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Comment by lfuller | Hacker News Reader

G100UL, the current unleaded alternative to 100LL, has been tested to act as a drop-in replacement for any and all aircraft engines that run on 100LL.

If you can run, 100LL, you can run G100UL. The main issue is with mogas (standard car gas) - many aviation engines can't run it.

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I would imagine that the gelling problems ethanol can sometimes cause in automobiles is magnified when used for aircraft due to the thermal dynamics present in aviation (the temperature drop while in-flight) that aren’t a factor for most automobiles. That’s just my intuition, I’m sure there are better informed people here who know better.

Yup, also, there are specific chemistries used in some composite wet-wing designs that can be damaged by ethanol. It's the final reason I became less interested in the DarkAero 1 while following its development.

That's irrelevant considering G100UL doesn't contain ethanol.

Is that due to the Ethanol fraction or some other factor?

From memory octane levels have a huge bearing on engine performance and that affects take off distance. Aviation has had a much higher octane rating than your typical car gas.

A myriad of things: Octane, ethanol, and also additives used in the winter vs the summer.

That's part of it. Ethanol is incompatible with materials commonly found in airplane fuel systems.

Another aspect is that many piston airplane engines need high octane because they have relatively high compression ratios: WWII era engines need 130 octane to develop full horsepower (they can be operated at reduced manifold pressure on currently available 100 low-lead gas), and even many post-war civilian engines require 100 octane.

Still another aspect is that the FAA is relatively conservative and doesn't want to approve something that might lead to, for example, vapor lock or fuel freezing issues.

>airplane engines need high octane because they have relatively high compression ratios: WWII era engines need 130 octane to develop full horsepower

I mean, high octane and compression relative to other 1960s engines. The common engines in nearly any single prop cessna have about a 9:1 compression ratio, which was massive back when it was built, but laughably bad compared to anything manufactured after the advent of Fuel injection and better piston geometry.

Modern cars regularly have over 12-1 compression ratios on 87 octane.

1960s era carbs and top ends were just abysmal and did a terrible job of mixing the fuel and air charge and controlling the flame front, because we just didn't have the kinds of computer controls and fluid dynamic simulations we have now, to dynamically prevent knock.

Rotax engines are modern and can reach identical performance figures or better, with the same weight or lighter, simply by using modern techniques like fuel injection or a small turbocharger. They do this while running on 91 octane

Except none of that can be used until someone goes through the millions of dollars to do all the design work, safety testing, and mountains of FAA paperwork. Oh, and take on the liability of a mistake killing a bunch of people.

In the mean time, rebuilds have to meet the existing design. Which has specific standards.

It's not like Cessnas themselves can't be wildly improved on in general!

As an old Boeing engineer friend of mine used to say 'when the weight of the paperwork exceeds the gross weight of the plane, it will fly.'.

Rotax 900 series also have a maximum rpm of 5800, substantially higher than old school aero engines, which also helps improving power/weight.

Low lead is a misnomer, you might as well call it LL.

1) Regulations (vehicle fuel comes from sources/supply chains generally not willing to do the paperwork)

2) Lack of solid quality control relative to aviation fuels (see #1)

3) Power to weight ratio matters a LOT in aviation, and aviation engines run at typically far higher elevations for at least part of their flight time. So aircraft engines generally push things harder, have higher compression, and can be damaged more easily with 'junk' or contaminants.

As to #1... I may know a guy who can help.[0,1]

[0]: https://www.bloomberg.com/news/newsletters/2023-10-12/fake-p...

[1]: https://www.bloomberg.com/news/features/2023-10-11/fake-part...

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