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无阀脉冲喷气发动机

2013-07-24 35页 pdf 617KB 63阅读

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无阀脉冲喷气发动机 Valveless Pulsejet Engines 1.5 -- a historical review of valveless pulsejet designs -- by Bruno Ogorelec The idea that the simplest engine an enthusiast can make at home is a jet engine will sound strange to most people -- we perceiv...
无阀脉冲喷气发动机
Valveless Pulsejet Engines 1.5 -- a historical review of valveless pulsejet designs -- by Bruno Ogorelec The idea that the simplest engine an enthusiast can make at home is a jet engine will sound strange to most people -- we perceive jet engines as big complex contraptions pushing multi-million dollar aircraft through the skies. Yet, this is completely true. In its most basic form – the valveless pulsejet -- the jet engine can be just an empty metal tube shaped in a proper way. Everyone able to cut sheet metal and join metal parts can build one in a garage or basement workshop. Due to peculiar historical circumstances, this interesting fact has escaped popular attention. It is not familiar even to enthusiasts of jet propulsion. You are not very likely to see or hear jet engines roaring in people’s back yards on Sunday afternoon. Few if any people can be seen flying aircraft powered by jet engines they have built themselves. This document aims to help change that. However, it is not a how-to primer. It is an attempt to describe and explain the valveless pulsejet in principle. It also offers a rough sketch of the amazing variety of layouts the inventors and developers have tried during the long but obscure history of this device. My aim is to inspire, rather than teach. My goal is to demonstrate that jet power is accessible to everyone in a great variety of simple ways. Should you find the inspiration, plenty of information on the practical steps towards jet power will be available elsewhere. 2 HOW DOES A VALVELESS PULSEJET WORK? The picture below shows one of the many possible layouts of a valveless pulsejet engine. It has a chamber with two tubular ports of unequal length and diameter. The port on the right, curved backwards, is the intake pipe. The bigger, flared one on the left is the exhaust, or tailpipe. In some other engines, it is the exhaust pipe that is bent into the U-shape, but the important thing is that the ends of both ports point in the same direction. When the fuel-air mixture combusts in the chamber, the process generates a great amount of hot gas very quickly. This happens so fast that it resembles an explosion. The immediate, explosive rise in internal pressure first compresses the gas inside and then pushes it forcefully out of the chamber. Two powerful spurts of hot expanding gas are created – a big one that blows through the tailpipe and a smaller one blowing through the intake. Leaving the engine, the two jets exert a pulse of thrust – they push the engine in the opposite direction. As the gas expands and the combustion chamber empties, the pressure inside the engine drops. Due to inertia of the moving gas, this drop continues for some time even after the pressure falls back to atmospheric. The expansion stops only when the momentum of the gas pulse is completely spent. At that point, there is a partial vacuum inside the engine. The process now reverses itself. The outside (atmospheric) pressure is now higher than the pressure inside the engine and fresh air starts rushing into the ends of the two ports. At the intake side, it quickly passes through the short tube, enters the chamber and mixes with fuel. The tailpipe, however, is rather longer, so that the incoming air does not even get as far as the chamber before the engine is refilled and the pressure peaks. One of the prime reasons for the extra length of the tailpipe is to retain enough of the hot exhaust gas within the engine at the moment the suction starts. This gas is greatly rarified by the expansion, but the outside pressure will push it back and increase its density again. Back in the chamber, this remnant of previous combustion mixes vigorously with the fresh fuel/air mixture that enters from the other side. The heat of the chamber and the free radicals in the retained gas will cause ignition and the process will repeat itself. The spark plug shown on the picture is needed only at start-up. Once the engine fires, the retained hot gas provides self-ignition and the spark plug becomes unnecessary. Indeed, if spark ignition is left on, it can interfere with the normal functioning of the engine. It took me more than 300 words to describe it, but this cycle is actually very brief. In a small (flying model-sized) pulsejet, it happens more than 250 times a second. The cycle is similar to that of a conventional flap-valve pulsejet engine, like the big Argus (which powered the V-1 flying bomb) or the small Dynajet used to power flying models. There, the rising pressure makes the valve flaps snap