The whole diesel aero engine saga is a sorry tale of botched design, derailed commercial progress and a simple lack of economic payback for most solutions.
For the last 50 years or so, the GA industry has supported two main aero engine manufacturers delivering relatively simple products. How is it going to support a bunch of small purveyors of complex diesel engines? The honest truth is, it won't, because the (shrinking) industry isn't big enough to support even one new player. If Europe, China and India open up their skies to more GA and private operations, it may change the economics, but don't hold your breath. Put simply, diesel aero engine economics don't add up because of volume - that's all.
In my opinion, there has only been one technically promising aero diesel engine so far, which is the Wilksch Airmotive WAM-120. Unfortunately for reasons far from explainable, the commercial direction that WAM seems to have taken has pretty much destroyed the potential of the market adopting this engine or more powerful derivatives of it. It is, in my opinion, one of the best near-term, low-risk design concepts out there. The designer Mark Wilksch has
published a paper demonstrating the theory that a DI version of the same engine could be pushed to 150hp and offer a substantial fuel consumption benefit to boot. All this for an engine that is weight-competitive to an O-235! That would be impressive!