd expect there to be.

I've seen a few comments about the fit of the metal tube, that the the feed hose pushes onto, at the pump, being a less than perfect fit into the pump body. Any idea if it's a push fit, shrink fit, or sealant fit, assuming it's a different metal to the pump body. Maybe the way a new pump is now built is different from back in the day? Different materials even.

Sorry, lots of questions from the armchair mechanic.
Mac.
 
I've corrected errors made in my earlier, middle of the night post here, and deleted the original.

Corrected version, hopefully fairly accurate.
I was meaning the difference between potential pressure, into a dead end, and actual pressure when flowing. If the return was closed, then when the relief valve opened pressure in the return would be full pump pressure, (and the relief function wouldn't work). But, as the return is not closed, (ignoring any check valve, which will open at much less than pump pressure), then there will never be a high pressure in the return.
The pressure limiting valve relieves excess pressure to the suction side of the pump, not the return line , as I had written. The flow is small, as it's only has to be enough to relieve the excess, above 7.5 Bar. The need for the regulating valve to open at all, will depend on the pressure the pump produces, which, in turn, depends on engine speed. My point is that the flow from pressure side to suction via the regulating valve is, I think, going to be quite small, and probably intermittent, depending on engine speed.
With regards to an air leak, it only needs a tiny gap for air to be sucked in by the pump vacuum.
But such a small gap may not let fuel out. That's how the bypass/restrictor in the air separator works, passes, (thin), air, not, (thicker), fuel.
Seems to work very well too!
The relief valve only "vents" enough fuel into the return, to drop the system pressure to the safe level, so only a small volume of flow.
My second thoughts anyway.
Mac.
Little Dog said:
Yes that is what I mean. It is there on the right. A slight flat on valve seat. I sealed off the base of the valve and blew air through that gap.
I don’t think it is there though to separate air. I think it is to allow pressure to equalise throughout the return when not running. The valve sets the back pressure on the injectors when running.

Edit. SSP thinks it is an air bypass, I’m not convinced. Need to do some thinking.

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I can see the tiny bypass path, (which I'd call a restrictor, and has to be very small to work), passing air, but not fuel.
The pressure limiting valve keeps the return line at 1 Bar, fair enough, prevents fuel drain back.
Not sure about the "force equilibrium" though, where is the solenoid needle valve referred to located?
Mac.
 
PlasticMac said:
Or, is the solenoid needle valve, actually the injectors?
Mac.
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You do know how to temp somebody out of their comfort zone. I think the injectors are hydraulically triggered to inject diesel. The hydraulic trigger is diesel under pressure operating a piston and releasing the ultra high pressure diesel generated by the cam. The trigger flow of diesel is controlled by a solenoid needle valve giving precise control over the timing of, duration of and number of injections. I think?

Edit, pg 6 onwards of this document http://www.volkspage.net/technik/ssp/ssp/SSP_209.pdf needs reading and understanding. The solenoid needle valve controls the changing of the high pressure chamber. As far as my pre breakfast quick read understanding goes the actual injection a mechanical pressure release.
 
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@PlasticMac another case of I was thinking about this one but haven't concluded yet, but here goes. You suggested the flow back through the pressure relief to the supply side of the pump would be small to maintain 7.5 Bar.
I’m not comfortable with this feature and would prefer to see it relieving to the return line. I also think the relief flows may not be insignificant especially with a 3 cylinder engine. How and where it is relieved is what I’ve not worked out.
I thought about it like this. Imagine a 1 cylinder PD. Two revs of the crank drives one rev of the cam. All the time the cam is turning the tandem pump is delivering diesel. But the diesel is only consumed by the engine on the power stroke. On induction, compression and exhaust the diesel delivered by the pump has to go somewhere.
Add three more cylinders then there are 4 injections per rev of the cam. The tandem pump can be sized for a suitable flow rate.
To the best of my knowledge the same pump is used on 4 and 3 cylinder engines so its capacity must be oversized for the three cylinder engine by as much as 25%
That extra capacity has to be relieved somewhere and I’ve not worked out if it is back to supply through the pump or through the injectors back to return.

Edit pg 17 http://www.volkspage.net/technik/ssp/ssp/SSP_209.pdf diesel flushes through the injectors to keep them cool. So the pressure relief in the pump is probably just that. Only relieving when the injectors do not flush enough excess flow.
 
