”Captain.Jack” said:
So GAMI100UL, at 20-40% xylenes, couldn't pass D910?
No, GAMI G100UL can’t meet D910. For starters, D910 requires lead, and G100UL is, well, unleaded! For that and other reasons, a NEW spec is required for the unleaded avgas formulations. The history of D910 is interesting, it goes back to ~1936 when the War Department decided there might be a war, and that the seven (!) competing avgas formulations would be unwieldly to support in a wartime aviation supply process. So the War Department led an effort to settle on a single spec. That spec was written around the ways folks made avgas then, 90 years ago… and the distillation end point was set to prevent Houdry cracked gasoline, which was unstable and would form gums over the course of a couple months, from being included in the gasoline.
The spec evolved over the decades to become the D910 we have today… but it’s a lot of tradition, and not a lot of fundamental thought about why the spec should be the way it is… avgas hasn’t been important enough since the introduction of the Boeing 707 in 1957 to merit the engineering effort. And we’ve come to understand a lot more about gasoline since 1957!
The FAA’s unleaded avgas effort, which they were intent on leading, began in 1991, assuming from the get-go that all they needed to do was come up with an unleaded formulation that in every respect except lead met D910. It took hundreds of millions of dollars and decades to determine that was undoable. In the meantime, Swift, GAMI, and others decided to take a *different* tack… determine *first* what might work in actual aircraft engines, THEN write a spec around the successful fuel. The FAA came to that realization quite late, unfortunately.
GAMI spent quite a bit of effort trying to understand all ~40 constraints on avgas imposed by D910… and why they might or might not be actually important to performance in an actual aircraft engine. The end distillation spec, for instance, was excessively restrictive, since Houdry cracked gasoline hasn’t been made since about 1970 when the last such cracker was torn down. And yet… the spec attribute remained. Tradition!
”Captain.Jack” said:
It would have been helpful to list it as a test procedure.
Apologies. It’s difficult for me to guess what misunderstanding of industry terms different folks might harbor. Thanks for making your thinking on this evident.
”Captain.Jack” said:
The Shell SDS pages … listing compounds … that aren't there can change the hazard classification for shipping which can dramatically change shipping costs and shipping methods.
Perhaps. But remember, this is an evolutionary work of almost 90 years. Back in the 80’s when lead was being phased out of mogas, folks ran their naphtha reformers at unprecedently high severities to generate octane… 102 octane reformate wasn’t uncommon (Research octane in this case). If you do that, you make some xylenes. And the light and heavy reformate were pretty good stuff, so some would make its way into avgas as a blend component. So avgas might be as much as 5% xylene. However, with the advent of the Congressional force of ethanol into the mogas pool, the overall pool was too HIGH on octane, and folks not only reduced reformer severity, but spent bucks modifying their reformers to run at unprecedented lower severity (and lower liquid volume loss). Thus, reformate became too low of an octane rating to include in avgas. But… the spec artifacts remain, in honor of the way refineries were operated in the 1980s. That spec is truly a time machine!
”Captain.Jack” said:
It doesn't matter where they make or buy it. They sell it, with xylene. They sell several products and all but one contain xylene.
No, the SDS says xylene. It doesn’t mean the product contains xylene. I’m not arguing whether Shell distributes avgas, my friend. I merely observed that they don’t make any in the US. And somehow, you thought pointing out that they sell it somehow refutes the observation that they don't make it. Huh. There’s not much incentive for them to spend time and money modifying their SDS… it’s inclusive enough. I’m just pointing out that if you grab a sample and obtain a fairly inexpensive gas chromatogram, you’re not going to find xylene content anything like that.
Paul
