E-85 as a piston aviation fuel, thoughts...

[Cujet]

Over the years, there has been discussion of ethanol (E-85) based aviation fuels. There are quite a few downsides, that's for sure. However, for each negative, there are those who seem to overcome the issues. There are a few pilots in Brazil who use ethanol based fuels with what appear to be good results.

Some of the downsides when compared to 100LL avgas include: significantly Lower energy density and lower specific energy, corrosion potential, ethanol is hygroscopic (it attracts and holds moisture) and possible wear/oil related issues.

Some interesting positive points: Very high knock resistance (some consider it "always" knock free), roughly 114 octane equivalent, near zero risk of detonation under boost, A diesel-like high thermal efficiency when direct injection, high compression and boost is employed (45% thermal efficiency is possible under higher loads) and it is highly oxygenated, resulting in clean combustion with very low exhaust soot, and of course, the cooling effect, which is responsible for the knock resistance and subsequent volumetric efficiency boost of alcohols.

My thoughts are not to consider it as a replacement for 100LL, but whether it's a viable aviation fuel at all. If, for example, we were to utilize a modern direct injected/turbocharged, high compression engine, configure it properly for efficient E-85 use, the results may be in some ways acceptable, vs. using 100LL in say, a TSIO 360 or 520 Continental.

Some additional items. No fuel enrichment is required under boost/high loads. So in a proper engine design under high loads, the heating value disadvantage is largely or completely offset by no need for enrichment, as would be common in turbocharged, low compression, air cooled aircraft engines.

 

[Cujet]

From a fuel consumption standpoint, a TSIO 520 will consume 33 gallons of 100LL per hour to make 305HP. A conventional, non optimized, turbocharged engine running on E-85 will consume 29-32 gallons per hour to make 305HP. An optimized engine, with a diesel like compression ratio and robust construction can match or slightly exceed a diesel's thermal efficiency and will consume less than 29gph.

An optimized E-85 engine can extract the same percentage of the fuel's energy as a diesel.

 

[Surestick]

Given how a big part of the piston-engined fleet is operated (privately owned, not flown that frequently) the hygroscopic nature of ethanol is likely the biggest issue.
That's somewhat solvable with a completely sealed fuel system and fuel injection where fuel doesn't sit in a float bowl absorbing moisture while the airplane isn't in use (like that employed in the Chevy Volt) but that means a lot of $ and extra weight for the conversion.

 

[Cujet]

Given how a big part of the piston-engined fleet is operated (privately owned, not flown that frequently) the hygroscopic nature of ethanol is likely the biggest issue.
That's somewhat solvable with a completely sealed fuel system and fuel injection where fuel doesn't sit in a float bowl absorbing moisture while the airplane isn't in use (like that employed in the Chevy Volt) but that means a lot of $ and extra weight for the conversion.

Bladders are common in aviation, and could be both pressure refueled and have managed venting. Sure, a wet wing is sometimes better, but there are advantages to each system. The guys in Brazil operate without significant issues. Although I'm not sure just how much water the fuel can absorb without operational or separation problems. I don't believe the ethanol used is anhydrous, and may contain 2% water already.

 

[stonny9]

There are a few guys down here in Florida running e85 on airboats in the Everglades. I know one has an 8-cylinder. Search on southern airboat there's discussions on it over the years it's a slow forum.

 

[440_Magnum]

The low energy density is probably a killer. Not many general aviation operators would accept a ~25% reduction in range at the same takeoff weight. Of course you could modify the engine to gain back a portion of the loss, but then you’d have certification issues. And corrosion, and water absorption, and....

 

[meadows]

I think water is the problem. The FAA did not want you to use fuel with any alcohol. Cujet-do you know how much lead is in 100LL currently?

 

[Cujet]

The low energy density is probably a killer. Not many general aviation operators would accept a ~25% reduction in range at the same takeoff weight. Of course you could modify the engine to gain back a portion of the loss, but then you’d have certification issues. And corrosion, and water absorption, and....

That's the thing I noted above. The lower energy is largely or completely offset by the very high thermal efficiency of an optimized E-85 engine.

