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A pulse jet engine is an extraordinarily 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 both easy and cheap enough for an amateur to build, requiring only basic skills and equipment. Pulse jet engines have become especially popular among model airplane enthusiasts.
There are two distinct pulse jet engine designs. Some engines rely on a set of one-way intake valves to regulate the airflow, but the valveless pulse jet engine has no moving parts at all and instead relies on the engine’s shape to regulate airflow. Both designs, though, take advantage of the same principles and operate in the same way. Fuel ignited 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 appears that the fuel in the chamber would explode, the engine would lurch forward a bit, and that would be it, but this explosive burst is only the first stage of the combustion cycle. As the exhaust is blown out, pressure in the combustion chamber is reduced. Inertia keeps the air flowing out even after that pressure drops below the surrounding air pressure, and low pressure in the chamber causes fresh air and fuel to be drawn into the tube, where it ignites and begins the process again. In large engines, this cycle is completed 45 times per second, but a small pulse jet engine might pulse as often as 250 times per second. A spark plug typically is required to start the engine, but once ignited, the cycle is self-sustaining, and no further ignition is required.
In a valved pulse jet engine, the one-way valves prevent exhaust from blowing out of the intake, but valveless engines avoid this problem by pointing the intake and exhaust pipes in the same direction. On ignition, exhaust may blow from the intake, but as the pressure drops, air flows through the shorter intake pipe into the combustion chamber. Inertia helps keep this direction of flow, and the exhaust blows in the intended direction. Valveless engines have the advantage of working without any moving parts, making them more reliable by avoiding damage to valves because of rapid fluttering and heat stress.
These engines are perhaps most often associated with the German V-1 bomb of World War II, known as the buzzbomb because of the engine’s distinctive noise. After the war, advances in turbojet and rocket technologies meant an end to the pulse jet’s military application. In the years following, pulse jet engines have been used in recreational model airplanes, fog generators and home heating.