We have had various threads on why car engines can’t be used in planes. It is a persistent question in GA. Car engines seem to be incredibly reliable and just go on for ever. So why not in planes?
Expanding on my points above about low power density and low practicality, it’s worth looking at why such engines cannot be overhauled, in general, and must be replaced. One reason is that despite being heavy due to extra parts like overhead cams etc (which have little redeeming value for any aircraft engine), structural stresses are still high and fatigue life becomes a consideration. The automotive derived diesel (that after development has half the parts custom made) may seem like such a good idea viewed from afar but it becomes less so when you understand for instance that the aluminum cylinder head could not be overhauled, maybe even less so than a turbo-Lycoming. It’s intrinsically a throw away, as is the rest of the engine, and you pay $75-100K or whatever for a brand new and very complex 150-200 HP engine every so often, from a sole source supplier, whether you like it or not.
Meanwhile how many of the planes doing the roughly 1500 Lycoming and Continental powered operations within 50 miles of my house today will ever have a factory new engine, other than the one they may have come with? At least for now, and for years to come, not many of them when the engines can be field overhauled three times or so, and they don’t have to go back to factory to have it done. There are about six overhaul shops within 150 miles of me, not counting A&Ps doing their own engine overhauls. Lycoming may have screwed themselves building relatively light engines that can be run for 6000 hours with only wearing parts being replaced, but their attention to practicality benefits the operator in the field a lot. They do sell some of the replacement parts in competition with PMA suppliers, and some very expensive factory overhauls for those who are in a particular hurry, so they soldier along and stay in business instead of taking so much of the frugal operator’s money.
Who in that circumstance would buy a brand new factory engine, or an STC’d auto engine, or a new diesel powered plane or a plane needing two new Rotaxes every 2000 hrs just to make 200 HP combined? Just a few private individuals, mainly those who are forced into it by local gasoline (tax) prices, fuel unavailability or by living in an area where there is very little use of practical light aircraft requiring circa 200 HP, and so limited access to GA infrastructure that could overhaul what they have.
Aircraft engines are quasi-industrial devices, and those that pile on the hours are owned by hard nosed people with their eye on the life cycle cost bottom line and very little taste mental meanderings or image over substance. The planes that pile on the hours need to be tough and long lasting, and that requires enough power and maintenance at the airport. This is not really Porsche territory.
For the OP, here are photos comparing the six-cylinder, overhead cam, fan cooled, geared 212 HP PFM Mooney installation, requiring valve adjustments every 25 hours plus a longer fuselage to balance the extra mass, and the lighter four cylinder, direct drive 200 HP Lycoming that reliably makes 2000 hrs TBO and doesn’t require valve adjustments nor much of anything else except oil changes. Approximately the same power on both.
Some interesting info on the PFM at the link. There were valve spring issues and reduction gear issues. Porsche (VW) apparently paid the owners $90K or $120K when they stopped supplying parts and abandoned the TC to FAA, for which the owners agreed to leave Porsche out of any future problems resulting from continued use (there is no connection between manufacturer ‘support’ and FAA certification: in the absence of an unresolved AD or inability to source approved parts new or used, once FAA certified always FAA certified).
I understand the fuselage stretch later came in handy when Mooney installed much more powerful engines in the aircraft.
RV14 wrote:
did those aircraft loose airworthiness certificate like the Concorde?
No, but when the engine needs repair/overhaul/replacement and there are no parts, then what do you do?
In a suitable jurisdiction, you can manufacture any part.
On a certified aircraft this is obviously harder. In FAA-land you produce a DER 8110 design package (these start at $10k, from jobs I have seen). In EASA-land I guess it would be a Part 21 design company (similar cost).
In FAA-land you have the owner produced parts concession but the value of that depends on the part you want to make. I looked into something like this years ago, for a landing gear part IIRC. If you can establish the exact alloy used, you can copy the dimensions. Otherwise you need to do a lot more work i.e. establish the original design criteria, and you obviously won’t get manufacturer support for that.
I reckon for the money Porsche offered you could just walk away from the plane.
when the engine needs repair/overhaul/replacement and there are no parts, then what do you do?
“certified” aircraft engines are being field overhauled here and there without much of a fanfare and any trace, i.e. certified VW clones come to mind
The simplest explanation is usually the correct one. Looking at the sales trend of certified general aviation, Wikipedia
And the trend for later years:
The number of aircraft sold (in/from the US) went from about 17k (SEPs mostly) each year ca 1980 to about 2k ca 1985 (about equally divided by SEP and jet/TP), and has stayed there ever since. The other aspect is average unit billing going from a few 100k to 6 mill. Thus, jets and TPs selling at min 5 mill a piece is more than 90% of the market. Less than 1000 (about 700) certified piston aircraft are sold each year.
Where Porsche envisioned a trend based on the last 3 decades and started developing the engine, the reality showed something very different when they started selling it: a total collapse of the market.
Even if Porsche got, lets say an optimistic 20% of the market, this would only amount to about 100-150 units sold each year. For Porsche to continue pouring resources into a product for which the market has collapsed, it would never happen no matter what. We can talk about the engine in detail, but it’s irrelevant. Porsche would have pulled the plug regardless. There no longer existed a market for a tiny 200 hp piston engine. The market (in terms of $) became almost exclusively jets and TPs. And it happened over night in an engine development perspective.
