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Most commercial jet engines today would be those large jet engines on large passenger craft like a Boeing 747. Can these run on straight ethanol without any problems?

Would more maintenance be required? Would the engine lifetime be reduced? Would it be more susceptible to flameouts or other mechanical risks? Would the airplane never make it off the ground? (I imagine thrust reduction will be limit the takeoff weight.)

One reason I'm asking is because eventually petrol will be gone. (I'm not an apocalyptic saying that will happen next year, but somewhere down the line it will certainly be gone.) Will jet engines require a major redesign, or could they just use ethanol for a few years/decades and be fine?

Note: I'm talking about jet engines (turbofans, but turbojets are okay too), but not piston engines and turboprops.

EDIT: A lot of talk has been generated that ethanol contains less energy than kerosene. I assure you I'm aware of this. Ethanol is like 25 MJ/kg and kerosene is more like 40 or 45. I'm not asking this question with energy efficiencies in mind, but rather out of curiosity if the same engine geometries will work well with both fuels.

A rephrasing: Would current jet engine geometries (blade angles, compression ratios, volumetric flow, etc.) work well with ethanol instead of regular jet fuel?

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    $\begingroup$ Probably extensive testing is required. Ethanol can destroy rubber seals and so on. If you want to run a car on ethanol special modification to the fuel system is required $\endgroup$ – Andrius Apr 13 '16 at 8:08
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    $\begingroup$ I want to question the belief that "petrol will be gone" - why would it be? As it gets harder to produce (or more heavily regulated), the price rises. As the price rises it becomes economical to partially replace it with other energy sources, or reduce consumption, or synthesize it (including growing it). At some point extracting it from the ground stops because it's no longer profitable, but we never 'use it all up' and we always have petrol - price goes up, consumption goes down on some kind of curve, just like any other commodity. $\endgroup$ – Spike0xff Apr 13 '16 at 20:46
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    $\begingroup$ No one has yet mentioned a redesign of the combustion components, but my guess is that differences in the fuels' stoichiometric mixture ratios, volatility, viscosity, reaction speed, flame propagation rates and other issues that I haven't thought of would require changes to the nozzles and combustion chambers. The fuel delivery and control system would have to be at least re-calibrated for ethanol, and may not be adjustable to the extent needed. I know that industrial gas turbines can be run on a variety of fuels, but I believe these components have to be adapted to the fuel. $\endgroup$ – sdenham Apr 13 '16 at 21:38
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    $\begingroup$ @Spike0xff yes I know it will deplete as a bell curve and never still be literally all gone. Some will remain trapped and unextractable except at huge costs. I was being simplistic in saying it will be "gone" because this question isn't really about that, but about current engine geometries possibly working with ethanol. $\endgroup$ – DrZ214 Apr 14 '16 at 0:21
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    $\begingroup$ @sdenham Yes exactly. I edited the OP to include a thing about engine geometries. $\endgroup$ – DrZ214 Apr 14 '16 at 0:21
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Although turbines could be designed to run on ethanol it's actually a lousy aviation fuel (actually it's a lousy fuel in general). Petroleum-based jet fuel is high grade kerosene which has lubricant properties, which ethanol lacks, and ethanol has different characteristics which mean you can't pour it in and use it as a replacement.

Some problems with ethanol as a fuel are:

  • It's a solvent which dissolves some types of seals and hoses
  • It evaporates very easily, and can cause vapor lock
  • It is hygroscopic, meaning it attracts and mixes with water very readily. Water contamination would not settle to the lowest point, instead it would be suspended with the fuel
  • It has significantly less stored energy per unit of weight. Jet fuel has an energy density of 46 MJ/kg (megajoules per kilogram) and pure ethanol has 26.4 MJ/kg; that's only 57% of the energy density of jet fuel! Even if a jet was able to burn it at its maximum efficiency it would get just over half the oomph from it.

There are bio-fuel replacements for jet (and diesel) fuels based on oilseed crops like rapeseed (canola), palm oil, etc. These are processed to remove their OH atoms using transesterification to create fuels which are almost identical chemically. Modifying jets to use ethanol would be hugely expensive if it is even possible to do, so there's no point given there's a suitable replacement.

