| Pulsejet | |
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| Diagram of a pulsejet | |
| Part of a series on Aircraft propulsion |
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A pulse jet engine (or pulsejet) is a type of jet engine in which combustion occurs in pulses. Pulsejet engines can be made with few [1] or no moving parts,[2][3][4] and are capable of running statically.
Pulse jet engines are a lightweight form of jet propulsion, but usually have a poor compression ratio, and hence give a low specific impulse.
One notable line of research of pulsejet engines includes the pulse detonation engine which involves repeated detonations in the engine, and which can potentially give high compression and good efficiency.
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There are two main types of pulsejet engines, both of which use resonant combustion and harness the expanding combustion products to form a pulsating exhaust jet which produces thrust intermittently.
Valved engines use a mechanical valve to control the flow of expanding exhaust, forcing the hot gas to go out the back of the engine through the tailpipe only, and allow fresh air and more fuel to enter through the intake. The other type of pulsejet, valveless pulsejets, have no moving parts and use only their geometry to control the flow of exhaust out of the engine. Valveless engines expel exhaust out of both the intakes and the exhaust, most try to have the majority of exhaust go out the longer tail pipe, for more efficient propulsion.
The valved pulsejet comprises an intake with a one-way valve arrangement. The valves prevent the explosive gas of the ignited fuel mixture in the combustion chamber from exiting and disrupting the intake airflow, although with all practical valved pulsejets there is some 'blowback' while running statically and at low speed as the valves cannot close fast enough to stop all the gas from exiting the intake. The superheated exhaust gases exit through an acoustically resonant exhaust pipe. The valve arrangement is commonly a "daisy valve" also known as a reed valve. The daisy valve is less effective than a rectangular valve grid, although it is easier to construct on a small scale.
The valveless pulse jet engine operates on the same principle, but the 'valve' is the engine's geometry. Fuel as a gas or liquid vapour is either mixed with the air in the intake or directly injected into the combustion chamber. Starting the engine usually requires forced air and an ignition method such as a spark plug for the fuel-air mix. With modern manufactured engine designs, almost any design can be made to be 'self-starting' by providing the engine with fuel and an ignition spark, starting the engine with no compressed air. Once running, the engine only requires input of fuel to maintain a self-sustaining combustion cycle.
The Swedish inventor Martin Wiberg has a claim to having invented the first pulse jet in Sweden, but exact details of the patent are unclear. It is also unclear whether any working models were made.
The first working pulsejet was patented in 1906 by Russian engineer V.V. Karavodin, who completed a working model in 1907. The French inventor Georges Marconnet patented his valveless pulsejet engine in 1908, which many commentators argue greatly influenced the V-1 flying bomb through engineer Paul Schmidt, who pioneered a more efficient design based on modification of the intake valves (or flaps), earning him government support from the German Air Ministry in 1933.[5]
In 1934, Georg Madelung and Munich-based independent designer and inventor Paul Schmidt proposed to the German Air Ministry a "flying bomb" powered by Schmidt's pulse jet. Madelung co-invented the ribbon parachute, a device used to stabilise the V-1 in its terminal dive. Schmidt's prototype bomb failed to meet German Air Ministry specifications, especially owing to poor accuracy, range and high cost. The original Schmidt design had the pulsejet placed in a fuselage like a modern jet fighter, unlike the eventual V-1 which had the engine placed above the warhead and fuselage.
The Argus Company began work based on Schmidt's work. Other German manufacturers working on similar pulsejets and flying bombs were The Askania Company, Robert Lusser of Fieseler, Dr. Fritz Gosslau of Argus and the Siemens company, which were all combined to work on the V-1.[5]
With Schmidt now working for Argus, the pulsejet was perfected and was officially known by its RLM designation as the Argus As 109-014. The first unpowered drop occurred at Peenemünde on 28 October 1942 and the first powered flight on 10 December 1942.
The pulsejet was evaluated to be an excellent balance of cost and function: a simple design that performed well for minimal cost.[5] It would run on any grade of petroleum and the ignition shutter system was not intended to last beyond the V-1's normal operational flight life of one hour. Although it generated insufficient thrust for takeoff, the V-1's resonant jet could operate while stationary on the launch ramp. The simple resonant design based on the ratio (8.7:1) of the diameter to the length of the exhaust pipe functioned to perpetuate the combustion cycle, and attained stable resonance frequency at 43 cycles per second. The engine produced 500 lbf (2,200 N) of static thrust and approximately 750 lbf (3,300 N) in flight.[5]
Ignition in the As 014 was provided by a single automotive spark plug, mounted approximately 75 cm (30 in) behind the front-mounted valve array. The spark only operated for the start sequence for the engine; the Argus As 014, like all pulsejets, did not require ignition coils or magnetos for ignition — the ignition source being the tail of the preceding fireball during the run. Contrary to popular belief, the engine casing did not provide sufficient heat to cause Diesel-type ignition of the fuel, as there is insignificant compression within a pulsejet engine.
