The DA40 is probably more akin in performance to the TB9 or TB10 and the 140hp version of the Robin 400.
I meant DA40 having the same speed as a DA42 
It should have half the fuel flow (for same engine(s)) but it must be much slower because the cockpit has similar dimensions AFAIK.
BTW, here is a great diesel TB20 thread. Some info there on what happened.
Weight and cooling drag are the 2 banes of aero diesels – liquid cooled and turbo engines put out a lot of heat, clean sheet design aircraft should be able to overcome both but the retrofit market is just hard, but hard can be overcome with money, but as Peter pointed out the market is just not big enough.
And even clean sheet designs do not get it right – the DA-42 looks like its pushing 2 garden walls through the sky
One can only wonder. Each Austro engine weigh 100 kg more than a Rotax 915, and has at least 2, if not 3 times the projected frontal surface area. The Austro has 165? hp, the 915 has 140. With 915s the DA42 would be at least 200 kg lighter, certainly much more if the structure would be designed from scratch, let’s say 250 kg. Drag would go down drastically.
According to Wikipedia the empty weight would go down from 1251 to 1000. 25% reduction. Max HP would go down from 168 to 140, 20% reduction. Drag would be much less due to less weight and less frontal area. Less weight, higher power to weight ratio, less drag, cheaper and much more used engine. Sounds like a better aircraft to me , at least when disregarding Jet A1.
And if the 915 engine longevity is a good as the 912 then it will be good for 5000 hours.
The austro diesel either puts out 170Hp or 180 HP depending on the vintage – I know that the aerodynamic design of the continental diesel on the DA-42 was cleaner, but as you said you are looking at 155hp versus 140Hp in the rotax then.
The rotax 915 has been challenging to design for as well – its taking time to perfect the cowls and performance – its a small relatively light package but it puts out a lot of heat.
180 hp? Hmmm that means finally a diesel with the power output of a O360…
There was a 2×180HP DA42 in the early days. It was pretty quick. @pilot_dar will tell you
Re https://www.euroga.org/forums/hangar-talk/1194-will-we-all-be-flying-diesels?page=11#post_198254
Ouch, those are huge/ugly! Would love to see a pair of Rotax 915is there – but that will never happen. :-(
Diesels should in theory be easier to cool (better thermal efficiency = less cooling needed), but those engines are simply too large & heavy.
Wait a minute! Real men would call these large engines masculine!
