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Old 04-27-2001, 02:53 PM
Posts: n/a
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.

<<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? >>

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.

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

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?

91 300SE

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