Pulse jet engine

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.

History

Martin Wiberg (1826-1905) 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 Triebflügel, 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 was in the V-1 flying bomb, the 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.

Functioning

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 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 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."

Valveless Design

The second type of pulsejet is known as the valveless pulse jet. This name is really a misnomer. These engines have no mechanical valves, but they do have aerodynamic valves, which, for the most part, restrict the flow of gases to a single direction just as their mechanical counterparts. Indeed they have no mechanically moving parts at all and in that respect they are similar to a ramjet.

With these engines, the intake and exhaust pipes usually face the same direction. This necessitates bending the engine into a "U" shape (the Lockwood-Hiller design is made this way) or placing a 180 degree bend in the intake tube. When the air/fuel mixture inside the engine ignites, hot gases will rush out both the intake tube and the exhaust tube, since the aerodynamic valves "leak". If both tubes weren't facing in the same direction, less thrust would be generated because the reactions from the intake and exhaust gas flows would partially cancel each other. This idea was the brainchild of a French propulsion research group named SNECMA.

The advantage of the aerodynamically valved pulsejet is simplicity. Since there are no moving parts to wear out, they are easier to maintain and simpler to construct. However, they are more difficult to optimize.

Outlook

Pulsejets survive today in target drone aircraft, flying control line model aircraft, 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.

See also

• Pulse detonation engine
• Valveless pulse jet

References

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.

• Pulsejets in aeromodels