Pulsejet Engine Overview
Part of a Series on Jet Engines
The pulsejet is an air breathing jet engine. The pulsejet is different from most other types of jet engines as it operates in discrete cycles or pulses, where as most jet engines run smoothly and continuously.
At the beginning of a cycle in a standard valved pulsejet, air is mixed with fuel at the mouth of the engine. This mixture then enters open valves at the mouth of the engine. The valves slot closed and the fuel-air combination is combusted forcing hot gas out the tail pipe, providing thrust. The valves at the mouth then open and the vacuum caused by the expelled gas fills the combustion chamber with air.
In a valveless pulsejet, the moving valves at the mouth of the engine are eliminated. This type of engine uses the vaccum created by escaping air to cycle the engine.
At the start of the cycle, a chamber with two pipes connected to it, one short and one long, is filled with a fuel air combination. The mixture is combusted and hot gas accelerates out of both pipes. The escaping gas creates a vacuum in the combustion chamber. The shorter pipe is evacuated more quickly and air rushes through it to fill the combustion chamber. This air is mixed with fuel and the combustion process starts all over again. In order to ensure an optimized fuel-air mixture and a smooth cycle, the lengths of the two pipes must be very carefully tuned.
Both varieties of pulsejets are incredibly simple and largely eschew moving parts (excepting the valves on the valved pulsejet.) Additionally, pulsejet, if properly tuned, can be very efficient. The reason that pulsejets do not see wide use in the aerospace industry is that they produce what is considered an unacceptable amount of noise and vibration. This is a result of their discrete cycles which range from about 50 cycles per second in large pulsejets to 250 in smaller models. Additionally, at high speeds, ram air pressure can prevent valves from fully closing, drastically reducing power and efficiency.
The pulsejet’s most notable use was on the German V-1 cruise missile. This missile was nicknamed the “Buzz Bomb” as its engine, cycling at 45 cycles-per-second emitted an audible buzz. Today, some military target drones and civilian model aircraft use pulsejet engines. The military has experimented with pulsejets as tipjets for helicopters, even going so far as to produce a number of working models.
The engines were found to be far to loud and the project was abandoned. Pulsejets are also used in commercial drying and heating operations as the pulsejet produces heat from fuel very efficiently.
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