How Does a Jet Engine Work? Understanding the 6 Main Types

Jet engine technology is one of the most impressive marvels of the aviation and aerospace industry. Without it, none of us would have a career!

Whatever aircraft type you have flown, maintained, or admired over the years, the technology inside those engines gets crazier every day ⚙️

Manufacturers are pushing the boundaries of what is possible. Currently, there are 6 main categories of jet engine.

From TurboJets in your average plane, to ScramJets that could launch us into hypersonic passenger travel in the future 🚀

We’re going to keep things as simple as possible, and go over exactly how all these jet engine types work! ⤵️

We don’t publish all our Notes from the Cockpit (like this one) publicly, some are shared only by email. Get the next one sent straight to your inbox ⤵️

What is a Jet Engine?

A jet engine is the part of an aircraft that generates thrust or power. In most cases it uses Newton’s 3rd law to create forward motion.

Let’s refresh Newton’s 3rd Law:

So, if we’re able to collect a lot of air and push it away behind us, it will propel us forwards or generate kinetic energy: Action = reaction!

So how do we achieve this for aircraft in use today?

Jet engines use a very simple (but hard to perfect) principle that involves 4 steps:

1) Suck 🌪️(Collect as much air as possible to convert it into energy later)
2) Squeeze 🙏🏼(Compress the air to increase its pressure and temperature)
3) Bang 💥(Combine the compressed air with fuel and ignite it to get massive expansion)
4) Blow 💨 (Accelerate the air backwards through a turbine to drive the compressor, and create thrust acting forwards)

To get a birds-eye view of this process, we can map the process on a pressure – volume graph.

• Point A is the air at atmospheric pressure as it enters the inlet 💨
• Point B is the compressed air after it has gone through the compressor, entering the combustion engine as it is ignited under a relatively constant pressure 🙏
• Point C is the volume increase that results from the ignition in the combustion chamber🔥
• Point D is the reduction of pressure by letting the air go through the turbine stages (which drives the compressor and sometimes other systems), and exhaust back into the atmosphere 🚀

This 4 step process is relevant for all 6 main types of Jet Engine, which are:

So how do the 6 main types of Jet Engines in use today accomplish the same 4 steps?

How Does a TurboJet Engine Work?

A TurboJet is considered the most basic type of Jet Engine.

For the 4 step process we discussed earlier, we need:

1) The Air Intake (or Inlet)
2) The Compressor
3) The Combustion Chamber
4) The Turbine
5) Exhaust

1) The Air Intake

A Jet Engine sucks in air via the air intake (or inlet) by using a fan, or the front stage of the compressor. For large passenger planes, the amount of air getting sucked in can reach over 1.3 tonnes of air every second, depending on the size of the engine!

The main job of the inlet is to provide the compressor with a constant, smooth supply of air. Air doesn’t always enter the inlet in a ‘straight’ line, so one of the objective of the inlet is to redirect the air at the correct angle 📐

If the air supply isn’t constant and at a decently steady velocity, we can get problems like compressor stall, or other starting issues.

2) The Compressor

The compressor has 1 goal: Increase the pressure and temperature of the air going into the engine. This is to make sure the air reaches an optimal condition for mixing with fuel and ignition 🔥

The higher the difference between the pressure before and after the compressor, the higher the overall efficiency and thrust in general. This is called the pressure ratio: high pressure ratio = efficient engine!

Engine manufacturers are pushing the boundaries every year on how much they can increase this ratio.

There are 2 main options here: a centrifugal or a axial flow compressor.

A centrifugal compressor (also called an impeller), is a rotating disc that spins around it’s own axis. It is forcing the air to flow radially outwards to the edge, and causing a pressure rise at the end before it enters a diffuser, which will convert the energy of the flow into pressure, like the graphs show:

However, most compressors in TurboJets are axial flow compressors. For these, there are a number of alternating stages between stationary stators, and rotating rotors.

The rotor stages draw in the air and push it towards the next stage of compression. The stator stages have 2 main objectives:

1) Reduce the velocity of airflow that was increased by the rotor, to increase pressure

This looks like the image below, note how every time velocity reduces each stage, it increases overall pressure:

2) Act as an airflow guide, to make sure the air hits the next rotor stage at the correct angle of attack.

It’s a delicate balance between the angle of each blade, and the speed of the engine. If the speed of the rotor is too low or too high compared to the flow velocity through the engine, the angle could be too high and a stall can occur, which could ruin our day!

3) The Combustion Chamber

The Combustion Chamber introduces fuel, mixes it with the compressed air, and ignites it.

This creates a huge increase in temperature and pressure, which is what goes through the last stage of the engine.

It can be a difficult task to burn large quantities of fuel with large volumes of air. The aim here is to release the heat in such a way so that the air expands and accelerates, and gives a smooth stream of uniformly heated gas at all operating conditions. This is required for the next stage (the turbine) to work correctly.

The temperatures in the combustion chamber can reach up to 2100 °C, which is pretty insane!

