I think it is safe to say that none of these engines, turbo’ed ones included (except maybe a very few special variants) are octane-critical, so you can use RON95 with no problem if you keep RPM and mixture in the normal range
That, presumably, means not climbing with it?
Incidentally, is there a water testing kit for avgas? I looked for such a while ago, and there is a thread on it here, but never found anything compact which could be carried. We also had a thread here describing funnel-type filters one could use for avgas and which claimed to separate out water, but they were very large and slowed down the tank fill rate so much that it could be an issue in some scenarios where the bowser is busy.
The vapor lock issue is very real. Numerous Sonex aircraft flying with mogas has issues. This may be exaggerated by the particular fuel system lines, tank and carburettor, but none using avgas has any issues whatsoever.
Flying at 10k or higher, and I can imagine problems also with more “conventional” fuel system in certain cases.
The solution is dead simple and literally foolproof; A return bleed line after the pump, the way most Rotax engine are set up. That is, a pump and a return line is required, at least for conventional carburettor. With FI and high pressure pumps, the situation is probably different depending on how the systems are designed.
The real problem is of course there is no money in GA, yet it is has regulations like the big money making jets. Nothing is changed because no one is willing to pay for changes involving years of certifications, and the authorities are just staying put with their feets in the mud.
I still don’t see the difference between mogas and “car petrol” though. EN228 defines up to 15% oxygenates content. This may be a blend of alcohols and ethers, but up to 3% methanol and 5% ethanol, and more of other alcohols. There is no practical difference between ethanol and methanol as far as I know, only methanol is more toxic.
There is no practical difference between ethanol and methanol as far as I know, only methanol is more toxic.
I beg to differ. Methanol also has a much stronger swelling effect on some types of rubber. Back in the 1990s, when California mandated low-sulphur diesel fuel, which caused gaskets in the fuel system to shrink, lots of diesel cars started seeping fuel around injectors. The remedy was to add a cupful of methanol (which you could buy as automotive fuel, too) per tank.
As LeSving mentions, vapour lock is a risk with non Avgas gasoline. That is the basis of the altitude and temperature “limitations”. Vapour lock risk is a product of the airframe fuel system arrangement. It also is increased or decreased by the seasonal and geographical differences in “Mogas”, which are not present in 100LL. I have vapour locked aircraft during testing, and will 50:50 result in your gliding.
Cars do not suffer vapour lock, since the manufacturers caught on to installing the fuel pump in the fuel tank, so there is never low fuel pressure in front of the pump, and fuel lines with bends near warm areas. Airplanes don’t have the fuel supply lines pressurized, as it would annoy cabin occupants, should they leak.
Cars do not suffer vapour lock, since the manufacturers caught on to installing the fuel pump in the fuel tank, so there is never low fuel pressure in front of the pump, and fuel lines with bends near warm areas.
This is what I could never understand… WHY is the fuel pump so far downstream? It will obviously try to create vacuum in the pipe before it. It seems a stupid design. It does (see below) avoid having to have two (or more) pumps though.
Especially the bit about switching on the fuel pump while changing tanks. The fuel selector is also before the pump, so by switching on the pump you are more likely to create bubbles while changing tanks.
Airplanes don’t have the fuel supply lines pressurized, as it would annoy cabin occupants, should they leak.
I thought it was done this way to avoid having two (or more) pumps. Otherwise it amounts to an admission that the manufacturer doesn’t know how to make a bit of pipe which doesn’t leak. Even in WW2 they figured that out.
Back to the original Q, is there any data on how much “petrol” you can put into avgas before you can no longer fly normally i.e. 20000ft ceiling / -40C or so? Does the petrol dissolve evenly in avgas, and changes the properties of the whole tank just a little, or does it remain in solution and just turns into vapour when you climb above whatever altitude (I believe 14000ft is the figure) and escape, leaving behind the original avgas?
Dissolving properties I wouldn’t know but having driven up the Yungas road in Bolivia (with much less panache than Jeremy Clarkson) and refueled from jerrycans, I can attest that the “summer jungle grade” gas didn’t evaporate up top :-)
With regards to pipes and technology – I guess we know the answer :-) but having a selector downstream of the pumps that activates the selected tank pump shouldn’t be too difficult to design in this day and age. Say like Polak valves on off-road jeeps with multiple tanks.
Does the petrol dissolve evenly in avgas, and changes the properties of the whole tank just a little, or does it remain in solution and just turns into vapour when you climb above whatever altitude (I believe 14000ft is the figure) and escape, leaving behind the original avgas?
From general physical considerations, it dissolves evenly. The properties of the two fluids are very close and neither of the two has a fixed boiling point – each one is actually a mixture of a great many components and boils out in a wide range of temperatures. Actually, it’s possible to mix even more dissimilar fuels: Phil Croucher writes in one of his pilot textbooks that a mixture of 1/3 mogas + 2/3 diesel fuel is a decent emergency substitute for Jet-B (which, unlike Jet-A, is a wide-cut fuel containing petroleum components from fairly light from fairly heavy).
The certification requirements for FAR 23.863 “Flammable fluid fire protection”.would become much more difficult to meet if there were pressurized fuel in the cabin.
If people care about additives, isn’t UL91 the answer?
