K-Jetronic, why a distributor?

[sixto]

If all the lines out of a K-Jetronic fuel distributor are at the same pressure and have the same flow rate, why not just have a common rail?

Thanks,
Sixto
91 300SE

Six-to

The injectors have constant pressure on them, but only open when the pressure increases to a certain point. The distributor sends the opening pressure to the specific cylinder that is on compression stroke. This way, they accomplish by mechanical means what a pulsed injection system accomplishes by electric solenoid.

The fuel distributor is the "metering" section of the fuel system. It controls fuel flow based on the position of the sensor plate.
Regards,
Randy D.

[sixto]

Jason,

I have to question that. There is no linkage between the fuel distributor and the crankshaft; so the fuel distributor can't know if a cylinder is in its compression stroke. My understanding is that a CIS injector is open as long as there's sufficient fuel pressure, regardless of what the valves and piston are doing. That's my understanding of the basis of 'continuous.'

I believe, though I haven't tried, that one can swap lines between the fuel distributor and the cylinders with no negative effect. If this is true, then I raise the original question again, why not have a common rail?

The only thing I can think of is that there is some dynamic during the various engine cycles that individual fuel lines isolate.

Randy,

Good point to clarify. I understand the metering function of the fuel distributor, just not why fuel is essentially distributed at the point of metering and not later. Why have six metering slots on a six cylinder engine if all six metering slots do the same thing at the same time?

Thanks,
Sixto
91 300SE

[stevebfl]

Good question. I believe it has to do with response time. I remember years ago a Saab technician told me about a problem he had been chasing.

It turns out that Saab uses plastic fuel line between the distributor and the injector. Two lines had been remade with a common line used for such. The black line used was common to VW and Volvo. The clear lines used on Saab and BMW were half again as big in diameter. He fixed the problem by putting the lines back to original and the factory rep that helped with the diagnosis said that the diameter of the lines changed the quantity of fuel in the line which changed the ability of the fuel to respond to sudden changes in flow rate.

I would also presume that the design theory of the differential pressure valves internal to the distributor would be changed by the common rail.

[Jack500]

Steve, I have a take off of your last response dealing with the diameter size of the fuel lines. Question, when monitoring the system and operating pressures on a CIS system, is it necessary to use a test gauge that has the same diameter lines as the fuel line being circumvented in the test (the line going from the fuel dist. to the warm up regulator)? Theoretically the pressure would be the same regardless but would the actual fuel delivery be the same?
Jack

Originally posted by sixto
<
I have to question that. There is no linkage between the fuel distributor and the crankshaft; so the fuel distributor can't know if a cylinder is in its compression stroke. My understanding is that a CIS injector is open as long as there's sufficient fuel pressure, regardless of what the valves and piston are doing. That's my understanding of the basis of 'continuous.'>>

RD: That's right. There isn't any likage between the fuel distributor and the crankshaft. It isn't timed or pulsed injection so there is no need. It really doesn't care if the intake valve is open or closed. Or where the pistons are. The critical function of the control piston is controlling volume, not pressure.

<>

RD: Yes. You can swap the lines between cylinders. It makes no difference(and it can also be a handy diagnostic tool). The injectors spray a "constant" fine mist(of variable volume) at the intake valves. A mechanical system with a common rail would create several problems.The most important being that even a slight difference in injector opening pressure would affect the entire engine instead of just one cylinder. Electronic systems don't rely on injector opening pressure. The injectors are either off or on.

<>

RD: Not really, it's just the way the system was designed. With a single compact metering piston which must(of course)be connected to the air-flow housing, I think they did a great job. I am glad that they eventually went with a more efficient electronic system. Mercedes was the last to hang on to the "mechanically based" CIS/K-jetronic system.

<
Good point to clarify. I understand the metering function of the fuel distributor, just not why fuel is essentially distributed at the point of metering and not later. Why have six metering slots on a six cylinder engine if all six metering slots do the same thing at the same time?

Thanks,
Sixto
91 300SE

>>

Sorry for the confusing format. The original quotes are between the "<>" marks, the responses are not. Duh...next time I'll think more and type less.

W. Durance

Thanks for clearing that up for me. I was assuming that it metered like a pulsed system and delivered like a mechanical, I was mistaken. Thanks again

[stevebfl]

Mercedes discontinued CIS-E because of the US emission requirement for sequential control of fuel. The fuel must be turned off on a cylinder that is misfiring to protect the catalyst.

To get an idea why MB was the last to leave CIS one might look to the Porsche Carrera engine. In the car the motor h
as Motronic - One control unit for fuel and ignition. When the same engine was used in a Mooney (sp?)airplane the fuel system from the earlier 911SC CIS was used.

cis is not a pulsed system. cis is constantly injecting fuel when the engine is running. the distributor adjust the amount of fuel based on the input from the air flow sensor plate.
it has zero relationship to the position of the crankshaft or the cylinders. the cis system could care less where the crank or the cylinders are at any given time.
sounds odd that the injectors are constantly injecting, right? where does all the "extra" fuel go? it doesn't go anywhere, it sits in the intake manifold behind the intake valve, for that microsecond before the intake valve sucks it into the cylinder.
as you can see, its not the greatest from a precision or efficiency standpoint. which is why its not used anymore. it is, however, kind of an interesting system, it is not electrical at all (except the later ones that use an o2 sensor for some fine tuning), in its pure, early form, it is entirely hydromechanical.
btw, motronic does *not* have one control for fuel and one for ignition, motronic uses one integrated ECU to control both.

[stevebfl]

Looking back, who said Motronic had a control unit for each. I seem to be the only one to mention Motronic and I said: "One control unit for fuel and ignition". Maybe you misunderstand the English. As I obviously didn't say one control unit for fuel and one for ignition.

Hmm, "looking back," the wording in the post now doesn't seem to be the same wording that was in the post when I responded to it. I remembered it pretty clearly, and it seems different now. Even the spacing seems different.

So, I really have no way to repond. Not really fair under those circumstances to suggest I can't understand English.

[Edited by Brian K on 05-01-2001 at 05:09 PM]

[stevebfl]

Whatever, I didn't change it and I sure wouldn't have said it any other way. It was the point I was making that they used one controller.