Jet Fuel is different. Or somewhat similar. Most of the non-aviation geeks and those who’re just starting out in aviation, must have wondered, What is Jet Fuel? How is it different from other fuels? What are the different grades/ types of jet fuels? And most importantly, is it sustainable?
We must answer all of these in this blog!
History Of Aviation Fuels
Aviation earlier was dominated by Piston Engines. The standard piston engines that were used in cars were tweaked in order to be used in aviation. The tweaks included making bigger bores, larger displacement area, longer strokes in order to provide more power to drive the heavy propellers that propel the aircraft.
Now that’s all good, but these engines can’t run on your standard gasoline or diesel. I mean yes they can but not quiet as efficiently.
AV Gas is a alternative to standard gasoline that can be used in aviation. AV Gas powered pretty much all the piston engines on the planet. Okay that’s fine, what about Jet Fuel though?
Why Jet Fuel?
In order to understand what is jet fuel? We must understand why did the need even arise?
With advent of air warfare, there was huge demand for faster flying, highly maneuverable aircrafts, but piston engines weren’t suited for these demands. They were bulky, heavy, didn’t produce much thrust. Now although this seems like a problem of aircraft engines but jet fuels played a major role in a few aspects of these problems. And then the Jet Engine came into existence.
Seriously! WHAT IS JET FUEL?!
Jet Fuel in the most simple sense is the fuel used in turbojet engine powered aircraft. Compared to standard gasoline or diesel it provides better efficiency, more power generation and better physical properties in hostile conditions.
But how is it different from other fuels? Answered in the next section.
How Is Jet Fuel Different From Other Fuels?
Based on the following criteria’s we can distinguish a jet fuel from any other fuel.
Jet fuel is colorless to straw-colored. Petrol and Diesel on the other hand are Pale Yellow and Yellowish Green in color respectively.
Jet fuels have a quiet bit more heat content than other fuels. Which means, these fuels can heat up more than other fuels.
How does that lead to good performance? Remember, more heat means more vibrating particles to extract energy from, hence, the turbines of the jet engine can extract more energy out of the flowing hot air, further producing more work.
Cold Temperature Performance
With increasing demand for better pans and more complicated war tactics, planes were to fly faster and at higher altitudes. With higher altitudes came colder temperatures.
Colder temperatures started freeing your standard AV Gas, and ice crystals formed in fuel tanks.
So Jet Fuel fixed it. It provided much lower freezing temperatures. Meaning, airplanes could fly longer at lower temperatures.
Hot Temperature Performance (Volatility)
Jet Fuels better volatility characteristics. Now what does volatility mean?
Volatility in most simple terms is, ability of an liquid to form vapors. I’m pretty sure you’ve smelled petrol at an petrol pump/station, that’s due to the vapors that petrol releases.
Now, another term Flash Point refers to the temperature at which a liquid produces enough vapor that when a source of fire is brought near it, the vapors burn/flash very briefly.
From the above table we can easily see that petrol starts forming vapors at temperatures above -43 degree Celsius compared to Jet A which starts producing vapors at temperatures above 38 degree Celsius.
Meaning, planes can fly longer in high temperature regions without having to worry about fire caused from fuel vapors.
Now, these differences are on the very basic level and do not dive much into detail. But hey, I can assure you these are enough* to get you started with the basic differences and grasp the understanding of why jet fuels are preferred over standard gasoline or AV Gas.
*More detailed study of other differences is highly recommended
Different Grades & Types Of Jet Fuels
Jet fuels are broadly classified into two main categories: Commercial Use Jet Fuel, Military Jet Fuels
Commercial Use Jet Fuels
Jet A is the most commonly used in aviation. It has flashpoint above 38 degree celsius. It has an energy density of 45 MJ/Kg.
Jet A1 on other hand is pretty much same as Jet A1. It’s just that Jet A1 is limited for use in The United States, and the rest of the world uses Jet A1. The properties between both fuels is pretty much identical.
Now I know, its tough to grasp these units so easily. But here’s a way to understand them and my favourite way too.
If you directly convert 45 MJ/Kg energy density of Jet A1 to Cal/Kg it gives us a value of 10,748,065.3482 Calorie/Kilogram, which means a Kilogram of Jet A has enough energy to heat 1 gm of water to 10.7 million degree celsius.
JET B is a mix fuel of naptha and kerosene. Naptha imparts higher volatility to fuel. Wait, wasn’t high volatility bad for aircrafts? Yes. But not in cold weather.
Cold weather thickens the fuel, suppressing its ability to produce vapors. If you know the basics of fuel volatility, as much as it is bad for aircraft safety it is also equally important for diffusion with air in the gas turbine diffusion chamber.
B’s highly volatile nature limits its use to very cold regions (Russia, Canada, Alaska) and for military applications.
A question now arises, what about commercial aircrafts that fly in cold temperatures, shouldn’t the Jet A freeze? or thicken at the very least?
Yes, it does thicken but not a lot due to fuel additives like Anti-freeze and the temperature is regulated using Fuel Cooled Oil Coolers (FCOC), hence, the temperature is maintained at the right level.
Military Application Fuels
The earliest fuel used in military aviation. It has a higher flash point, allowing longer and safer flights in hot regions.
JP-3 was introduce to improve availability. JP-1 has tight tolerances in production and use standards, in order to ease those standards, JP-3 was introduced.
50-50 kerosene-gasoline blend, with lower flash point and also better availability.
JP-5 has a higher flash point, meaning lower volatility and is cleared for use on Aircraft Carriers. JP-5 was specifically formulated for use on aircraft carriers.
JP-7 was used with J58 turbojet used on the fastest aircraft in history the SR-71 Blackbird.
Is the fuel which is widely used by US military currently.
Also known as High Energy Fuel (HEF) is the most energy dense fuel. Means, it can provide most energy for least amount. It was used in the XB-70 Valkyrie, XF-108.
There are other variants of JP namely the JP-2, JP-6 but weren’t discussed above due to their less relevance.
These fuels have been the hero of aviation for so many years, but have come under heavy criticism in the recent times, due to their emissions. Now, although the engines have been upgraded and researched a lot on to make them efficient, the fuel research industry remains stagnant.
This has led the aviation to look into new directions including Bio-Fuels. Which are far cleaner and low emissions fuels. Bio-Fuel industry has been booming in the recent years, and remains as the future of aviation. Airbus’s Zeroe aircraft series just seem to be pushing the green aviation initiative. As of now the future of jet fuel remains in the dark.
This blog is in no way to be considered a complete comprehensive guide on the topic. I wish to provide the basics of the topic in order for the readers to establish the basic foundation for further studies.
Alternative Jet Fuels – Stanford University http://large.stanford.edu › courses › greenbaum1
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