Thanks for the link, jxk, but it is not that interesting to me. Too much bla-bla, little factual information. I didn’t even see basic specs, or were they buried too deep..? And they seem only to be where Delta Hawk was five years ago, and Zoche ten years ago, so let us not hold our breath yet.
Interesting though to see the comparison graphs showing all the diesel engines very close, this seems to indicate they have realised everything that can be.
“Whereas if there was a direct IO540 replacement, with the same overall dimensions so it could use the existing mounting frame and cowling, and the price was at/below a new Lyco engine (c. $70k) they would sell loads. I would buy one. […] Unfortunately I just don’t see any sign that anybody is going to do a reasonably priced drop-in-replacement diesel. And Lyco are not likely to compete with themselves by doing it – even if it is only outside the USA where they would be competing with themselves.”
@Peter: I agree with you. Even more: the first producer of a Diesel engine making a reasonable replacement for all the SR22, Columbias, TTX etc. possible must be in a position to really make a lot of money. For me, this is only a question of a short time. Diamond is trying with the bigger cabins already (DA50 / 52), and the one important piece missing in that story is the correct Diesel engine with the AE being too small actually for launching a competitive product. Let’s wait and see what the will be doing …
An interesting research paper on a proposal for a new two-stroke diesel with rotary inlet valves can be found here. A bit left field perhaps but interesting nontheless.
www.sciencedirect.com/science/article/pii/S1876610214000800
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Hit the pdf on the page to download the full fesearch paper.
Really interesting. Nice to read all the engineering choices made to reach/justify the specific design. At this stage it is purely a theoretical engine inside of GT Power with some CFD on the side, but hopefully they will build a prototype. Regarding weight, I am not that optimistic. The max power is 150 kW or 200 HP. The assumed weight is about 95 kg, and this only includes the core engine, not the turbo, inter-cooler, water cooling or fuel pumps or electronics or not even the reduction drive. All this will add at least 50 kg, so the engine will weigh at least 150 kg, or 330 lb. This is almost the exact same weight as a 200 HP Lycoming 360. A 200 HP ULPower 520iS weighs 108 kg ready to fly.
The engine is also considerably more complex than a 360 or the ULPower (turbo, inter-cooler, common rail, reduction drive), so it will cost much more.
All this will add at least 50 kg, so the engine will weigh at least 150 kg, or 330 lb. This is almost the exact same weight as a 200 HP Lycoming 360. A 200 HP ULPower 520iS weighs 108 kg ready to fly.
Replacing a simple, four cylinder air cooled GA engine with a complicated water cooled turbo-diesel makes absolutely no sense to me unless you have no choice but to do it, because you can’t buy suitable gasoline. The smell and sticky diesel residue would also be a significant factor for me, but many people wouldn’t be as bothered as somebody who works on their own stuff. Anyway, I’d agree that UL Power’s approach makes more sense, however the UL Power weight advantage is not exactly what is seems: as I understand it, the six cylinder UL Power engine makes 165 HP at 2600 rpm, where people are actually running them for takeoff with a fixed pitch prop. A parallel valve four cylinder O-360 Lycoming (not the heavier angle valve 200 HP version) makes about the same power in fixed pitch climb and weights 117 Kg versus 108 kg if I have my numbers straight Link
A hydraulic constant speed prop for the UL Power engine would make it more interesting in terms of takeoff power, because you could rev it over 3000 rpm… although with some extra propeller noise, inefficiency and weight. The electric constant speed prop would be a turn-off to me: my own Lycoming O-320 turns an electric CS prop at 2700 rpm, with a pretty good take-off power to weight ratio, but given the choice I’d trade it in a heartbeat for a Lycoming with a hydraulic prop driven through a hollow crankshaft.
Anyway, I’d agree that UL Power’s approach makes more sense, however the UL Power weight advantage is not exactly what is seems: as I understand it, the six cylinder UL Power engine makes 165 HP at 2600 rpm, where people are actually running them for takeoff with a fixed pitch prop. A parallel valve four cylinder O-360 Lycoming (not the heavier angle valve 200 HP version) makes about the same power in fixed pitch climb and weights 117 Kg versus 108 kg if I have my numbers straight
Not entirely straight 
The specs of those engines.
