Re: Opened DI Casette - For those interested

Reply to
John Hudson
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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.

Yep. That's their method alright - and very neat too.

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.

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.

Well yes. It would have to be properly designed.

And there was me, trying to keep it down to a subtle smiley ;o)

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.

Cheers,

Colin.

Reply to
Colin Stamp

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).

Worthy of admiration. I mean that in a serious, engineer-geek kind of way, not a sarcastic way, in case that's not clear.

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.

Riiight, I think I see that.

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.

Sorry, I couldn't help it.

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

Reply to
Dave Hinz

That would cover the pressure up to the spark, but after that it all depends on how fast the charge burns - which is unknown. That's the bit they're trying to measure.

Yep, but only up to a point. Go too far, and the tail end of the pressure pulse gets wasted, either by trying to push the piston past BDC or by leaking out through the (by now open) exhaust valve. Not sure which - maybe both.

I'm pretty sure it could be made to work. Alfa manage to get their T-spark engines to work quite nicely with two spark plugs per cylinder. Also, the pressure sensor could be flush with the combustion chamber top. You're right though. There's no need to do it with the current emission standards. Ion sensing works well enough.

Damn - spotted again. I started out with electronics, but now I'm writing low level driver software for digital TV set-top-boxes for my sins. I'd like to be playing with car electronics, but it hasn't happened :o( I have to console myself by trying to build a car in my garage...

Cheers,

Colin.

Reply to
Colin Stamp

So, then, I think integrate the area under the pressure curve between TDC and whenever the exhaust valve opens. No, correct for the force imparted to the crank based on the crank angle and integrate _that_. Still reasonable math.

Well, by playing with cam profiles, I'm sure they're already imparting a swirl into the combustion chamber, so yeah they could probably work around something like this.

From a mile away.

Reallllllllly...are they a brand that I might recognize?

I've toyed with that idea. I have a '71 Sonett which would benefit greatly from a new Sonett, only out of stainless.

Reply to
Dave Hinz

Back on the "other cars with DI systems". My '88 Nissan had a DI system, coils were known to go around 75K. Nissan Maxima's (since

95, IIRC) have DI systems with the coils typically going out at 60K miles. Audi & BMW just had a big debacle - Audi coils were going at 15K at such rates that customers had to wait months for replacements.

DI systems are prone to failure - I don't think you can subject electronics to this kind of abuse and expect them to last.

Reply to
Retro-Bob

You wouldn't happen to be involved with the Pace Twin Digital? I am still wishing, some 20 months after buying the *** thing, that they'd fix its outstanding major bugs... Don't waste time on all this Saabish nattering! Focus on your programming! ;-)

Reply to
Andrew Stephenson

No spares in my boot either. From memory, the cars with coil-on-plug ignition that I've had over the years had covered around 230K miles between them before I lost track of them. I had the records for all of them, and none of them had needed anything other than routine spark plug changes on their ignition systems.

Cheers,

Colin.

Reply to
Colin Stamp

Could be a few. We only sell silicon. It's our customers that make the boxes. We've worked with quite a few people in the past, like Amstrad, Pace, Toshiba, Sony, Grundig etc.

Unfortunately, the Saab content in mine is limited to the brake-fluid reservoir...

Cheers,

Colin.

Reply to
Colin Stamp

Translation: They last *at least* 75K and give absolutely no trouble in that time. After that, a few of them fail and their owners make a lot of noise.

Translation: See above.

Translation: Audi and BMW had a quality control problem that could have affected any part. They were expecting virtually no failures (based probably on their experience with previous systems) so they had virtually no spare stock and got caught with their trousers down.

It's a hostile environment alright, but it's not that bad. It should be perfectly possible to do, and it has been done for some years now by a lot of manufacturers.

FWIW, my last Saab had done 105K and never needed a replacement. My current one is still fine at 55K.

Cheers,

Colin.

Reply to
Colin Stamp

Thankfully, we didn't have anything to do with the Twin ;o). I can sympathize with you though - I've got one myself. I have a bit of a love-hate relationship with it. When it works, it's the dogs-danglies and I can't do without it, but there's times when I feel like driving the 9-3 repeatedly backwards and forwards over it.

I'm with you. If only they'd finish off the software...

Cheers,

Colin.

Reply to
Colin Stamp

There is not yet an answer as to the failure modes involved in the DI failures. Nobody has identified any "electronics" in the units and even if there is, that might not be the cause of the failures.