shut, leaving only one way for the hot gas to go -- into the exhaust tube. In the J-shaped and U-shaped valveless engines, gas spews out of two ports. It does not matter, because they both face in the same direction. Some valveless pulsejet designers have developed engines that are not bent backwards, but employ various tricks that work in a similar fashion to valves -- i.e. they allow fresh air to come in but prevent the hot gas from getting out through the intake. We shall describe some 3 of those tricks at a later point. You may wonder about the sharp transition from the intake tract into the chamber. It is necessary to generate strong turbulence in the incoming air, so that it mixes with injected fuel properly. A gentler, more gradual entry would not generate the necessary swirling of gases. In addition, turbulence increases the intensity of combustion and the rate of the heat release. THE BEGINNINGS The idea of using the elastic properties of air to generate power pulses is very old. The first pulsejet engines were built in France at the very beginning of the 20th century. They found only very limited use at the time and were soon forgotten for all practical purposes. In the 1930s, however, German engineer Paul Schmidt rediscovered the principle by accident while trying to develop a detonation engine. He built a series of impressive pulsejets with valves. At roughly the same time and in the same country, engineers at the Argus engine company were working on a valveless device that used compressed air. The circumstances were much more propitious now. The world was preparing for a big war and the war machines were gearing up. The German War Ministry brought Schmidt and Argus together, which resulted in the development of the first mass produced jet engine. Like the Schmidt engines, it used valves and natural aspiration, but its mechanisms were greatly modified by Argus. Thus, while the opposed sides in World War II were still trying to put together their first jet-powered fighter aircraft in 1944, the Vergeltungswaffe 1 (or V-1 for short) was regularly buzzing its way to England with a 1,870-lb load of explosives. Its Fieseler airframe was powered by the Argus As 109-014 pulsejet engine. You can see one flying over the English countryside on the photo on the right. The utter simplicity, low cost and demonstrated effectiveness of the pulsejet impressed the Allies so much that they badly wanted to have something similar. It looked amazing to everyone that a device that simple could power a serious flying machine. Captured examples of the Argus were carefully studied and copies built and tested. It soon became obvious that the pulsejet had certain drawbacks and limitations, but the basic principle still looked very attractive and ideas for improvement abounded. Various uses for the device were contemplated. Ford Motor Company built a proper assembly line to manufacture Argus copies. With the end of the war, some of the projects were scuttled, but the Cold War started soon and the quest for a better pulsejet continued. Unfortunately, progress was very slow and purely incremental. In the mid 1950s, after a decade of effort, developers were not that much better off than their wartime German predecessors. In total contrast, the advances in turbojet design over the same period were 4 tremendous. By that time, turbojet-powered fighters already had the Korean War behind them. Turbojet strategic bombers were carrying nuclear weapons in their bomb bays and turbojet airliners were getting ready to earn their money carrying businessmen and the idle rich from continent to continent. It was becoming completely clear to everyone that the turbojet was the jet engine of the future. Engineers were still excited by the promise of the pulsejet, but the reality was not to be denied. During the 1950s and 1960s, most pulsejet researchers gradually abandoned their efforts and turned to other things. THE ADVANTAGES What originally attracted and excited the researchers and developers most of all about the pulsejet engine was a peculiar property of pulsating combustion – it can be self-compressing. In the pulsejet, the fuel-air mixture does not burn steadily, at a constant pressure, as it does in the other jet engines. It burns intermittently, in a quick succession of explosive pulses. In each pulse, the gaseous products of combustion are generated too fast to escape from the combustor at once. This raises the pressure inside the combustor steeply, which increases combustion efficiency. The pulsejet is the only jet engine combustor that shows a net pressure gain between the intake and the exhaust. All the others have to have their highest pressure created at the intake end of the chamber. From that station on, the pressure falls off. Such a decreasing pressure gradient serves to prevent the hot gas generated in the combustor from forcing its way out through the intake. This way, the gas moves only