Laitch wrote: Beamer wrote:. . . I don't understand this explanation.
Does this mean that when running low on fuel you are more likely to draw unfiltered fuel via this line ?
I have not checked this assumption but if not it must be drawing in air from the top of the tank. I can not see how this tube could be pulling in fuel.
I think the purpose of this system is not to draw air from the top of the tank into the system but to
aid in evacuating air and vapor that would otherwise enter the pump at severely low fuel levels. Vapor entering the pump can be compressed then its sudden expansion within the unit can damage the components. I believe that is what rbm meant by "directing fuel around the pump inlet." The hose provides a means of expelling vapor that might block fuel entrance into the pump by providing a path for vapor to exit from the basket to the pipe inlet inside the tank then to its outlet beneath the tank and from there to the ground via another hose or a catch cup/hose combination.
The absence of that hose from the pre-filter basket in my bike's tank made no difference in the bike's performance because I haven't chronically run my bike when it was extremely low on fuel. This feature might be another example of BMW over-building. I'm ok with that.
Anyway, that explanation works for me. I'm not trying to sell it. I'm giving it away.
Great. This is the best explanation I have read here about the fuel pump intake. Thanks for your generous gift Laitch
I cannot see why this tube (and I resist calling it an Anti-cavitation tube) would be used to introduce air to the pump inlet so I therefore accept Laitch’s explanation that its purpose is to "
aid in evacuating air and vapor that would otherwise enter the pump at severely low fuel levels”. I've had this happen and the sound is loud and screeching.
Cavitation is often talked about at this point but I can't see how it has anything to do with the fuel screen basket or the integrated tube. I have often wondered about cavitation in K-bike fuel pumps, as my late father, an ex British naval engineer had long ago,
lovingly explained to me the effects of cavitation on propellers of Destroyer class frigates (thanks Dad. I knew that bit of information would come in handy.... one day
).
Firstly, my understanding is that cavitation will not occur outside the pump but inside on metal surfaces where pressure differentials are far greater. During cavitation, the bubbles of vapour which form in the low pressure zones within the pump, do not explode, they implode, as Rawdon mentioned, and with some force when met with denser higher pressure liquid immediately above the surface. Their existence is very tenuous and not sustainable in such a volatile environment and so they disappear as quickly as they formed, never leaving the metal surface. This implosion is enough to cause serious erosion damage to the metal surfaces on which they formed. Cavitation has another effect, the generation of sound. A distinct problem for submarines. Props are designed for a specific rpm range at low speed to avoid cavitation.
If you’re still wondering how this cavitation works, it can be easily seen at the bottom of a saucepan of water as it comes to the point of boiling. Heat is the catalytic force here instead of vacuum. Bubbles of vapour appear but then quickly disappear never really leaving the bottom of the pan. You may also remember that science experiment we all did in High School where we boiled water at room temperature by placing a beaker of water inside a glass vacuum bell and evacuating the air. Vacuum forces pockets of vapour to form on the sides of the glass beaker. Just a heads up. Don't enter the hard vacuum of space without your pressure suit. You'll die from cavitation within about 15 seconds.
And have you ever wondered why your electric kettle increasingly makes so much noise as it comes to the boil?…. cavitation. That is the sound of vapour bubbles imploding. It soon stops as all the water reaches boiling point and the bubbles of vapour are able to sustain themselves in the evenly heated water and make it to the surface.
Your post above, Laitch, has urged me to research this a little further in relation to fuel pumps and I've found some interesting facts.
- Cavitation creates heat and the hotter the fuel becomes, more cavitation is produced. Fuel can reach temps of around 120ºc due to cavitation. This sounds very familiar to a K-biker. Heating of fuel is of course aided by engine and ambient temps but if hot fuel facilitates cavitation then this is a good case for fitting a fuel cooler especially in hotter climates like ours here in Australia.
- Cavitation in the fuel pump can produce loud noise. We’ve heard a few owners reporting pump noise before too. I think that our K fuel pumps are fairly reliable but an especially noisy pump may be a good indication of cavitation and impending failure sometime down the road.
- Also, as a means to reduce the occurrence of low pressure zones at the pump inlet which can facilitate cavitation inside, pump inlets are wide and mostly unobstructed. It’s for this reason they are located inside the fuel tank. To have a hose connected to the inlet of the pump would create a vacuum which would promote cavitation.
Take from that what you want.... it's free. And
take a read of this excellent article… it’s a good read. Cheers
Also interesting....
Death of the External Fuel Pump