Pulse jet engine

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A pulse jet engine (or pulsejet) is a very simple form of internal combustion engine based jet engine where combustion occurs in pulses.

[edit] Types

There are two main types of pulse-jet engine, working on exactly the same principle: one uses valves to accomplish via mechanical means what the other variant accomplishes via resonance (differences in air pressure).

The valved pulse-jet 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 air-flow. The super-heated exhaust gases exit via an acoustically resonant exhaust pipe. The valve arrangement is commonly a "daisy valve" also known as a reed valve.

The valveless pulse jet engine operates on the same principle, but the 'valve' is commonly a u-shaped tube. This tube via pressure differences and resonance forces exhaust gas to exit the resonance tuned exhaust pipe. 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. Once running, the engine only requires input of fuel to maintain a self-sustaining combustion cycle.

[edit] History

The Swedish inventor Martin Wiberg has a strong claim to have 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 pulse-jet was patented in 1906 by Russian engineer V.V Karavodin, completing a working model in 1907. The French inventor Georges Marconnet patented his valveless pulse-jet engine in 1908, which many commentators argue greatly influenced brilliant V-1 engineer Paul Schmidt. Paul Schmidt, a Munich-based independent designer and inventor began work on a new design of pulse-jet engine. Schmidt's design pioneered with a new efficient design based on modification of the intake valves (or flaps)- earning him government support from the German Air Ministry in 1933.^[1]

[edit] Argus As 104

Discussion of pulse-jet engines necessitates detailed discussion of the most famous and greatest volume-produced engine- the Argus As-104.

In 1934, Professor Georg Madelung and Paul Schmidt proposed a "flying bomb" powered by Schmidt's pulse-jet to the German Air Ministry. Professor Madelung co-invented the ribbon-parachute- the device that stabilised the V-1 in its terminal dive. Schmidt's prototype bomb failed to meet German Air Ministry specifications- especially due to poor accuracy, range and high cost. The original Schmidt design had the pulse-jet placed in a fuselage like 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 pulse-jets and flying bombs were the 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.^[2]

With Schmidt working now for Argus, the pulse-jet was perfected and was officially known as the Argus As-014. The first unpowered drop flight occurred at Peenemuende on October 28, 1942 and first powered flight on December 10, 1942.

The Pulse-jet was evaluated to be an excellent balance of cost and function: a simple design that performed well for minimal cost.^[3] It would run on any grade of petroleum and the ignition shutter system was not intended to last beyond the V-1 normal operational flight life of one hour. The V-1's resonant jet could operate while stationary on the launch ramp, contrary to popular belief. The simple resonant design based on 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 Hertz. The engine produced 500 lb (2,224 Newtons) of static thrust and approximately 750 lbs (3,336 N) in flight.^[4]

Ignition to the Argus was provided by a single automotive spark plug, mounted approximately 75 cm (30 inches) behind the front-mounted valve array. The spark only operated for the start sequence for the engine; the Argus As-104 like all pulse-jets did not require ignition coils or magnetos for ignition - the ingnition source being the tail of the preceeding fireball during the run, contrary to popular belief, the engine casing did not provide sufficient heat to "dieselize" the fuel as there is no "compression" when a pulsejet runs.

The Argus As-104 valve array was based on a shutter system which operated at the 43 to 45 Hertz frequency of the engine.

Three air nozzles in the front of the Argus As-104 were connected to an external high pressure source to start the engine. Ignition fuel was acetylene, with the German 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 attained, external hoses and connectors were removed.

Pulse-jet engines create low thrust at rest, thus the Argus As-104 was launched on an inclined ramp powered by a steam chemical reaction of hydrogen peroxide and potassium permanganate (termed T-Stoff and Z-Stoff).

The origins of the term "Chinese Copy" actually originated from a Wright Field reverse-engineered V-1, built from the remains of a V-1 that had failed to detonate in Britain.

General Arnold of the US Navy was concerned that this weapon could be built of steel and wood, in 2000 man hours and approximate cost of USD $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 Argus engine's characteristic droning noise earning 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 over-stretched industry at that stage of the war. Modern cruise missiles do not generally use pulsejet engines but true rocket or gas turbine engines.

[edit] Operation

Pulsejet engines are characterized by extreme simplicity, low cost of construction, poor fuel economy and very high noise levels. The high noise levels make them impractical for other than military and other similarly restricted applications. ^[6]

Pulsejets have been used to power experimental helicopters, the engines being attached to the extreme ends of the rotor blades. As an aircraft propulsion system, pulse-jets have the distinct advantage over conventional turbine engines by not producing the usual reaction torque upon the fuselage. A helicopter may be built without a tail rotor and its associated transmission and drive shaft, greatly 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.^[7] Rotor-tip propulsion is estimated to reduce the cost of production of rotary-wing craft to be 1/10 of conventional powered rotary-wing aircraft.^[8] Pulsejets have also been used in both control line and radio-control model aircraft. The speed record for control line model aircraft is greater than 200 miles per hour (323 km/h)

[edit] Functioning

Pulse jet schematic. First part of the cycle: air flows through the intake (1), and mixed with fuel (2). Second part: the valve (3) is closed and the ignited fuel-air mix (4) propels the craft.

