I've been following some of this stuff, not because I have any interest in Thielert and Diamond, but because several years ago I looked very very hard at marine and automotive diesels as well as writing, in ancient times, an expert report for one of those "new engine technology" start ups (which floated and failed).
The basic issue with engines is getting rid of heat. The more heavily loaded the engine, the harder that becomes, so the idea that Thielert was going to pull 135HP out of a 1.7 litre automotive diesel engine intrigued me.
I know the Centurion has a billet crank, and various fancy pistons/internals, but that doesn't really change the equation. The engine was designed for automotive use, and even with autobahn time thrown in, it would not be designed to operate at continuous power levels of more then 20 - 30% for any length of time, let alone the 55% plus required by aircraft cruise.
This translates into basic engine dimensions that Thielert can't alter - the sizes of bearing surfaces, the capacity of the cooling system and suchlike. It also translates into individual power pulse shape and magnitude, which is probably at the heart of gearbox issues. It translates into distortion of castings etc. that Theilert has no control over.
In my musings years ago a marine engineer put me right. He pointed out that my marine diesel in my yacht weighs 185KG and produces a measly 13 HP. But that's 13 HP continuous, all day, all night, at 35 degree angles of heel. It never misses a beat, and has been doing this most weekends for 25 years with nothing more than an occasional oil change, injector rebuild and valve clearance adjustment.
Of course I could get an engine from a Subaru WRX, weighs half as much and could squeeze 400HP out of it....but not for long. That's the dilemma.
The "old" direct drive engines look deceptively simple and antiquated, but as I learn more about them I realise they aren't. They appear to be one of the better solutions for extracting motive power out of liquid fuels in an aviation environment.
I wish Thielert well. They are travelling down the same road many others have followed....and from which none to my knowledge have returned - the one that starts with brave statements of "aircraft engines are crap compared to today's automotive engines." Seems they aren't.