Pulse Detonation Engine

A pulse detonation engine is an unsteady propulsive device in which the combustion chamber is periodically filled with a reactive gas mixture, a detonation is initiated, the detonation propagates through the chamber, and the product gases are exhausted. The high pressures and resultant momentum flux out of the chamber generate thrust.

A pulse-detonation engine, or "PDE", is a type of propulsion system that can operate from subsonic up to hypersonic speeds. In theory the PDE design can produce an engine with a burn efficiency higher than other designs, with considerably fewer moving parts. The burn efficiency of a PDE can exceed even turbojets and turbofans. However, the impossibility of including a bypass fan in a PDE (which increases the overall efficiency of turbofans above the theoretical maximum for no-bypass engine) restricts the competitiveness of PDE to applications where turbofans are impractical or impossible.

To date, no practical PDE has been put into production, but several testbed engines have been built and one was successfully integrated into a low-speed demonstration aircraft that flew in sustained PDE powered flight in 2008.

Quasi-steady thrust levels can be achieved by repeating this cycle at relatively high frequency and/or using more than one combustion chamber operating out of phase.A pulse detonation engine has a detonation chamber with a sidewall. At least two fuel ports are located in the sidewall, spaced longitudinally apart from each other. An oxygen fuel mixture is introduced into the forward port and detonated. This creates a detonation wave which propagates with an air fuel mixture introduced into the rearward fuel port.

After the detonation, purge air passes through the chamber before the next detonation. A rotating sleeve valve mounted around the detonation opens and closes the fuel ports as well the purge ports.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). The exact details on many of the PDE designs currently being developed are rather sketchy -- mainly because they have the potential to be extremely valuable so most of companies researching in this field are not about to tell us what they're doing. It seems that nobody yet has the PDE developed to the point of being a practical propulsion device (or at least if they have, they're not telling anyone). 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 doesn’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. Many developers have high hopes that the PDE will ultimately become the most cost-effective method of propelling supersonic sub-orbital craft. The ultra-high compressions obtained by detonation offer the potential for much better fuel-efficiency than even the best turbojet, and the fact that they are an air-breathing engine reduces the fuel-load and increases safety when compared to rocket motors.

All regular jet engines and most rocket engines operate on the deflagration of fuel, that is, the rapid but subsonic combustion of fuel. The pulse detonation engine is a concept currently in active development to create a jet engine that operates on the supersonic detonation of fuel.

The basic operation of the PDE is similar to that of the pulse jet engine; air is mixed with fuel to create a flammable mixture that is then ignited. The resulting combustion greatly increases the pressure of the mixture to approximately 100 atmospheres, which then expands through a nozzle for thrust. To ensure that the mixture exits to the rear, thereby pushing the aircraft forward, a series of shutters are used to close off the front of the engine. Careful tuning of the inlet ensures the shutters close at the right time to force the air to travel in one direction only through the engine.

The main difference between a PDE and a traditional pulsejet is that the mixture does not undergo subsonic combustion but instead, supersonic detonation. In the PDE, the oxygen and fuelGeneral Electric, the shutters are eliminated through careful timing, using the pressure differences between the different areas of the engine to ensure the "shot" is ejected rearward. combination process is supersonic, effectively an explosion instead of burning. The other difference is that the shutters are replaced by more sophisticated valves. In some PDE designs from

The main side effect of the change in cycle is that the PDE is considerably more efficient. In the pulsejet the combustion pushes a considerable amount of the fuel/air mix (the charge) out the rear of the engine before it has had a chance to burn (thus the trail of flame seen on the V-1 flying bomb). Even while inside the engine the mixture's volume is continually changing, which is an inefficient way to burn fuel. In contrast the PDE deliberately uses a high-speed combustion process that burns all of the charge while it is still inside the engine at a constant volume. This increases the burn efficiency, i.e. the amount of heat produced per unit of fuel, above any other engines, although conversion of that energy into thrust remains inefficient.

Another side effect, not yet demonstrated in practical use, is the cycle time. A traditional pulsejet tops out at about 250 pulses per second due to the cycle time of the mechanical shutters, but the aim of the PDE is thousands of pulses per second, so fast that it is basically continual from an engineering perspective. This should help smooth out the otherwise highly vibrational pulsejet engine – many small pulses will create less volume than a smaller number of larger ones for the same net thrust. Unfortunately, detonations are many times louder than deflagrations.

The major difficulty with a pulse-detonation engine is starting the detonation. While it is possible to start a detonation directly with a large spark, the amount of energy input is very large and is not practical for an engine. The typical solution is to use a deflagration-to-detonation transition (DDT) - that is, start a high-energy deflagration, and have it accelerate down a tube to the point where it becomes fast enough to become a detonation. Alternatively the detonation can be sent around a circle and valves ensure that only the highest peak power can leak into exhaust.

This process is far more complicated than it sounds, due to the resistance the advancing wavefront encounters (similar to wave drag). DDTs occur far more readily if there are obstacles in the tube. The most widely used is the "Shchelkin spiral", which is designed to create the most useful eddies with the least resistance to the moving fuel/air/exhaust mixture. The eddies lead to the flame separating into multiple fronts, some of which go backwards and collide with other fronts, and then accelerate into fronts ahead of them.

The behavior is difficult to model and to predict, and research is ongoing. As with conventional pulsejets, there are two main types of designs: valved and valveless. Designs with valves encounter the same difficult-to-resolve wear issues encountered with their pulsejet equivalents. Valveless designs typically rely on abnormalities in the air flow to ensure a one-way flow, and are very hard to achieve a regular DDT in.

NASA maintains a research program on the PDE, which is aimed at high-speed, about Mach 5, civilian transport systems. However most PDE research is military in nature, as the engine could be used to develop a new generation of high-speed, long-range reconnaissance aircraft that would fly high enough to be out of range of any current anti-aircraft defenses, while offering range considerably greater than the SR-71, which required a massive tanker support fleet to use in operation. (See Aurora aircraft)

While most research is on the high speed regime, newer designs with much higher pulse rates in the hundreds of thousands appear to work well even at subsonic speeds. Whereas traditional engine designs always include tradeoffs that limit them to a "best speed" range, the PDE appears to outperform them at all speeds. Both Pratt & Whitney and General Electric now have active PDE research programs in an attempt to commercialize the designs.

Key difficulties in pulse detonation engines are achieving DDT without requiring a tube long enough to make it impractical and drag-imposing on the aircraft; reducing the noise (often described as sounding like a jackhammer); and damping the severe vibration caused by the operation of the engine.

The exact details on many of the PDE designs currently being developed are rather sketchy -- mainly because they have the potential to be extremely valuable so most of companies researching in this field are not about to tell us what they're doing.

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.