Date: 19 Jan 2005 19:34:45 GMT From: Dave Hinz <DaveHinznospamcop.net> Subject: Re: Opened DI Casette - For those interested
On Wed, 19 Jan 2005 19:07:52 +0000, Colin Stamp <colinnospamp.plus.com> wrote: > On 18 Jan 2005 22:55:53 GMT, Dave Hinz <DaveHinznospamcop.net> wrote: > >>Well, conductivity, which is a function of a few things, but yes. > > The way I read it, they only measure conductivity because that's all > they can measure using the spark plugs. They can get a rough idea of > the pressure from the conductivity but, like you say, they get a few > other variables mixed in. I guess that's a bad thing, but not bad > enough to make the system non-viable. Thing is, if pressure is all you wanted, you can get that from manifold pressure, crank position, intake air mass, and intake air temperature. I think. (let's see..pressure, quantitiy, temperature, known volume, yup). >>> The goals of sensing knock, misfire, ppp >>> etc. are all achieved by interpreting the pressure waveform. The trick >>> then, is to find a fast-response pressure sensor that's reliable and >>> rugged enough to be used inside the combustion chamber. >> >>Right, which would be the "combined sensor/actuator" which looks >>very like a spark plug. > > Yep. That's their method alright - and very neat too. Worthy of admiration. I mean that in a serious, engineer-geek kind of way, not a sarcastic way, in case that's not clear. >>Well, the timing of the cycle helps a lot. When they're measuring, they're >>not firing the spark, and the other way around. They know _about_ when >>these things will happen, just not _exactly_ when. Measuring the >>conductivitiy of the air:fuel mixture gives them that. > > I think you're right. What they seem to be most interested in is how > many degrees after TDC the pressure reaches it's peak - the PPP. > Thankfully, that's well out of the way of the spark. I'm thinking that the peak pressure would ideally come as late as possible, so the cosine of the angle is the highest - most downward force turns into the most rotational force that way. >>I'm not convinced that pressure is the only variable that they care about. >>You could have the same pressure with wildly different air:fuel mixtures, >>just by changing charge air temperature, humidity, and probably another >>handful of things. > > The actual value of the pressure at it's peak doesn't seem to be > important, just the crank angle at which the peak occurs. Riiight, I think I see that. >> A second sensor, of any type, would also change >>the head considerably, and possibly screw up the combustion chamber >>geometry, flow patterns, and a bunch of other things I can't even imagine. > > Well yes. It would have to be properly designed. It might be a fundamentally flawed idea, though. It either breaks the symmetry of the combustion chamber, or it puts the spark plug off-center. Not sure there's enough reason to justify it. >>Yeah, 'cuz we'd need that like we'd need a hole in the head. >>(had to do it; you understand I hope). > > And there was me, trying to keep it down to a subtle smiley ;o) Sorry, I couldn't help it. >>Goes a long way to explaining why they're not using a dizzy and coil >>any more, eh? > > Ah well probably not actually. coil-on-plug ignition seems to pre-date > spark plug ion sensing. When it first started to come in (can't > remember by which manufacturer), the elimination of unreliable dizzys > and leads was given as the reason. Of course, it makes ion sensing a > lot easier, but that seems to be accidental. One of those "OK, since we're here, what else can we learn from this new technology" kind of things, yup. You're an engineer or technician of some sort, I assume, Colin? Dave Hinz
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