I think water is the problem. The FAA did not want you to use fuel with any alcohol. Cujet-do you know how much lead is in 100LL currently?

Looking it up, E-85 can hold a lot of water, possibly as much as 20% without separation. That's both good and bad. It's also good to note that isopropyl alcohol is currently used in aviation fuel as a "dry gas" so alcohols are allowed to be added if necessary.

100LL can have as much as 2 grams Pb per gallon. However, it's good to note that a good portion (possibly most) of today's 100LL meets the 100VLL specs (very low lead) and will contain about 1.2 grams per gallon in real world terms.

 

[George Bynum]

From a fuel consumption standpoint, a TSIO 520 will consume 33 gallons of 100LL per hour to make 305HP. A conventional, non optimized, turbocharged engine running on E-85 will consume 29-32 gallons per hour to make 305HP.

You are a real pilot and airplane owner, but these numbers puzzle me. 100LL is assumed to weigh 6 lb/gal, so your consumption numbers are in the 0.65 lb/hp-hr, the normal domestic unit for BSFC. This is lots higher than what I see more commonly used for NATURALLY ASPIRATED engines, in the 0.45 range. Does turbocharging (well, probably turbo-normalizing in an airplane) increase consumption that much, or am I out in left field? I don't have density numbers at hand to compare E-85; that's not considered in my question.

 

[meadows]

I had a STC on my piper, and it said no alcohol premium fuel. If you get enough water in an alcohol based fuel, the alcohol will dropout. this causes a problem with the octane (lower) of the fuel. Of course the alcohol/water mix will also cause a problem. The E-85 will also cause corrosion problems with your carb., etc.

 

[meadows]

I used to work for Chevron and we used to make 100LL with 80% alkylate, 20% toluene, and 2 g of TEL in the 70's and 80's.

 

[Cujet]

You are a real pilot and airplane owner, but these numbers puzzle me. 100LL is assumed to weigh 6 lb/gal, so your consumption numbers are in the 0.65 lb/hp-hr, the normal domestic unit for BSFC. This is lots higher than what I see more commonly used for NATURALLY ASPIRATED engines, in the 0.45 range. Does turbocharging (well, probably turbo-normalizing in an airplane) increase consumption that much, or am I out in left field? I don't have density numbers at hand to compare E-85; that's not considered in my question.

I apologize, as I misspoke in my post above. I should have said “non conventional, high compression, direct injected, turbocharged and optimized E-85 engine”

The takeoff BSFC comparison numbers are accurate. Turbocharged, air cooled, aviation engines have low compression ratios and use less ignition timing than non turbo engines. This leads to very poor overall, BSFC numbers, especially at high loads. Efficiency comes largely from the thinner air turbocharged planes can operate in.

Note: This is not to discount the fact that some turbo pilots can lower the cruise fuel flows by leaning and reducing power to keep Turbine Inlet Temps (exhaust temp) low enough to save the turbo while they achieve good BSFC in cruise. This is not always possible.

Put another way, the High load fuel flow rates can be very similar (which is to say poor) between the two engine types.

Normally aspirated, higher compression aviation engines can be very efficient, some have cruise BSFC numbers below 0.4 pounds of avgas per hour. This has been achievable since the late 1930’s, but is easier today with electronic ignition, matched fuel injectors and other tricks.

Brazil launches alcohol powered plane - Bellona.org

Agence France-Presse English Wire via NewsEdge Corporation informed that in March a Brazilian firm delivered the world’s first …

bellona.org

.

 

[LeoStrop]

Following this interesting thread Cujet.

When i get home, i will try to look for something around here, if there is any more info that could be usefull for you.

I know that we had a couple of diesel trucks converted to use E100 on the 80's :



But the BSFC was not even close to the diesel siblings.

I know that Scania now has some ethanol trucks running inside some factories to reduce the carbon footprint

 

[Cujet]

I know that we had a couple of diesel trucks converted to use E100 on the 80's :
But the BSFC was not even close to the diesel siblings.

