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.
A valveless pulsejet (or pulse jet) is the simplest known jet propulsion device. Valveless pulsejets are low in cost, light weight, powerful and easy to operate. They have all the advantages (and most of the disadvantages) of conventional valved pulsejets, but without the reed valves that need frequent replacement; a valveless pulsejet can operate for its entire useful life with practically zero maintenance. They have been used to power model aircraft, experimental go-karts,[1] and unmanned military aircraft such as cruise missiles and target drones.
A pulsejet engine is an air-breathing reaction engine that employs an ongoing sequence of discrete combustion events rather than one sustained combustion event. This clearly distinguishes it from other reaction engine types such as rockets, turbojets, and ramjets, which are all constant combustion devices. All other reaction engines are driven by maintaining high internal pressure; pulsejets are driven by an alternation between high and low pressure. This alternation is not maintained by any mechanical contrivance, but rather by the natural acoustic resonance of the rigid tubular engine structure. The valveless pulsejet is, mechanically speaking, the simplest form of pulsejet, and is, in fact, the simplest known air-breathing propulsion device that can operate "statically", i.e. without forward motion.
The combustion events driving a pulsejet are often informally called explosions; however, the correct term is deflagrations.[2] They are not detonations, which is the combustion event in pulse detonation engines (PDEs). 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. It is this expansion that performs the main work of moving air rearward through the device as well as setting up conditions in the main tube for the cycle to continue.
A pulsejet engine works by alternately accelerating a contained mass of air rearward and then breathing in a fresh mass of air to replace it. The energy to accelerate the air mass is provided by the deflagration of fuel mixed thoroughly into the newly acquired fresh air mass. This cycle is repeated many times per second. During the brief mass acceleration phase of each cycle, the engine's physical action is like that of other reaction engines — gas mass is accelerated rearward, resulting in an application of force forward into the body of the engine. These pulses of force, rapidly repeated over time, comprise the measurable thrust force of the engine.
Some basic differences between valved and valveless pulsejets are:
Valveless pulsejet engines have no mechanical valve, eliminating the only internal "moving part" of the conventional pulsejet.[citation needed]
In valveless engines, the intake section has an important role to play throughout the entire pulsejet cycle.
Valveless engines produce thrust forces in two distinct but synchronized mass acceleration events per cycle, rather than just one.
Basic (valved) pulsejet theory
In a conventional "valved" pulsejet, like the engine of the infamous V-1 "buzz bomb" of World War II, there are two ducts connected to the combustion zone where the deflagrations occur. These are generally known as the "intake" (a very short duct) and the "tailpipe" (a very long duct). The function of the forward-facing intake is to provide air (and in many smaller pulsejets, the fuel/air mixing action) for combustion. The purpose of the rear-facing tailpipe is to provide air mass for acceleration by the explosive blast as well as to direct the accelerated mass totally rearward. The combustion zone (usually a widened "chamber" section) and tailpipe make up the main tube of the engine. A flexible, low mass one-way valve (or multiple identical valves) separates the intake from the combustion zone.
At the beginning of each cycle, air must be pulled into the combustion zone. At the end of each cycle, the tailpipe must be reloaded with air from the surrounding atmosphere. 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). This temporary low pressure opens the metal valve and draws in the intake air (or air/fuel mixture). It also causes a reversal of flow in the tailpipe that draws fresh air forward to re-fill the pipe. 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.
Valveless pulsejet operation
The valveless pulsejet is not really valveless — it just uses the mass of air in the intake tube as its valve, in place of a mechanical valve. It cannot do this without moving the intake air outward, and this volume of air itself has significant mass, just as the air in the tailpipe does — therefore, it is not blown away instantly by the deflagration but is accelerated over a significant fraction of the cycle time. In all known successful valveless pulsejet designs, the intake air mass is a small fraction of the tailpipe air mass (due to the smaller dimensions of the intake duct). This means that the intake air mass will be cleared out of contact with the body of the engine faster than the tailpipe mass will. The carefully designed imbalance of these two air masses is important for the proper timing of all parts of the cycle.
