Date: Tue, 18 Jan 2005 22:42:51 +0000
From: Colin Stamp <>
Subject: Re: Opened DI Casette - For those interested

On 18 Jan 2005 20:17:59 GMT, Dave Hinz <> wrote: >Check out this article: > > > >> , 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. 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. 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. 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. 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). 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... Cheers, Colin.

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