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leadfoot · Posted: Mon Mar 22, 2004 11:50 am Post subject: Fuel rail pressure

I was reading an interesting article on getting evenly balanced pressure to each injector in a fuel rail.
Taking a single rail with four injectors the first injector would receive more pressure than the last in the chain.
The solution they used was to have a tree like setup and to have the single rail split into two rails.
Taking the setup for one of these rails, the origional single fuel inlet would split into two, to feed the new rail in the centre- thus supplying even fuel pressure to both injectors in the rail.
Then each end of the rail would have an outlet that would then be joined with a "Y" fitting to form a single outlet.
Therefore each of the two rails would have one input and two outputs with the two outputs ending up being joined.
Lastly these two output would also be joined with a "Y" fitting to form a single output.
This setup would stop any cylinder from not recieving the right pressure and going lean.
Leadfoot.. article from ZOOM magazine

Manning · Joined: 29 Feb 2004 Posts: 151 Location: Akron O-Hi-O

Kind of sounds like CIS injection, where all the injectors are fed by hoses from the fuel distribution block.
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Michael Manning
1988 924S Undergoing weight loss program

Richard · Joined: 04 Nov 2002 Posts: 617 Location: Pacific N.W.

I may be wrong, but I thought having the pressure regulator on the return side of the fuel rail prevented this. I think I read it somewhere.

Lizard · Posted: Tue Mar 23, 2004 1:30 pm Post subject:

umm, this only applys to EFI cars, so forget you even read this for the CIS cars,
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kaffine · Joined: 13 Jun 2003 Posts: 644 Location: Las Vegas

Okay now that is getting a little picky on fuel pressure. I know on diesel engines that have a seperate fuel injection pump that the lines have to be the same length but the pressure is 2000psi and higher and the fuel is metered at the pump the main problem is the line expanding. On common rail diesels where the fuel rail pressure is up to 26,000psi but the fuel is metered at the injector the lines aren't always equal length.

I could understand it if we are talking top fuel dragsters that consume more fuel in a pass than a 747 on takeoff but on a street car I think that is a little overkill since the injectors aren't going to be matched that closely and there is little fuel flow.

Having the pressure regulator on the return wouldn't insure all the injectors had the same pressure. Pascall's law that pressure in a contained space is equall every where only applies to a static system if the is flow then there are frictional losses in the lines.

On a CIS I wouldn't tie all the injectors together that may cause an even bigger inbalance of fuel delivery. The injectors will have slightly diffrent opening pressures and flow rates combing all the injector lines would likely magnafiy that. Although I could be wrong.
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Manning · Joined: 29 Feb 2004 Posts: 151 Location: Akron O-Hi-O

leadfoot · Posted: Wed Mar 24, 2004 8:24 am Post subject:

Yeah sorry... This article was in relation to efi... and i'm converting to this soon... I just found it an interesting article and it makes sense as the relationship between running series and parallel seem to run across the board from applications from electricity to garden watering systems, and considering in this application a new design fuel rail could be a simple performance part to be designed.
I was thinking that three tubes could be used, two as fuel distributors and one as a rail. Weld all three tubes together in an "L shape" (and for convenience call these top bottom and right tubes) but pre drill with the following...
top tube one input hole in the centre, two on the opposite side aligned inbetween injectors (1 and 2) + (3 and 4)
bottom tube has two input holes on the top and four injector mount holes on the bottom plus two outlet holes adjacent to the inlet holes.
The right tube has two inlet holes and one outlet hole.

This is a basic idea but it could easily be refined so that the bottom tube is segmented into two independant rails and the fuel flow holes can be increased in number to obtain increased flow. Symetry is the key to obtaining even flow and by using tubing and drilling holes costly fitting can be avoided. If you really wanted to get serious between each of the tubes you could mount flow restrictors with a metering point.
Leadfoot

Richard · Joined: 04 Nov 2002 Posts: 617 Location: Pacific N.W.

Wait a minute. I deal with manifolds all the time and I've always been under the impression that if the manifold (fuel rail) is of suffcient diameter(volume) then friction losses and pressure drops acrosss the length of the manifold is nil. This is engineered into the fuel rail upfront.

If you are not taking out (through the injectors) more gas then is going in, the fuel rail pressure stays close enough to constant. Since after leaving the fuel rail, exceess gas goes back to the tank, there is an abundance of pressure and flow. No injector in a 18" fuel rail will be lean. This a non-problem and the author of the article is just trying to earn a paycheck. About 8 trillion cars on the road today use a common fuel rail for a bank of cylinders and they all have pressure regs on the outlet and returns to the tank.

leadfoot · Posted: Thu Mar 25, 2004 1:04 pm Post subject:

Yeah paychecks are nice...
thoughts regarding this....i may be wrong but just trying to logically work things through.
common rail fuel lines are run on alot of cars... so was cis... this doesn't change the fact that they might be able to be improved on.
In a common setup a regulator is run after the pump before the rail regulating input pressure not output, this would be only regulated by tube size from rail to tank. If the fuel injectors are asked to run a bigger duty cycle, then it is possible to have a pressure drop by the end of the rail.
The problem being i guess the diference in flow consumption as opposed to regulated input, this might not be noticable at normal driving as the pressure input would probably compensate for this, but at higher flow of the injectors the pressure drop across the rail may be aggravated in this scenario.
As an example of clever manifold design, Greddy have a tapered design so that airflow pressure delivers as evenly as possible.
Tapering a fuel rail might also be a design considerarion.
Leadfoot

Richard · Joined: 04 Nov 2002 Posts: 617 Location: Pacific N.W.

Leadfoot, Just in case you haven't seen this http://www.sdsefi.com/tech.html is a wealth of info on efi. What are you planning? I've made a Megasquirt ECU that bench tests can't get the time to put it in the car.

I really only think this fuel rail problem is a problem if you are pushing hundreds of horses. I just don't have the hands on experience to be sure.

Lizard · Posted: Fri Mar 26, 2004 12:24 pm Post subject:

leadfoot · Posted: Mon Apr 05, 2004 3:06 pm Post subject:

great website... lots of info there.
I particularly like the DIY info...
Are the 944 injectors a 14mm o ring ??
and if anyones made a fuel rail for efi is it necessary to have the rail bolted down to the block.
I couldn't see any evidence of this on alex roy's project 924.
what psi would it be necessary for adequate sealing??
Leadfoot

Alex Roy · Posted: Tue Apr 06, 2004 3:37 am Post subject:

On my fuel rail, I started out with a pontiac grand am fuel rail, cut off the injector "bungs" and welded them onto an aluminum bar. The bungs had holding clips that clampped onto each injector directly. I would recomend bolting the whole piece down to the head in some fasion because otherwise there is nothing holding the injector into the head except for the vacuume of the engine.