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Could you run a jet with gasoline? Why do all jet engines use kerosene?

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yippy_yay
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  • Posting a comment because I only have a partial answer: "The PT6A engine is approved for operation with all commercial jet fuels, JP-4, JP-5 and for a maximum of 150 hours during any overhaul period with all grades of aviation gasoline. Specific grades of diesel fuel are approved as alternate fuels for restricted use."

    http://www.pwc.ca/files/en/Know_your_PT6A.pdf

     – Brian Mar 04 '15 at 23:18
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    A better question is why do cars run on petrol rather than kerosene. – Aron Mar 05 '15 at 05:38
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    Cars run on petrol or diesel (which is similar to kerosene). As both are piston engines, the differences are quite big, e.g. diesel engines have twice the compression of petrol engines, different ignition and so on. You can not run the one with the other fuel. But it is not so clear why it's different for jet engines. – sweber Mar 05 '15 at 08:45
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    @sweber the ignition for jets is different (spray into already ignited chamber rather than compress into piston) – ratchet freak Mar 05 '15 at 08:47
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    Of course. I mean: For piston engines, there's a big difference between petrol and diesel. However, it seems to be no big difference if you spray kerosene/diesel/petrol into an already burning flame. So, while the difference for piston engines is obvious, it is not for jets. We already know, the PT6A takes avgas and kersosene. So, I don't understand Arons comment. – sweber Mar 05 '15 at 09:33
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    Every time I hear people say "kerosene" as jet fuel it just seems weird. I grew up on a farm; I know what kerosene is. It's the stuff you put in a lantern so you can see to go out and milk the cow at night. It's much less volatile than, say, gasoline. It burns slowly and (relatively) cool, which makes it great for a lantern, but--I would imagine--horrible for motor fuel. Is this a different type of kerosene? – Mason Wheeler Mar 05 '15 at 11:13
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    @MasonWheeler it's the same stuff (take a look at the third paragraph) – raptortech97 Mar 05 '15 at 11:47
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    @raptortech97: So why is something that--at first glance at least--seems to be a much lower-quality fuel than gasoline used in an application that poses a much higher energy demand than even the heftiest gasoline motors face? – Mason Wheeler Mar 05 '15 at 12:40
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    @raptortech97: I didn't say energy density. I said that the energy demand (the required output) is much higher for aviation than for ground transportation, so why use a lower-density fuel? That seems inefficient. – Mason Wheeler Mar 05 '15 at 12:47
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    Military turbine engines are designed to operate on everything from avgas to heating oil but are optimized to run on JP4 / JP5 (roughly the weight of kerosene). If you use one of those other fuels, the engines will require extensive maintenance afterwards. Jet fuel does have a higher energy density than gasoline: http://en.wikipedia.org/wiki/Energy_density#Energy_densities_of_common_energy_storage_materials – Jim2B Mar 05 '15 at 14:53
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    @raptortech97 - Jet fuel does have a slightly higher energy density than gasoline, both by weight and by volume: 46 MJ/kg and 37.4 MJ/l versus 44.4/32.4 for gasoline (and 48/35.8 for diesel fuel) – Johnny Mar 05 '15 at 20:56
  • @Johnny thanks for the correction. I really need to stop running my mouth so much – raptortech97 Mar 05 '15 at 21:32
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    @MasonWheeler They're planning to make paraffin (candle wax) rockets; the stuff in a match head is just as incendiary bombs in the military. Just because something looks innocent, doesn't say it is in all applications. A lantern can burn for hours - imagine that energy being released (with)in seconds. – Sanchises Mar 06 '15 at 10:08
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    @Johnny Looking on the web gives me all kinds of different specific energy #'s. This site shows gasoline at 45.8 and kerosene at 46.3, but jet fuel lower at 43.3. – TomMcW Aug 10 '15 at 15:30
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    This site shows the same #'s. This site shows several different numbers from different sources. They all seem to show a different - and lower - number for jet fuel than for kerosene. – TomMcW Aug 10 '15 at 15:40
  • @MasonWheeler: Liquid kerosene burns much more slowly than liquid gasoline. It also has a much lower vapour pressure and much higher flash point than gasoline. When sprayed as a fine mist into what is essentially a raging fire, it burns just as fast and as readily as gasoline. Thus, kerosene works just as well as jet fuel as gasoline would (actually, it probably works considerably better, due to its lower vapour pressure, and thus lower boiloff, reducing fuel losses), and is far safer to handle to boot. Trying to use gasoline for an airportfull of jets would be hideously dangerous. – Vikki May 27 '18 at 02:52

