Why do jet engines use kerosene rather than gasoline?

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

    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

    A better question is why do cars run on petrol rather than kerosene.

    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 the ignition for jets is different (spray into already ignited chamber rather than compress into piston)

    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.

    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?

    @MasonWheeler it's the same stuff (take a look at the third paragraph)

    @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?

    @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.

    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

    @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 thanks for the correction. I really need to stop running my mouth so much

    @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.

    @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.

    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.

    @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.

  • paul

    paul Correct answer

    6 years ago

    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

    Couldn't turbines just run on rampies ?

    The larger engines have been known to snack on them occasionally.

    Makes a change from birds I suppose. Pity that the environmentally-friendly free-range bird fuelled jet engine hasn't really had much success.

    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.

    @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.

    @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!

    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: 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).

    @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.

    What's a rampy?

    @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)

    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. :)

    I thought the biggest problem with coal dust as a turbine fuel was that it produced ash that damaged the blades.

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