How Does a Jet Engine Work?

Whoosh.

Feb 11, 2021

Here it is, the moment I’ve all been waiting for. The crown jewel of this newsletter’s existence. So please silence your cell phones and fasten your seatbelts.

Jet engines are pretty cool. They can provide power that very few other things can. They have the ability to get hundreds of people into the air and can operate in the worst conditions. Their thrust capabilities are matched by nothing but rockets. (we’ll get into that later) The new GE-9X engine has a takeoff thrust of 110,000 pounds. [1] With great power, however, comes great complexity. These engines have multiple parts rotating at the speed of sound and must withstand temperatures over one third the surface of the sun. As a result, these machines take a whole lot of time and even more money to develop. But how exactly do they work, and how can they be improved? [2]

Engines can best be understood by following the flow of air through them. All jet-propelled aircraft start by sucking the air in with a fan. This is the front part you’re used to seeing on passenger planes. [2]

The fan’s primary purpose is to suck air into the engine. Once the air passes through this portion, it goes into the compressors. These typically have multiple stages and many blades, but they all serve the same purpose: to take the regular air and increase its pressure. Note here that increasing a gas’s pressure also increases its temperature. This stage is important because it is more efficient to burn the air at a higher temperature. Resultantly, we want to increase the pressure as much as we can. But we run into a problem: heat. Materials that we have can only withstand so much heat, so we are limited in how hot we make the air.

Once the air is pressurized, it goes into the combustion chamber, which speaks for itself. Jet fuel is sprayed into this chamber to mix with the air and is combusted. This greatly increases the temperature and energy of the gas. Again, though, we must worry about material properties because the components downstream of combustion can only handle so much. Typical temperatures at this phase can be up to 2000 degrees Celsius. The metal in the engine can only withstand 1300 degrees Celsius. [3] Resultantly, extremely complicated cooling methods must be used so that the components themselves don’t get as hot as the air flowing past them.

After combustion the air has much more energy. Part of this energy is captured in the next stage: the turbines. The turbines (usually there are two of them) are what make the compressor and the fan upstream spin. The air passing through this phase generates power much like a wind turbine or a gas generator. High energy air passes through the blades and makes them spin. This process actually takes a lot of energy out of the air, but it is required for the previous components to work.

Once through the turbine, the air exits out of the rear nozzle. This component looks like a cone, and pushes the air through a smaller opening, which accelerates it. This exhaust of air is what generates thrust, and the faster the air is going when it exits, the more thrust the engine generates. Think of this like a hose. A normal hose has decent water pressure, where the water comes out relatively quickly. But, if you put your thumb over the exit, and only allow a small area for the water to escape, it comes out much more quickly. The logical conclusion is to then make the nozzle of the engine tiny, to accelerate the air as much as possible. We can do this, but only up to a point. The limit is when the flow reaches the speed of sound, which is referred to as the flow being “choked”. Past this point we get into the realm of shock waves, di-con nozzles, and rocket engines, which we’ll get into in a later newsletter.

So now that we have a general idea of how the engine works, let’s start applying these phases to make them as efficient as possible. Looking back at the first stage of the engine, the fan, most modern-day commercial jet engines have a fan that is way bigger than the inner section of the engine. These engines only have a certain portion of the air go through the engine itself (compressors, combustion chamber, turbines, and nozzle), and the rest passes around the outside of it. This not only cools the engine, but it increases efficiency. Only the air that passes through the engine itself uses fuel. As such, to increase efficiency, we want to reduce the amount of this flow, called “hot thrust”. Note that the air that goes through the fan but around the engine is called “cold thrust”. This produces much less power but is more efficient because it doesn’t get combusted with fuel. There is a significant amount of research and design that goes into finding the optimal ratio of cold to hot thrust. This is often referred to as the bypass ratio. The higher the bypass ratio, the more cold thrust is used, and the more efficient the engine is. But with a decrease in hot thrust comes a decrease in pure thrust capability. Thus, depending on the application, the bypass ratio is different. For a commercial jet, where cost and efficiency are paramount, bypass ratios can be up to 12:1, but for fighter jets, where thrust and maneuverability are more important, the bypass ratios are closer to 1.5:1.

While increasing the bypass ratio improves efficiency by reducing hot thrust, and advanced cooling methods increase efficiency by allowing the engine to run at higher temperatures, commercially, engines have come to a sort of plateau. At this point there is only so much room for improvement. The next big idea is to turn to biofuels and electric powered planes.

These options are promising because they reduce the carbon emissions of the engine itself and have more sustainable production methods. The feasibility, however, all comes down to specific energy and energy density. Specific energy is simply the amount energy per unit weight of fuel and energy density is the amount of energy per unit volume. For example, jet fuel has a specific energy of 43 MJ/kg. This is a lot. Additionally, it has an energy density of 35 MJ/liter. [5] To put this into perspective, the most advanced batteries that we have today have a specific energy of 0.9 MJ/kg. [6] This would mean that in order to be carrying the same amount of energy, the batteries would weigh 48 times more than traditional jet fuel. Not to mention all of the additional weight of the batteries that isn’t usable chemical energy. Now this number is made slightly less daunting when we consider that electric motors are much more efficient. As a result, the weight probably increases more like 30x. With more weight comes decreased range. This is why electric propulsion in planes has only really been done on a smaller scale, where range is less important, and the aircraft don’t have to carry as many people.

Biofuels have a similar problem to batteries, where most of them cannot compete with jet fuel in terms of energy density. So, the aircraft has to be much larger, relative to what it’s carrying, than its competitors or it must sacrifice range. Additional issues with biofuels arise in material selection, because when they burn, they produce different chemicals that may corrode the currently used materials in the engines, necessitating a full redesign. The last large factor for both of these fuel sources is cost. Currently they are much more expensive than traditional jet fuel so until the cost can be competitive, fuel efficiency must be improved through continual optimization of the current engine designs.

I hope you found this description interesting and please let me know if there are more details that you would like to see explained. Thanks for reading and I’ll see you next time!

It's Not Rocket Science

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