In aid of a RAeS conference on light aircraft design that I'm helping put together, I did a phone around about a month ago of all the people advertising aero-diesels.
It appears very much that so far as certification is concerned nobody is very far down the line - the aircraft written about in Flyer and others all seem to be only at the late-prototype stage is concerned. Probably the best new-technology engine, the British Wilsch engine simply doesn't seem to be pursuing certification (although does appear to work well on a few PFA types such as the Europa).
Needs another year or two methinks. But ultimately, they do - at least in the 100-400hp bracket - show every signs of doing a lot to cut running costs down.
G