The Argus As 014 valve array was based on a shutter system that operated at the 43 to 45 cycles-per-second frequency of the engine.
Three air nozzles in the front of the Argus As 014 were connected to an external high pressure source to start the engine. The fuel used for ignition was acetylene, with the technicians having to place a baffle of wood or cardboard in the exhaust pipe to stop the acetylene diffusing before complete ignition. Once the engine ignited and minimum operating temperature was attained, external hoses and connectors were removed.
The V1, being a cruise missile, did not have any landing gear, thus the Argus As 014 was launched on an inclined ramp powered by a piston-driven steam catapult. Steam power to fire the piston was generated by the violent exothermic chemical reaction created when hydrogen peroxide and potassium permanganate (termed T-Stoff and Z-Stoff) are combined.
Wright Field technical personnel reverse-engineered the V-1 from the remains of a V-1 that had failed to detonate in Britain. The result was the creation of the JB-2 Loon, with the airframe built by Republic Aviation, and the Argus As 014 reproduction pulsejet powerplant made by Ford Motor Company.
General Henry Harley "Hap" Arnold of the United States Army Air Forces was concerned that this weapon could be built of steel and wood, in 2000 man hours and approximate cost of US$600 (in 1943).[5]
The principal military use of the pulsejet engine, with the volume production of the Argus As 014 unit (the first pulsejet engine ever in volume production), was for use with the V-1 flying bomb. The engine's characteristic droning noise earned it the nicknames "buzz bomb" or "doodlebug". The V-1 was a German cruise missile used in World War II, most famously in the bombing of London in 1944. Pulsejet engines, being cheap and easy to construct, were the obvious choice for the V-1's designers, given the Germans' materials shortages and overstretched industry at that stage of the war. Designers of modern cruise missiles do not choose pulsejet engines for propulsion, preferring turbojets or rocket engines.
Pulsejet engines are characterized by simplicity, low cost of construction, and high noise levels. Pulsejet fuel efficiency is a topic for hot debate, as efficiency is a relative term. While the thrust-to-weight ratio is excellent, thrust specific fuel consumption is generally very poor. The pulsejet uses the Lenoir cycle which lacking an external compressive driver such as the Otto cycle's piston, or the Brayton cycle's compression turbine, drives compression with acoustic resonance in a tube. This limits the maximum (pre-combustion) pressure ratio, to perhaps 1.2 to 1.
The high noise levels usually make them impractical for other than military and other similarly restricted applications.[6] However, pulsejets are used on a large scale as industrial drying systems, and there has been a new surge to study and apply these engines to applications such as high-output heating, biomass conversion, and alternative energy systems, as pulsejets can run on almost anything that burns, including particulate fuels such as sawdust or coal powder.
Pulsejets have been used to power experimental helicopters, the engines being attached to the ends of the rotor blades.[7] As an aircraft propulsion system, pulsejets have the advantage over turbine engines of not producing torque upon the fuselage. A helicopter may be built without a tail rotor and its associated transmission and drive shaft, simplifying the aircraft (though it is still necessary to rotate the fuselage relative to the rotors in order to keep it pointing in one direction). This concept had been considered as early as 1945. The Hiller rotor-tip helicopter, known better as the Hiller Powerblade, was the world's first hot-cycle pressure-jet rotor in 1949, however the Hiller YH-32 Hornet was ram jet and not pulsejet powered.[8] Rotor-tip propulsion is estimated to reduce the cost of production of rotary-wing craft to 1/10 of conventional powered rotary-wing aircraft.[6] Pulsejets have also been used in both control-line and radio-controlled model aircraft. The speed record for control-line model aircraft is greater than 200 miles per hour (323 km/h) although the long control lines create 70% of the drag of the system.
A free-flying radio-controlled pulsejet is limited by the engine's intake design. At around 450 km/h (280 mph) most valved engines' valve systems stop fully closing owing to ram air pressure, which results in performance loss. One company, Beck Technologies, has produced a valved pulsejet design with variable intake geometry, allowing the intake to open and close to control ram airflow, and letting the engine produce full power at any speed. Valveless designs are not as negatively affected by ram air pressure as other designs, as they were never intended to stop the flow out of the intake, and can significantly increase in power at speed.
Another feature of pulsejet engines is that their thrust can be increased by a specially shaped duct placed behind the engine. The duct acts as an annular wing, which evens out the pulsating thrust, by harnessing aerodynamic forces in the pulsejet exhaust. The duct, typically called an augmenter, can significantly increase the thrust of a pulsejet with no additional fuel consumption. Gains of 100% increases in thrust are possible, resulting in a much higher fuel efficiency. However, the larger the augmenter duct, the more drag it will produce, and it may only be effective at certain speed ranges. Most vehicles will be drag-limited at a much lower speed than the speed at which a small to moderate-size augmenter will stop producing positive thrust increase.
