A rather interesting article.
SMA expect to sell about 300 4-cylinder engines within the next two years. That is probably mostly the Cessna 182 JT-A. They think the technology can scale up to 450hp but not go beyond 500hp. The 6-cylinder they're currently working on (ca 400hp) is supposed to have a better power/weight ratio. The currently SMA engine looks very sturdy and this is probably the reason it started with an impressive TBO so it will be interesting to see how they improve weight based on experience. A safer approach than Thielert who started with a very ambitious low weight design only to realize it was not solid enough (but who have come a long way and now show good reliability).
Also very interesting to read that the 182 JT-A's SMA engine is 10 pounds heavier than the TIO-540 it replaces. That is very good.
They also state that when cutting off fuel, the prop will always continue to spin, regardless of altitude and the engine will immediately start firing once fuel is injected. I find that remarkable because the avgas engines don't do that at all altitudes and it should be even harder for diesels due to the much higher compression (i.e. resistance).
I personally believe there is going to be a major shift towards piston diesel engines, not just for the low end GA but in commercial operations. Turbine engines are very inefficient compared to piston engines and as fuel consumption will get more and more important, R&D will eventually return to diesels. Before WW II we had highly efficient 900hp aircraft diesels, a technology more advanced than what we have today (SFC almost half that of a PT6). Development went up to 2400hp but was stopped when turbines came to market. Back then fuel consumption was irrelevant and it continued to be until a few years ago.
Diesels are interesting and likely to be more prevalent in the next few years in the GA fleet. But won't replace turbines. SFC is not that significant - turbines have other advantages including reliability that a piston diesel engine won't have.
SFC is not that significant - turbines have other advantages including reliability that a piston diesel engine won't have.
That's the changing mindset I'm referring to. In the future you won't be able to say "SFC is bad but..." because SFC will be the one factor that trumps everything. Reliability is not an inherent technical limitation. The 2-stroke valve-less opposed piston engines are far simpler than today's piston engines.
Turbine SFC gets better the larger they are and the the higher one flies. However, the Jumos were flown at up to 15 000 meters (yes, that is FL500) and their SFC was still considerably better than today's best turbofans.
Maybe in the next 20-30 years we will see piston engines replace turbines in the 500-5000hp category.
Maybe in the next 20-30 years we will see piston engines replace turbines in the 500-5000hp category.
I highly doubt it.
There are as yet no diesels to fit planes in the 600 kg max gross category. Nor can it be easy to create one. Also, the engines that are now used there (predominantly Rotax) can run on Mogas, which is bound to remain available for a few decades. Long enough for me, I think.
There are as yet no diesels to fit planes in the 600 kg max gross category.
And there most likely won't be. Diesels in that category tend to have a poor power/weight ratio. The larger the engine, the more interesting the technology. Development stopped at 2500hp around 1942 but it might soon become a focus again. 40%-50% lower fuel consumption is enough motivation to overcome all technical issues.
So you think they will become common in transport category aircraft?
Yes.
The question is how long it will take and up to which power output. Reliability is not an inherent limitation. The market up to 1500hp is very small so the goals will have to be more ambitious to fund major R&D. The Jumos were in airline service in the 1930s and considered to be reliable (TBO 1000h). This was very early technology.
Ships have the best SFC of all fossil fuel driven vehicles and they use big 2-stroke diesels with virtually no unscheduled maintenance. Obviously weight is not such a big issue there but the same principles apply to aviation.
Passengers will never accept diesels. They struggle with Turboprops.
There is reportedly a lot of research going on on turbofans whose fan is open i.e. like a turboprop but with many blades.
There are problems with noise and with the thing looking really scary, apparently...
But the SFC is much better than current turbofan jets.
