IMO they get at least 5x of their 10x by using a diesel engine, vs. a jet (inefficient at a range of powers) or avgas (expensive fuel, old designs, inefficient). Just putting a diesel SMA, Thielert, Deltahawk, etc. engine into a Cessna 172 would give you substantial fuel economy and cost savings vs. 100LL. 100LL is also taxed, while Jet-A (which a diesel can burn) has no excise taxes by international treaty (there are some other taxes, especially in the US). It's maybe 2x fuel economy improvement and 2.5x on per-BTU cost savings.
So, it's actually kind of plausible -- the problem is the diesel aviation engines have been VERY slow in coming to market. The main market so far has been non certified use (military UAVs), where eliminating 100LL from the supply chain has huge savings on top of performance.
You probably have a pretty valid point. One of my aspiration toys, the Diamond Star DA42 (http://en.wikipedia.org/wiki/Diamond_DA42) burns diesel or Jet-A in a internal combustion engines. I forget the economy figures, but I think this plane does quite well when throttled back.
I'm not a big fan of twins for infrequent pilots (just based on observing and talking to pilots, not actual flying myself except in a simulator) -- they seem to bring with them a lot of problems. You obviously have more cost and more potential maintenance downtime from two engines, and when one goes out, the asymmetric thrust and other issues often end up killing pilots who would otherwise have probably been able to deal with no power. There are exceptions like over water use, but I would have preferred a super reliable single engine (Cessna Caravan) to the less-reliable twins (King Airs and Shorts 330s) I was stuck on -- they had to use a twin for regulatory reasons, but even the pilots thought a single would be better. And those are professional (military) pilots; for someone who flies infrequently, the risks of an engine failure are lower, while the proficiency in dealing with engine out is probably lower as well. Single + ballistic parachute might be the best compromise, unless you're routinely operating over water.)
Maybe with FADEC it makes more sense; eventually it will de-skill some of the engine management and you'll just pay the higher operations and maintenance costs of the twin engines, while getting the reliability benefit if ones goes down.
I really want a CH-801 (extreme STOL, rough field, slow), or a Maule.
So, it's actually kind of plausible -- the problem is the diesel aviation engines have been VERY slow in coming to market. The main market so far has been non certified use (military UAVs), where eliminating 100LL from the supply chain has huge savings on top of performance.