So, what I gather from Paul's article is that the technology isn't bad in cars, or not adaptable, but just not economically viable.
I'd agree. Because the hoops to jump through would be HUGE to certify such an engine, even though the "certified" ones have any number of HUGE problems.
Look at the AD lists for some of these. Look at the SDR's, and all the parts that are sold to repair these engines. Swapping "jugs" on piston aircraft is almost sport on some models. Dropping valves, breaking crankshafts, grinding of cam lobes, oil pumps that sheer off and quit, cracked crankcases, mechanical magnetos failing in any number of ways, cracked exhausts burning the wing off (admittedly rarely part of the engine package, but instead the airframe mfg "engineers" these). And I'm just touching the surface of all the "features" of these FAA certified masterpieces.
I agree that the decades of advancements in the piston engine world (not just cars, for sure) would be difficult to economically bring to the piston aircraft world.
Not technical... economic. And the single largest cost might just be the certification effort that motorcycles, boats, cars, trucks, lawn mowers, farm equipment, etc, don't have to engage.
And please have Paul answer this question; if the conversion of automotive engines "can't" be done for whatever reason, why is it being successfully done now with Mercedes Benz diesel engines?