I'd appreciate some clarification of "Diesel aircraft are either propeller or turboprop, because diesel fuel's stability and need for high pressure are prohibitive to use in a turbine, (like on a 747)."

A turboprop uses both a propeller and a turbine so contrasting it against propeller and turbine aircraft doesn't make sense to me. Does biodiesel only work in compression ignition engines (and not turboprop/turbofan/turbojet engines)? Is it an issue of the turbine, the fuel pump or the (cold) conditions where those planes typically fly (thus requiring fuel additives)?

Also, most of the aircraft used by Private Pilots have spark-ignition engines so you might want to investigate AGE85.
http://www.age85.org/

---------Author's response------------
Kyler- Sorry for the confusion. I actually meant to write "Piston" and instead wrote "turboprop". My mind is mush.

But your comments on AGE85 are awesome. From this site:

http://www.biodiesel.org/resources/reportsdatabase/reports/gen/19990615_gen-215.pdf

It looks to be an ethanol/biodiesel blend which has suitable ignition temp, vaporization properties, and freezepoint for use in commercial aircraft. Sounds like a winner.

It takes a lot of heat and pressure to ignite diesel fuel.Jet A-1 and Jet A are kerosene type fuels, which are heavier than gasoline but lighter than diesel. Diesel engines ignite injected fuel at a compression ration of around 25:1. Anything much lower and the fuel will not detonate. Jet engines currently run at 44:1 at higher speeds and altitudes. Compression ratios in continuous combustion scenarios are not comparable to cycled combustion, however a sustained detonation may be possible at the pressures found in modern jet engines using diesel fuel. Jet fuel contains many additives to deal with low temperatures, static from high pressure/high flow fuel pumps, bacteria and probably other stuff. All of this would have to be similarly addressed with biodiesel.
Breaking diesel down into lighter fuels is also possible. Ethanol type fuels might also be feasible in turboprops and turbines. These can be produced sustainably.

-----------------Author's response-----------------

Thanks for the information about compression ratios. Your numbers for passenger jets are particularly interesting. Would that make some sort of hybrid-fuel engine possible, which got up to speed using conventional fuels, and then switched over to Biodiesel when the compression was high enough for ignition? Or maybe some sort of variably blended fuel where ethanol is bleed into the engines in the beginning, tapering off as higher speeds are reached?

In any case, you're absolutely right, that some sort of sustainable option (be it ethanol, biodiesel, or a blend) is achievable, and worth pursuing.

That's part of why high-speed electric rail is so much cheaper.

In Brazil ethanol is about half the price of gas and they're running planes on it. I know there are few manufacturers of ethanol planes and they are back-ordered for several years.

"That's almost 8 times as much fuel per passenger mile as a bus, and over twice as much fuel/passenger mile as a car."

Here's the actual US data from 2002 (BTU per passenger-mile):

Commercial air - 3,703
Bus, transit - 4,127
Bus, intercity (Greyhound etc) - 932 (year 2000 data)
Automobile - 3,581

Since 1975, commercial aviation has improved the most of those four modes - decreasing BTU per passenger-mile by 53%. Automobiles decreased 24%, and Intercity Bus decreased 6%. Transit Bus, however, increased its BTU per passenger mile by 47% over that period.

Also, the author of that paper says that a campus bus has capacity for 80 passengers, with an average ridership of 64 people?

For comparison, on a Greyhound, during one of my marathon rides on that mode (50 hours), I got bored enough to count seats, and I believe there are around 47 of them. Standard transit buses have about the same capacity, with articulated buses having closer to 60 seats.

So, his campus buses must be pretty big, pretty full, and with lots of people standing -- very much not representative of bus travel in the US in general.

You also have to keep in mind that the plane is going about 10 times faster than an intercity bus, and probably 30-40 times faster on average than a campus bus, so it's akin to comparing a child crawling with automobiles doing 70 MPH on a freeway.

Even so, real-world data shows commercial air uses about 4 times the energy per passenger-mile as intercity bus, and 10% LESS energy per passenger mile compared to transit buses. Also, it has almost the same energy intensity as automobiles, not twice as much. If you factored in light trucks/SUVs, commercial air is more efficient than light vehicle travel - again, at 10 times the speed.

ref. http://cta.ornl.gov/data/tedb24/Edition24_Chapter02.pdf

------------------Author's Response-----------------

Thanks for the updated figures for fuel consumption. I'd imagine that mine were probably skewed because of their age, and because the came from only one airline (the linked paper, northwest airlines, 2000).