The combustion cycle comprises five or six phases: Induction, Compression, (in some engines) Fuel Injection, Ignition, Combustion and Exhaust.

Starting at 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.

It is the inertial reaction of this gas flow that 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 underway, fuel in atomized form is being inducted into the combustion chamber to replace 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 preceeding 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.

[edit] Valved Design

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 typically around 250 pulses per second — whereas for a larger engine such as the one used on the German V1 flying bomb, the frequency was closer to 45 pulses per second. The low frequency sound produced resulted in the missiles being nicknamed "buzz bombs."

[edit] Valveless Design

The second type of pulse-jet is known as the valveless pulse jet. Technically the term for this engine is the acoustic-type pulse-jet. The engine has two tubes, joined together. The shorter tube is bent into a "U" bend, and the orifice faces the same direction as the exhaust pipe, which is longer.

The combustion process creates two shock-wave fronts, one traveling down the long upper tube and one down the short lower tube. By properly 'tuning' the system, a resonating combustion process can be achieved. Fuel consumption is very high, as is the noise generated.^[9]The valve-less pulse-jet was first built by French propulsion research group SNECMA (Societe Nationales es Etudes de Moteurs d'Aviation), in the late 1940's. Its first use was the Dutch drone Aviolanda AT-21^[10]
The advantage of the acoustic-type pulse-jet is simplicity. Since there are no moving parts to wear out, they are easier to maintain and simpler to construct. The thrust output is also considerably lower than the valved design. Roughly half the thrust is produced compared to the equivalent sized valved design. ^[11]

[edit] Future Uses

Pulsejets survive today in target drone aircraft, flying control line model aircraft (as well as RC), fog generators and home heating equipment. Some experimenters continue to work on improved designs. The engines are difficult to integrate into manned aircraft design due to high fuel consumption, noise, and vibration.

The pulse detonation engine (PDE) marks a new approach towards non-continuous jet engines and promises higher fuel efficiency compared even to turbofan jet engines, at least at very high speeds. Pratt & Whitney and General Electric now have active PDE research programs.

[edit] See also

• Pulse detonation engine
• Valveless pulse jet
• Ramjet

[edit] References

1. ^ George Mindling, Robert Bolton: US Airforce Tactical Missiles:1949-1969: The Pioneers, Lulu.com, 200: ISBN 0557000297. pp6-31
2. ^ George Mindling, Robert Bolton: US Airforce Tactical Missiles:1949-1969: The Pioneers, Lulu.com, 200: ISBN 0557000297. pp6-31
3. ^ George Mindling, Robert Bolton: US Airforce Tactical Missiles:1949-1969: The Pioneers, Lulu.com, 200: ISBN 0557000297. pp6-31
4. ^ George Mindling, Robert Bolton: US Airforce Tactical Missiles:1949-1969: The Pioneers, Lulu.com, 200: ISBN 0557000297. pp6-31
5. ^ George Mindling, Robert Bolton: US Airforce Tactical Missiles:1949-1969: The Pioneers, Lulu.com, 200: ISBN 0557000297. pp6-31
6. ^ Jan Roskam, Chuan-Tau Edward Lan; Airplane aerodynamics and performance DARcorporation: 1997: ISBN 1884885446: 711 pages
7. ^ Joseph Lawrence Nayler, Ernest Ower; Aviation: its technical development, Published by Dufour Editions, 1965, 290 pages
8. ^ Jan Roskam, Chuan-Tau Edward Lan; Airplane aerodynamics and performance DARcorporation: 1997: ISBN 1884885446: 711 pages
9. ^ Jan Roskam, Chuan-Tau Edward Lan; Airplane aerodynamics and performance, DARcorporation: 1997 ISBN 1884885446: 711 pages: pp255-56
10. ^ Jan Roskam, Chuan-Tau Edward Lan; Airplane aerodynamics and performance, DARcorporation: 1997 ISBN 1884885446: 711 pages
11. ^ Jan Roskam, Chuan-Tau Edward Lan; Airplane aerodynamics and performance, DARcorporation: 1997 ISBN 1884885446: 711 pages

• Aeronautical Engineering Review, Institute of the Aeronautical Sciences (U.S.): 1948, vol. 7.
• George Mindling, Robert Bolton: US Airforce Tactical Missiles:1949-1969: The Pioneers, Lulu.com, 200: ISBN 0557000297. pp6-31

[edit] External links

• http://www.pulse-jets.com/ - An international site dedicated to pulsejets, including design and experimentation. Includes an extremely active forum composed of knowledgeable enthusiasts.
• http://www.Beck-Technologies.com/ - A site for hobby jet propulsion, specifically valved and valveless pulsejet engines. They offer many plans, and have a lot of useful information including pictures and video.
• http://www.frenchgeek.com/pulsejet.php - A detailed guide documenting all the steps required to build one's own Pulsejet. The example created on this site is eventually mounted onto a home-built kart and tested.
• Pulsejets in aeromodels
• Aviastar information on Hiller rotor-tips[[1]]
• Popular Rotocraft Association [[2]]
• Pulse Jet Bike [[3]]