The amount of fuel added to the air depends on the temperature rise required. Only about 20% of the compressed airflow enters the combustion chamber at the entry section. The flow enters the chamber via swirl vanes, which optimise the airflow for combustion.

The other 80% flows between the outer and inner casing. This is mainly designed like this because of cooling, but some of this flow also gets mixed in at a later point. The purpose of this is to stabilise the airflow, and protect the flame in the combustion chamber.

Have you ever seen a plane leave large smoke trails as it takes off from a runway? This is the result of inefficient combustion within the combustion chamber, and has a huge impact on both fuel efficiency and overall pollution. This is why this entire process is constantly monitored and controlled by the engine control units.

4) The Turbine

The main objective of the turbine is to extract enough kinetic energy out of the hot expanded airflow from the combustion chamber, and convert it into rotational energy 🔄

The expanded hot air goes through a series of turbine stages, which are connected to the compressor. If there are multiple turbine stages, the other stages are usually connected to other systems, such as a propeller, a rotor system, or other subsystems.

So how do we efficiently convert this kinetic energy from the hot air, into rotational energy?

Well, the hot gas from the combustion chamber forces its way into the turbine. The flow is guided through the turbine blades via nozzle guide vanes.

These vanes make sure that the angle at which the flow hits the turbine blades is optimal. Because of the turbine blade’s convergent shapes, they are accelerated to tip speeds of up to 1400 kts!

So don’t they break the sound barrier?

Yea! It would if the temperature was 15° C. But at over 1000 °C, the local speed of sound is above 1400 kts, so we’re all good on that front!

The power generated by the turbine stages is dependent on the speed of the airflow.

Faster airflow = more power ⬆️

Slower airflow = less power ⬇️

So the sequence for an increase in thrust would be:

Pilot demands more power ➡️ Increased fuel flow ➡️ More kinetic energy from combustion chamber ➡️ higher turbine speeds ➡️ higher compressor speeds ➡️ further increased airflow, etc.

This flow of steps is also why there are numerous ways in which starting a Jet Engine can fail.

5) The Exhaust

The exhaust is usually constructed in a way that utilises a converging duct. This accelerates the airflow further, while static pressure reduces. After the airflow has reached the throat (the smallest area), the airflow usually then enters a diverging duct.

This further increases the airflow’s velocity due to the built up momentum, but increases pressure at the same time (we’ll go into why this is in the future). This increases thrust of the engine as much as possible.

The velocity and pressure of the exhaust gas generate the thrust for TurboJets. For TurboProps and TurboShafts though, only a small amount of thrust is generated by the exhaust gases. This is because most of the energy has been absorbed by the turbine for driving the propeller or rotor system.

Is the case of TurboJets, the velocity of the exhaust gas is only subsonic at low thrust settings. During most conditions, the airflow can reach the speed of sound (based on the gas temperature!).

When this happens, the nozzle is then considered ‘choked’. That means that no further increase in speed can be achieved, unless the temperature is increased. This is because a temperature increase will also increase the local speed of sound, as per this equation (T is the temperature in Kelvin):

The nozzle size is extremely important, and is carefully designed to obtain the correct balance
of pressure, temperature and thrust. This is also one of the reasons why Jet Engines generally are not very good at going at speeds higher than Mach 3.

With a nozzle that’s too small, these values will be high, but there is a possibility of the engine surging. On the other hand, if the nozzle is too large, the values obtained are too low (and therefore the thrust isn’t high enough).

How Does a TurboFan Engine Work?

There are 2 main differences between a TurboJet and a TurboFan. The first thing you’ll see in a TurboFan is the larger fan at the front.

The reason the inlet fan is larger here, is to make way for something called a ‘bypass duct’. The bypass duct allows for bypass air to flow around the core of the engine, without ever entering the combustion chamber at all!

This improves the engine in 2 ways:

1) An increased amount of flow through the engine, increasing the total thrust
2) The bypass flow adds to the thrust, but doesn’t require combustion, increasing the overall efficiency of the engine

Often, the fan is part of the low pressure compressor, but it can also be fully separated from the compressor stage. If this is the case, the engine usually consists of multiple separate turbine stages, that each are connected to their own compressor stage or fan. This is called a two spool (or sometimes three spool) engine.

Because the fuel flow rate for the whole engine is changed only a little bit with the addition of the fan, a TurboFan creates a lot more thrust for almost the same amount fuel compared to a TurboJet.

This is the reason why a turbofan is very fuel efficient, and why so many airlines are using them. In fact, high bypass ratio turbofans are nearly as fuel efficient as turboprops, but up to much higher speeds!

The inlet fan can operate efficiently at higher speeds than a simple propeller. This is why TurboFans are found on faster planes, and propeller (usually TurboProps) are used on slower planes.

A TurboFan engine has two gas streams to eject into atmosphere: we have the cool bypass airflow, and the hot gasses that flowed through the engine core.

In an engine with a low bypass ratio, the two flows are combined by a mixer unit. This allows the bypass air to flow into the turbine exhaust gas flow in a way that thoroughly mixes the two streams.

