The environmental issue is actually a non-issue, as worldwide consumption is so small as to be environmentally irrelevant and it's also destined to get smaller and smaller as GA more or less dies off, or moves to ultralight planes that simply use mogas with no issues. The problem is that as production decreases the production costs really don't and it becomes economically no longer viable, leading sooner or later to cessation of production.

As to the original question, I doubt we'll ever see turbines small enough to replace Lycosaurs and Contisaurs actually certified and fitted. The certification costs alone for such a small market would put off even the most risk-prone manufacturer. The small jet turbines the OP has mentioned that are used in gliders are not certified and can be operated only as sustainer engines, they're not allowed to autonomously take off and most couldn't even pull it off for performance reasons.
It will be a lot cheaper and easier to develop certified piston engines that can simply run on standard mogas, which will still be available for some time to come. They will also have the added advantage that they won't require your average PPL holder to get a turboprop rating to be able to operate them, they'll just be another (more modern) type of piston engine.

Only relevant in EASA-land, not in the US which is after all the biggest market for GA, and by a huge margin too.

It will take a lot of time for sure. Look at Thielert (Centurion) with their diesel engines. It takes ages to get the technology and certification to a certain level Thielert started their production in 2002 and it is still not a real alternative to Lycoming and Continental engines, unfortunately (relative expensive to install the retrofit and the insolvency of Thielert did not help either). Fortunately Cessna is now building the C182 with a diesel (an SMA I believe?).

As well as the Chipmunk G-ATTS I mentioned, I have a vague recollection of a Currie Wot being flown with a turbine engine. Can't remember the reg, but I think it was in a Meccano Magazine c1963

It's NOT the lead that unleaded is primarily here for.
Unleaded fuel has lower overall emissions because it can run through a catalytic converter. Lead kills the cat.

Therefore Unleaded...

I'm not saying lead is a good thing or even desirable, just that the lead is not the primary reason for unleaded.

The pelicans perch has a really interesting story on the future of aviation engines on AvWeb.

Thielert trashed the diesel market with their stupid accounting antics which led to their bankrupcy.

They also had constant reliability issues, which is not to bad for a flying school (I am sure most DA42s were bought by FTOs) doing short trips, but is no good for a private owner doing serious flying, especially to places where there isn't much avgas which is, ahem, probably why one would buy a diesel in the first place :ugh:

Time will tell, but it won't be tomorrow before one can be sure they are proven engines.

And people I know very well who do Diamond maintenance tell me the build quality is still poor, with simple stuff like corrosion of simple metal fittings within a year. Again, OK for a school...

The GA business has only itself to blame for the lack of progress.

I managed to get along; the 5" turbo-electric loco was indeed amazing.

What my discussions with the aforementioned "highly knowledgable people" did confirm was the relative inefficiency of these small turbine powerplants & whilst the idea of using these model motors to create a 4-engined 152 does seem strangely appealing, the endurance would be measured in minutes rather than hours.

Hi, I might be showing my age here, but I remember when Leaded Petrol was replaced by Unleaded.

There was a conversion of engine design done that just upgraded the material of the Inlet and Exhaust Valve seats, and maybe the Valves as well, to allow Unleaded fuel to be used in existing engines. Any good engineering company could do the conversion.

So surely it would be possible for Lycomming to do the same; When your engine reaches TBO the newer parts could be fitted... It should not cost too much either.

Not all Lyco and Conti engines can be upgraded to take mogas, as of now. Some may never be able to, either because of technical issues or because of the limited number still in service. I still see replacing those engines with equivalent engines that can operate on mogas as the easiest option, certainly much less painful than replacing them with a completely different technology (be it turboprop or turbine hybrid or whatever).

I seem to remember the Conti 0-200 in the C150s I learnt on used to operate on 80 octane MON, so why not 95 RON UL, it must be about 87 MON? Also a lot of Gypsy engines designed to run on 80 octane had to have new valve seat inserts for 100LL.
Pity the Mid-West AE110 never caught on; it could use anything from 91 octane (RON)upwards and could have been re-developed as a diesel and used paraffin.

I am told by a respected US engine shop which does a lot of Experimental work is that 9.5:1 and below is OK for lead-free, but above that it's a no-go.

The big issue with turbines is the cost of an unscheduled maintenance. For instance to replace a compressor on a 250 Allison could cost more than a piston engine overhaul.
A complete engine failure could cost you more than you might spend on piston engine overhauls for a very long time. You need deep pockets to run a gas turbine.

That's where a wankel rotary diesel would have the advantage; no turbines and the fuel provides lubrication for the rotor tip seals.

peterh337:
American 'lead free' mogas isn't the same as our 'unleaded' and is only about 86 octane, hence it can't sustain high combustion chamber pressure without detonation. Now if you used a Bruce Crower designed camshaft with a late closing intake valve, this would allow up to as much as 14:1 CR as it doesn't produce excessive combustion chamber pressure.

Hi.... Turbo-Props are available, but maybe costly to buy and run.... What about a turbo-Prop for a Cessna 172/182....

http://www.rolls-royce.com/Images/br...cm92-12978.pdf

Might need to change the A.S.I. as well.

Does anybody know the price of an m250?

I looked for it but could not find it.

Somewhat into 6 figures.

The good news is there is a solution, an unleaded Avgas, I have seen, smelled tasted.....well almost.

It has passed all the required FAA testing just recently, I am a few weeks out of touch, but the component compatibility testing was ongoing last I checked. This is very important, as it must be proven acceptable in all the old fuel systems.

I have read the Dixie Labratories test reports, and this unleaded Avgas is a higher performance (in terms of octane) fuel than the old purple 115/145 and is most likely to be known as a 100/150 or 160 as it regularly tests over 160 mon.

It is 100% compatible with Avgas, in any mix you create. Take 999 gallons of Avgas and mix in 1 gallon of G100UL and you have have 1000 gallons of G100UL conforming fuel. Sure it has a TEL content still but it is a conforming fuel certified to go fly. So tank mixing, either in the ground or wing is not an issue.

Just how long it takes to get the piles of reports finalized and ready to produce I can't tell you, but it should be long before 2018 and I suspect maybe before the nd of 2013.

If the Ameicans could bestowe a knighthood honor on somebody for their services to General Aviation, I know one deserving and worthy receipient.

So don't panic about losing avgas, in fact I can't wait to see it go! This new stuff is way better. Ohh yeah, and it has about 2-3% more BTUs so you will pick up a bit of extra range, for me thats about 7 minutes which when Ifr could be a fuel planning deal breaker some times.

Time will tell, but don't give up just yet.

Someone has been tinkering with the concept.

And another one