The other thing that happened was the “UL movement”. It’s no coincidence that the “UL movement” started in the late 70s, early 80s. It was almost exclusively a European thing from the start. Since 1989 when the first Rotax 912 series was sold, they have sold more than 50,000 units by now. In comparison the total of certified engines cannot be more than 15-20,000 and includes everything from Lycoming to Austro and everything in between. To put a perspective on this, the number of new certified engines sold the last 30 years is about the same that was sold in one single year in the late 70s. I can’t find any numbers of 912s sold each year, but common sense say it must an upward trend from zero in 1987. Perhaps 20-30k are sold in the last 10 years alone. Still a way to go to match the late 70s, which probably never will be matched, but nevertheless comparable numbers, and it’s all mostly in Europe.
The things about the Rotax 912 engines is that 1/ new they are a lot cheaper than Lycosaurs. You can buy 2 and still have a lot of change from one 200hp Lycoming. AIUI that’s not even a new one.
2/ Whilst in certified world you AIUI you can only overhaul a Rotax engine once it is still a TBO of 2000hrs.
3/ There is a ready market in the European ULM and homebuilt field for 2nd hand Rotax engines even having reached full potential ie 4000hrs including one overhaul.
4/ They are still readily available and clubs have very short downtime.
5/ In France at least Rotax have a good maintenance network.
6/ For the ULM and homebuilt market as well as for those who take advantage of pilot maintenance rules, Rotax run courses around France in maintenance and overhaul for anyone who is interested.
7/ Parts are readily available and can be obtained by mail order in a couple of days.
8 / Peripheral parts are also available and some can be picked up at your local motorcycle dealership. (France has a large and active motorcycle community of all types.) One could perhaps see ULM as an extension of the motorcycle community just as much as a lead to Annexe 1 or annexe 2 ownership. Like motorcycles Rotax started with 2 strokes.
9/ Rotax were quicker at recognising future fuel problems in Europe and Africa, certainly quicker than Lycoming. So nearly all 912 Rotax engines and those that came before are built to run on Mogas UL95 or UL98 which you can buy at the local supermarket. This in turn cut the number of oil changes needed from those running on Avgas. I don’t know the cost of Aviation engine oils in the USA but here in France it is blooming expensive.
So
10/ Rotax engines use motorcycle oil and not.having the word aviation in front of it makes it a lot cheaper. At least that’s the case here.
11/ Rotax engines are quieter and so there are less complaints from the neighbours.
To get a Lycoming engine Robin down to the same dB level as it’s Rotax counterpart needs an expensive exhaust system.
Please note I write this from a French club point of view as at the moment the question of their fleets and how to keep costs down both for club budgets and price per hour is something which most club presidents and treasurers are constantly looking at. Aircraft with Rotax engines are very much to the fore of what they are looking at.🙂
Commenting only on that which I can do so usefully 
The things about the Rotax 912 engines is that 1/ new they are a lot cheaper than Lycosaurs
I bought my 150 HP Lycoming O-320 for $35K, and it came with a plane attached. The point there is that relatively few people buy new ones, because they don’t have to do so, and that then leads to the new ones being expensive. Mine BTW has still never been apart after 53 years, although this is not typical, so when it is eventually overhauled it will be a first run engine. I suspect this situation will remain the same for quite a while to come, but eventually some kind new-manufacture certified clone Lycoming will come along and fill the eventual demand for new engines when all the cores are past being rebuilt someday.
Rotax engines use motorcycle oil and not.having the word aviation in front of it makes it a lot cheaper. At least that’s the case here
This is the oil I use in my O-320, $59 for a gallon and a half Link
This is the 10W-60 synthetic oil spec’d for use in my latest motorcycle, and which is typical for newer European motorcycles (the old ones use cheap oil), $56 a gallon link so about 40% more expensive per gallon than the oil for the plane.
There is just one outfit (of many) near me that is currently doing training with Rotax powered Slings, but they are doing a lot of hours after several tried and failed over the last 15 years with similar but different aircraft types. I think that’s great, and apparently they’ve found an operating model and aircraft type that works for their customer and program. The rest use mostly C172s, because that works better for their customers and program.
As I mentioned above there are about 1500 total GA operations per day within 50 miles of where I live so it’s not exactly a dead market. That is a perception from the hinterlands 
That may all be true @Silvaire but I was writing about what is happening here and why.
Our clubs need to keep prices down for our certified aircraft or we are not going to attract new members and keep our existing ones.
So club presidents and treasurers are investigating all manner of ways to solve the declining hours flown.
You cannot compare personal ownership costs in the USA with aeroclub costs in Europe. They are so different. Fuel being a major factor plus the availability of parts because there is nothing worse than downtime to de-motivate club pilots. It is up there with the weather.
Returning to the Porsche engine, it was an experiment for Robin. The advantage being European and Porsche a well known brand.
You need to remember that back after the end of the war Renault engines became de rigueur. Limbach (derivative of VW engines) were also tried. So Porsche was just another try. Always looking for a way forward from the old Lycosaurs which owned the market but both Lycoming and Continental became complacent. Whilst the Porsche engine didn’t work out whereas reports from clubs with the Rotax conversion to their wooden Robins are promising large reductions in costs and reduced downtime. On top of this in Frnce Elixir and Issoire are building interesting (to club treasurers) annexe 2 aircraft with Rotax engines. Outside of France there are many other Rotax powered certified aircraft on the market already or due to get CS23 certification soon.
fwiw, and completely off topic, the Rotax DR400 is completely anemic. I have quite a few hours in the very first one (F-GORZ). As a training plane it’s fine, though you have to careful with coolant temps. Its climb performance is awful.
