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Old 01-30-2020, 01:24 AM   #1 (permalink)
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Passive prechamber spark plug (DIY??)

So you may have heard of Mahle Jet Ignition that's used in F1, and Honda CVCC from the past. They ignite a rich prechamber mix to then shoot jets of flame out into the main chamber instead of relying on the flame front propagating from the spark plug.

Lean burn or charge dilution tolerance isn't the only efficiency benefit of fast combustion though, as we've seen Toyota and Mazda demonstrate. Since mechanical compression and thus expansion ratio are quite limited on a gasoline engine, fast combustion gets you as much effective expansion as possible.

They are obviously not retrofittable since they involve adding a direct injection system and a prechamber.

However I noticed these: https://prometheus-at.com/prechamber-spark-plugs/

They're used on large natural gas engines which would be a little different from a gasoline engine since natural gas would diffuse into the prechamber faster than gasoline vapors, but it should still work, because fuel/air mix gets forced into the chamber on the compression stroke. The prechamber will have slightly more residual exhaust lowering its EGR tolerance, but it'll also be a little hotter which might negate that. The smaller the prechamber volume, the less residual exhaust will remain in it.

What if you were to fabricate a cap that goes over the spark plug forming a small prechamber? Typical charge temperature at max compression is something like 800K, and the flame burns at something like 3000K, so you can kind of guesstimate the jet orfice size. It would need to withstand something around 800psi and operate at high temperature so stainless steel seems like a good material.

E.g. 0.5cc chamber on a 500cc cylinder gets you about 1% of the total fuel air mix into the prechamber, seems approximately on the same order of magnitude as what the properly designed stuff has. Copy the 6 hole pattern, copy what looks to be approximately 1mm diameter jets.

Since it's a passive prechamber it won't help very much with increased EGR or lean burn, but it should be able to facilitate increased compression ratio. Getting a MPFI engine up to say 13:1 compression ratio on pump gas like the Toyota A25A-FKS would be a really big deal, that would be a massive fuel efficiency gain!

I fully intend on trying this someday when I have time but wanted to put it out there in case someone else thinks this is easy to do. How hard can it be?


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Old 01-30-2020, 02:35 AM   #2 (permalink)
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Pre-chambers were prevalent in gas ICE's from 100 years ago, and older diesels too. I've got an ancient B&S with one.
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Old 01-30-2020, 11:33 AM   #3 (permalink)
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Built into the head right? That's not an option for a retrofit, so it would have to be somehow welded to the end of the spark plug. I'm not really sure how welding something that tiny works.
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Old 01-30-2020, 03:00 PM   #4 (permalink)
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Should work. Jewelers do it all the time.
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Old 01-31-2020, 09:06 AM   #5 (permalink)
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I've got prechambers from the factory in my 2010..... GEP 6.5L Diesel.

My understanding is the prechamber orifice size, shape, and orientation would need specifically designed for the combustion chamber it is mated to. All attempts I am aware of on the 6.5L engine to modify the prechamber orifices has led to worse performance.

The prechambers are usually made of exotic materials (e.g. inconel) so welding one on there may be more difficult than it sounds.
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Old 01-31-2020, 11:57 PM   #6 (permalink)
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Quote:
Originally Posted by aardvarcus View Post

My understanding is the prechamber orifice size, shape, and orientation would need specifically designed for the combustion chamber it is mated to. All attempts I am aware of on the 6.5L engine to modify the prechamber orifices has led to worse performance.

The prechambers are usually made of exotic materials (e.g. inconel) so welding one on there may be more difficult than it sounds.
Yes on the first part, but I think it can be guesstimated. MPFI engines from the last 2 decades are pentroof shape chamber with spark plug slightly offset to the exhaust side, and a flat piston top. The central jet could be eliminated to reduce risk of overheating a spot on the piston, and the jets can be aimed towards the ends of the "pentroof" since that's where most of the fuel/air is furthest from the plug.

You're right about the material, now that I think of it, stainless steel might not cut it. The jets would need ceramic insulation since the temperature is definitely hot enough to slowly melt away the material.

It would be a lot easier to ask Prometheus AT to make some car sized plugs instead haha.
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Old 02-01-2020, 06:17 PM   #7 (permalink)
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I weld inconel all the time.

Quote:
Originally Posted by aardvarcus View Post
I've got prechambers from the factory in my 2010..... GEP 6.5L Diesel.

My understanding is the prechamber orifice size, shape, and orientation would need specifically designed for the combustion chamber it is mated to. All attempts I am aware of on the 6.5L engine to modify the prechamber orifices has led to worse performance.

The prechambers are usually made of exotic materials (e.g. inconel) so welding one on there may be more difficult than it sounds.
Inconel 625 is common and filler rods of this alloy are too. Tig welding with back shielding.

You can weld to other steel alloys with 625.

Yes, I used to work in the nuclear industry.
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Old 02-01-2020, 06:25 PM   #8 (permalink)
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Thinking on this a bit more. . .

Having a standard spark plug threaded into a heavy external chamber would work.

Injectors from a Direct Injection system that run at several hundred to thousands of PSI can feed the chamber with a side injection and would provide cooling.

Couple this with port injection of a lean mixture and your DIY system might be a possibility.
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Old 02-01-2020, 10:03 PM   #9 (permalink)
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Quote:
Originally Posted by RustyLugNut View Post
Having a standard spark plug threaded into a heavy external chamber would work.

Injectors from a Direct Injection system that run at several hundred to thousands of PSI can feed the chamber with a side injection and would provide cooling.

Couple this with port injection of a lean mixture and your DIY system might be a possibility.
Oh you were talking about a side injector, those are always aimed at the piston, and even if they were aimed at the prechamber, the orfices would be too small for fuel to get in. Stratified injection works late in the compression stroke when lots of lean mix has already gone into the prechamber.

I was thinking about an external chamber too. There's not a lot of space to do that, but one way would be to screw a threaded inconel chamber from the inside (the threads don't need to hold much force), and then have the usual spark plug screwed part of the way in to form a bigger chamber. You can use a surface gap plug to make some more space.

One thing this helps with is cooling, heat would conduct to the head to some degree. Stainless steel/inconel aren't the best conductors of heat but it's worth something. If the jets are very close to the surface of the head, then the hottest part would be conducting some heat away to the head.

One more thing working in favor of the passive prechamber is that the residual exhaust in it mitigates detonation (compression stroke is crucial for getting fuel/air mix into the prechamber).

It's possible that the fuel spray coming from the intake ports washes over the hottest part of the prechamber and provides sufficient cooling, but that's maybe not something you want to count on.

Last edited by serialk11r; 02-02-2020 at 12:31 AM..
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Old 02-02-2020, 04:31 AM   #10 (permalink)
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Why not have small spikes on the piston and try to get an arc from the top to the bottom?

Maybe even multiple arcs?

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