The point is actually moot though because crop-based biofuels are never going to replace petroleum. There simply aren't enough crops grown to feed people and engines at the same time, in fact even if we used all the worlds farmable land to grow bio-fuel crops instead of food crops we still would not have enough of it to replace fossil fuels. We have enough trouble feeding people already and the biofuels craze is not helping that. Bio-fuels based on non-food crops from marginal land may some day become economically viable and partially replace petroleum fuels.

After some searching I found a link to an article I wrote on biofuels for AOPA UK magazine which goes into more detail on biofuel issues.

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    $\begingroup$ ..even if we used all the worlds farmable land to grow bio-fuel crops instead of food crops we still would not have enough of it to replace fossil fuels... Do you have a source for that? It is a terrific way to put humanity's dependence on fossil fuels in perspective and I would love to use it. $\endgroup$ – dotancohen Apr 13 '16 at 9:55
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    $\begingroup$ That statement is based on weeks of my own research, which I put together in a blog article here: tetsui.net/gdblog/2011/06/17/why-biofuels-make-no-sense $\endgroup$ – GdD Apr 13 '16 at 10:12
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    $\begingroup$ @FreeMan, I'm not dismissing bio-fuels at all, I'm saying that you have to have a sustainable source of biomass, and that Ethanol is not a good choice for aviation. If cellulose-based alcohol fuels can be made economic and algae based diesel produced in quantity then we won't have pressure on food crops. Getting off-topic though $\endgroup$ – GdD Apr 13 '16 at 12:43
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    $\begingroup$ Anither example of what GdD is saying about how growing fuel crops is not a long-term answer: The palm oil crops which are growing at a breakneck pace in southeast Asia are destroying enormous amounts of ancient peattand which ace the best carbon sinks on the planet and take centuries to reform. The net effect of palm oil plantations on CO2 is really bad. $\endgroup$ – TomMcW Apr 13 '16 at 17:21
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    $\begingroup$ "[Ethanol is] a solvent which dissolves some types of seals and hoses" So, just like kerosene. The salient point is presumably that ethanol might dissolve the seals and hoses of an engine designed to run on kerosene, (just as kerosene might dissolve the seals and hoses of an engine designed to run on ethanol). $\endgroup$ – David Richerby Apr 13 '16 at 21:16
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Yes, jet/turbine engines can run on ethanol... but ethanol has only 60% of the energy content per mass unit of Jet-A fuel which means you have to use 67% more of it by mass to get the same energy output.

You'll have to increase the additional fuel carried (due to having to lug around a lot more low density fuel) of every aircraft by quite a margin if you were to fuel them with ethanol.

Alternative energy use for turbine engines has been studied extensively. The problem is always that they pack much less energy density than Jet-A, making them unfeasible.

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  • $\begingroup$ Yes I know ethanol has much less energy content and thus is not economical today. I won't downvote since nothing you said was incorrect, but I'm really looking for the details of using ethanol in today's engines, like maintenance changes, not efficiency reduction. $\endgroup$ – DrZ214 Apr 13 '16 at 7:58
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    $\begingroup$ While I guess this answer implies it, it's probably worth explicitly pointing out that carrying all of that extra fuel mass around means that the plane needs to produce more power to remain airborne. Thus, the fuel requirements to move the same payload the same distance in the same amount of time grow superlinearly as fuel energy density decreases. This also generally means more pollution for the same work accomplished. $\endgroup$ – reirab Apr 13 '16 at 14:49
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I'm not an expert of real jet engines. I think it should in general be possible to run a jet on ethanol, though some modification might be necessary. Some materials might not be resistant to ethanol and need replacement.

But the main reason for not using ethanol is that it contains by far less energy: Just 27MJ per kilogram instead of 43MJ for jet fuel. For the same energy demand (i.e. an entire flight), the aircraft would need 60% more fuel, plus additional fuel to carry the additional fuel...

Here is some data about the A380 from wikipedia (fuel weight calculated by a factor of 0.8kg/l):

Maximum take-off weight    575,000 kg (1,268,000 lb)
Operating empty weight     276,800 kg (610,200 lb)
Max. fuel capacity         256,000 kg (564,000 lb) (44% MTOW)
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Calculated payload:        299,000 kg

If the aircraft would use ethanol, this changes to:

Maximum take-off weight    575,000 kg (1,268,000 lb)
Operating empty weight     276,800 kg (610,200 lb)
ethanol equivalent:        407,000 kg (900,000 lb) (70% MTOW)
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Calculated payload:       -108,000 kg (-240,000 lb)

Even if you manage to load the additional fuel, the aircraft is waaay to heavy to take off, even without payload.

