Types
There are two main types of pulsejet engines, both of which use resonant combustion and harness the expanding combustion products to form a pulsating exhaust jet which produces thrust intermittently.
Valved engines use a mechanical valve to control the flow of expanding exhaust, forcing the hot gas to go out the back of the engine through the tailpipe only, and allow fresh air and more fuel to enter through the intake. The other type of pulsejet, valveless pulsejets, have no moving parts and use only their geometry to control the flow of exhaust out of the engine. Valveless engines expel exhaust out of both the intakes and the exhaust, most try to have the majority of exhaust go out the longer tail pipe, for more efficient propulsion.
The valved pulsejet comprises an intake with a one-way valve arrangement. The valves prevent the explosive gas of the ignited fuel mixture in the combustion chamber from exiting and disrupting the intake airflow, although with all practical valved pulsejets there is some 'blowback' while running statically and at low speed as the valves cannot close fast enough to stop all the gas from exiting the intake. The superheated exhaust gases exit through an acoustically resonant exhaust pipe. The valve arrangement is commonly a "daisy valve" also known as a reed valve. The daisy valve is less effective than a rectangular valve grid, although it is easier to construct on a small scale.
The valveless pulse jet engine operates on the same principle, but the 'valve' is the engine's geometry. Fuel as a gas or liquid vapour is either mixed with the air in the intake or directly 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. With modern manufactured engine designs, almost any design can be made to be 'self-starting' by providing the engine with fuel and an ignition spark, starting the engine with no compressed air. Once running, the engine only requires input of fuel to maintain a self-sustaining combustion cycle.
Read more about this topic: Pulse Jet Engine
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