The comparison I make is between the turbocharged aero engine and an optimized E-85 engine. But another very valid comparison is thermal efficiency. The optimized E-85 engine can achieve identical 'thermal' efficiency to a modern diesel. Specifically, it converts 42-44% of the fuel's energy into work. But it's good to remember that alcohol contains about half the energy of diesel. So almost twice as many gallons of E-85 are consumed for the same amount of work.

It's still worth thinking about. As the super high 'effective' octane of E-85 allows all sorts of engine power density tricks that save weight. reduce size, and reduce drag. Especially when compared to an aero diesel.

I could not translate the video, but from what I gather, they may not have been optimized ethanol engines. As that knowledge is fairly new.

 

[Danno]

Such an engine seems to be such a specialized duty cycle, would it even be worth it?

 

[Cujet]

Such an engine seems to be such a specialized duty cycle, would it even be worth it?

How so? Aviation piston engines generally are required to produce a high percentage of peak power under most aspects of flight. That seems to be a good fit for an E-85 engine which achieves it's best thermal efficiency at higher loads.

My airplane, the mighty thunder Cardinal, takes off at 100% power and the only loss of power during climb is due to the lapse rate of thinner air (the RPM is not reduced or throttle pulled back in climb) . Cruise power is generally 60-70% of peak power, with only a slight reduction in RPM, 2700 down to 2550, If it were turbocharged, it's very likely that the engine would be operated at 72-77% of peak power. Again, it seems like this is a good fit.

Interestingly, https://pubs.acs.org/doi/10.1021/acscatal.9b05319. and https://www.popularmechanics.com/science/a33573417/carbon-dioxide-ethanol-catalyst/ Which is what got me thinking about ethanol use in the first place.

 

[George Bynum]

Indy cars used a 98% ethanol (2% denaturant) from about 2007 to 2012. Since, something similar to the E85 is used. It may be "standard" E85; Speedway supplies the fuel. In 2022, according to Wikipaedia, they'll be getting 900 HP from 2.4 liters. These engines would be great for an airplane if their (MY GUESS) TBO of 4 hours is satisfactory. Coast to coast with 1 rebuild .

Methanol and methanol mixtures were used earlier.

One advantage of alcohol is that fires can be extinguished with water; that's a reason for alcohol stoves on "small" boats. I don't know about E85.

My point ... there is knowledge and experience with alcohol fueled engines.

 

[Danno]

How so? Aviation piston engines generally are required to produce a high percentage of peak power under most aspects of flight. That seems to be a good fit for an E-85 engine which achieves it's best thermal efficiency at higher loads.

Low energy density would be difficult to overcome, to show advantages over traditional engine builds.
And significant added R&D expense in trying to overcome the energy density problem, since it could be a low production engine.
As I see it, same challenge as faced by battery powered planes. Low energy density, high development costs - unsure of ROI 5 - 10 years out.

 

[supton]

Cujet, what is your take on using rotary engines in aircraft? I know one evening I came across a site that was very pro-rotary, stating that this engine likes being under load, and furthermore, when abused, is more likely to not catastrophically fail when overheated. Yet it clearly isn't widely used so I have to wonder about the veracity of the argument.

As for E85, dunno. Water and corrosion would have to be overcome. Seal the fuel system off when not in use? I wonder if somehow you could pressurize unused takes with something dry, either clean dry air or nitrogen (probably want to seal the tank off fully).

 

[john_pifer]

Cujet, what is your take on using rotary engines in aircraft? I know one evening I came across a site that was very pro-rotary, stating that this engine likes being under load, and furthermore, when abused, is more likely to not catastrophically fail when overheated. Yet it clearly isn't widely used so I have to wonder about the veracity of the argument.

As for E85, dunno. Water and corrosion would have to be overcome. Seal the fuel system off when not in use? I wonder if somehow you could pressurize unused takes with something dry, either clean dry air or nitrogen (probably want to seal the tank off fully).

Off the top of my head, I’d say an advantage to rotaries is the low vibration when compared with piston engines.

But I’ve read that rotaries aren’t very efficient, in comparison to piston engines, either. And then those pesky apex seals...

There are a couple of airplanes that have used Wankels, though. I know at least one of them was a Russian-built amphibious aircraft. Or was it a mini-Ekranoplan... Can’t recall.