Peter wrote:
Isn’t hydrogen via electrolysis so energy inefficient that it makes sense only on Mars where you have just solar panels and that’s it?
According to Wikipedia the best currently available methods achieve around 80% efficiency.
I am not doubting the achievable thermodynamic efficiency of the H/O separation process, but it is still a very expensive way to go about making hydrogen. Electricity is expensive, and dirty especially in countries which for domestic reasons avoid nuclear, or ones which don’t care. Then you have to store the stuff, which in an aircraft would probably need to be in liquid form and that adds yet more cost.
I am fairly positive about 100UL arriving when actually needed. It won’t happen before. IMHO the US will have to do it first, on a large scale.
There is always the fear of Brussels suddenly outlawing 100LL. This is constantly dragged out. I think it is very unlikely, due to the widespread and very visible economic damage) but one can never be 100% sure. They outlawed lead in solder despite it causing huge technological issues and despite there being no evidence that lead gets out of landfill-dumped electronics (same can be said for the other ~200 substances which have been banned from electronics). The lead-free issues (whiskers) were eventually solved by a different direction in high density packaging (BGA) but still the military refuses to touch LF solder. However, the lead ban in electronics was not a “publicly visible” thing, so it was politically doable because all the “victims” were “nasty corporations and mostly American ones” with Swatch being one loudly protesting exception but they were not EU based 
I read a couple of airports in California are banning 100LL but will offer UL94.
Does anyone know anything about UL94?
How is it different to UL91?
It’s just political skullduggery to shut down an airport that the city has wanted to close for decades (and sell off to property developers, of course). Sad, but not entirely unpredictable.
Peter wrote:
Then you have to store the stuff, which in an aircraft would probably need to be in liquid form and that adds yet more cost.
The problem with storing hydrogen is that it’s tragically un-dense.
You may hear that per kg hydrogen is very energy dense. This is true, but hydrogen is also the smallest, lightest atom – and a kg of the stuff requires a lot of volume even when compressed. To get any kind of density you have to put the hydrogen under immense pressures. However hard you compress it, hydrogen can’t exist as a liquid above 33K (-240 C) – it’s not like propane which can be compressed enough to become a liquid.
As an example, hydrogen at around 70 megapascals (700 bar, or about 10,000 psi) only gets you 38kg per 1000L. Although hydrogen per kg is 3 times as energy dense as petrol, 1000L of the stuff at 70Mpa is only the equivalent of 114kg of avgas (about 150 litres) so by volume even at this immense pressure, hydrogen is only 15% as dense as avgas per litre, and a fuel tank that can withstand 10,000 psi and fit in in an aircraft isn’t going to be all that light. And of course EASA would inisist that such a pressure vessel be lifed so you’d have the expense of frequent replacement (especially, given the small size of the H2 molecule, it also tends to leak out of things and embrittle them in the process). Not to mention other problems of storing gasses at those sorts of pressures. I respect the pressure stored in our small portable compressor at the hangar, and it’s only 120 psi!
100ll harmful to Californian kids..
Discussed here, [ posts moved ] its being implemented by airport management as a defense against ongoing local politics that have been trying to shut down the affected airports for a long time. The tactic may work for a while as a defense, but the manipulative ‘environmental’ angle is typical Bay Area stuff, and why the Area has lost its appeal for many people in the US. That, and maybe the feces in city streets that forms a well known part of the local Bay Area environment 
The pressure tanks in Toyota’s hydrogen tanks still store more energy than an equivalent electric battery. It’s also much lighter. For aircraft I think cooling it down is the envisioned way of storage rather than pressure vessels. More or less the same technology is used for storing huge amounts of natural gas, although at higher temperatures than is required for liquid hydrogen. It’s a bit hard to imagine how this is going to work in practice though. All stranded aircraft will “leak” hydrogen that can ignite at any time. Certainly no solution for GA.
In a practical sense biofuel is the only viable alternative for GA in the long run IMO. Ethanol can be used for all aircraft in existence today with only minor modifications. Except those with diesel engines of course
This is an avgas thread – 100LL or 100UL etc. Other topics belong elsewhere e.g. here
Does anyone know anything about UL94?
How is it different to UL91?
It is probably one of the specially blended fuels available in specific markets – Sweden? I thought they have 96UL or some such.
If you take the lead (TEL) out of 100LL you end up with 91UL. So “94UL” or “UL94” must be that but with some octane enhancer added.
I found something on the US Socata forum: An STC is required and available from Swift Fuel for $100 to use UL94.
The problem is that the US engine makers are not going to certify a whole range of engines for a fuel used in a small market in some corner of Europe. Currently, most non-turbo Lyco engines are certified for 91UL; the exceptions are IIRC some higher compression engines. One US engine shop advises me that anything above 9:1 compression will not run on 91UL. Turbo engines are not certified for 91UL (any exceptions?).
What is really needed is a “100UL” fuel of some sort so GA can continue. The engines which need 100 octane are also biggest avgas users.
So does that mean anything with an appearance crane rating of less than 9:1 will run on UL91?
I don’t think anything with such a high octane rating has been “approved” to run on UL91 in EASA land.
I’d be very surprised that turbo/high compression are the biggest uses of avgas in the UK. Certainly at the 3 airfelds I frequent by far the biggest uses of Avgas are the flying schools (90% of sales) and all their fleet can run on UL91. And if it was one pence a litre cheaper than Avgas that’s what they would use.