towards the exhaust nozzle in which pressure is converted to speed. The great intake pressure is usually provided by some kind of compressor, which is a complex and expensive bit of machinery and consumes a great amount of power. Much of the energy generated in the turbojet engine goes to drive a compressor and only the remainder provides thrust. The pulsejet is different. Here, the exhaust pressure is higher than the intake pressure. There is pressure gain across the combustor, rather than loss. Moreover, the pulsejet does it without wasting the power generated by combustion. This is very important. According to some rough figures, a 5-percent gain in combustion pressure achieved by this method gives about the same improvement in overall efficiency as the 85-percent gain produced by a compressor, all other things being equal. Now, that’s rather impressive. Personally, I am interested in the pulsejet for another reason -- because it brings the jet engine back to the people. It is a back-to-basics kind of machine, so simple to be accessible even to enthusiasts with rudimentary skills and simple tools. Turbojets and fanjets are at the opposite end of the complexity scale. In most cases they employ inaccessible, cutting-edge technology. Just look at the collection of pulsejets on the picture on the right. They were built by Stephen Bukowsky, a high-school student, purely out of fun. 5 If I remember it right, the three valveless engines (second, third and fifth from left) each took him about a couple of days to make. This is just a part of Steve’s collection! Cost is another advantage. Pulsejets are cheaper than even the simplest piston engines of comparable output. In contrast, turbojets are frighteningly expensive. THE DISADVANTAGES So, given the advantages, why did the pulsejet disappear from view? There are several reasons. A big problem is that the gain in efficiency offered by pulsating combustion is not at all easy to utilize for propulsion. Paradoxically, the central problem here is the same as the source of the benefit – namely, pulsation. The very means of increasing combustion efficiency makes it difficult to take advantage of the result. The real potential for the pulsejet has always been in its use as the combustor for a turbine engine, rather than as an engine in itself. Its ability to generate pressure gain is greatly multiplied in a high-pressure environment. Compared to the more usual constant-pressure combustor, it can either give the same power with much smaller mechanical loss and lower fuel consumption, or much greater power for the same amount of fuel. Alas, a turbine demands steady flows to function efficiently. Unsteadiness generates loss. Also, pulsations are dangerous for the brittle axial turbine blades. Radial turbines are tougher in that respect, but they are less efficient, especially so with intermittent flow. They are mostly used to exploit waste heat, as in a turbocharger, rather than as prime movers. Researchers have toyed with converting pulsations into a steady flow, but most methods proved inefficient. But, how about simplicity? In a manner of speaking, a pulsejet is what remains when you remove all the complex and expensive parts from a turbojet and leave only the simple and cheap combustor that is hidden in the middle. Well, yes, simplicity is attractive, but it also has its disadvantages. The promise of the pulsejet on its own, outside a turbojet, is less significant. The pressure gain is still there, but in the atmospheric pressure environment, without the multiplication offered by the compressor, it does not amount to very much. The average pressure in the working cycle is low, the specific power unimpressive and fuel efficiency poor. The power ‘density’ is much lower too. For the same engine bulk, you get less thrust than with the competing jet engines. Pushing the pulsejet further down the scale of desirability in the postwar era was the fact that even with the improvements arrived at in the 1950s and 60s; the pulsations still produced horrible noise and mad vibration. Pulsejets depending on reed valves were also short-lived and unreliable. OK, they were cheap, but in the Cold War era that was certainly not a prime consideration. Finally, there was little that pulsejets were really good for. For a while, it looked like they would power small helicopters. Some spectacular-looking prototypes were built, especially in France. In the end, however, they never made the grade, mostly for aerodynamic reasons. 