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Probably one of the most complicated machines we have ever invented, the jet engine is a marvel of ingenuity.
It elegantly frames the relations between physics and engineering, theory and application. And yet most people have no idea how they work. Let us give you a quick tour. |
"To some people, there is no noise on earth as exciting as the sound of three or four big fan-jet engines rising in pitch. The very danger in the situation is inseparable from the exhilaration it yields. You are strapped into your seat now, there is no way back, you have delivered yourself into the power of modern technology. You might as well lie back and enjoy it." - David Lodge, Small World: An Academic Romance (1984)
First and foremost, let me say that the topic we will be covering is a tad outsized. Jet engines after all includes several subgroups of which will be mentioned in passing. However we will be concentrating mostly on turbofans, a specific type of jet engine which I find to be the most interesting, in terms of its history, intricacy, and the adherence to the engineering concept of technical trade-offs.
Although the principle of expelling pressured gas from a vent to extract mechanical power has been understood since Grecian times, by Hero of Alexandria no less, it has never caught on until the 20th century.
Since the Wright brothers made their great leap into the air in 1903, propeller-driven aviation had been the only option to getting off the ground. The key to the leap from an action-based (pulling the plane forward) to a reaction-based (pushing it forward) propulsion is to a system that is continuous and self-contained, that draws in air from the atmosphere as it pushes it behind the engine.
Although the principle of expelling pressured gas from a vent to extract mechanical power has been understood since Grecian times, by Hero of Alexandria no less, it has never caught on until the 20th century.
Since the Wright brothers made their great leap into the air in 1903, propeller-driven aviation had been the only option to getting off the ground. The key to the leap from an action-based (pulling the plane forward) to a reaction-based (pushing it forward) propulsion is to a system that is continuous and self-contained, that draws in air from the atmosphere as it pushes it behind the engine.
Something is missing here....
It was only in in 1928 that a young military engineer, Frank Whittle, solved this problem by merging the two processes together – a mechanism that extracts power from the hot gasses in the back, and use it to suck in more of it from the front. And he managed it by using a technology that have been around since the Ancient Romans: the turbine. Whittle basically linked the compressor and the turbine with a shaft; when the fuel ignites, hot gasses passes through the turbine, turning it, which then turns the compressor, which pulls in more air into the engine.
The Shaft
You may think that this solved everything, however, limitations in technology – especially metallurgy – requires for the shaft to be tough enough to withstand the high pressures from the ignited gasses. This leads to a design where the combustion chamber is limited in size, reducing the overall efficiency of the engine. This is called a “centrifugal flow” engine, due to the fact that air is pushed to the periphery of the protected shaft and ignited there.
As better materials were developed, the shaft itself became narrower and allowing the combustion chamber to occupy the axis – thus the “axial flow” engine. Air flows directly into the chamber, which leads to less drag, higher power output and fuel efficiency.
You may think that this solved everything, however, limitations in technology – especially metallurgy – requires for the shaft to be tough enough to withstand the high pressures from the ignited gasses. This leads to a design where the combustion chamber is limited in size, reducing the overall efficiency of the engine. This is called a “centrifugal flow” engine, due to the fact that air is pushed to the periphery of the protected shaft and ignited there.
As better materials were developed, the shaft itself became narrower and allowing the combustion chamber to occupy the axis – thus the “axial flow” engine. Air flows directly into the chamber, which leads to less drag, higher power output and fuel efficiency.
The Intake
To put it in the simplest terms, the intake is there to draw in air from the atmosphere. It’s probably the least complicated part of the jet, but then as requirements develop so does its design.
As explained earlier, earlier-generation jet engines uses centrifugal compressors, which pushes air to the periphery of the engine shaft. Intakes in these engines have to be narrower than the compressor itself as a wider ‘mouthed’ intake will allow too much air to flow in at too fast a speed. This leads to the dilution in air-fuel mix in the combustion chamber which at best affects output efficiency, and at worse resulting in no combustion at all.
In axial compressed engines air goes through the compressor and the speed and volume of air is controlled by stators (to be explained in the next section). As these more modern designs have the ability to regulate airflow, wider mouthed intakes are adopted, which is also more aerodynamic as you can see.
To put it in the simplest terms, the intake is there to draw in air from the atmosphere. It’s probably the least complicated part of the jet, but then as requirements develop so does its design.
As explained earlier, earlier-generation jet engines uses centrifugal compressors, which pushes air to the periphery of the engine shaft. Intakes in these engines have to be narrower than the compressor itself as a wider ‘mouthed’ intake will allow too much air to flow in at too fast a speed. This leads to the dilution in air-fuel mix in the combustion chamber which at best affects output efficiency, and at worse resulting in no combustion at all.
In axial compressed engines air goes through the compressor and the speed and volume of air is controlled by stators (to be explained in the next section). As these more modern designs have the ability to regulate airflow, wider mouthed intakes are adopted, which is also more aerodynamic as you can see.
The Compressor
The first part of the combustion trifecta. The compressor’s job is to increase the amount of air to combust in as little volume as possible; more air equals more oxygen, which will react to more fuel, resulting in more gasses to power the jet.
Centrifugal compressors (also known as ‘impellers’) rely on the shape of the engine to reach sufficient pressures, imagine pinching a hose so that the volume of whatever fluid coming out of it is restricted to a small opening, same principles apply. This has a downside of limiting the volume of air that can be combusted, and thus the efficiency and power output of the engine. We can blame the oversized shaft protector for this, which also serves as a reminder that in engineering EVERYTHING is connected.