6 Answers6

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You can persuade a turbine engine to run on just about anything that can burn. So the decision of which fuel to actually use depends on the side factors including, but not limited to:

  • availability
  • cost
  • emissions
  • hot section temperature
  • chemical reactions with engine parts

Specific examples:

  • Coal dust is rather difficult to pump around, and the rampies don't like shovelling
  • liquid hydrogen (used in the Space Shuttle) requires a lot of storage and has the nasty habit of freezing anything it touches, like rampies.
  • ethylacetylenedecaborane is unpleasantly toxic (rampies union again) and the combustion byproducts were rather abrasive to the engine's innards
  • trimethylaluminum would reduce the engine complexity (no igniters needed) because it has the nasty habit of igniting instantly upon contact with air, so leaks are rather dangerous.
  • natural gas is commonly used as a turbine fuel in pumping stations: it's already there and thus is "free". The required pressure vessels make it impractical to use as an aircraft fuel.

So kerosene basically became the standard turbine fuel because it's:

  • cheap: kerosene makes up a rather large fraction of crude oil. When you measure your fuel load in tons a few cents per litre makes a difference.
  • safe to handle: relatively non-toxic, doesn't ignite all that easily
  • storable and transportable in common structural metals
  • doesn't clog up the engine
paul
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    Couldn't turbines just run on rampies ? – NobodySpecial Mar 05 '15 at 02:38
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    The larger engines have been known to snack on them occasionally. – paul Mar 05 '15 at 02:51
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    Makes a change from birds I suppose. Pity that the environmentally-friendly free-range bird fuelled jet engine hasn't really had much success. – NobodySpecial Mar 05 '15 at 02:58
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    Industrial gas turbines can and do run on just about anything. There's a stream called "refinery gas" which translates to "anything lighter than pentane that the refinery doesn't want & can't store". This includes hydrogen, carbon monoxide, and hydrocarbons. Frequently they are mixed with inerts like nitrogen (mixtures of hydrocarbon and nitrogen are a pain to separate.) The same turbine will be expected to run on kero or gas oil when that is in excess, or for startup. Water is also injected for NOX control. The only issue, as you say, is not to leave a solid residue that will clog the engine. – Level River St Mar 05 '15 at 08:35
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    @NobodySpecial : Give me some rampies too, I love all kinds of pies... A tad more serious: Is kerosine really used as a lubricant? I heard this the first time but it sounds logical. – Patric Hartmann Mar 05 '15 at 21:17
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    @PatricHartmann -- Kerosene and diesel need to have lubricity properties to keep the high-pressure fuel injection pumps in compression ignition (gas turbine, diesel piston) engines from falling apart -- they can't be oiled otherwise! – UnrecognizedFallingObject Mar 06 '15 at 01:17
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    Perhaps nice to mention why piston engines are so picky: gasoline should only ignite when you hold a spark to it; diesel should ignite immediately when it's sprayed into a hot cylinder. In a jet engine, fuel is sprayed into apocalyptic burning conditions anyway, so one can have a less 'picky' fuel. – Sanchises Mar 06 '15 at 10:13
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    @sanchises: Gasoline is easier to ignite than diesel or kerosene. The reason spark-ignited engine can only run on gasoline is that the spark would not ignite the heavier fuel at all or it would burn too slowly. In the compression-ignition engine the temperature is much higher so it ignites even diesel. Incidentally this also increases the thermodynamic efficiency (which is why diesels have about one third lower consumption than gasoline engines). – Jan Hudec Mar 06 '15 at 13:11
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    @UnrecognizedFallingObject : Thank you very much for clarification! It does make sense now, I just never had really though of this issue. Thanks for making this clearer. – Patric Hartmann Mar 06 '15 at 19:43
  • What's a rampy? – MarcusJ Sep 22 '15 at 19:59
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    @MarcusJ slang term for ground crew; i.e the guys who load and unload the planes, fill them with fuel, etc. on the ramp (or apron, depending where in the world you are) – Nigel Harper Sep 25 '15 at 12:33
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    I see, I'm 'Murican, just never heard that term before, but I don't know too much about aviation or airports, but I'm interested. :) – MarcusJ Sep 26 '15 at 01:33
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    I thought the biggest problem with coal dust as a turbine fuel was that it produced ash that damaged the blades. – Peter Green Sep 22 '17 at 13:23
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In a modern turbofan engine, fuel is not only burned in the engine and used to lubricate parts such as fuel pumps and controls, it is used as a hydraulic fluid as well -- this is used to power things like inlet guide vanes and variable stator vanes in many engines, as well as more exotic accessories such as movable nozzles and inlet ramps.

