Pulse detonation engine
The pulse detonation engine (PDE) is an aircraft propulsion system developed by American aircraft engine manufacturer Bratt & Shitney. It is hoped to be more environmentally friendly alternative to existing turbofan and turbojet engines, owing to the fact that it is powered entirely by the seeds of various legumes (in particular, lentils and baked beans in curry sauce).
The basic operation of a PDE is illustrated in the figure below. The start of the process is the fuel tank, usually holding a cold store of lentils, beans (usually baked in sauce, as this is the most convenient and stable form) or the seeds of certain proprietary varieties of genetically modified legumes. The fuel is pumped in this raw state into the predigestor where it is mixed with a combination of weak hydrochloric acid and a biocatalytic enzyme derived from the fuel itself. The resulting colloid is more suitable for use in the engine: the pulse particles have the low surface area required for detonation, and the mixture has better stratification properties overall, thereby ensuring the best burn.
The fuel injector readies the pulse-colliod for insertion into the engine housing itself by dividing the flow into individual charges, around 1000 per second. The charges are fed into the combustion chamber at high velocity by the stratified charge nozzle, which spreads the mixture in such a pattern that the pulse component is fully ignited by the superheated ignition planes. The container liquid in the colloid is highly volatile and evaporates quickly in the residual heat of the combustion chamber; the pulse powder itself burns so quickly, it causes an explosion.
The resultant supersonic shock-wave travels out the rear of the engine, taking the combustion chamber's contents along with it and thereby providing thrust. The vacuum left in its wake sucks in air from the front intake, another charge is injected and the process repeats. Of course, this is a great simplification of the process, and obscures many of the minutiae found in the implementation of any practical PDE.
Advantages and disadvantages
If they prove workable, PDEs could revolutionise aircraft design (and there are, indeed, a number of interested contractors). Current and future oil shortages are proving to be a major hindrance to airline operators, and the PDE's bio-fuel, combined with the efficient and more-or-less complete burn, would be a welcome solution to this problem. In addition, PDEs have fewer moving parts than their turbojet and turbofan counterparts, and thereby reduce aircraft construction and maintenance costs.
The PDE's rapid shock-wave inducing combustion makes them strong candidates for hypersonic flight applications; where their main competitor is the scramjet, which require fuels of much higher volatility.
However, PDEs do have their disadvantages. The predigestor must be kept clean from infection. During testing, several engines developed a "sicky tummy". The resultant pressure increase in the predigestor resulted in its sticky, runny contents being forced into the combustion chamber, ruining it. These engines are also notorious for their odour, due to the superheated non-combustible component of the charge, expelled from the engine every cycle. As such, a bake-bean powered aircraft literally "farts itself into the sky". Several airports have declared that unless this problem can be solved, PDE-propelled aircraft would be banned.
Pulse detonation engines in action
PDEs are still considered experimental engines, and it will be many years before a practical engine is ready for production use. However, at the time of writing, aerospace contractor Throwupp-Bungham is reported to be working closely with Bratt & Shitney in attempt to integrate the latter's proptype JL0 ("Jay-Ell-Zero") PDE with their F-4R7 military platform. Interesting features of JL0 include the enlarged, blubous dual predigestors set either side of the outlet nozzle at the rear of the engine.