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Old 20th March 2012, 13:11   #1
T-Cut
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Default How To - Understand Fuel Tanks

How To - Understand Fuel Tanks (Last edited 21 August 2014)

This is another bit of work in progress and there are probably things I've got wrong. Hopefully these will be corrected through informed comment. I'll edit it as additional info is gathered and mistakes are corrected.

Over the years, I’ve read loads about what happens in the fuel tank if for example, a diesel pump fails, or Fuel Filter Syndrome (FFS) develops in a petrol system. Thankfully there are only a few different problems, but they do happen frequently and they get discussed a lot. However, I’ve often felt the explanation of cause and effect is little more than guesswork or misinterpretation. So, I decided to learn what I could about the way tanks work and did some research. I admit I was surprised. Boiling it down, I thought a simple fluid flow diagram for the fuel tanks would be useful. Basically, if you understand the flow paths you can explain the things that happen. What surprised me is the fact that all fuel tanks contain not one, but three pumps.

In addition to the electric pump located in the RHS of the tank, there are two static pumps located deep inside the plumbing system. These transfer fuel within the tank and ensure the electric pump inlet is always submerged, no matter what the fuel inventory or the driving conditions. Now this seems contrary to many things I’ve read on the forums, especially regarding diesels. The ‘quarter tank’ rule for diesels has always intrigued me and I now see the explanation of it. Similarly, the idea that under certain circumstances the RHS of the tank will have all its contents transferred into the LHS seems an illogical design feature. So I made some drawings of the petrol and diesel tanks to simplify the workings.





These show only the liquid flows. Vapour and pressure handling are omitted. The saddle tank and interior pipework are shown diagrammatically, so all this stuff is a bit different in reality. The idea behind the internal plumbing becomes apparent when the fuel level is below the central dividing hump. It makes most sense when the fuel inventory is very low, so the drawings show that situation. Diesel or petrol, the system is identical in principle, but the way it’s done is slightly different.

The primary electric pump, whether diesel or petrol, is a centrifugal (impellor)type. The diesel one generates around 30psi at the outlet when in top condition. The single petrol pump generates around 50psi. The UBP in the diesel is a positive displacement type with a 'pigtail' gear design.

A centrifugal pump can run against a closed outlet while maintaining its delivery pressure, but it must be adequately primed. To ensure this, the pump is fed from a permanently flooded pickup or swirl cup. It’s the job of those other two pumps to keep the swirl cup full. When there's plenty of fuel in the tank, the electric pump and its swirl cup are completely submerged, so while there are transfers happening within the tank, they only become important when the inventory falls. At very low inventory, any air getting into the pump inlet is vented off at the upper end of the pump through a scavenger/priming valve which closes when the pump is hydraulically locked. Make note of this valve.

The two secondary in-tank pumps are static inductors based on the Venturi principle. These get their motive power from pressurised fuel generated by the electric pump (petrol) or the diesel’s HP injector pump back-flow from the engine. Their sole purpose is to ensure the swirl cup remains full under all conditions. Since they will only work when pressurised fuel is fed into them, any circumstance which reduces this pressure will cause a tank malfunction. For example, the diesel HP backflow passes through the cooler mounted externally near the tank. A fuel leak from the cooler may inhibit delivery from the inductors. Similarly, FFS in the petrol tank may inhibit feed to the inductors. This will also cause tank malfunction and explains why FFS is sometimes accompanied by an absence of fuel in the RHS.

Diesels with two electric pumps (ITP and UBP) may under certain conditions fail to get sufficient fuel if one fails. The ITP has adequate delivery capacity when assisted by the UBP. So, when it has to work alone, it needs to do the work of the UBP as well. Whether it copes with the load depends very much on the lift it has to cope with. The lift of a pump is determined by the outlet pressure. The required lift is the vertical height between the fuel surface in the tank and the HP pump inlet on the engine. While the height remains below some critical point, fuel will get to the engine. The only variable in this equation is the fuel level. As the level drops, the required lift increases, until a point is reached where delivery fails. I assume this is at the nominal quarter tank level. As fuel delivery reduces, the volume of HP backflow from the engine will fall. This backflow is vital for the correct handling of fuel by the Venturi pumps. As noted, these scavenge fuel from the left side of the tank into the right and from there into the ITP swirl cup. Clearly, if the scavenger pumps are inoperable due to lack of HP backflow, the RH side of the tank will run dry.

If the ITP fails before the UBP, this has to lift fuel from the tank and deliver it to the HP pump on the engine. I would guess then, that the UBP is a different type from the ITP and is logically positive displacement. (This has subsequently been confirmed and there are some nice photos (somewhere) of the interlocking spiral worm gears). It must have sufficient 'suction' to raise fuel from the tank (via the dead ITP) on startup. It would be interesting therefore to compare the innards of each. Again, the quarter tank rule seems to apply, so that the UBP can cope with the lift and engine requirements. As the UBP eventually fails, the scavenger pumps fail and fuel seemingly becomes transferred the LHS of the tank.

Later diesels only have a single primary pump (ITP). I believe this is a higher spec'd type and presumably has the lift and capacity to cope with the demands of the engine and the tank scavenging system. When this pump starts to fail, you'd expect the outcome in terms of fuel distribution to follow a similar pattern.

It would seem logical in cases of double pump failure to replace the ITP with the later type and simply bypass the UBP. Apparently this isn't possible, I assume because of design differences between the two. (??)

The following discussion raises the question of the fuel cooler fitted in the diesel system. This isn't shown in the above drawing to keep it simple, but there are some implications if it goes wrong. An example of this can be found here: https://www.the75andztclub.co.uk/for...d.php?t=193016

TC

Last edited by T-Cut; 16th July 2017 at 13:48.. Reason: Images released from the Photobucket ransom plot.
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