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The amazing thing about valveless pulsejet engines is that they are the simplest known engines and that’s because they have zero moving parts, essentially they are just a hollow pipe of varying shape and diameter. That’s it. Just like our glass jar.

Despite their extremely simple anatomy their working principle is in fact pretty complex and can be difficult to explain and understand and that’s because inside a pulsejet we have three distinct processes that occur simultaneously to ensure their continued operation.

The first process involves the fluid dynamics caused by the combustion of the air and fuel inside the combustion chamber of the engine
The second process is acoustic and involves the sound waves created by the combustion
The third process is thermodynamic and involves the rapidly changing temperatures of the gasses inside the engine

So as you can see a lot of stuff is going on making things kinda complicated, but if you do manage to properly understand a pulse jet I promise that you will gain a new multidisciplinary appreciation and understanding of the amazing things that physics can do, even with zero moving parts.

The pulsejet shape we will be using for our explanation is the Lockwood Hiller which comes from the early 60s is sort of a culmination of a hundred years of trial and error and is probably the most popular design by far today and that’s because it offers a really nice balance of ease of starting, thrust, efficiency, and reliability.

The main parts of any pulsejet engine are the intake, the exhaust and the combustion chamber.

To start the engine we will inject fuel, in most cases this will be propane from a tank that will be injected either directly into the combustion chamber or in front of the engine at the intake. We will also install a spark plug inside the combustion chamber. The spark plug is needed only during the starting phase and is not used later.

So let’s imagine we have opened our propane valve and allowed fuel into the chamber. Because the engine is a hollow tube we already have air inside it which means that now we have an airfuel mixture in the chamber. We ignite that airfuel mixture which causes it to combust. As the combustion flame front expands it increases the pressure and temperature dramatically. This expansion of the combustion forces and accelerates the masses of air in the intake and the exhaust out of the engine. Now a jet engine is a reaction engine. As you know for every action there is an equal and opposite reaction. So by accelerating the air mass in one direction the reactionary force moves the engine in the other direction and if we attach that engine onto something that something will move together with the engine. So what we have inside a pulsejet is something that was called thermal breathing by Francois H. Reynst, who is considered one of the most important pioneers of modern pulsed combustion. So if pulsejets are so simple and they can produce thrust why aren’t they more widespread?

Well, first of all, they have poor fuel efficiency. One of the reasons behind that is that we are igniting the air fuel mix with the heat of the exhaust gas. That means that we always have a mix of exhaust gas air and fuel upon ignition which is less then ideal leading to an incomplete burn of the fuel which then gets spit out the intake and exhaust. The other reason we have poor efficiency is that there is no active compression. A turbojet or a turbofan have a compressor section consisting of several stators and rotors which easily increase the pressure of the air five times over or even more. A pulsejet only has atmospheric pressure at it’s disposal. When we ignite compressed air and fuel we achieve a much higher combustion temperature which not only helps to burn the fuel more completely but it also achieves much higher combustion pressure leading to higher thrust. Another reason why pulsejets produce less thrust is the intermittent nature of the combustion which is simply less capable of producing high thrust compared to the constant combustion inside a turbojet or a turbofan.

The final drawback is the noise. Pulsejets are incredibly loud compared to most other engines. Despite their drawbacks, pulsejets are by far the simplest and cost effective way of achieving powered propulsion or flight making them an ideal candidate for RC planes and other unmanned aircraft.


A special thank you to my patrons:
Daniel
Peter Della Flora
Dave Westwood
Toma Marini
Zwoa Meda Beda
valqk
Cole Philips
Marwan Hassan11
RePeteAndMe
Sam Lutfi

00:00 Glass Jar Jet Engine
03:46 Operating Principle and Fluid Dynamics
12:16 Acoustics
17:28 Thermodynamics
18:55 Drawbacks and Benefits

#d4a #pulsejet