Valveless pulsejet

This clearly distinguishes it from other reaction engine types such as rockets, turbojets, and ramjets, which are all constant combustion devices.

This alternation is not maintained by any mechanical contrivance, but rather by the natural acoustic resonance of the rigid tubular engine structure.

The deflagration within the combustion zone of a pulsejet is characterized by a sudden rise in temperature and pressure followed by a rapid subsonic expansion in gas volume.

The combustion zone (usually a widened "chamber" section) and tailpipe make up the main tube of the engine.

Both of these basic actions are accomplished by a significant drop in pressure that occurs naturally after the deflagration expansion, a phenomenon known as the Kadenacy effect (named after the scientist who first fully described it).

When the next deflagration occurs, the rapid pressure rise slams the valve shut very quickly, ensuring that almost no explosion mass exits in the forward direction so the expansion of the combustion gases will all be used to accelerate the replenished mass of air in the long tailpipe rearward.

When the deflagration begins, a zone of significantly elevated pressure travels outward through both air masses as a compression wave.

This low pressure region returning to the combustion zone is, in fact, the internal mechanism of the Kadenacy effect.

Also, the eventual flow reversal will take place much sooner in the intake, due to its smaller air mass.

The actual breathing of the engine as a whole will not begin in earnest until the major low pressure wave from the tailpipe reaches the combustion zone.

As air flows rapidly into the combustion zone, the rarefaction wave is reflected rearward by the front of the engine body, and as it moves rearward the air density in the combustion zone naturally rises until the pressure of the air/fuel mixture reaches a value where deflagration can again commence.

One simple method is to turn the engine around and then put a U-bend in the tailpipe, so both ducts are spouting rearward, as in the Ecrevisse and Lockwood (also known as Lockwood-Hiller) types.

However it works fairly well with a simple instrument such as jam jar with a pierced lid and fuel inside, hence the name.

The bottle is far less efficient than the jam jar versions and is unable to sustain a decent jet for more than a few seconds.

[citation needed] Successful valveless pulsejets have been built from a few centimeters in length to huge sizes, though the largest and smallest have not been used for propulsion.

Medium and larger sized engines can be made to burn almost any flammable material that can be delivered uniformly to the combustion zone, though of course volatile flammable liquids (gasoline, kerosene, various alcohols) and standard fuel gases (LPG, propane, butane, MAPP gas) are easiest to use.

Because of the deflagration nature of pulsejet combustion, these engines are extremely efficient combustors, producing practically no hazardous pollutants, other than CO2[citation needed], even when using hydrocarbon fuels.

Up to the present, the physical size of successful valveless designs has always been somewhat larger than valved engines for the same thrust value, though this is theoretically not a requirement.

Working mechanism of a valveless pulsejet engine. The basic idea is that the column of air in the long exhaust pipe functions like the piston of a reciprocating engine . From another point of view, the engine is an acoustic resonator internally excited by resonating combustions in the chamber. The chamber acts as a pressure antinode which is compressed by the returning wave. The intake pipe acts as a kinematic antinode which sucks and exhausts gas. Note the longer length of the exhaust pipe—this is important as it prevents oxygen from entering the wrong way and igniting the system the wrong way. It does this because when the pulse ignites, there is still some exhaust gas in the exhaust pipe. That is sucked in before any additional oxygen is sucked in. Of course, the air intake pipe has already supplied the oxygen by that point and the pulse reignites.
Work mechanism of jam jar jet. (b) Mixture of air and fuel vapors could ignite using external igniter or by residual free radicals from last work cycle. (a) The previous jet expelled more air than conform to equilibrium pressure in chamber, thus some of the fresh air is sucked back. The pressure drop in this case is caused more by cooling of the gas in chamber than by gas momentum. Gas momentum can not be used well in this design because of lack of exhaust (resonator) pipe and very dissipative aerodynamics of the aperture.