TheDieselStop.Com Forums: MPG AND CACKLE

DZacc and Frobozz (Duncan) raise a good point about the split shot. After all, prior to the split shot, there was no California Cackle... so the split shot technology ought to be in the line up with all the other suspects. I am glad this point was made.

Duncan is right of course, just one energized actuation of the injector solenoid, and the "split-shot" occurs mechanically within the fuel portion of the injector.

Which leads to my question...

How does the IDM, as controlled by the PCM, respond electrically to pressure fluctuations in the fuel rail? Is the FDCS modified in real-time in correspondence to fuel pressure fluctuations in the rail?

For clarification, "FDCS" stands for Fuel Delivery Control Signal, which is the signal that the PCM sends to the IDM to instruct it to fire. If the FDCS instructs the IDM when to activate which injector solenoid for how long, might a change in the FCDS make a corresponding change in the high voltage signal from the IDM to the injector... a corrective change in real time? That is one way I can imagine the CPS affecting the IDM, and is also one way that I am guessing the split shot fuel flow described below might be relevant.

Pressure fluctuations in the fuel rail (the fuel rail machined into the head, not an external supply rail) have been measured and recorded, right? When Coach Davis et al was busy measuring pressure spikes, they were measuring at the head port, right?

What is it then about the mechanical method of producing a split shot fuel delivery, that is consistent with fuel pressure fluctuation?

It being established that there are two vastly different fuel pressures... the pump produced line and rail pressure of the fuel supply, which ranges within a relatively low bracket of 30 to 100 psi, and then there is the intensifier piston produced fuel injection pressure, which ranges within a relatively high bracket of 2,100 to 21,000 psi... what happens when the high pressure is momentarily introduced into the relative vacuum of the low pressure?

Will a spike in the low pressure fuel rail signal a cue for a compensatory electronic action? Is there a case when the action-reaction cycle cannot keep pace, or is ahead of pace with actual conditions?

Split-shot within an injector mechanically occurs due to a gap in the fuel plunger, a plunger that was at one time solid, prior to split-shot. The plunger is both a piston and a valve, pushing fuel out as well as allowing fuel in, depending on its position. A solid plunger pushes fuel down creating high pressure, closing off the entry (and exit) of fuel from (or into) the supply rail. On the other hand, the split shot plunger (with a gap) starts to push fuel down, creating high pressure, but then, because of the gap in the plunger, it momentarily opens the high pressure fuel resident within the injector up to a low pressure port common to the rail. There are tiny channels in the lower part of the plunger that allow the fuel in the injector to escape back into the rail, where the pressure is lower. So, 2,100 to 21,000 psi fuel is momentarily sharing a common channel with 30 to 100 psi fuel. Which way is the fuel going to flow?

For those thinking that there should be a check ball to prevent the obvious backflow, yes there are check balls at the main injector inlet, but note that there is no check ball at the upper release port in the latest split-shot injector diagram supplied by International. The fuel flow arrows at the end of the first shot indicate fuel at high injector pressure flowing OUT of the injector INTO an upper rail port where the fuel is at the lower pump pressure. Furthermore, since some fuel has thus escaped to find pressure equilibrium, that is just that much less fuel to go into the cylinder, right? And, the high pressure has to ramp up again before the end of the injection process. The gap in the plunger is small, so very quickly, as the plunger continues down it's path, the escape hatch is closed off, pressure is redeveloped, and the second shot of the injection is completed.

I am not trying to second guess the merits of this design, I in no way qualified to even render an opinion. Besides, the new system obviously works, delivering adequate power to get the job done, while reducing smoke and emmissions. My question: Does what I assume to be these tiny rail pressure "spikes" induced by the injectors releasing high pressure fuel back into the rail between the first and second shot... have an effect on cackle that ultimately ripples through the electronic sensor action/reaction system... enough to be measured electrically? And, ultimately, does that mean it is possible to manipulate or contol such reactions electrically-- as a compensatory measure to a fuel flow and plunger characteristic that cannot be easily changed?

The same question stands for if the cackle is more to do with the fuel loss in the injector from having lost either a tiny amount of expected fuel between the shots, or having to re-ramp up pressure again (having released it momentarily) to complete the second shot, within the timing that the cylinder is alloted.

And, the same question stands for the long standing discussion about the fuel shock wave causing starvation in the rail.

Or, expanding on my comments above, the same question stands for where any injector in the "fill-stage" of operation expands the volume (hence the low pressure vacuum) of the rail, allowing more fuel out of an injector that is inbetween shots, more so than other injectors that are inbetween shots when the other three injectors common to the rail are not in the fill stage.

Sorry, I didn't mean to ramble in questions like this, but I have often wanted to discuss the specifics of the split shot process, since it is directly and unquestionably linked to the cackle, and to be quite honest, this is the first decent cackle thread that has come about in a long time... where there is an opportunity to get some openly shared feedback. I could be way off base here, but I would like to learn of any measurable corresponding links between the electrical signals being measured by TexasTowncar, and these split shot fuel flow dynamics.

Edited by Robyn (12/28/01 09:24 PM)