Cars use gasoline but jets use something closer to diesel, maybe kerosene, that’s less combustible. Wouldn’t they need a _more_ volatile or explosive fuel to get the most thrust for performance?
In: Engineering
Jet engines run much hotter than a car engine, so they actually need fuel that is more stable at higher temperatures so that it doesn’t ignite until it’s time for it to ignite.
And believe it or not, the kerosene-like jet fuel can hold more energy in it for the weight than car gas. Weight is an important factor for aircraft, for obvious reasons.
You wouldn’t want your fuel to actually blow up, then the pods on those engines would explode every time you introduced fuel! You want a controlled burn to accelerate the gases and turn the turbine which turns a shaft that turns a huge fan in the front which puts basically *just enough* air through the low and high pressure compressors while the vast majority of the air moves around ducts and is never combusted. That is the principle operation of a turbofan engine.
That is the difference between a turbine reaction and a typical piston combustion engine. In a piston engine the ‘power’ phase needs to combust the fuel and air mixture with enough force to blow the piston down and the other ‘side’ of the crank up (thus causing another piston to go into compression or exhaust) and have enough twisting power so that if you connect something to the crankshaft you can impel motion. To do that you do need considerable explosive force. A turbine engine produces forward motion by moving air molecules that were in front of the engine to the back of the engine. The fan in front of the engine gobbles up a bunch of air and farts them out the back. It is why we sometimes call them ‘air eaters’. In order to do this we need to figure a way to turn the turbine blades which power the big fan in the front and the compression stages. A wind or water turbine can do this just by the pressure of the wind or the water, the mechanical force comes from gravity or the uneven heating of the earth, but a gas turbine slung to an airplane wing has to create its own. So, if you light up some pressurized air and fuel, you can blast the turbine with hot gases with enough energy to turn the whole shaft. It is more desirable to have a slower and more predictable burn than a fuel that will easily explode.
Octane ratings are not a measure of how much energy is stored in the fuel. They are a measure of how stable the fuel is: which is to say, how difficult it is to get it to combust. The higher the octane, the more energy input required to start the combustion. Different engine designs require a different octane rating for the fuel they run on.
In terms of actual energy per gallon, all these fuels you mention are relatively similar. Jet fuel is higher than gasoline but lower than diesel. But even that isn’t really related to the power output/performance of an engine. An engine design that can put to use 1 gallon per minute of jet fuel will obviously be way more powerful than a diesel engine that can only burn 1/10 of a gallon per minute, assuming absolute efficiency. Different fuels are chosen (or really, designed) for their behavior in certain conditions rather than just their energy density.
In a jet engine you want: lots of energy, high safety, easy operation.
As other commenters said, all petroleum fuels have pretty close energy density (it varies a bit but not enough to care). So you could use gasoline, diesel, kerosene, whatever.
For high safety you want *low* volatility. You want it to light when you want it to and *not* otherwise. Gasoline is volatile AF. Poor safety. Jet fuel (which is near kerosene or diesel) is great…very hard to light accidentally, and jets run so hot and continuous that lighting it in the engine isn’t an issue. Yay safety.
For easy operation you want a fuel that stays stable over wide ranges of time and temperature and pressure. Jet fuel is great for that. Gasoline sucks.
Now, you do want these features in cars too, so you really should be asking why cars use gasoline, rather than why do jets use jet fuel. Because basically everyone *except* cars uses something diesel-like for all the reasons above. And here we get to ignition…Otto cycle engines are the only ones that use a spark plug to run continuously. So the fuel ignition properties are *really* important to them, and basically nobody else. And so they’re stuck with gasoline.
Crude oil is made of several different chain of carbon, the chain in the 5 to 7 range are very light so they vaporize first and those are used as solvents and other chemical. Chain 7 to 11 is a bit heavier so they evaporate next and that’s gasoline. Next is chain 12 to 15 which is kerosene, then you have a bunch of other product like diesel, lubricant, oil, etc. They are not more or less refined than the others, they are just lighter or heavier chain of carbon, each with their own physical properties.
Gasoline have a decent energy density of 124 thousand BTU per gallon, it also have an high octane which make it work well in compression engine. What this mean is a low octane fuel would ignite in the high pressure of a piston engine before the fire from the ignition would reach it, this would create issues called knocking, which you don’t want.
Kerosene have an even higher energy density of 132 thousand BTU per gallon. It have a low octane, which mean it would be bad in compression engine like a pistol. But in a turbine, you don’t have this problem. On top of this, Kerosene have a lower freezing temperature than gasoline which is extremely important for an aircraft that might be going in high altitude where the temperature is low.
That’s why you can have a turbine on a land vehicle (like the Abrams) able to use several type of fuel The turbine make it ok for kerosene and since it’s not in high altitude, gasoline won’t freeze.
When you say volatile and explosive, you have some misconception. Volatile is how easy it evaporate at normal temperature, but engine don’t operate at normal temperature, making this physical property completely irrelevant to the energy content of the fuel.
As for explosive, well there is a misconception here. Explosiveness is the rate of oxidation (aka the rate at which it burn) and all those fuel have a low rate of oxidation, that’s why if you lit up gasoline on the floor it will only burn. It’s only when you a pressure that you can create an explosion out of fuel. Contract that to real explosive which oxide extremely fast and can create explosion on their own at ambient pressure.
