Diesel for turbines

The world’s best-selling helicopter is currently the Robinson R44. Now Robinson are selling the turbine-powered R66. If the success of the R44 is any measure, one might expect the R66 to do similarly well.

Reciprocating aircraft engines are designed to use leaded fuel, and there are efforts to ‘get the lead out’. New fuel formulas must be compatible with existing engines. There are a few Diesel engines flying, but they are relatively few. And there are jets. Jet-A is what turbines run on, and it’s very close to Diesel fuel. The bad news is that turbines burn more fuel (23 gph in the R66 vs. 15 gph in the R44). The good news is that it’s cheaper than avgas. The bad news is that the supply of petroleum is finite. The good news is that Diesel fuel can be made from plants and animals. ISTM that what may become the world’s best-selling jet-powered helicopter might be a good consumer of biodiesel. It would demonstrate the viability of the fuel and contribute to the ‘greening’ of aviation.

How hard would it be to certify a turbine engine, specifically the Rolls-Royce RR300, to run on biodiesel fuel? I’m asking about actually running the engines reliably, and not just the official certification. What are the technical/chemical issues?

I believe biodiesel has gelling issues at low temperatures (not really very low in some cases), which could be a problem at any altitude or if the 'copter were used for cold-weather rescues, etc. It seems unlikely to me that you could certify any aircraft for use with biodiesel given these limitations. B10 or other blends with low proportions of biodiesel would probably be okay.

I’d forgotten about the gelling issues. Helicopters generally operate at low altitudes. But they are also operated in cold temperatures in many parts of the country/world.

I question the accuracy of that article because bio-jet fuel has already been successfully tested on commercial airliners and may go into use on naval ships in 2012. Of course, the tests are all 50/50 mixtures from what I can tell.

Well, the article (and it is Wikipedia, after all) does mention that different flavors of biodiesel gel at different temperatures, so it’s entirely possible that a very careful formulation of biodiesel (Jet-Bio?) could be made that is functionally equivalent to petro-diesel for aircraft use. Or, I presume, tank heaters could be built to keep the fuel warm, but that’s just another thing to fail.

But as far as run-of-the-mill biodiesel, I think the problems are large enough to be of concern, and that those concerns are large enough to keep an engine or aircraft from being certified for use with biodiesel. If there is developed a biologically sourced fuel for these turbine engines that has these problems worked out, I think it will be called something else and that the aircraft in question will (could) be certified to use that, but not biodiesel as we know it today.

That’s what they’re working on now. Certifying a 50/50 bio-jet fuel.

Google - air force certification of biofuel - for information. Certain aircraft (engine) certifications have already been made.

Overall there are two approaches: make a new bio-derived fuel which works in existing engines or make new engines which work with an existing bio-derived fuel. Or a mix of each.

Because of the tremendous installed base of engines, and their multi-decade useful life, almost all the effort to date has been to try to make a bio-based fuel which matches the existing fuel spec in all regards. That would make it “drop-in” replacement from the fuel consumer’s point of view.

Along the way there has been some surveying of the existing fuel specs to see if the far corners of some of the specs could be relaxed to ease that process.
When 100LL avgas was introduced a similar engineering / bureacratic process was used. It didn’t match the current fuel specs, so they tested both engines & actual usage patterns to discover how far they could move the spec while still being compatible with the actual use of the actual installed base of engines. A compromise was reached that the engines could safely use & the refineries could cost-effectively manufacture.

The end result was existing engines & aircraft were re-certified *en masse *(grandfathered in effectively) to use a fuel which they were never tested with. And it’s worked out fine, some GA old wive’s tales notwithstanding.

The limitations of that approach are apparent in the current effort to switch to no-lead avgas: There’s no known chemistry which meets the operationally relevant minimum needs of the installed engine base.

What did 100LL replace?

100/130 fully-leaded avgas.

I thought it replaced 80. I don’t think I’ve ever seen 130. (Is that the blue stuff?)

80 is still around if you can find it.

‘If you can find it.’ AOPA article from 2004.

The way I remember it is that the EPA wanted to get rid of leaded fuel, and 80 octane is (was?) full of it. So they came out with 100LL. I remember a lot of pilots being concerned in the late-80s that they would be unable to get 80 octane and that 100LL might not be acceptable for their older aircraft.