It seems that there is still come confusion of just why diesel cycle (Compression Ignition = CI) is infinitely superior to aspirated charge (Spark Ignition = SI) engines for aviation, marine or heck ANY use.
The number one issue is that SI engines (with a few rare exceptions) must aspirate their charge to have it dispersed within the combustion chamber when it is ignited by a spark (or, in far, far better designs, plasma from an igniter such as the aspirated charge compression ignition engines built by Mark Cherry at Smartplugs). ANY ICE is limited only by the amount of air and fuel you can stuff into the cylinder. Problem is, when you must aspirate a really bad fuel such as gasoline, it reaches a limit from the temperature and pressure while on compression stroke when instead of burning controlably from the point of spark igniting, the whole thing explodes at once (detonation). You can solve SOME of that problem by aspirating far higher octane fuels (such as ethanol, methanol, etc.) but even at the very extreme (fuel dragsters that make near 10,000 HP from less than 10 litres) spark plugs only get the engine working under very light loads. In that installation, they are totally destroyed the instant power goes up and the engine is once more a compression ignition (but in this case aspirated charge) diesel. Look at what happens at altitude with these engines. You need more air to burn the avgas, so you hang on forced induction (super or turbochargers). Problem is, you can now burn some more fuel to maintain power, but to do so, you flirt constantly with detonation, while the air availble to cool the engine is getting much less dense, so now you need to do something to limit detonation in the cylinder. The normal solution (look at Aerostar Lycs and P210 Conti) is to dump a bunch more gasoline into the aspirating airstream to cool not only the combustion chamber, but the whole bloody engine. Then comes the spark thing: as you get into lighter air, you get into poorer insulating air, so the magnetos (talking ox cart technology here - i.e.what most of us actually fly) start arcing all over the place so need to be pressurized or eliminated.
Now, Dr. Diesel's CI engines (in long past referred to as "constant pressure" engines, due to how you CAN manage the BMEP curve) simply don't have any of these limits. The gasses coming into the cylinder contain no fuel, so the only thing that limits how much air you can jam in is your ability to deal with the heat and of course keep the whole thing together. Ignition happens when you inject fuel, and if you have enough strength and cooling, there is no limit until you literally fill the combustion chamber to hydro-locking levels (as IS the case with top fuel dragsters). So: in what technically sane world would you waste time building engines around a bad fuel and combustion concept when a much better one is right under your nose?
The first successful aviation diesels were, IIRC, Packard radials, that set a mess of records in the '20s. The problem was that the materials and technologies of the day meant that injection pressures were too low to get really good atomisation, and lack of understanding of how dissolved air was knocked out of solution while supplying to injectors meant that timing was imprecise and not predictable, and the injected blobs of fuel might burn controllably on injection, or splat into the piston or cylinder only to explode with a big bang at the wrong time (the clatter and knock of old diesels that thus required HUGE amounts of cast iron to tolerate the shock wave and pressure spikes that could literally blow big chunks of cast iron off of the outside of the cylinder (cavitation).
Even with all of those old-tech limits, Hugo Junkers (no slouch at this aviation engineering thing) built some incredibly successful diesel engines that used the 19 07 tech of opposed piston, uniflow 2 cycle to make some really nice engines. Now, you would ask where was the US in dealing with such nice tech? Well, if you take a ride in a submarine where performance, reliability, fire safety, energy density, smoothness, quiet, etc. are SUPER important (kind of like in airplanes) it will likely have an opposed piston diesel either as prime power or backup power - built from licenses by Fairbanks-Morse from Junkers patents and licenses.
Today, we have a couple of huge advances that make aviation diesels inevitable. First, we have materials science that allows injector nozzles to live at 25,000 psi and up without eroding in a flash. THAT allows heavy fuel atomisation that dramatically improves cold starting and combustion efficiency. Second: we understand far better how to control timing, to the extent we can "rate shape" the injection events to first give a little pre-shot (into which the main injections can be fired at the resulting fireball that keeps fuel from reaching a quenching surface such as the piston crown). You will notice that modern automotive diesels are FAR lighter, cleaner and quieter than their predecessors (thanks to VW for starting down the road in 1978, and a pox on VW for screwing up royally a few years ago).
On top of all of that, the diesel cycle of CI engines are extremely tolerant of fuel diversity (something that gasser SI engines really are NOT).
What we need, though, is for someone to work with EXISTING mass market hardware to build small diesels for our relatively small power market. Small and very high performance (i.e. the whole power/weight issue - that automotive diesels do NOT cover well).
BTW: from an earlier post about automotive engines no being in marine applications: take a look at the HUGE lineup of VWAG marine diesels - they are the full lineup of every automobile diesel they produce(d). Ditto, almost the entire world of stern drives are an auto gas or diesel on one side of the transom, and what is finally happening the outboard world sometimes uses auto engines plucked right out of a car (all gassers so far). In that world, there are now a few outboard diesels that are already on the market - i.e. VERY light, high performance diesels with almost identical use profile to aviation.
https://boatdiesel.com/Engines/Volkswagen/Volkswagen.cfm
https://www.mercurymarine.com/en/ca/engines/diesel/mercury-diesel/42l-tier-3/
https://www.mercuryracing.com/wp-content/uploads/2015/05/2557_DieselOB_SS_NC1.pdf
and i loved that engine most of all. (God it was cheap on fuel, no power... But beat the hell out of driving a civic)