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Maybe only slightly relevant, but I was amazed how quickly 5 liters of fuel was transferred from one can to another when I had fuel fed directly into the pump. Within a few minutes on tickover the fuel can was empty. There must be a lot of fuel going around in circles on these cars..
 
DaveySprocket said:
Maybe only slightly relevant, but I was amazed how quickly 5 liters of fuel was transferred from one can to another when I had fuel fed directly into the pump. Within a few minutes on tickover the fuel can was empty. There must be a lot of fuel going around in circles on these cars..
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In your temporary setup there is virtually no resistance in the return line, so flow rate is high. In the original setup, there is significant resistance, smaller pipe, and whatever is in the return side of the sender, so the flow rate will be much lower, I think.
From the recent @Little Dog edits, the whole circuit is much more complicated than it seemed.
I still think that air leaking in can only come from the suction side though.
Thanks to @Joga for putting me right
Edit: Should read:
I still think that air leaking in can only come from the suction side though, unless there is evidence of a fuel leak on the pressure side. An air leak on the pressure side will only allow air in, when the engine is off, and pressure has dropped to atmosphic, so likely a small leak, not much air in. A leak on the suction side will be constant, whenever the engine is running, so lots of air, I think ...
Mac.
 
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PlasticMac said:
I still think that air leaking in can only come from the suction side though.
Mac.
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I am thinking air should be able to enter also on the non-suction side, when the car is off and the pressure has declined to around atmosphere pressure?
 
Joga said:
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I am thinking air should be able to enter also on the non-suction side, when the car is off and the pressure has declined to around atmosphere pressure?
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Yes, I agree, but diesel would have to leak out to make room for the air. So I'd expect to see evidence of that. Such a leak would, probably, leak under pressure,(engine running), and be more obvious.
Thanks, I've edited my original post.
Mac
 
Joga said:
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I am thinking air should be able to enter also on the non-suction side, when the car is off and the pressure has declined to around atmosphere pressure?
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That depends what happens in the fuel filter. If return is mixed with supply then the pressure in the return could be less than atmosphere when switched off. Air could then find its way in and just like the supply diesel would drain back to tank.
 
Little Dog said:
That depends what happens in the fuel filter. If return is mixed with supply then the pressure in the return could be less than atmosphere when switched off. Air could then find its way in and just like the supply diesel would drain back to tank.
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Depends a bit on what in the return side of the sender too.
A negative pressure in the return, could draw fuel from the tank, perhaps ...
Mac.
 
I started this thread because I had rebuilt a tandem pump and based on what I had seen I posted some incorrect advice. A much deeper understanding of how the pump works was required to give advice.
It has since become obvious that pump can only be fully understood when its interaction with all components in the fuel supply are also understood. So the functionality of the injectors, fuel filter, fuel pickup and any yet to be found check valves have to be understood and taken into consideration. I’m months away.
An understanding I have developed is the fuel pressure at the pump inlet. Consider diesel an incompressible fluid. If you attempt to compress it you can’t but its pressure increases. Give up on your attempts to compress and there is no noticeable expansion the pressure decreases.
Take a healthy 3PD, no air leaks into fuel switched off with an almost empty fuel tank on level ground. The pump is above the level of fuel so there is negative pressure at the pump relative to atmosphere. It is sealed so there is no flow in either direction. Start the car and the pump draws diesel from the tank. Losses through the filter and pipes increase the negative pressure at the pump. Switch off and the pressure at the pump inlet increases but is still negative. There is no flow back of diesel
 
So what? If your TDi won’t start because the fuel supply is full of air but it runs fine otherwise then air has leaked in while it is parked up. The negative pressure will draw air in that will collect at the pump as fuel flows back to tank. Check it out, fill the tank and park facing down hill. Possible positive pressure at pump. If there is no air in the fuel and it starts on the button you have a leak of air into the system to find and fix.
All pipe connections need checking.
There is a seal in the pump that could allow air from the crank case in.
The pump may not be sealed to the head.
I believe, but have not concluded yet, that when switched off ( parked facing up hill with an almost empty tank) the injectors are also subject to negative pressure. So then the injector seals and nozzles are possible sources of air.
Clever things can the be done with clear pipes and tubs of diesel to determine where and when the air is getting in.
 
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