Regardless of how much, or relatively little gasoline is burned by planes each passenger mile, they are still a very appealing target for switching to sustainable fuels, because there are so few airlines controlling all the planes. For example, imagine the impact in passenger miles/year that switching United Airlines to a biofuel would have on the world biofuel marketplace. Now, imagine how many individual car owners you would have to switch for the same impact. It might be easier to switch your car, but once one airline makes the switch, it could trigger some incredible changes in fuel prices or everyone.

some good information of aviation diesel engines here
http://www.deltahawkengines.com/article00.shtml

My buddy Craig wrote a piece about using liquid hydrogen (which can be, but mostly isn't renewable) in commercial aviation. It's certainly not a drop-in replacement, but it has some compelling advantages in terms of specific energy.

Thanks for the response.

Of course I agree that it's easier to move markets with larger consumers. So I wasn't questioning the asking of whether biodiesel would be appropriate for airplanes.

It seems to me, though, that the low hanging fruit are semis. In the US, biig trucks consume twice as much energy in diesel fuel alone than all of air travel and transport combined. And since there is no measurable consumption of diesel for air travel/transport in the US, you're also talking about a more radical fuel substitution than would be required to modify semis.

Granted, you're dealing with more entities with big trucks, but I would hazard a guess that the bulk of the consumption comes from trucks owned by a handful of firms.

As for relative scale to personal vehicles, I believe the ratio is about 8:1 in terms of energy use (light vehicles to air). So porting to cleaner fuels for all of aviaton would be like converting roughly 23 million personal vehicles.

Oh, and just because everyone here seems to love a good hybrid, here's something Boeing recently tried.
http://www.wheeltug.gi/

Perhaps this will give us another way to reduce aviation fuel consumption.

I don't know exactly how much fuel consumption they represent, but piston-powered general aviation aircraft seem like a great candidate for alternative fuels, for two reasons: First, their engines need to be overhauled frequently compared to other vehicles (which should mean that engine technology can be cycled over by attrition fairly rapidly). Second, they currently burn leaded gasoline, which in addition to being an environmental disaster, is getting harder and more expensive to make every year.

Unfortunately, aviation in general and GA in particular, is absurdly slow to adopt new technology (most private airplanes still have mechanical fuel-injection systems, or carburetors), but if diesel aircraft engines get just a little cheaper the savings will be too much to ignore.

There are a number of companies, most notably Thielert and the aforementioned Deltahawk, making aviation diesels now - which brings me to my final point: all of them are designed to run on Jet-A, which is already available at airports small and large. As I understand it, diesel and Jet-A differ primarily in their additive packages (maybe someone else can shed some more light on this), but the point is: make an ecomonical, bio-based or blended direct replacement for Jet-A and you've solved both problems at once.

Best. Thread. Ever.

Holy shit guys, nice work. Not only is this thread filled with eye-popping statistics and inspiring green potential, it's also a been a phenomenal discussion to follow.
Joeseph - amazing comment, thanks for all the interesting stats.
I'm not sure who the author is, your name's not showing up at the top, but it was great to see your responses - this has become a really well flushed out post/subject.

Re: "Here's the actual US data from 2002 (BTU per passenger-mile):
Commercial air - 3,703
Bus, intercity (Greyhound etc) - 932
Automobile - 3,581"

I would be curius to know how full they were assuming cars were for this study. And how full the planes were.

One thing that makes planes so enengy-intensive is that people tend to fly very long distances. The figures above are btu PER mile. I would guess that many people go 10 times further by plane than they would go by car or bus. All those miles add up!

"I would be curius to know how full they were assuming cars were for this study. And how full the planes were.

One thing that makes planes so enengy-intensive is that people tend to fly very long distances. The figures above are btu PER mile. I would guess that many people go 10 times further by plane than they would go by car or bus. All those miles add up!"

They used the National Household Travel Survey (2001)
for the load factors for light vehicles (autos 1.10, light trucks 1.72, motorcycles 1.22). Pretty sad that motorcycles have more passengers on average than cars.

They got the load factor for commercial air travel by dividing passenger-miles by aircraft-miles, which are both published in "Air Carrier Traffic Statistics Monthly" put out by the Bureau of Transportation Statistics.

You can find all those source details in Appendix A of that "Transportation Energy Data Book" I referenced earlier.
http://cta.ornl.gov/data/tedb24/Edition24_appendix_a.pdf

NHTS
http://www.bts.gov/programs/national_household_travel_survey/

Air Carrier Traffic Statistics
http://www.bts.gov/xml/air_traffic/src/index.xml#MonthlySystem

How about our waste motor oil? I run my conditioner, furnace and hot water heater for my home on it. It is free for the asking.

If you're out searchign, I believe Baylor University's aviation program is pretty heavily into this; the EPA at least for a while was, I believe, operating a high-performance piston (a Beechcraft or King Air or something) on renewable fuels - in one of its engines! - so that they could put an air monitoring rig on that side and get cleaner data.