6 The French briefly used pulsejet power on motor gliders and flying drones, too. Cheap flying drones and missiles were built in several countries, including the US, Russia and China. The picture above shows the French Arsenal 501 target drone, powered by a valved engine. The color picture on the first page of this document shows a Chinese target drone with a valveless engine. That was about it. Given the ample defense budgets, most of the real-life applications that required a jet engine were better satisfied with a turbojet or with rocket power. Civilian industry did not look upon the pulsejet with any greater kindness. Turbojet development was intense and engineers had little time for the exotic pulsating things that few people understood properly anyway. The difficulty of defining the processes inside the pulsejet mathematically was a major problem for most researchers and engineers. Modeling the semi-chaotic pulsating combustion was far too much for the computing abilities of the time. It meant that pulsejet design was unpredictable -- part science and part black art. Industry tries hard to avoid such tricky propositions. By the mid-1960s only a few isolated enthusiasts still considered the pulsejet as a potential aircraft powerplant. The noisy tube was in a blind alley and relegated to the role of model aircraft engine and such humdrum applications as an efficient combustor for central heating systems, a power unit for agricultural spray dusters and a blower and shaker for industrial slurry drying machinery. CHANGE OF CIRCUMSTANCES So, why look at pulsejets now? Well, my reason is the change of circumstances. Sometime in the early 1980s, ultralight fun flying started getting increasingly popular due to the availability of good, simple and affordable flying platforms – hang gliders and paragliders. When provided with motor power, these machines offered unprecedented freedom of flight to anyone interested. In addition, with the fantastic development of modern electronics, a whole new class of unmanned flying machines appeared, designed as utility platforms for a variety of telecommunications, surveillance, measuring and sensing devices. All those new flying machines, whether designed for fun or utility, are powered by piston engines that drive propellers. Jet engines only appear at the very top end of the price scale – on machines costing several hundred thousand dollars apiece. All the piston engines currently used in ultralight flying are relatively heavy and cumbersome, even in their simplest form. They also require much ancillary equipment, like reductors, prop shafts, propellers etc. etc. Having all that gear mounted on a lightweight flying machine almost defeats the original purpose. A simple lightweight pulsejet seems much more appropriate. Turbojets, on the other hand, are terribly expensive – far out of enthusiasts’ reach. Things are not likely to get much better in the near future, either. Because of the very high technological requirements, the cost of turbojet engines has always remained high. Only the small turbojets based on old turbocharger parts are relatively inexpensive, because their most precious parts are taken off scrapped truck engines, but even their prices are not pleasant. In contrast, the humble low-technology pulsejet is laughingly cheap by any standard. Besides, in the engine sizes likely to be used by enthusiasts, the best pulsejets can compete in performance with the other jet engines, especially in the power-to-weight stakes. I am often told that a jet engine will never be good for recreational purposes. Jet propulsion is really efficient only at relatively high airspeeds, seemingly making it unsuitable for low-speed devices such as hang gliders. However, maybe a niche for a simple jet engine 7 can be found at the top end of hang-glider performance – possibly with rigid wings. Also, the rule does not seem to be very strict. For instance, a British Doodlebug harness powered by a Microjet turbojet engine has been tested with delightful results with a regular foot-launched hang glider (see the picture). This bodes well for pulsejets. When equipped with a thrust augmenter, a good pulsejet can be optimized for speeds much lower than those of other jet engines. It can hardly fail to perform at least as well as the Microjet in a similar application. In terms of thrust to weight it is already superior. Tote up those points and the lightweight, simple, cheap low-speed pulsejet engine suddenly starts making a lot of sense. Its admittedly high fuel consumption, noise and vibration need not be of major importance for the applications I have in mind -- or may perhaps be alleviated or designed out of the concept. The enormous advances in computing power over the past few decades have made modeling of pulsating combustion more realistic, too. It is still not easy even for the supercomputers, but it can now be done. This can cut down development time drastically and make it much more straightforward. Finally, our understanding of pulsating combustion has advanced to the point where these engines can be designed on paper with performance predictability much closer to that of the other engine types. It is perhaps time to blow the dust off the old tube. WHY VALVELESS? The ordinary pulsejet is already a very simple engine. It is jus
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