Self Dampening Fuel Rails 2

[alans]

I am interested in Self Dampening Fuel Rails for Low* (2-5 bar) and High Pressure (150 bar plus) Rails, in particular what makes a good (dampening) design of rail and why?

 

[AboveRedline]

Unfortunately we've never instrumented this, but the trend in racing seems to be using a large cross section (15mm Ø) main tube with a flow through design, ie the regulator on the end of the rail.

 

[Retracnic]

I only have experience in making standard 3 bar fuel rails. Due to the size of the injectors and fuel flow rates that I work at, an adjustable FPR is up to the task of handling my injector pulse dampening needs.

For more expert help I'd suggest hitting the guys and gals up on the DIY_EFI mailing list. This is a group of individuals dedicated to nothing but fuel injection. They've helped me out more than once!

Try them at:

http://www.diy-efi.org/

Good Luck

Bryan Carter

 

[enginead]

Fuel rail pressure oscillations can be a significant issue, particularly on returnless systems. A standard wave is set up by the injector closing and the inertia of the fuel being bounced back through the rail as a pressure wave. This pressure wave can then effect subsequent injections depending on when it occurs in relation the opening time. One of the really tricky bits to this is the pressure wave and its effect is a complex interaction of engine speed, injection time and fuel temperature (effects the velocity of the pressure wave). The complexity of it and the fact that fuel temperature in the rail is not measured makes it very difficult to compensate for in software so a hardware fix is normally required. There are some design guidelines to try and reduce the effect such as not having sudden changes of section shape or area in the rail (from box section to tube for example) so the parallel faces are reduced (this helps to prevent the wave 'bouncing' off features in the rail). There are additional dampers you can add to the rails (literally a sprung mass) to try and absorb the standing wave, I've even seen flexible pipe used between banks on V engines to try and reduce the effect (you have to be very careful about durability on this kind of approach). It is a tricky one though and I've seen production programmes significantly delayed because of this.

 

[Andy330hp]

Visteon makes self dampening rail made of SS. I'm not sure how it works. I thought I remember reading automotive engineering int'l that the body is thin-wall and can flex to absorb pressure spikes, but I'm am NOT positive about that and I can't find the article right now.

 

[andyv8]

I've also run into this problem with low pressure returnless fuel rails (3-4 bar).
Enginead is quite right that it is due to oscillations set up by the injectors & closing. The problem seems to be that these can start up a standing wave that causes resonance related to the injector firing frequency.
I had no problems with a large bore single rail for a V6, but when I had to change it to a twin rail system due to a reposition of the injectors, up to 14% fuelling deviations cylinder/cylinder were the result!
Fuel in ---->=========
Simply modelling the fuel rail as a helmholtz resonator using bulk moduli for fuel & injector rail gave quite good correlation to the problem fuelling areas that were related to engine speed.
The solution was to use a rubber connector between the 2 ends of the fuel rail which solved the problem (so did a braided goodridge hose that had much less damping than the plain rubber hose).
Fuel in ----->========)

I know that returnless V8 systems with twin fuel rails also require a connection between the rails at the opposite end to the fuel inlets.

If you get really stuck, then Bosch in Germany have a test rig that can measure individual fuel flows through the injectors whilst they are fired with a control signal. (I did my validation with them).

Hope this helps!