However, for engines with high bypass ratios, the two airflow streams are usually exhausted separately.

Very often, you’ll see the back of TurboFans have what look like little pointy bits. These are called Chevrons:

The reason these are added to the more modern engines nowadays, is to make them quieter. The fact that airflow with a high velocity and pressure from the engine core, has to mix with much slower air with a lower pressure, is one of reasons that Jet Engines are so crazy loud!

They help with what’s called aerodynamic mixing. They help reduce the amount of turbulent airflow and small local shockwaves, which all contribute to noise. The result is a smoother airflow that mixes in a less disruptive way, and therefore less of that annoying noise!

How Does a TurboProp Engine Work?

A TurboProp Engine is a Jet Engine where the low power turbine (also called the power turbine here) is directly connected to the propeller, which generates the thrust we need.

TurboProp engines are ideal for planes that require a combination of relatively low speed and fuel efficiency. Compared to TurboJets, TurboProps have an easier time generating high amounts of thrust at lower speeds as well. This makes the planes that use TurboProps much better suited for taking off and landing on shorter runways.

The actual speed range where TurboProps can outperform TurboJets is roughly 400 kts or less. After this, they massively reduce their efficiency, as we’ve shown above.

One of the reasons for this is that the propeller causes a disturbance of the airflow caused by the high blade-tip speeds of the propeller at these speeds. This is why you usually only really see slower cargo planes like the C130 Hercules or regional jets with this type of engine.

How Does a TurboShaft Engine Work?

Similar to a TurboProp, the low pressure turbine for a TurboShaft Engine provides drive to another system. In this case, it is connected to a rotor system via a main rotor gearbox.

For most helicopters, a free power turbine is used to drive the rotor system. This is so that the rotor system is independent from the rotation of the compressor.

This makes sure that rotor inertia can keep the rotational velocity of the blades constant during manoeuvres like autorotation. Variations in engine speeds, or engine problems, have less impact on the rotor system this way.

From the power turbine, the shaft usually connects to the main rotor gearbox, which uses reduction stages to go from the relatively high rpm of the engine shaft, to the lower rotor rpm of the main rotor system.

How Does a Ramjet Engine Work?

A ramjet has quite a few significant differences compared to a conventional TurboJet Engine.

Like a conventional Jet Engine, a Ramjet still relies on capturing incoming air, or ‘ramming’ it. However, the main difference here is that it doesn’t need any compressor stages or blades!

A Ramjet is designed to use the movement of the aircraft to compress incoming air, by using diffusers that create shockwaves. The air is slowed down from supersonic to subsonic speeds. This raises the overall pressure of the airflow, before entering the combustion chamber.

The problem with this, is that we need a huge amount of air coming into the engine. So much air in fact, that we need to reach a speed of at least Mach 3 before this thing can give us any usable thrust at all!

This is what the SR71 blackbird solved by being able to transition its engines in flight from TurboJets at lower speeds, to Ramjets at higher speeds. This is called a TurboRamJet. It’s either this, or having the plane ejected from another plane that flies to speeds of at least Mach 3 by using more conventional jet engines.

The maximum operating speed of a Ramjet ends at about Mach 6. After this, the compression effects of the airflow are too extreme for the engine to handle properly.

One huge benefit that Ramjets come with, is the lack of any moving parts! Imagine if you could get rid of all those turbine stages, compressor blades, and rotating parts. That is exactly what Ramjets provide, but only between Mach 3 and 6!

How Does a Scramjet Engine Work?

Like RamJets, ScramJets rely on the incoming air being compressed by the inlet section. The main difference here is that for Scramjets, the airflow remains supersonic throughout the entire engine!

A scramjet uses a special inlet that is designed to use shockwaves to compress the air as it travels through the inlet, into the engine. The shockwave itself bounces around the inlet numerous times, increasing the inlet pressure around itself, as the airflow travels further into the engine.

The operating range of a Scramjet is Mach 5 and above, which is pretty insane! The fastest demonstrated speed of a Scramjet aircraft is the X-43A flight that went up to Mach 9.6. However, the theory suggests that these engines can go much faster than Mach 15, but this limit in still undefined.

Once the air is compressed enough, the air enters the combustion chamber while still at supersonic speeds. From here onwards, the operating principle is very similar to a Ramjet, other than the presence of much much higher speeds!

Resources

The Jet Engine, by Rolls Royce (1986)

Jet Engine Performance, by Jet-X

NASA Jet Propulsion Laboratory

Conclusion

Jet Engine technology is an amazing piece of human engineering. With six main categories ranging from TurboJets to ScramJets, each with its own unique design and capabilities, we are still figuring out how to push forward as an industry.

The next uncharted territory will be hypersonic passenger air travel, which will most definitely be enabled by technologies such as ScramJets!

We don’t publish all our Notes from the Cockpit (like this one) publicly, some are shared only by email. Get the next one sent straight to your inbox ⤵️