Fourteen of the air chargers in the form in use, is an effective decent maximum for my engine compartment. I could put more on, but not neatly at all, and I don't want total spaghetti-tangle, right now it's as close to that as I want to get.
One option might be to replace the existing units with more powerful ones. For just a few days on AliExpress, I did see a unit advertised as producing 125M/cm3, costing approximately US$15. (The current ones are rated at 30M/cm3.) This big one ran on 110VAC only, and I found myself pondering a VAC-->VDC inverter under the hood, wondering about heat tolerance. But it vanished from the web site within a few days.
So then my thoughts went to building my own. Here's the test rig:
The little black box on the left (notice the warning!) is an EMCO F40, a component which takes 15VDC input and puts out 4000VDC output: DC to DC, quite the neat trick, not sure how it works, but possibly perfect for this, because I was able to obtain several independent reports (reportedly born of experience) stating that to minimize ozone and maximize ionization, 4000VDC is the desired. The output is clipped to an Amstat Industries eight-inch carbon fiber brush, of a sort designed to take static charges off or out of things; kind of exactly the reverse of this application, but it seemed a good place to start, and Amstat was great to work with, unlike the other companies I contacted for this. The white block is craft foam I bought from Michael's, which tests as having a very high electrical resistance (for safety).
The most important challenge unmet so far, is measuring results of the above and similar experiments. There are very cheap devices being called "ionization meters" out there, less than $50, but their literature is clear, these are neither accurate nor precise. There is a large jump between those and industrial meters measuring ionization of air; the professional ones with the needed capacity, seem to start at about $800 and go up quite a lot, and I cannot justify such an expenditure in the known future! However, researching a lot and asking people in the field, I learned that these air chargers are doing what they do as a result of something called "corona discharge", which exists as a result of an artificial static electric field. So I bought an Electro-Static Discharge Meter, which is much less expensive, and I am using it; it does confirm corona discharge, so I'm not completely barking up a wrong tree.
And for a while the above is where it sat. I turned over and over the problem of mounting an EMCO F40 or other related options into or onto the air cleaner, tried to find appropriate parts to make that happen, and did not. Clearly a much more prepared fabricator than I, would be needed. In parallel with this I kept on searching for more powerful air chargers similar to the ones we have been using, and for quite a while, no go at all. And then I found this:
The description lacks most details, e.g., output power. But following the rabbit-trail just a bit, the above seems to be this:
And the details on the Amazon page are very interesting. The originals reportedly put out 30M/cm3 negative ions at standard testing distance; this thing, supposedly, puts out 1.9T (trillion!) negative ions per cubic centimeter at the same test distance.
That does seem high, but, well, why not. I can always shut the things off. So I bought two of them, mounted them outside and the 8" carbon-fiber brush inside (pix and more details on the site soon), and have been testing for a few months now. When we calculate, we find that the twelve originals are theoretically putting out 360M/cm3 at test distance, which means that the new ones should be producing (1900 * 2)/360 = 10.5 times as much as the first twelve combined. I am definitely not seeing results of this magnitude that I can identify, though certainly I noticed some. In general I am taking this to mean that either my 8" carbon-fiber brush isn't emitting as we'd prefer, or there is some sort of saturation point reached. This may be due to the fact that we're still delivering the ions pre-filter; a post-filter delivery would certainly be interesting, but I'd want a lot of assurances towards preventing anything evil being sucked into the engine, and that gets sticky. By many reports, the negative ions leave the device by way of sharp tips and edges of the business ends. So, perhaps something like a very rough-textured graphite plate replacing a 1x1" square of intake hose...?