Pulse jet engines are simple and inexpensive devices that produce thrust by burning fuel in rapid pulses. They are popular with hobbyists and model airplane enthusiasts. There are two designs, valved and valveless, but both work by igniting fuel in the combustion chamber and forcing it through the exhaust to produce thrust. The cycle is self-sufficient and requires no further ignition. Valveless engines have no moving parts, making them more reliable. They were used in the German V-1 bomb during World War II but are now used in recreational model aircraft, fog generators, and home heating.
A pulse jet engine is a remarkably simple device, little more than a hollow tube that produces thrust by burning fuel in a series of rapid pulses. This very basic design makes the pulse jet engine easy and inexpensive enough for a hobbyist to build, requiring only basic skills and equipment. Pulse jet engines have become especially popular with model airplane enthusiasts.
There are two distinct designs of pulse jet engines. Some engines rely on a set of one-way intake valves to regulate airflow, but the valveless pulse-jet engine has no moving parts at all, and instead relies on the shape of the engine to regulate airflow. air. However, both designs take advantage of the same principles and work in the same way. Ignited fuel in the combustion chamber expands explosively and is forced through the narrow exhaust. The force of that exhaust produces the thrust that moves the engine forward.
At first glance, it seems that the fuel in the chamber would explode, the engine would move forward a bit, and that would be it, but this explosive explosion is only the first stage of the combustion cycle. As the exhaust is expelled, the pressure in the combustion chamber is reduced. Inertia keeps air flowing even after the pressure drops below the surrounding air pressure, and the low pressure in the chamber forces fresh air and fuel into the tube, where it ignites and starts the process all over again. . In large engines this cycle is completed 45 times per second, but a small pulse jet engine can pulse up to 250 times per second. A spark plug is usually required to start the engine, but once ignited the cycle is self-sufficient and no further ignition is required.
In a valved pulse-jet engine, one-way valves prevent exhaust from exiting the intake, but valveless engines avoid this problem by pointing the intake and exhaust pipes in the same direction. On ignition, the exhaust can blow from the intake, but as the pressure drops, air flows through the shorter intake tube into the combustion chamber. Inertia helps maintain this direction of flow, and the exhaust blows in the intended direction. Valveless engines have the advantage of running without moving parts, making them more reliable by preventing valve damage from rapid flutter and heat stress.
Perhaps these engines are most often associated with the German V-1 bomb of World War II, known as the hum bomb due to the distinctive engine noise. After the war, advances in turbojet and rocket technologies spelled the end of the military application of the pulse jet. In subsequent years, pulse jet engines have been used in recreational model aircraft, fog generators, and home heating.
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