Ball: Pulse jet engine

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Pulsejet

Part of a series onAircraft propulsion

Shaft engines turning propellers, rotors, Ducted fans and propfans

Reaction engines

External combustion engines

Rocket-powered aircraft

Steam-powered aircraft

Motorjet

Internal combustion engines

Pulsejet

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Ramjet

? Wankel engine

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Turbine engines

? Turbojet

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Human-powered aircraft

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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.

A typical pulsejet comprises an air intake fitted with a one-way valve, a combustion chamber, and an acoustically resonant exhaust pipe. The valving is accomplished though the use of reed valves or, in a valveless pulse jet engine, through aerodynamics. Fuel in the form of a gas or liquid aerosol is either mixed with the air in the intake or 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 an input of fuel to maintain a self-sustaining combustion cycle.

Contents

1 History

2 Functioning

2.1 Valved Design

2.2 Valveless Design

2.3 Outlook

3 See also

4 References

5 External links

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History

Martin Wiberg developed the first pulse jet in Sweden. 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. Pulsejets have been used to power experimental helicopters, the engines being attached to the extreme ends of the rotor blades. One proposed design during WWII was the Focke-Wulf Fw Triebfl榛l, although the craft was never built. In this application they have the distinct advantage of not producing the usual reaction torque upon the fuselage and the 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). Pulsejets have also been used in both tethered and radio-control model aircraft. The speed record for tethered model aircraft is 186 miles per hour (299 km/h), set in the early 1950s.

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.

In 2001, Bob Maddox, a cabinet maker and mechanic in Medford, Oregon, began conducting experiments with pulse jets. He eventually began bolting the engines to bicycles. By 2008, he had achieved speeds in excess of 70 mph.[1]

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 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 complete a reflected high pressure wave from the tailpipe compresses the charge, which is ignited by residual heat from the previous cycle.

Valved Design

There are two basic types of...(and so on)