After monitoring this thread for a week or so, I'm beginning to wonder if the DI is a case of suboptimization. The distributor, HV coil, and sparkplug wires have been eliminated, but these were relatively inexpensive items and rarely failed. These were replaced by multiple expensive items which are just as prone to failure if not more so.

Reply to
Everett M. Greene

What makes you believe that there is no electronics inside a DI cassette?

AFAIK the DI cassette contains two major functions, a capacitive discharge ignition system and an ion sensing circuit.

Capacitive discharge ignition system

One 12V to 400V voltage converter. One large capacitor charged to 400V. Four (one for each spark plug) capacitive discharge spark generators with - A thyristor (red DI) or a triac (black DI) - drive electronic for thyristor/triac for each spark plug - HT coil for each spark plug The DI connector has one pin for each spark generator. The ignition timing is done by the Trionic ECU.

Ion sensing circuit

One 12V to 80V voltage converter. Four (one for each spark plug) ion sensing circuits using the 80V to ionize the gasses in the spark plug gap. The DI connector has two pins for the ion signal, one for cylinder 1 and 4, one for cylinder 2 and 3. The Trionic ECU knows which cylinder is in the combustion phase.

The circuit board containing the electronics is the same size as the expoxy filled area, i.e. the whole length of the cassette.

Reply to
Goran Larsson

I didn't say there was none. I said nobody had identified any. You now have.

Reply to
Everett M. Greene

There's roughly the same number of components in a coil-on-plug system as in a comparable dizzy setup. They both have roughly the same electronics. Coil-on-plug has more coils, but dizzy systems have the dizzy itself and the HT leads. I wouldn't be surprised if those two items don't hold first and second place for the component that's brought the most petrol engines to a standstill. Getting rid of those two is *really* worth doing. I remember canceling journeys in my first car (a '77 Mini) just because it was raining. I used to buy WD-40 in great big cans, and I wasn't the only one either. In winter it would sell like hot cakes. (It probably still does but I wouldn't know - I dropped out of the market when I got my first car with coil-on-plug ignition.) I really don't miss any of that. Mind-you, I might look after the system a bit better if I had one these days ;o)

Cheers,

Colin.

Reply to
Colin Stamp

Not saying it can't happen, but I've never once had a distributor or plug wire(s) fail in a manner that suddenly makes the car undriveable. I've had one carbon button break off inside a distributor cap and make the engine run poorly, I've also had a plug wire fail and do the same, but in either case I could drive the car home. I like the idea of DI, but the particular system Saab uses appears to have a glaring weak spot, it simply shouldn't fail in a way that renders the car completely undriveable in the typical lifespan of the car. A few may fail here and there, but from what I gather this is something that *will* fail, and I maintain that that's unnacceptable.

Reply to
James Sweet

Although that would be fine on an older car. It'd be a really bad idea to do it on a car with a catalyst.

That's the crux of it, I suppose. I've not come across anyone yet who has any real, solid data about how reliable the DI system is. Certainly reading newsgroups like this one won't get you anything like a balanced view.

Cheers,

Colin.

Reply to
Colin Stamp

I think that any tendency of the engine to run hot will exacerbate DI problems. The DI on my 9000 CSE failed at the time when the temperature was on 3/4 instead of normally 1/2, possibly due to other engine problems at the time. Since the DI was replaced 5 years ago, I've had no problems. I was very embarrassed by the DI failure as the car had to be transported home on the back of an AA lorry. That had never happened with my previous FIAT Croma that I owner for 10 years. Even then, the 9000 is a much better car.

Reply to
Johannes H Andersen

You have to go pretty old here to find a car without a catalyst, in this case it was a '79 (with catalyst) Volvo that had the center contact button fail, didn't seem to hurt anything and I replaced it as soon as I figured out what the problem was. I've had plug wires fail on my '84, where the wire came right out of the boot, but I was able to jam it back in there well enough to get home.

Reply to
James Sweet
[sorry for the slow followup, I've been busy]

Not so. Many of them fail, starting at 75K. Trust me, it was the number one answer to "my car is running rough".

Again, not so. These fail reliably. Most Maxima owners have replaced at least one after 60K, many have replaced the entire set because they got tired of diagnosing them one by one. Very common problem.

I agree... but the point is that other manufacturers have failures. It was my understanding that 60-75K miles was the norm for these coils, just like other makes.

I have yet to see a version of this where the coils reliably lasted beyond 75K miles. I'm sure it is "possible", but I haven't seen it done. I'd be interested in any particular designs that seem to last longer.

Good luck. Mine was fine too until one day when it wasn't - with little warning.

Reply to
Bobo

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