Jet Fuel

[surface wind]

One of my kids wants to know

1. What fuel is used in an a/c
2. What is the difference between Jet A1 and AVGAS
3. What are they refined from....and which fuel i.e. Diesel or Petrol do they most resemble

Thanx

SW

[Genghis the Engineer]

(1) Many different types of fuel, depending upon the engine.

(2) Jet A1, also called AVTUR, is jet fuel. It is basically a heavy oil which after mixing in the right way with air can be made to burn in a controlled way giving off a lot of energy in the form of heat - this causes gas to expand, which in turn is used to push the aircraft.

AVGAS is the fuel for a "spark ignition" engine, this is what you'll find on most light aircraft. Mixed in the right way with air it can be made to burn when it's exposed to a spark (from a spark plug). It is a much more "volatile" fuel, that is it will combust at lower temperatures and pressures than are needed for AVTUR.

(3) Jet-A1 is very similar to Diesel, in fact although it doesn't do either engine a lot of good you can run a jet engine on diesel, or a diesel engine on Jet-A1.

Avgas is basically petrol with a high octane number and a lot more lead. Again, the settings aren't quite right for this in some cases, but you can run a petrol engined car on Avgas or a piston engined aeroplane on ordinary petrol.

G

[Intruder]

Jet A probably resembles kerosene more than Diesel, if your son is familiar with kerosene or "lamp oil." Jet A1 is Jet A with additional anti-icing additives to reduce the freezing point from -40 deg (C or F, your choice) to -47.

Commercial jets and turboprops use Jet A, A1, or A50 (further reduced freezing point), whatever is available at the airport where they refuel. Northern airports like Anchorage, Alaska pump Jet A50, most US airports pump Jet A, and many other countries primarily pump Jet A1.

Avgas is high-octane gasoline/petrol. While petrol for cars is 87-92 octane (US rating system) and no longer contains lead, Avgas ranges from 100 to 115 octane and is still leaded. Only piston-powered aircraft use Avgas, and only the smaller engines use the 100 octane version.

[Old Smokey]

Valid responses surface wind, but if your kid's question related to the chemical composition, then consider the following -

Petrol engined cars and piston engined aircraft run on gasoline, whilst jet engines run on kerosene. Both gasoline and kerosene are derived from the Methane Hydrocarbon series. Gasoline is typically a mixture of Septane and Octane (7 and 8 Hydrogen atoms per molecule respectively), whilst kerosene covers the band from Nonane to Hexadecane (9 through to 16 Hydrogen atoms per molecule respectively).

Methane, at the start of the chain is totally gaseous, and as we progress further up the chain, volatility decreases, and calorific content increases. Septane was fine for early low compression piston engines, but increasingly higher proportions of Octane had to be added with increasing compression to avoid detonation, thus the Octane rating, with 100 Octane fuel being 100% Octane. To go beyond a 100 Octane rating, Nonane could have been added, but wasn't, as the combustion temperatures were too high for piston engines, thus the addition of Tetra-Ethyl Lead to supress detonation.

Wide cut kerosene has the disadvantage of a relatively low flash point, particularly if there is Nonane or Decane included, and civil operators have shied away from this, opting to go "further up the chain" for less volatile (safer) fuel with increased calorific value. The higher you go, the more energy available per unit of mass of fuel (1 Kg of kerosene contains more energy than 1 Kg of Dynamite). Narrower 'cuts', higher up the series form the normal variations in the commonly used jet fuels today. Mention has been made by earlier posts of fuel freezing point, and the 'native' freezing point of a particular fuel will depend upon the fuel cut. Although only 8 molecules exist between Nonane and Hexadecane, these form thousands of isomers, each with their own distinctly different freezing point.

No mention has been made here of the different cuts, JP4, Jet A1 etc., that's another story, and, as Genghis has said, the jet engine will run on just about anything. Your car will run on kerosene too, for the first few miles............

Regards,

Old Smokey

[The SSK]

I'm told (I'm no expert...) that jet fuel is very similar to domestic heating oil, and the refineries can switch production between the two, according to demand.

So if the US Northeast has a cold winter, demand for heating oil surges, there's less refining capacity available for jet fuel, consequently the price goes up.