It's not the cost (or availability), it's the bad energy density which makes ethanol unsuitable.

It would be better to use something more similar to jet fuel like Biodiesel, which is similar to fossile diesel. Biofuels typically also have a lower energy density, but not to an extent that it makes them technically unsuitable.

There are different kinds of such fuels for aviation, and wikipedia has an impressive list of test and commercial flights using biofuels. In most cases, a mixture of fossile fuel and biofuel was used, but on some flights, one engine (or the entire aircraft?) was powered by pure biofuel.

The main problem with biofuel of any kind still is availability. We simply can't produce enough to replace fossile fuels completely.

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The straight conversion of fuel for gas engines is a change in air/fuel ratio. Other factors come into play such as fuel supply, lubrication, corrosion, etc. These are a non-issue in the cars I have converted. One aspect that should be addressed here is the power to weight ratio of the resulting system. You can make more power in a car using the same engine by switching to ethanol. Yes, you have to fill up more often. Yes, there might be something with seals in older cars. Bottom line: it's faster, cheaper, and the exhaust does not stink. There is a new technique for making ethanol developed by scientists at Oak Ridge National Laboratory that converts industrial smoke stack exhaust to ethanol. This is a new source for fuel that cleans up pollution at the source while providing a less polluting fuel for use. No grain involved so no impingement on food sources. NASA is running tests on a new biofuel made from false flax. The Blue Angels are flying 50/50 with this new stuff. The Navy is supposed to be switching over by 2030. I'm calling for that to happen faster. Anyone interested in biofuels should check it out. The plant has never been bred for fuel production so there is an opportunity. Interesting side note, the area around Fukushima is planted with the stuff. Genetic mutations most likely have occurred. Among those mutations will be something that affects fuel production capability.

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    $\begingroup$ Welcome to the site and thanks for the info. If you can, post links to reference things. $\endgroup$ – DrZ214 Mar 4 '17 at 14:20
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NO! Both as originally asked and revised. NO!

Maintenance? Jet-engines are LUBRICATED with their FUEL. Ethanol does not have the necessary viscosity and lubricity to protect the moving surfaces. How about replacement after every flight!

The High-bypass fan at the front of the engine requires a certain amount of Shaft Horse Power delivered from the “Jet” portion at the back. The existing fuel lines can't deliver enough ethanol to achieve either full power or peak efficiency.

Burning ethanol will require re-engineering not limited to:

  • recirculating oil lubrication pumps
  • oil coolers (which increase drag)
  • larger fuel lines
  • ethanol safe seals

Ethanol's lower density alone does not prevent its use as the vast majority of commercial fights do not take off with full fuel and as such there has always been extra volume available. However, the longest direct flights such as L.A. to Sydney will require a fuel stop. Adding extra landings and takeoffs to an existing routes as well as fuel depots almost define the word problem.

Ethanol? NO!

Peanut oil? Yes

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    $\begingroup$ Adding extra landings and takeoffs to an existing routes as well as fuel depots almost the define the word problem. ehum, what? could you clarify? $\endgroup$ – Federico Apr 14 '16 at 7:36
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    $\begingroup$ @Federico current airliners and routes are built around the ability for the aircraft to get from point A to point B on its own fuel tanks - switching to a lower energy density fuel means you can no longer get from A to B on an airliners own fuel tanks (or rather the margins for the route are decreased, potentially past unsafe levels). This means you have to start thinking about either changing the routes so the aircraft makes a fuel stop part way there, or you build bigger fuel tanks into the aircraft. $\endgroup$ – Moo Apr 14 '16 at 10:33
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    $\begingroup$ @Federico Fuel stops mean more time, more wear and tear and more cost (you have to descend from cruise altitude, depressurise the cabin, land, take off, repressurise and get back to altitude), while bigger tanks mean more weight to carry, meaning more fuel needs to be burned to fly the same distances. $\endgroup$ – Moo Apr 14 '16 at 10:33
  • $\begingroup$ ah, ok. So there is seems to be an extra the in that sentence (the one in front of define) $\endgroup$ – Federico Apr 14 '16 at 10:48
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    $\begingroup$ Real jet engines are not lubricated by fuel. They have a separate oil system which (contrary to reciprocating engines) main purpose is to cool the bearings, only part of its purpose is to lubricate them $\endgroup$ – Chris V Apr 18 '16 at 7:20

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