This means that gasoline is often not tolerated by larger aviation turbines, as it boils at such a low temperature that it could boil off inside fueldraulic (or other fuel system) parts and interfere with their operation, atop the lubricity and lead fouling issues that it obviously would pose. Even wide-cut jet fuels such as JP-4 and Jet-B are prohibited for service in some larger turbofans due to the volatility issues they pose (this is a quote from the 777 QRH Limitations section):

The use of JP–4 and Jet B fuels is prohibited.

UnrecognizedFallingObject
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From my training, the limits on the PT6 use of avgas is related to its ability to lubricate the engine's fuel pumps, and the lead fouling of the hot section which will result from the avgas. I can't say about other engine's tolerances, but some military jet fuels have much more volatile components than straight kerosene and marine gas turbines run on diesel. A turbine's fuel isn't always decided by what it can burn, but by what it's practical and economic to feed it.

Thomas McKelvey
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Apologies if this is tangential but other properties of kerosene (aka kerosine) as turbine fuel were brought up. To my knowledge, all "Jet fuels" (intended for aircraft use) are based on kerosene.

Another property of jet fuel that was not mentioned is freeze point where viscosity drops because of wax formation and pumps and filters begin to clog. Ordinary kerosene (as used in lanterns and space heaters) rarely has to deal with sub-zero temperature (e.g. -40C) and 30,000 feet altitude.

Also important is volatility which can be reduced at low temperatures and impede combustion.

see http://www.shell.com/global/products-services/solutions-for-businesses/aviation/shell-aviation-fuels/fuels/types/civil-jet-fuel-grades.html for different fuels and their freeze points.

diogenes
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We ran Olympus Gas turbines for fast power and speeds when I was in the Royal Navy. These happened to be the same turbines that Concord used when she was in service. We ran them on Marine Quality Diesel and had no problems.

Again may be its the - temps could be a real issue, and the fact you get more power from Higher octane fuels, with all the technology these days you would think their would be a cheaper alternative.

Paul
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The very high temperatures of jet engines cause gasoline to be a poor fuel because it tends to burn too fast. Kerosene, which is routinely called "Fuel OIL" some places, avoids pre-ignition problems (and some safety hazards) just like higher-octane gas avoids spark-plug knocking. The ultimate control of ignition comes from using Diesel Fuel (which ALSO is routinely called Fuel Oil some places), and that's why big trucks use Diesel: that control gives them the best fuel efficiency their engines can have; but Diesel wont' run a jet engine. Gasoline is too volatile for a jet engine; Diesel fuel is not volatile ENOUGH for a jet.

Tom Johnson
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    The problem with running diesel in a jet engine is the igniters -- the SR-71's triethylborane system would ignite diesel no problem! (Diesel won't work in the SR-71 through its envelope for various other reasons, but the ignition system in that aircraft isn't one of them, and I suspect a J58 would run on diesel in an emergency, albeit with a severely limited flight envelope.) – UnrecognizedFallingObject Mar 06 '15 at 01:02
  • Gasoline's extreme burn rate makes it a poor fuel for a much simpler reason: the difference in burn rates is even more conspicuous in the liquid fuels. A flame will spread over the surface of a pool of gasoline something like six times as fast as over a pool of kerosene, and gasoline also has a much higher vapour pressure and a flash point below most normal ambient temperatures (!). Even Jet B, which is a mixture of kerosene and gasoline, has safety problems due to gasoline's overenthusiasm for catching fire; pure gasoline is far too dangerous to use as jet fuel. – Vikki May 27 '18 at 02:58