Jet Engines, for People in a Hurry

As they say, a simple way to recall how a jet engine works sounds like lovers having a good time; Suck, Squeeze, Bang and Blow. That is:

1. Suck: Air is sucked into the engine from the surrounding as a source of the oxygen for combustion in the later stages.
2. Squeeze: The incoming air is compressed to reach a higher pressure and temperature. This enables a bigger escape velocity after combustion hence more power output.
3. Bang: The compressed air is mixed with fuel such as JET A and then combusted to achieve a very fast expansion of the mixture - hence the bang.
4. Blow: The expanding exhaust from the combustion process is released through the engine nozzle over a diverging aperture which produces a very fast moving exhaust flow. This fast flow is used to produce a force that propels the aircraft.

1.0 Operation

The type of jet engine determines how a resultant force from the exhaust is utilized to produce the propulsion. This engine, also known as the power plant in industry speak, is usually the main source of energy for the aircraft. Also, in its role as the power plant, some extra energy from the engine may be converted and used to maintain supplementary operations for the comfort of the passengers and not necessarily for propulsion. This includes features like cabin air-conditioning or recharging electronics.

The compressor can be centrifugal (as shown in fig 1 above) or axial flow (as shown in fig 2 below). Most jet engines have axial flow compressors because they are more efficient at handling the large volumes of air sucked into a jet engine. Thus, even if the centrifugal versions achieve higher pressure increase in fewer stages, their increase in drag relegates them to smaller volume airflow and compact engine designs. The axial flow types make up for their deficiency in pressure increase through having multiple compression stages as shown below.

In general, jet engines use Newton’s third-law of motion to generate the thrust. The fast moving exhaust leading to a force in the opposite direction (reaction). Next we examine the differences in how each type of engine then uses this force to propel the aircraft.

2.0 Types

2.1 If the jet engine is using an axial compressor with multiple stages and directly applying the exhaust output to the propulsion of the aircraft, then it is either a turbojet or turbofan:

2.2 However, if the jet engine is mainly used to drive another system which actually propels the aircraft, the engine is either a turboprop or turbo shaft. This kind of engine is fuel efficient but the aircraft is slower.

2.3 Finally, if the jet engine depends on the speed of the aircraft to compress the air, then it doesn’t need compression chambers or blades and is most likely a ramjet or scramjet. This type of engine can only be operable at very high speeds and hence mostly used in conjunction with one of the other engine types or separate vehicle to first build up to a speed at which it can work.

3.0 Industry Trends

3.1 Noise reduction: A lot of noise is created by jet engines due to aerodynamic mixing between the high-velocity jet exhaust and a much slower ambient air, creating vortices and turbulent streams. Efforts are continuously being made to reduce the noise level and the governments around the world are gradually tightening noise requirements which forces more innovation in this area.? Efforts include using chevron nozzles, high bypass airflow around the noisy engine and acoustic liners in the fan ducts, among other innovations.

3.2 Fuel efficiency: An ongoing effort to reduce the fuel required to achieve the same or more power output has been a major boon for turbofan engine type and the reason for its success in the aviation industry. Of late, a major push has been made for the open fan concept, led by the CFM Rise engine.?

3.3 Sustainable fuel: Research is pushing alternatives to the traditional Jet engine fuel made from kerosene mixtures (such as Jet A-1). Proposed fuel sources include hydrogen, LNG (liquefied Methane) and biofuel from algae as options that are less harmful to the environment.

3.4 Manufacturing efficiency: With the advancements in technology such as 3D printing, engine innovations and designs with less parts that were previously not feasible are being considered for lighter, cheaper and more efficient engine solutions that were otherwise too complex and unsafe to build.

3.5 Operation efficiency: As in many technology innovations, the growth of AI, and computational power has led to an industry push for operation cost improvements on jet engines and their aircraft platforms, for shorter learning curves and easier maintenance. This includes trends like:

• Augmented reality (AR) to provide easier training for engineers, easily visualize and diagnose issues or improvements
• Predictive maintenance where data analysis is increasingly being applied to catch and prevent engine failures before they even happen, leading to less downtime and costs.

That's it! We’ve covered a brief introduction to jet engines and provided pointers to common trends as indicators to where the industry is headed. Please follow any additional links to learn more about the various aspects mentioned. Feel free to drop a comment below with any questions that you have about this topic and I’ll add more info - if you are not in a hurry.

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