Lycoming O360A (according to Lycoming):
Bore: 130 mm Stroke: 111 mm Bore/Stroke: 1.17 Displacement: 5911cc
Weight 285-301 lb = 129-136 kg Max Power: 180 HP at 2700 RPM 135 HP at 2450 rpm
ULPower 520iS (according to ULPower)
Bore 105.6 mm Stroke: 100 mm Bore/Stroke: 1.06 Displacement: 5254cc
Weight: 108 kg Max Power: 200 HP at 3300 rpm 180 HP at 2800 RPM 155 HP at 2450 rpm
The UL 520iS is 25% lighter, It is a longer stroked engine (lower bore/stroke ratio), it has 15% more power at typical cruise rpm (2450 rpm), higher max power, FADEC
Even the lower compression UL 520i has 150 HP at 2450 rpm. ULPower started out making engines for helicopters. They started with the 260i, which basically is a much improved Jabiru 2200 with larger bore, it has the same stroke. The 260 has bore of 105.6 mm and stroke of only 74 mm. Then they just increased the stroke to 100 mm and the UL 350i was created. The UL 350 is similar to Lycoming 235 or Continental 240/200 regarding displacement and bore/stroke, only much lighter, FADEC, more HP. The next step was to add two extra cylinders and the 390 and 520 was created.
In my opinion ULPower is doing what Lycoming and Continental should have done ages ago. They are using top modern production methods and material technologies to produce a series of engines in small batches that are simple variations of one single basic design. The FADEC is identical for all of them. These engines surpasses Lycoming/Continental in every respect, and they are comparable in price. A 350iS is actually cheaper than the uncertified Lycoming 233. Who knows if ULPower ever will be certified engines, right now they cannot produce enough 350’s to supply the demand on the American experimental market. If the 520i will start to get a foothold in RVs is i bit early to say, it depends on the price of the 520i compared to Lyc 360 and clones.
The UL Power data indicates a 23% decrease in power with a 26% decrease in rpm, while the O-360 data indicates a 25% decrease in power with a 10% decrease in RPM… Those Lycomings are very highly tuned peaky engines, eh? 
It might be nice to see the UL Power mid range power data from somebody other than themselves, and I’m sure that will happen in due course…
The stuff I wrote above comes from a propeller manufacturer who is supplying ground adjustable props for the UL engines, and the feedback he’s getting from his customers. He’d also be supplying his CS props to the UL guys if they could fit a hydraulic CS prop to the engine.
Although I think four cylinders are better than six whenever practical, when it comes to crankcases I think its definitely true that you can lose a little weight using the better casting alloys now available, or CNC machining from solid. I also think the all-aluminum Lycoming cylinders posted here before are quite interesting – screwed together bimetallic cylinders were originated by Wright in the 20s and assuming no cooling fin or strength issues they aren’t necessarily needed any more – the bore no longer needs to be steel to provide a reliable wearing surface.
P.S. The question of power versus rpm for the 180 HP Lycoming O-360 got me curious, so when I had time I checked the Operator’s Manual. At 2450 rpm, or anything close, peak power is not much reduced from that at peak rpm, as I’d expected from an engine in a relatively mild state of tune.
Call it 168 HP at 2450 rpm depending on available full throttle manifold pressure, and I think you’ll be pretty close: Link
Roughly a 7% reduction in power from a 10% reduction in RPM. The 135 HP number at 2450 rpm number is apparently the 75% throttled power at 2450 rpm and 24.5 inches manifold pressure, not the full throttle power which is much higher. When UL Power issues a document with a similar amount of data, it will be useful for propeller manufacturers
Looks like 168 HP at 2450 RPM is the correct number for full throttle. It is the bigger displacement coming into play. Lycoming engines are very good designs, just too bad they have stopped development the last 40-50 years.
Nothing for the big guys:
The April issue of the LAA UK Magazine brought an article about a new diesel being developed by a British company Weslake.
The opposed piston two stroke diesel engine shall compete with the Rotax 912.
The marine version of the engine exists, the prototype of aero version of this engine is being manufactured now.
The announced performance is 80BHP and 220NM at 2500RPM.
As per a look at their web site www.weslake.eu they are not too good on marketing.
However, Google says Weslake has a long history with engines.
Looks like Weslake has a reason to believe that (their) diesels will take off, at least in the light aviation.
Well, that would make an easier comparison: a complex geared Rotax vs. a complex geared diesel.As others have said: weight will be the difficulty.