The combustion cycle comprises five or six phases: Induction, Compression, (in some engines) Fuel Injection, Ignition, Combustion, and Exhaust.
Starting with ignition within the combustion chamber, a high pressure is raised by the combustion of the fuel-air mixture. The pressurized gas from combustion cannot exit forward through the one-way intake valve and so exits only to the rear through the exhaust tube.
The inertial reaction of this gas flow causes the engine to provide thrust, this force being used to propel an airframe or a rotor blade. The inertia of the traveling exhaust gas causes a low pressure in the combustion chamber. This pressure is less than the inlet pressure (upstream of the one-way valve), and so the induction phase of the cycle begins.
In the simplest of pulsejet engines this intake is through a venturi which causes fuel to be drawn from a fuel supply. In more complex engines the fuel may be injected directly into the combustion chamber. When the induction phase is under way, fuel in atomized form is injected into the combustion chamber to fill the vacuum formed by the departing of the previous fireball; the atomized fuel tries to fill up the entire tube including the tailpipe. This causes atomized fuel at the rear of the combustion chamber to "flash" as it comes in contact with the hot gases of the preceding column of gas—this resulting flash "slams" the reed-valves shut or in the case of valveless designs, stops the flow of fuel until a vacuum is formed and the cycle repeats.
There are two basic types of pulsejets. The first is known as a valved or traditional pulsejet and it has a set of one-way valves through which the incoming air passes. When the air-fuel is ignited, these valves slam shut which means that the hot gases can only leave through the engine's tailpipe, thus creating forward thrust.
The cycle frequency is primarily dependent on the length of the engine. For a small model-type engine the frequency may be around 250 pulses per second, whereas for a larger engine such as the one used on the German V-1 flying bomb, the frequency was closer to 45 pulses per second. The low-frequency sound produced resulted in the missiles being nicknamed "buzz bombs."
The second type of pulsejet is known as the valveless pulsejet. Technically the term for this engine is the acoustic-type pulsejet, or aerodynamically valved pulsejet.
Valveless pulsejets come in a number of shapes and sizes, with different designs being suited for different functions. A typical valveless engine will have one or more intake tubes, a combustion chamber section, and one or more exhaust tube sections.
The intake tube takes in air and mixes it with fuel to combust, and also controls the expulsion of exhaust gas, like a valve, limiting the flow but not stopping it altogether. While the fuel-air mixture burns, most of the expanding gas is forced out of the exhaust pipe of the engine. Because the intake tube(s) also expel gas during the exhaust cycle of the engine, most valveless engines have the intakes facing backwards so that the thrust created adds to the overall thrust, rather than reducing it.
The combustion creates two pressure wave fronts, one traveling down the longer exhaust tube and one down the short intake tube. By properly 'tuning' the system (by designing the engine dimensions properly), a resonating combustion process can be achieved.
While some valveless engines are known for being extremely fuel-hungry, other designs use significantly less fuel than a valved pulsejet, and a properly designed system with advanced components and techniques can rival or exceed the fuel efficiency of small turbojet engines.
In 1909, Georges Marconnet developed the first pulsating combustor without valves. It was the grandfather of all valveless pulsejets. The valveless pulsejet was experimented with by the French propulsion research group SNECMA (Société Nationale d'Étude et de Construction de Moteurs d'Aviation ), in the late 1940s.
The valveless pulsejet's first widespread use was the Dutch drone Aviolanda AT-21[9] A properly designed valveless engine will excel in flight; as it does not have valves, ram air pressure from traveling at high speed does not cause the engine to stop running like a valved engine. They can achieve higher top speeds, with some advanced designs being capable of operating at Mach .7 or possibly higher.
The advantage of the acoustic-type pulsejet is simplicity. Since there are no moving parts to wear out, they are easier to maintain and simpler to construct.
Pulsejets are used today in target drone aircraft, flying control line model aircraft (as well as radio-controlled aircraft), fog generators, and industrial drying and home heating equipment. Because pulsejets are an efficient and simple way to convert fuel into heat, experimenters are using them for new industrial applications such as biomass fuel conversion, boiler and heater systems, and other applications.
Some experimenters continue to work on improved designs. The engines are difficult to integrate into commercial manned aircraft designs because of noise and vibration, though they excel on the smaller-scale unmanned vehicles.
The pulse detonation engine (PDE) marks a new approach towards non-continuous jet engines and promises higher fuel efficiency compared to turbofan jet engines, at least at very high speeds. Pratt & Whitney and General Electric now have active PDE research programs. Most PDE research programs use pulsejet engines for testing ideas early in the design phase.
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