Science Fair Project Encyclopedia
Pulse jet engine
A pulse jet engine is a very simple form of internal combustion engine wherein the combustion occurs in pulses and the propulsive effort is a reaction to the rearward flow of hot gasses.
A pulse jet comprises an inlet arrangement, a one-way air inlet valve, a combustion chamber, and an (acoustically) resonant exhaust tube (tailpipe) with integral propelling nozzle. It also has a means of admitting and mixing fuel with the intake air (or injecting fuel into the combustion chamber), and a means of ignition when the engine is started. Once the engine is running there is no need to provide further ignition. There is also some means of providing combustion air to start the engine from a compressed air supply.
Martin Wiberg (1826-1905) developed the first pulse-jet in Sweden.
Pulse jet engines are characterized by extreme simplicity and low cost of construction, high reliability, poor fuel economy and a very high noise level. The high noise level makes them impractical for other than military applications and other restricted applications. Pulse jets have been used to power experimental helicopters, the engines being attached to the extreme ends of the rotor blades. 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. Pulse jets 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 principle military use of the pulse jet engine was in the V-1 "buzz bomb". This was a German pilotless aircraft used in World War II, most famously in the bombing of London after mid-1943. The concept developed in the V-1 is still with us as the modern "cruise missile", although these do not generally use pulse jet engines.
The combustion cycle comprises several 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 pressurised 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 travelling 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 pulse jet 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.
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 valveless pulsejet. These engines have no valves; indeed they have no moving parts at all and in that respect they are even simpler than a ramjet. With these engines, the intake and exhaust pipes usually both face the same direction. This often necessitates bending the engine in half (the Lockwood design is made this way) or placing a 180 degree bend in the intake tube. This is necessary because when the air/fuel mixture inside the engine ignites, hot gases will rush out both the intake tube and the exhaust tube, there being no valves to stop them. If both tubes weren't facing in the same direction, little or no thrust would be generated because the reactions from the intake and exhaust tubes would cancel each other out.
The advantage of the valveless pulsejet is simple and obvious, there are no moving parts to wear out so they are far more reliable and a lot simpler to build.
The cycle frequency is dependent on the length of the engine itself and, for a small model-type engine 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.
Like most jet engines, the pulse jet engine is very simple in design -- consisting primarily of a long tube into which air enters and is mixed with fuel to create a combustible (stoichiometric) mixture. Where the pulsejet differs from other engines such as the Turbojet or Ramjet is that the combustion inside the engine is not continuous but occurs in the form of repeated deflagrations, hence the name "pulsejet".
However, despite this advantage, pulsejets are seldom considered to be practical power plant due to their high fuel consumption, low efficiency, noise, and significant vibration levels. Today, they survive as a powerplant for model aeroplanes.
See also: Pulse detonation engine
- 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.aardvark.co.nz/pjet/ - An extensive site on pulse jets, including the site maintainer's extensive development work on modern designs.
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