Date: 18 Jan 2005 22:55:53 GMT From: Dave Hinz <DaveHinznospamcop.net> Subject: Re: Opened DI Casette - For those interested
On Tue, 18 Jan 2005 22:42:51 +0000, Colin Stamp <colinnospamp.plus.com> wrote: > On 18 Jan 2005 20:17:59 GMT, Dave Hinz <DaveHinznospamcop.net> wrote: > >>Check out this article: >> >>http://www.vehicular.isy.liu.se/~larer/Projects/main.html >> >>> , but the Saab setup does do the job neatly and I >>> suspect more cheaply than using extra sensors. >> >>I'd venture to say much of what they're using the DI for can't >>be done in other ways, since the spark plugs are the only thing >>that can be used as sensors in the combustion chambers. There's >>just no other way to get that information. >> > > That's an interesting article. Cheers. > > It looks like the basic parameter that they are trying to sense is > combustion chamber pressure. Well, conductivity, which is a function of a few things, but yes. > 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. > Using the spark plugs as ion sensors is really compelling since the > plugs are in there anyway, but it's not the only way and I bet it has > a few problems. For a start, the plugs have to be designed primarily > to ignite the charge. That must make them far from ideal as ion > sensors. Then they have to break off their role as sensors every cycle > to produce a spark. 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. This is the reason that using a different flavor of spark plugs in a modern Saab is a bad idea. > One way round this would be to fit a second plug in each chamber. The > second plug doesn't need to do any sparking so it can be designed to > be a much better pressure sensor than a spark plug could ever be, and > it can operate over the whole cycle. It doesn't have to measure > ionization either. I've just done a quick Google for "combustion > chamber pressure sensor" and got quite a few interesting hits. 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. 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. > Most > seem to be peizo-resistive but there's also fibre-optic ones. Some are > even built into spark plugs or diesel glow plugs so you don't even > need an extra hole in the head, so to speak :o). Yeah, 'cuz we'd need that like we'd need a hole in the head. (had to do it; you understand I hope). > Using a dedicated pressure sensor would probably allow better control > than Saab currently get with their spark-plug ion sensing system, but > I bet they're more expensive and the emissions regulations mean we > don't need to resort to them yet, so I reckon Saab have the right > idea, at least for now... Goes a long way to explaining why they're not using a dizzy and coil any more, eh? Dave Hinz
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