In axially compressed engines, the linear alignment between the compressor, combustion chamber and turbine allows for a different method of compressing air. In axial compressors, there are two types of airfoils: rotors and stators. Rotors are the workhorse, spinning and pulling in air from the intake to ensure a continuous flow of pressurized air. Stators, as the name implies, are stationary, and are tasked to diffuse rotational force of the air into constant linear pressure. This is so that the airflow speed is predictable, allowing a consistent mix of fuel and air. Without the rotors, the engine can’t feed itself; without the stators the combustion process becomes unreliable.
The first part of the combustion trifecta. The compressor’s job is to increase the amount of air to combust in as little volume as possible; more air equals more oxygen, which will react to more fuel, resulting in more gasses to power the jet.
Centrifugal compressors (also known as ‘impellers’) rely on the shape of the engine to reach sufficient pressures, imagine pinching a hose so that the volume of whatever fluid coming out of it is restricted to a small opening, same principles apply. This has a downside of limiting the volume of air that can be combusted, and thus the efficiency and power output of the engine. We can blame the oversized shaft protector for this, which also serves as a reminder that in engineering EVERYTHING is connected.
In axially compressed engines, the linear alignment between the compressor, combustion chamber and turbine allows for a different method of compressing air. In axial compressors, there are two types of airfoils: rotors and stators. Rotors are the workhorse, spinning and pulling in air from the intake to ensure a continuous flow of pressurized air. Stators, as the name implies, are stationary, and are tasked to diffuse rotational force of the air into constant linear pressure. This is so that the airflow speed is predictable, allowing a consistent mix of fuel and air. Without the rotors, the engine can’t feed itself; without the stators the combustion process becomes unreliable.
The Combustion Chamber
The heart of the engine, without which would turn it into a random modern art piece. The chamber (also known as the ‘combustor’) is where all that air-fuel mixture is ignited to produce as much hot gasses as possible. In order to maximize its efficiency it has to withstand as much heat and pressure as possible.
As you may have learned in basic chemistry gases, by virtue of their more exited molecules, occupy more space than liquids and solids. This goes further as the temperature of the gas increases, agitated molecules need more space…just like people?
In a centrifugal jet engine, the combustor is spread along the periphery of the shaft, dispersing the air-fuel mixture along a larger space, thus not enough pressure is generated to fully utilize the energy released during combustion. As technology marches on, axial combustors and compressors allow for the constant flow and compression of the fuel mixture in a smaller, centralized space, resulting in more efficient combustion, and as an environmental bonus less unburnt fuel.
The heart of the engine, without which would turn it into a random modern art piece. The chamber (also known as the ‘combustor’) is where all that air-fuel mixture is ignited to produce as much hot gasses as possible. In order to maximize its efficiency it has to withstand as much heat and pressure as possible.
As you may have learned in basic chemistry gases, by virtue of their more exited molecules, occupy more space than liquids and solids. This goes further as the temperature of the gas increases, agitated molecules need more space…just like people?
In a centrifugal jet engine, the combustor is spread along the periphery of the shaft, dispersing the air-fuel mixture along a larger space, thus not enough pressure is generated to fully utilize the energy released during combustion. As technology marches on, axial combustors and compressors allow for the constant flow and compression of the fuel mixture in a smaller, centralized space, resulting in more efficient combustion, and as an environmental bonus less unburnt fuel.
The Turbine
The turbine ties the system together. It’s main job is to extract the power generated from the combustor into mechanical energy to drive the compressor, resulting in a closed and self-reinforcing cycle. Think of it like a windmill that blows the wind that turns it.
As the turbine have to take a constant flow of heat and pressure, it’s construction is probably the most complicated. In modern engines each turbine blade is made from a single heat and pressure resistant ceramic crystal. The keyword here is “single”, meaning that the whole thing is structurally flawless, resulting in greater strength and resistance to heat as well as structural deformation. In the past materials such as steel and titanium alloys had been used, though the requirements for weight reduction and the advancement of ceramic and composite materials allows for stronger, lighter and better turbines.
The turbine ties the system together. It’s main job is to extract the power generated from the combustor into mechanical energy to drive the compressor, resulting in a closed and self-reinforcing cycle. Think of it like a windmill that blows the wind that turns it.
As the turbine have to take a constant flow of heat and pressure, it’s construction is probably the most complicated. In modern engines each turbine blade is made from a single heat and pressure resistant ceramic crystal. The keyword here is “single”, meaning that the whole thing is structurally flawless, resulting in greater strength and resistance to heat as well as structural deformation. In the past materials such as steel and titanium alloys had been used, though the requirements for weight reduction and the advancement of ceramic and composite materials allows for stronger, lighter and better turbines.
Ponder this
- It seems rather wasteful that all the power extracted by the turbine to drive the compressor is wasted when the stators slow down the compressed airflow. How do we improve this while maintaining the output and purpose of each component?
- How do we improve the output in terms of materials used to construct the engine? What about the materials used to power it?
Discuss
If a hypothetical fuel exists that only produces combustion pressure, without the heat and light as from fossil fuel combustion, how would you change the engine to adapt to this new fuel?
Will the design be the same? What about the components themselves? What can we throw out? What do we add?
Further readings
Jet engine, at Wikipedia.
Frank Whittle, the father of the jet age.
Brayton cycle, the principle behind jet engine designs.
