Can someone help me understand how the pressure differential valves in a K-jet fuel distributor work, why is a contstant pressure differential cruical to measured fluid flow? And what is the relationship between pressure and flow through a restriction. I'm trying to get my head around how the innards of the fuel distributor works, in an effort to understanding why mine isn't behaving properly. I've got so far, but am falling short on the fluid dynamics bit. Can anyone help me???
http://theory.uwinnipeg.ca/mod_tech/node68.html Probabley does not help, but i feel better about myself (except i spelt Bernoulli wrong above). Edited by: gbolton
Yeah, I figured Bernoulli would come into it at some point, just still not too clear how this applies to the differential pressure valve..
the fuel pressure to each injector is proportional to slot exposed area, a long as the diff pressure across the slot is constant. the slot area changes proportionally to air flow, to get the right fueling. if the differential pressure changed as well, everything would be all over the place. hope that makes sense! Mark
So I can see how the differential pressure valve would maintain the pressure drop across the metering slots - but it seems to do this by ajusting the flow rate to the injector- is this right? (BTW I'm getting this from the pdf Bosch manual for K-jet - I can email/host if anyone's interested) Is there any way that ajusting the differential pressure valves would cause the injectors not to shut down properly?
directly from haynes "because the drop in pressure across the metering slot is unaffected by the length of slot exposed, the amount of fuel flowing depends only on the area of the slots". makes more sense than my last post! "So I can see how the differential pressure valve would maintain the pressure drop across the metering slots - but it seems to do this by ajusting the flow rate to the injector- is this right? " That is how gas control valves work on gas turbines, so it is a valid control theory. mail me the pdf and i will try to translate for you. The pressure regulator is supposed to drop the fuel pressure enough for the injectors to shut down properly. what car is it? what was the original fault? Mark
This whole headscratching excercise stems from a recent rolling road session, where it was proclaimed that my engine was overfuelling at high revs. This was not a total surprise as it was recently suffering from serious underfuelling, practically 0% CO and pinking under moderate load. This was remedied by Grant Motorsport by ajusting the pressure differential valves to increase fuelling across the rev range, at the time it was pointed out that theres still an underlying problem, but the 'tweak' has cured most the symptoms to make the car drivable again. There is still an underlying cold/warm start problem that seems to have gotten worse since these works, the usual suspect (the WUR) seems not to be the case as the problem was unaffected by swapping the WUR for an audi one (Injectors were also replaced at that time, to no avail). I'm thinking the fitting of an Audi WUR (with full load enrichment) is now causing the overfuelling. Before ripping out the Audi WUR, I'm looking into the possibility that water-in-fuel contamination from a leaking fuel filler neck (now fixed) has deposited crap in the fuel distributor, causing the underlying starting problem. Obviously I need to be sure, as fuel distributors are difficult to obtain in good nick. Would one from a KE-jet 16v work (just the head, not the airflow plate & funnel)? Bet you wished you never asked now... The car is a 91 scirocco, btw. The objective is to get the fuel system as sound, and as over-spec'd as possible for a (potentially) hairy 2.0l conversion that I'm picking at.
did you clean the gauze in the fuel tank after getting the filler neck fixed? they are usually full of rust and dirt if the filler neck is gone. Your problem could be poor fuel supply. When you are at high revs and using the most fuel, the system maybe can't keep up. Don't forget, due to the fuel pressure opposing the air flap, if you loose pressure the engine runs rich. Thats how the warm up regulator works. A good test would be to monitor the fuel pressure while running at full load to make sure it's not dropping off. I guess if it's a reputable place they will have covered all those bases though. When i get chance i will read the manual and see if i can come up with any other suggestions.Good luck Mark
The lift pump and gauze pickup strainer was replaced when the seriousness of the fuel contamination was first discovered (water in tank, very cold night - frozen lift pump ). My understanding of roller-cell pumps is limited, but apart from the noise (which I put down to ageing rubber mountings), I don't think there's any problems with the main system pump. I havent checked the actual delivery pressure personally, as I lack the tools, but I figure if there was a slight drop in system pressure it would not make too much difference to fuel flow as long as the injectors stay open: Whilst the control pressure is a function of main system pressure, the fuel delivery rate is also proportional, as it is dependant on system pressure too. i.e. if the system pressure drops the control pressure drops too, then the metering plunger rises, allowing more better fuel flow for the proportionally lower pressure that caused it, hence net delivery would be unchanged - unless the action of the pressure diferential valve messes this up.. Does this sound right? Edited by: SkyRocketeer
I've had a look at the manual and nothing really springs to mind. Although a full load enrichment warm up regulator AND a metering head that provides the full load enrichment function sounds like too much of a coincidence to me! Also, when the car ran lean, it sounds like the differential valves were adjusted to compensate for a fault, rather than fixing the fault. Which could have offset the system enough to produce this fault. i don't think it's normal for the differential valves to need that sort of adjustment. Good luck. M.
your saying; if the main fuel pressure drops, you have more air flap movement, due to less damping, but less pressure to the injectors, to cancel that out. right? M.
Kinda my thinking, but nobody else who's done the WUR mod seems to complain of this (unless they haven't noticed). Again I agree that the differential valve tweak cured the symptoms rather than the cause, but I'm still not entirely sure what the cause is, apart from the fact that it's a bugger to start in the mornings.. At least I'm a little clearer as to how it's all supposed to work. Maybe I'm just being fussy, and 6bhp off stock isn't too bad for a 12year old. But if something ins't right, I'm inclined to pick at it until it's either fixed or broken beyond redemption..
my gut feeling is that errors in fuel supply pressure do not cancel like that, but wait to be informed otherwise. With my mk1, i changed the filler neck, gauze, fuel pump, fuel filter, metering head (brand new), and injectors. I also made a fuel pressure measuring rig, but it still wasn't right! They can be a git.... You may be better of going for a s/hand electronic system, of a mk3 2.0 8v maybe, before ploughing too much dosh into this old mechanical system. It may cope with your future engine mods better and be cheaper in the long run, as modification on the rr is electronic rather than mechanical. M.
I reckon I'll stick with the K-jet for now, despite it's flaws, cos i understand how it works fairly well, and I don't have chips n stuff to worry about. I guess for ultimate tweakability something programmable is the way to go, but I'm far from that stage right now. So K-jet will have to do. I'll try and source a fuel distributor second hand, and see how swapping it over works. Cheers for your thoughts so far..
