Pulse jet engine
A
pulse jet engine (or
pulsejet) is a very simple form of
internal combustion engine wherein the combustion occurs in
pulses and the propulsive effort is a jet, this is, a reaction to the
rearward
flow of hot gases.
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. Once the engine is running it requires only an input of fuel, but it usually requires forced air and an ignition method for the fuel-air mix. Once running, the engine is self-sustaining.
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 applications and 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 Triebflugel, 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 after mid-
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.
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Pulse jet schematic. First part of the cycle: air intake (1), 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 six phases: Ignition, Combustion, Exhaust, Induction, Compression, and (in some engines) Fuel Injection.
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 most simple 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.
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Pulse Jet diagram 2 |
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 second type of pulsejet is the known as the valveless pulsejet. 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
SNECMAThe 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.
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.
Pulsejets survive today in target drawn aircraft, flying
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.
*
Pulse detonation engine*
Valveless pulse jet* http://www.pulse-jets.com/ - An international site dedicated to pulsejets, including design and experimentation. Includes an extremely active forum composed of knowledgeable enthusiasts.
