Pulsejet

thumb|300px|Diagram of a valved pulsejet. 1 - Air enters through valve and is mixed with fuel. 2 - The mixture is ignited, expands, closes the valve and exits through the tailpipe, creating thrust. 3 - Low pressure in the engine opens the valve and draws in air.

A pulsejet engine (or pulse jet) is a type of jet engine in which combustion occurs in pulses. A pulsejet engine can be made with few or no moving parts, and is capable of running statically (that is, it does not need to have air forced into its inlet, typically by forward motion). The best known example is the Argus As 109-014 used to propel Nazi Germany's V-1 flying bomb.

Pulsejet engines are a lightweight form of jet propulsion, but usually have a poor compression ratio, and hence give a low specific impulse.

The two main types of pulsejet engines use resonant combustion and harness the combustion products to form a pulsating exhaust jet that intermittently produces thrust.

The traditional valved pulsejet has one-way valves through which incoming air passes. When the fuel mix is ignited, the valves close, which means that the heated gases can only leave through the engine's tailpipe, thus creating forward thrust.

The second type is the valveless pulsejet. The technical terms for this engine are acoustic-type pulsejet, or aerodynamically valved pulsejet.

One notable line of research includes the pulse detonation engine, which involves repeated detonations in the engine, and which can potentially give high compression and reasonably good efficiency.

History

thumb|left|Ramón Casanova and the pulsejet engine he constructed and patented in 1917

Russian inventor and retired artillery officer Nikolaj Afanasievich Teleshov patented a steam pulsejet engine in 1867 while Swedish inventor Martin Wiberg also has a claim to having invented the first pulsejet, in Sweden, but details are unclear.

The first working pulsejet was patented in 1906 by Russian engineer V. V. Karavodin, who completed a working model in 1907.

French inventor Georges Marconnet patented his valveless pulsejet engine in 1908. It was the grandfather of all valveless pulsejets. The valveless pulsejet was experimented with by French propulsion research group Société Nationale d'Étude et de Construction de Moteurs d'Aviation (SNECMA), in the late 1940s.

Ramón Casanova, in Ripoll, Spain patented a pulsejet in Barcelona in 1917, having constructed one beginning in 1913. Robert Goddard invented a pulsejet engine in 1931, and demonstrated it on a jet-propelled bicycle. Engineer Paul Schmidt 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.

The valveless pulsejet's first widespread use was the Dutch drone Aviolanda AT-21

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.

The pulsejet was evaluated to be an excellent balance of cost and function: a simple design that performed well for minimal cost.

Design

right|thumb|350px|Animation of a pulsejet engine

Pulsejet engines are characterized by simplicity, low cost of construction, and high noise levels. While the thrust-to-weight ratio is excellent, thrust specific fuel consumption is 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 around 1.2 to 1.

The high noise levels usually make them impractical for other than military and other similarly restricted applications. However, pulsejets are used on a large scale as industrial drying systems, and there has been a resurgence in studying these engines for 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. In providing power to helicopter rotors, pulsejets have the advantage over turbine or piston engines of not producing torque upon the fuselage since they don't apply force to the shaft, but push the tips. A helicopter may then be built without a tail rotor and its associated transmission and drive shaft, simplifying the aircraft (cyclic and collective control of the main rotor is still necessary). This concept was being considered as early as 1947 when the American Helicopter Company started work on its XA-5 Top Sergeant helicopter prototype powered by pulsejet engines at the rotor tips. The XA-5 first flew in January 1949 and was followed by the XA-6 Buck Private with the same pulsejet design. Also in 1949 Hiller Helicopters built and tested the Hiller Powerblade, the world's first hot-cycle pressure-jet rotor. Hiller switched to tip mounted ramjets but American Helicopter went on to develop the XA-8 under a U.S. Army contract. It first flew in 1952 and was known as the XH-26 Jet Jeep. It used XPJ49 pulsejets mounted at the rotor tips. The XH-26 met all its main design objectives but the Army cancelled the project because of the unacceptable level of noise of the pulsejets and the fact that the drag of the pulsejets at the rotor tips made autorotation landings very problematic. Rotor-tip propulsion has been claimed to reduce the cost of production of rotary-wing craft to 1/10 of that for conventional powered rotary-wing aircraft.

Applications

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, and boiler and heater systems.

Proposed enhancements

Some experimenters continue to work on improved designs. The engines are difficult to integrate into commercial crewed aircraft designs because of noise and vibration, though they excel on the smaller-scale uncrewed 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.

Boeing has a proprietary pulsejet engine technology called Pulse Ejector Thrust Augmentor (PETA), which proposes to use pulsejet engines for vertical lift in military and commercial VTOL aircraft.

References

External links

pulse-jets.com: An international site dedicated to pulsejets, including design and experimentation. Includes an extremely active forum composed of knowledgeable enthusiasts
Video of 21st century-built German reproduction Argus As 014 pulsejet testing
Pulsejets in aeromodels
Popular Rotorcraft Association
Pulsejet Bike
Apocalyptic robotics performance group Survival Research Labs operates a collection of pulsejet engines in some of their creations, including the Hovercraft, V1, and the Flame Hurricane.
PETA (Pulse-Ejector-Thrust-Augmentors) article
Ramon Casanova's pulsejet
American Helicopter XA-5 Flight
PULSE JET ENGINE CAN BE USE IN BICYCLE TO RUN IT AT HIGH SPEED USING U-SHAPE TUBE
Pulse jet designs and valveless pulsejet configurations
Interaction behaviour of pulsejet engines
Theoretical and Experimental Evaluation of Pulse Jet Engine
US Air Force funds development of pulsejet powered air decoy