Rotating Detonation Engine

In the age of electrification, it is often easy to assume that we mastered chemical combustion. Many make this assumption, after-all, rocket engines fly into space, jet-liners move people around the globe, and internal combustion cars get us to work. Batteries along with solar panels have largely distracted us from digging deeper into combustion.?But the truth is that we merely began to scratch the surface on the topic. The machines that we are all so used to depend solely on a type of combustion known as “deflagration”. This means that the process of combustion occurs at a speed that is slower than the speed of sound. But there is another type of combustion called “detonation”. This is when the combustion process propagates faster than the speed of sound, and this process could offer significant increase in engine fuel efficiency.

The obvious question here is - why does one type of combustion offer improved fuel efficiency over another? Fuel efficiency is defined as the fraction of fuel in the engine that combusts at any given moment. Ideally, we want 100% of the fuel to combust and the engine ejects only by-products. In practice, this is impossible, but a detonation-based engine will get us much closer to this limit than a deflagration-based engine. The reason is simple, when cool fuel and oxidizer (in the form of a gas-mixture) get hot, they want to spread away from each other. So during a slow combustion process (deflagration) gas is given too much time to spread out further before reacting, which reduces overall efficiency. Detonation, on the other hand, moves so quick that it does not give the gas much time to expand. As a result, detonation is able to combust a significantly higher portion of the mixture at any given moment.

But detonation has one major pitfall - it propagates so quickly through the mixture that a continuous, steady burn process becomes very difficult to maintain. In a previous edition we discussed the “Pulsejet” which works by generating pulses of detonation waves through the mixture. But with such a design, the pulses were so strong, they produce incredible noise-levels which degrade the structure. Also, a lot of unburnt fuel was lost in between the pulses which pretty much negated any fuel efficiency advantages from using detonations.

Enter the RDE – Rotating Detonation Engine. The engine consists of a combustion chamber in the form of a donut. A detonation wave is continuously moving around in circles through the chamber combusting fuel with high efficiency as it moves through it. The RDE takes advantage of detonation-based combustion in a continuous burn process, without the pulses of a Pulsejet. The chamber often consists of many injection ports positioned radially. Fresh fuel-oxidizer mixture is injected in synchrony just ahead of the wave, so that only combustion products are left behind. Due to its chamber form-factor, the RDE cannot utilize an ordinary nozzle. Instead, it relies on the Aerospike nozzle (kind of like an inverted nozzle) which has the added advantage of allowing the engine to work optimally at all altitudes, sea-level to vacuum, ideal for launch vehicle applications. Of course, there is still much work to be done before detonation-based engines are implemented throughout our world. It is great to see NASA and Purdue University leading the development effort into this unknown field.

One thing is for sure. Humans began exploring the process of combustion long time ago with the first caveman lighting up a campfire. Jump 1-million years later and we are still nowhere close to knowing everything about it. ?

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