Actually, replacing leaded gasoline for general aviation has turned out to be a pretty tricky problem.

The big problem is that the fleet is old, and any major and even minor changes to a plane requires recertification, which is very expensive.

Ethanol isn't acceptable since it eats rubber seals on many older aircraft (can't change fuel system without recertification) and has lower energy content.

Replacing the octane from lead is also tricky since avgas has very high octane (no, even ethanol isn't good enough). High power supercharged engines that use 70 % of all the avgas consumed, are especially sensitive and require all the octane they can get.

See http://www.aviationnow.com/content/ncof/ncf_n40.htm for a thorough review of the issues.

On newer planes one can use diesel engines, turbines and gasoline engines that run well on car gasoline. The problem is the existing fleet.

Hi all,

Great posts. I have some clarifications I'd like to make.

If you look at the Baylor study, they ran biodiesel in a turboprop as a blend with Jet A fuel (basically kerosene) up to B20. That is, the study involved a gas turbine engine (Pratt & Whitney PTA6-6), not a piston engine. It does have a prop on it, but this is the most popular jet engine in the world. It's fundamentals are the same as any jet engine including those on the largest commercial aircraft. They actually flew with B20 in a King Air, which is really cool.

The person who was posting about ethanol use in Brazilian aircraft must have been referring to piston engines. These are similar to car engines, and in fact, most car engines can be run on ethanol (or at least high blends) with relatively minor adjustments.

Finally the new 555 passenger A380 is touted to burn only 3 liters /100 km per passenger. In english that's over 78 mpg per passenger. Pretty impressive!

Thanks for all the info. In addition to the age of the piston GA fleet a greater underlying problem exists. Not much stands in the way of a wealthy young persons ability to purchase a brand 'new' Cessna that spews lead using the same technology as when certificated decades ago. This entrenched level of ignorance plays right into the rampant conservatism that hides out nowadays behind the relative cultural safety of the country club like scene known as general aviation. Read the sentance over again if you think its too long. Most people involved in this area fully support whatever it takes to maintain the 'status quo' as a matter of tradition. The technical side of GA is mostly made up of ex military people who somehow seem to forget the EX part and glorify the stupidity of that whole scene while indoctrnating the useless anality into young technicians maintaining the problem. Educational forums such as this thread may help clear up some of the traditionalism so we can get on with a cleaner future. thanks for reading this

We GA pilots would very much like to switch to an alternative fuel. But aircraft aren't cars. If the airplane engine quits it's a big deal. That's why FAA certification exists, to make sure engines keep running.

It has proven astonishingly difficult to get past that certification. Every alternative currently has bigger problems than the existing 100LL avgas formulation. AGE85, like all alcohol fuels, won't work with existing fuel systems. (Ripping out and replacing fuel lines, tanks, pumps, and valves on thousands of aircraft is a nonstarter.) Diesel and kerosene fuels require new engines. Even pulling TEL out of 100LL would require big changes, like new electronic ignition systems in every engine. Unfortunately the few possible TEL replacements seem to be much more toxic.

One thing that might spur a switchover, despite the big obstacles, is a fuel tax "holiday." Currently aviation fuel carries an excise tax, much like motor fuels. Ethanol is exempt from that tax. (We can debate the merits of whether it should be. However, there's no question it means more ethanol and less petroleum stock.) Similarly, something like a 20 year tax moratorium (or excise tax rebate) on the new alternative fuel might be enough to encourage a switchover.

The current GA fleet of piston engine aircraft used 264 million gallons of 100 Low Lead in 2004 (http://www.faa.gov/data_statistics/aviation/aerospace_forecasts/media/table34.pdf). The estimated amount of tetraethyl lead released into the atmosphere by GA aircraft in 1998 was 1.39 million pounds. TEL cannot be removed from 100LL or the higher performance engines will experience severe detonation, retarding the timing (and horsepower) to deal with this problem is not a safe option.
AGE85 is currently being researched as a replacement for 100LL. AGE85 has been STC approved in the Cessna 180/182 with the Continental O-470 UTS engine. The fuel is currently being flight tested in a Mooney M20J, a Piper Seneca and a Grumman Ag Cat. In flight detonation testing has also been undertaken with promising results. The fuel is compatible with almost all hoses, bladders and sealants (not the ancient urethane fuel bladders). The fuel has proven to be less corrosive than 100LL to aircraft parts.
The modifications required to run AGE85 are to increase fuel flow by installing larger jets in the carburetor or recalibrating the fuel servo for a higher fuel flow. These relatively minor adjustments could be made for under $1000 per aircraft. Further information is available at www.age85.org