The valveless pulse jet is a very simple type of jet engine, which can run on a variety of fuels and contains no moving parts. These basic jet engines become very hot during operation and are also quite loud, but the lack of moving parts means they require substantially less maintenance than other jet propulsion devices. The most common valveless pulse jet design consists of a combustion chamber connected to two pipes, but it is also possible to construct a simpler version with only a single aperture. An initial ignition source is required to start this type of jet engine, after which a cycle of deflagration and air intake will repeat many times each second until the available fuel has been exhausted.
Early experiments with pulse jets occurred in the early 20th century, though the first practical designs did not begin to emerge until the 1930s. Some of the first pulse jets were built during the Second World War, and one famous design was used to power the German V-1 flying bombs. These engines were slightly more complicated than the valveless pulse jets, since they included shutters to control air intake and the release of combustion gasses.
Valveless pulse jets are able to operate without shutters to close off the air intake since both intake and exhaust pipes point in one direction. This is typically achieved with a U-shaped design, though other configurations are possible. The smaller of the two pipes in a valveless pulse jet functions primarily as the intake, though both will expel exhaust gasses during deflagration. Some air is typically also drawn in through the exhaust port during the intake phase, which can increase turbulence within the combustion chamber.
An external ignition source is required to start a valveless pulse jet, but not to keep it running. The initial deflagration expels exhaust gasses from both intake and exhaust pipes, after which new air can enter the intake. This causes the process to start again, and the cycle can repeat itself several hundred times each second.
Unlike many other jet engines, the valveless pulse jet requires no forward motion to operate, and contains no moving components that can wear out or break down. They can also operate on virtually any combustible substance, and many experimental and hobbyist designs use fuels such as gasoline, diesel and kerosene. The main drawbacks of the design include poor compression ratios and the great amounts of heat and noise that are typically generated.