Despite the fact that these two types of fuel bear seemingly completely different names, jet fuel and diesel fuel have more in common than one might think. Historically, as the fuel types developed to the wide variety that an engine operator might choose from today, jet fuel and diesel were quite similar in extraction, production, composition, and even their application.
However, the names for those two types of fuel are different for a reason
The main difference between diesel and jet fuels is their different composition, they are also more or less diverged in questions related to their extraction and more often as the time got by – in terms of composition, application, and broader use.
Here we will discuss all of them while also paying enough attention to the historical context and today’s application of these two different types of fuel.
So what exactly is jet fuel?
The definition of jet fuel represents aviation turbine fuel (which also often goes by the abbreviations such as ATF or avtur) which is just one of the types of aviation fuel and is specifically designated to power gas-turbine engines.
Aircraft with spark-ignition engines, including gasoline engines, on its part, is using aviation gasoline, also known as aviation spirit in the British English-speaking world.
They’re quite strict standards regarding how such aviation fuels are produced and distributed. And on such standards, the entire aviation industry relies on.
And what does it look like?
Depending on its characteristics, jet fuel is usually colorless or can have a pale yellow color. Such a type of aviation fuel is a mixture of different hydrocarbons. The exact ratio of specific hydrocarbons in a given type of jet fuel is virtually impossible to determine as it is mostly based on petroleum sources which, on its part, vary widely.
The specific molecular mass range between hydrocarbons is defined by the standardized requirements for such aviation fuel, such as its smoke point.
The main types of jet fuel
While there are many types of jet fuel alone, the most common ones which are used today in commercial aviation to power gas-turbine engines are Jet A fuel, along with the types of Jet A-1 and Jet B. The main difference between all three is attributed to different freezing points – Jet B has a lower freezing point than Jet A fuel or Jet A -1 fuel.
Another significant difference between those types of jet fuels is the required use of an anti-static additive in the composition of Jet A-1. In contrast, Jet A, Jet B, and other types of jet fuels are usually distributed without it and therefore are even cheaper to produce.
However, most commonly used jet fuel doesn’t mean the most widely used aviation fuel – as Jet A and Jet A 1 are, with a few exceptions, used only in North America.
For example, in other parts of the world, in post-Soviet countries, a TS-1 type of jet fuel is used to power aircraft engines. This type of jet fuel is known to have even higher volatility than Jet A-1, with its flash point being 28 °C (82 °F) and a freezing point even below −50 °C (−58 °F).
As the use of the alternative fuel in aviation is still rather in the phase of active research than on the path of practical implementation, it is probably too early to mention such experimental types of liquid fuel, except maybe biodiesel, which is known to be made from vegetable oil.
And what is diesel fuel?
Diesel fuel is comparable to jet fuel as a specific fractional distillate of petroleum fuel oil. However, as it is designed to power engines, specifically built to run on diesel fuels, there are some differences between diesel fuel and jet fuel.
So what about differences?
One of the main differences, as was already mentioned, is the application of these distinct types of fuel.
While, in theory, they may be used one instead of another quite often, the difference in specifications and legal requirements would make it highly inefficient or even dangerous.
Yet, in practice, there are a few successful examples where diesel fuel is used in gas-turbine engines and vice versa – jet fuel, even sometimes more expensive Jet-A type, is powering diesel engines.
Jet fuel in a diesel engine?
Exactly. The most frequent of such situations is related to the use of jet fuel in the ground-support fleets at airfields and airports where the same fuel goes into aircraft fuel systems as well as into compression ignition engines of diesel cars. Here different buses, pickup trucks, cars, and other vehicles burn jet fuel and sometimes do so in their fuel systems in large quantities.
The absence of the need to equip such facilities with diesel gas stations, as well as the opportunity to keep such vehicles within the airport premises without the need for them to leave for refueling elsewhere, makes such a decision economically viable despite higher jet fuel, especially a Jet-A type price in comparison to automotive diesel.
Despite higher operating costs – if such costs are put out of any broader economic context (such as the context in the example above) – using jet fuel in diesel engines or diesel gas fuel system has at least a few other disadvantages, and the reason for such penalties lies in the main differences between the diesel fuels and jet fuels.
And what are such differences?
Usually, the most widely noted difference in such context is the completely different lubricity. Jet fuel, especially a Jet-A type, is very ‘dry’, compared to diesel fuel. While the first one is considered a relatively high sulfur fuel, its lubricity is completely acceptable for gas-turbine engines.
However, a diesel engine isn’t built the same way
The injector system in the diesel engine needs lubricity additives to be added to road diesel fuel to be working correctly for a long time. Technical additives are also required for other parts of diesel engines. Without the refinements, any powertrain of such a vehicle is more likely to be worn out much faster than it was designed to.
Any other differences?
There are a lot more, for example, viscosity specifications. While both fuels can be made with the exact same viscosity, for the successful operation of the exact type of engines they are made for – be it gas -turbine-powered aircraft engines or heavy truck diesel engines – in practice, such two different types of fuel are made with entirely different viscosity.
While lower lubricity is becoming more likely as the viscosity decreases, using one fuel instead of another won’t necessarily cause instant damage to the engine. Still, it may cause long-term wear of pumps, injectors, and different powertrain parts.
Another example could be a cetane number. In different parts of the world, just like some higher octane fuel for the gas-powered car, pipeline diesel is manufactured with a required cetane number while Jet A and jet fuel of other types usually have no minimum cetane rating.
Can diesel fuel be used to power an aircraft?
Diesel fuel can be used, and it is used for different power types of aircraft. However, nowadays, putting diesel fuel into a plane is not common at all.
As of today, you most definitely won’t see a commercial aircraft, let alone, jet engine is, powered by diesel fuel under any normal circumstances.
Yet, historically, many diesel-powered engines were built by more than one engine manufacturer specifically for planes. And with technological development gaining pace, even in the last years, a number of on-road diesel engines have been converted to and even subsequently successfully certified for aircraft use.
The low popularity of diesel aircraft engines lies mainly in the comparatively low power-to-mass ratio of diesel engines.
Such engines are usually heavy, and the more power they produce, the heavier they need to be – if that is not a significant disadvantage for most ground transportation vehicles, such circumstance is rather crucial for any type of aircraft.
But there were a lot of diesel-powered airplanes before the war
In fact, before the introduction of petrol direct injection, the use of diesel engines in planes had several notable advantages. Namely, knowing all the dangers of fuel-air mixture, they were considered safer as such engines could not catch fire like any gas engine.
In addition, they were considered much more reliable than other types of engines used and required relatively very little maintenance.
Any other advantages?
Low fuel consumption was also a competitive advantage at the time as commercial aviation was in its early infancy and jumbo jets – along with biodiesel made from vegetable oil – still were nothing more than something that could be – at best – taken somewhere out of a science fiction novel.
Yet, just a couple of decades later, it became obvious that burning jet fuel much faster is still more economically viable than carrying a significant load of heavy diesel engines, even considering the rising fuel costs.
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Jet pilot @NASA
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