Jet engine technologies for interested amateurs
Last Updated: 6 April, 2002
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What is a Pulse Detonation Engine? One of the newest and most exciting areas of pulse-jet development is the Pulse Detonation Engine (PDE). While they work on similar principles to a regular pulsejet, the PDE has one very fundamental difference -- it detonates the air/fuel mixture rather than just allowing it to simply deflagrate (burn vigorously).
Detonation versus Deflagration The "woof" you get when igniting a dish of flammable liquid such as gasoline is a deflagration. Deflagration is, believe it or not, a relatively gentle process which is simply the rapid burning the fuel. When we cause an air/fuel mixture to deflagrate in a semi-enclosed space (such as a pulsejet or auto-engine) then pressure is generated and that pressure can be harnessed to perform some work -- eg: create thrust or turn a crankshaft. One of the primary attributes of deflagration is that the flame travels at a speed significantly lower than the speed of sound. Detonation however is a far more powerful reaction of the air/fuel mixture and results in such a rapid reaction that the pressure-wave created travels at super-sonic speeds. In effect, detonation is a violent explosion and as such it produces vastly higher pressures than the simple burning process of deflagration. Obviously, when we're looking to produce thrust, the more pressure we can generate within an engine the better -- that's why researchers are trying to build pulsejets that detonate their fuel charge rather than just burn it in a deflagrating reaction.
Turning Deflagration into Detonation One important component of triggering a detonation is pressure. It is much easier to detonate a mixture of air and fuel if it is highly compressed prior to (or during) the ignition process. Unfortunately, conventional pulsejets don't create enough pressure to produce detonation. In fact the level of compression created within a regular pulsejet engine is very low, even by automobile standards. Think of a pulsejet as having a compression ratio (CR) around 2:1 or less -- whereas your car probably has a CR of at least 9.5:1 or more. Now since the air-fuel mixture in your car doesn't detonate (except under exceptional circumstances) -- what chance do we have of getting a pulsejet to produce detonations?
So How do They do it? However, from the information that has been published, it appears as if most designs are using a two-stage ignition process to achieve detonation. Once a fresh air-fuel charge has been drawn into the pipe, a much smaller amount of a very volatile fuel (such as hydrogen) and an oxidizer (such as oxygen) are injected into a trigger chamber at the closed end of the pipe. This mixture is then ignited by an intensely powerful electrical discharge and made to detonate by forcing it through some carefully designed passages which create high levels of turbulence in the burning mixture. This tube is sometimes referred to as a DDT (Deflagration to Detonation Transition) tube and its job is to force the trigger charge to burn at a rate that creates a supersonic shockwave. Once it detonates, the small charge in the trigger chamber creates a very powerful shockwave that then hits the main air/fuel charge in the engine's secondary combustion chamber. It may sound odd that it is possible to compress the gas in a tube which has an open end -- but the incredible speed of the detonation shockwave means that the air/fuel simply doesn't have a chance to be pushed out of the tube before it is compressed. As, or because it is highly compressed, the air-fuel is also detonated by the intense heat of the shockwave. Now while this all sounds pretty simple in theory, there are clearly quite a number of practical problems to be overcome before a working PDE can be built. Firstly there's the issue of valving. The effective life of a traditional pulsejet tends to be measured in minutes rather than hours -- and that's even though they're only called on to handle the relatively low pressures generated by deflagration. If you tried to use the same fragile valves when detonating an air/fuel mixture they would instantly be destroyed. To get around this problem, some of the existing PDE designs appear to use robust rotary valves -- but this often requires a sophisticated synchronization system to ensure that the externally driven valves open and close at exactly the right times. Another alternative is to use a valveless setup and rely on a careful synchronization of the shockwaves produced to control the gas flows. Other problems with PDEs at this stage of their development include being able to inject and detonate the trigger charge at exactly the right moment to produce detonation of the main air-fuel charge. Too early and there won't be enough air/fuel to provide a good blast -- to late and the air/fuel will have already started leaving the tailpipe. Then there is the problem of structural integrity. What you're effectively doing with a PDE is repeatedly setting off a small charge of hi-explosive inside a metal tube. This obviously requires that a PDE be massively stronger than a pulsejet. It also means that the levels of noise and vibration are similarly far higher. One of the key players in the development of PDEs is NASA and they have a small but quite useful web page on the subject. CalTech have also published some excellent information on their own PDE research.
The Current State Of Development From what I've been able to gather, the main focus is currently being placed on researching and improving the detonation process. The current generation of PDEs don't seem capable of continuous running for any length of time -- they're more or less just single-shot devices requiring several seconds to recharge between detonations.
Amateur PDE Development The risks of containment failure (ie: a catastrophic explosion that turns your engine into a grenade) and the difficulties involved in obtaining reliable detonation of readily available fuels make both dangerous and likely to be quite disappointing. The sad truth is that you're unlikely to be able to coerce a mixture of gasoline and air or propane and air into true detonation using something you've built in your garage -- and if you do, the paramedics will likely have to take you away in a plastic garbage bag (that's if they can find all the pieces ;-).
The Future of PDEs Unfortunately there are still a number of negative issues that will need to be addressed. Firstly there's the noise -- if you think regular pulsejets are loud then you'll be absolutely blown-away by the noise levels created by a PDE. Then there's the issue of vibration. Although multiple engines could possibly be synchronized to fire in a manner that reduces vibration levels, they will still be significantly greater than those generated by turbojet or rocket motors. High levels of vibration place incredible demands on the materials from which motors and airframes are constructed.
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