Big R Posted January 10, 2006 Posted January 10, 2006 Guys I understand that I need to reduce the bore of my swirl pot overflow to fuel tank hose to stop the lift pump running flat out permanently. I’m going to make a reducer from aluminum and put it into the hose. Any advice on what size I should make the orifice? Thanks …. Quote
Gromit Posted January 10, 2006 Posted January 10, 2006 Most swirl pot return pipes are about 4mm ID. HTH Quote
Blatman Posted January 10, 2006 Posted January 10, 2006 I thought differently... The overflow won't/shouldn't be restrictive enough to drag down the speed of the lift pump, and if it did work like that then the fuel tank would also be providing pressure against the returning fuel. The restrictor needs to go in the pipe that feeds the swirl pot from the lift pump, and not the overflow from the swirl pot. Sadly, I have no idea what the bore of the restrictor should be... Quote
Westfieldman Posted January 10, 2006 Posted January 10, 2006 The restrictor goes in the pipe between the seperate fuel swirl pot [located at the top of the swirl pot] and the main fuel tank The fuel swirl pot provides A 1 ltr reserve of fuel [directaly above and closely linked to the HP pump A tank that exsess fuel to return to from the fuel rail A tank that the fuel can be deairated in So the fuel return pipe from the swirl pot to the main tank should be from the very top of the fuel swirl pot and have a small orifice in it or a restrictor in the pipe that returns to he main tank Dia of the pipe restrictor can be around 2 to 3mm If he swirl pot is full the lift pump [if the red top type] will stop and start as required Quote
Big R Posted January 10, 2006 Author Posted January 10, 2006 Thanks Paul - that's what I'd worked out from various threads and diagrams that I'd seen and it seems logical. I'll start out with a 2mm hole, which I can enlarge if required. Quote
Blatman Posted January 10, 2006 Posted January 10, 2006 So the fuel return pipe from the swirl pot to the main tank should be from the very top of the fuel swirl pot and have a small orifice in it or a restrictor in the pipe that returns to he main tank So the swirl pot is pressurised? What happens at tickover when most of the fuel is returning to the swirl pot faster than the engine is emptying it. At some point you must run the risk of having fuel backing up to the pressure regulator along the return line? I'm confused... Quote
Big R Posted January 10, 2006 Author Posted January 10, 2006 I'm assuming that the path of least resistance will still be the overflow from the swirl pot to the tank and all excess fuel will make its way down the pipe. Hence my question about what size of orifice - I want to slow down the flow rate, not increase the pressure. Quote
fordy Posted January 10, 2006 Posted January 10, 2006 surely you cant slow down the flow rate without increasing the pressure ?? Ive dont have a restriction in my return, and my redtop was fine all last year. Quote
Westfieldman Posted January 10, 2006 Posted January 10, 2006 At some point you must run the risk of having fuel backing up to the pressure regulator along the return line? I'm confused... The lift pump pumps fuel to the swirl pot at say 5psig this is under pressure the fuel fills the swirl pot. This fuel is dilivered to the HP pump by gravity and the 5psig the lift pump suplies. The fuel is now pumped by the HP pump at high pressure say 50psig the fuel is used on demand by the engine through the injectors the exsess is spilt back to the swirl pot, the spilt fuel will be at a pressure of 5psig as that is the pressure from the lift pump [the fuel regulator on the fuel rail maintains the fuel pressure on the HP side, the pressure drops quickly to 5psig as it is in effect an open to atmos pipe any overspill from the swirl pot returns to the main tank the restrictor gives the lift pump something to pump against when the swirl pot is full. Other wise the lift pump would rattle its self to bits [pumping what would be open circuit] Quote
Blatman Posted January 10, 2006 Posted January 10, 2006 That doesn't add up... The lift pump fills the swirl pot. Obviously. But the pressure in the swirl pot is determined by how fast the high presure pump empties it, and how quickly the overflow empties it should it fill up either by the lift pump and/or the overflow not flowing fast enough 'cos it has a restrictor. In IMHO the swirl pot should NOT be a pressurised vessel as the pressure *will* flow "backwards" towards the pressure reg down the return line, which is indeed gravity driven and not pumped. I maintain that you need to restrict the feed between lift pump and swirl pot. There will still be plenty of flow, but enough pressure will build up so that the lift pump won't kill itself. Of course, if you modify the fuel tank as I have to do away with lift pumps, external swirl pots and overflows, you won't have any of these issues at all... Quote
Westfieldman Posted January 10, 2006 Posted January 10, 2006 Heres the drawing of how it should look But the pressure in the swirl pot is determined by how fast the high presure pump empties it, The high pressure pump no who much or hard it pumps can only hope to pull a vacuum and how quickly the overflow empties it The overflow with a restrictor can only return fuel to the main tank when the swirl pot is full it should it fill up either by the lift pump and/or the overflow not flowing fast enough 'cos it has a restrictor. There is no restritor in these pipes In IMHO the swirl pot should NOT be a pressurised vessel as the pressure *will* flow "backwards" towards the pressure reg down the return line, The low pressure fuel delivery system has to be a positive pressure to work it won't work under a vacuum every thing from the HP pump to the fuel regulator is HP the rest is low pressure I maintain that you need to restrict the feed between lift pump and swirl pot. NO all fuid dynamics I have come across I industary are restricted on/in the return legs [NH3 fridge systems, Steam systems, Hydraulic systems and Air systems] Give me 6 months and I will explain how 650MW's of electicity is made where I now work Quote
Gromit Posted January 10, 2006 Posted January 10, 2006 Tend to agree with Paul on this one. If there is no restriction in the return pipe to the fuel tank then the lift pump could possibly be damaged. The issue about the high pressure regulator return line (breath) develoving back pressure would not happen unless the capacity (litres/min) of the lift pump is way to big for the maximum fuel flow requirements of the engine. Otherwise with the correct size lift pump there will probably be no more than 1-2psi (educated guess) developed in the swirl pot, unless the low pressure return to the tank is way to small. As with everything there are extremes of design that will mean the system will not work as planned but with the correct lift pump capacity and return (to tank) line there will be no problems. Quote
Blatman Posted January 10, 2006 Posted January 10, 2006 Paul. The diagram, whilst nice, doesn't demonstrate either of our theories, only a possible layout which I undestand perfectly. We are clearly talking at cross purposes, so I'll say no more. Quote
Big R Posted January 11, 2006 Author Posted January 11, 2006 I’m going to add the restrictor to my overflow hose – if the swirl pot explodes due to excess pressure and my car goes up in flames I’ll let you all know….. Quote
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