Just because it’s lower in the distillation tower doesn’t mean it’s less refined. The rules to store and deliver jet fuel are much stricter. The product has very precise and controlled properties, like conductivity, particulate content. The tanks holding it are lined internally, and cleaned much more often. I could go on but I’d have to pull out a standard and I’m tired.
Jet engines are not subject to preignition or knocking.
As others have mentioned, kerosene, diesel and fuel oil have more BTU per unit of mass than gasoline.
One of the biggest differences is intermittent deflagration, which is what cars do, and continuous combustion, which is what jet engines do.
Car engines intermittently ignite fuel air mixtures to produce a discontinuous sequence of explosions. This kind of explosive combustion is a lot less efficient and tends to be dirtier than continuous combustion. Continuous combustion can be tuned to cleanly burn much heavier molecules, like kerosene. Also, due to the extremely high temperatures achieved by jet engines, these big heavy molecules experience some thermal cracking before they oxidize. Thermal cracking is where exposure to extreme temperatures and a limited amount of oxygen breaks big heavy molecules into lighter molecules due to the violent molecular vibrations that occur at extreme temperatures. Thermal cracking doesn’t happen well in discontinuous explosive combustion, but it can be optimized in continuous combustion such as in jet engines.
Piston engines rely on the fuel burning at a specific time during the cycle, when the piston has reached the top of its stroke, and different fuels can tolerate different amounts of compression before spontaneously igniting. This means the compression ratio of the engine is tailored specifically for one fuel. (though some engines inject fuel directly into the cylinder to get a better compression ratio than premixed air and fuel will allow)
Turbine engines are much less picky about fuel, because the fuel is burning continuously. It doesn’t have to survive compression while mixed with air without detonating prematurely. Performance is mostly a question of compression ratio and combustion temperature. More of each of those will get you more efficiency and performance, but turbine engines basically always run lean to reduce temperature, in order to keep the turbine from melting and also because higher combustion temperatures increase NOX emissions.
One thing that’s sometimes done to increase power is inject water alongside extra fuel, so you can burn more fuel without melting the engine.
Volatility is the ability for something to give off a vapor. Water is usually the relative zero point. Ethanol, the alcohol in liquor, is more volatile than water meaning if gives off a lot of vapor compared to water under identical conditions. Something like honey is very nonvolatile, it’s mostly gluconic acid and is quite strongly attracted to itself so it prevents much from coming off as a vapor. An open bottle of ever clear will make a room smell like a bar in a short time but honey doesn’t fill a room.
Energy density is the amount of energy we get from a unit mass of fuel. Usually given as joules(or mega joules) per kg or sometimes calories per kg but it’s all the same. I won’t do any combustion analysis or calorimeter so take it on good faith that these ideas are disconnected from one another.
Kerosene has a little bit more energy density than gas, but this is only one of the many reasons it’s used. First off if you Google “energy density of fuels” you’ll see than Natural gas, primary methane (CH4) and Propane (C3H8) are towards the top with methane being higher than propane and hydrogen, H2 being the highest. This isn’t because of the volatility of each, you have cause and effect backwards.
Each of those with high energy density have bonds that take little energy to break and produce very stable bonds that release a lot of energy when they react with oxygen. Hydrogen- oxygen is the best. Elemental hydrogen is just one proton and one electron, this is the simplest element possible. If you have this hydrogen reach with hydrogen you get something with a mass of 18, where only 2 of those amu on the combustion by product came from the fuel almost all the mass came from oxygen. Whereas if we burn methane we get CO2 and H2O and co2 has a mass of 48 where 12 of those amu came from the fuel and the water that came off the fuel. So with H2 we had a 2:18 or 1:9 fuel to oxygen mass ratio and with methane we have a 12:48 or 1:4. We have the water present in both but a larger and larger portion of the mass of byproducts come from the carbon in heavier and heavier hydrocarbons rather than the hydrogens. This decreases energy density as this ratio grows closer and closer.
The key is that the hydrogen releases almost as much energy as the carbon when it oxidizes as the carbon, but because of how much lighter that hydrogen it plays a bigger role in effecting the energy density. As such, the lightest compounds tend to have a higher energy density.
Ok so with that background I can answer the question. Gas has a lot of stuff that’s lighter but releases less energy than the things in kerosene. Mostly Benzene but many other aromatics. These aromatics are heavy and very stable, meaning they require a lot of energy to break bons in but release a comparable amount of energy and so the net energy release is actually lower. These aromatics have especially stable bonds due to resonance meaning that electrons can flow more freely though their structure. They can be quite light but be very bad as far as energy density goes.
Tldr: volatility and energy density aren’t the same thing. Gas is more volatile but less energy dense than kerosene because it contains junk that is light but doesn’t release that much energy
P.s. also kerosene has a higher enthalpy of vaporization which means it absorbs more energy when it makes the phase change from liquid to gas this actually helps keep the engine within a safe temperature range. The sr-71 actually used the fuel as a coolant around the body of the aircraft.
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