When the deflagration begins, a zone of significantly elevated pressure travels outward through both air masses as a compression wave. This wave moves at the speed of sound through both the intake and tailpipe air masses. (Because these air masses are significantly elevated in temperature as a result of earlier cycles, the speed of sound in them is much higher than it would be in normal outdoor air.) When a compression wave reaches the open end of either tube, a low pressure rarefaction wave starts back in the opposite direction, as if "reflected" by the open end. This low pressure region returning to the combustion zone is, in fact, the internal mechanism of the Kadenacy effect. There will be no "breathing" of fresh air into the combustion zone until the arrival of the rarefaction wave.
The wave motion through the air masses should not be confused with the separate motions of the masses themselves. At the start of deflagration, the pressure wave immediately moves through both air masses, while the gas expansion (due to combustion heat) is just beginning in the combustion zone. The intake air mass will be rapidly accelerated outward behind the pressure wave, because its mass is relatively small. The tailpipe air mass will follow the outgoing pressure wave much more slowly. Also, the eventual flow reversal will take place much sooner in the intake, due to its smaller air mass. The timing of the wave motions is determined basically by the lengths of the intake and main tube of the engine; the timing of mass motions is determined mostly by the volumes and exact shapes of these sections. Both are affected by local gas temperatures.
In the valveless engine, there will actually be two arrivals of rarefaction waves — first, from the intake and then from the tailpipe. In typical valveless designs, the wave that comes back from the intake will be relatively weak. Its main effect is to begin flow reversal in the intake itself, in effect "pre-loading" the intake duct with fresh outdoor air. 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. Once that happens, significant flow reversal begins, driven by the drop in combustion zone pressure.
During this phase, too, there is a difference in action between the very different masses in the intake and tailpipe. The intake air mass is again fairly low, but it now almost totally consists of outside air; therefore, fresh air is available almost immediately to begin re-filling the combustion zone from the front. The tailpipe air mass is also pulled, eventually reversing direction as well. The tailpipe will never be completely purged of hot combustion gases, but at reversal it will be easily able to pull in fresh air from all sides around the tailpipe opening, so its contained mass will be gradually increasing until the next deflagration event. 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.
Practical design issues
In practical designs there is no need for a continuous ignition system— the combustion zone is never totally purged of combustion gases and free radicals, so there is enough chemical action in the residue in the combustion zone to act as an igniter for the next blast once the mixture is up to a reasonable density and pressure: the cycle repeats, controlled only by the synchronization of pressure and flow events in the two ducts.
While it is theoretically possible to have such an engine without a distinct "combustion chamber" larger than the tailpipe diameter, all successful valveless engines designed so far have a widened chamber of some sort, roughly similar to that found in typical valved engine designs. The chamber typically takes up a fairly small fraction of the overall main tube length.
The acceleration of air mass back through the intake duct doesn't make sense for engine thrust if the intake is aimed forward, since the intake thrust is a fairly large fraction of the tailpipe thrust. Various engine geometries have been used to make the thrust forces from the two ducts act in the same direction. 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. The Escopette and Kentfield designs use recuperators (U-shaped auxiliary tubes) mounted in front of the front-firing intakes to turn the intake blast and flow rearward. The so-called "Chinese" and Thermojet styles simply mount the intake on the chamber in a rear-spouting direction, leaving the front face of the chamber unbroken. The basic internal operation of the engine with these geometries is no different from that described above, however. The Lockwood is unique in one respect, namely, its very large diameter intake — the thrust from this large tube is no less than 40 percent of the engine thrust as a whole. The tailpipe volume of this design is quite large, though, so the imbalance of the contained masses is still clearly seen.
"Jam jar jet" design
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.
Most pulsejet engines use independent intake and exhaust pipes. A physically simpler design combines the intake and exhaust aperture. This is possible due to the oscillating behaviour of a pulse engine. One aperture can act as exhaust pipe during the high-pressure phase of the work cycle and as intake during the aspiration phase. This engine design is less efficient in this primitive form due to its lack of a resonant pipe and thus a lack of reflected compressing and sucking acoustic waves. However it works fairly well with a simple instrument such as jam jar with a pierced lid and fuel inside, hence the name.
Successful versions of the jam jar jet have been run in a plastic bottle. 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. It is theorized that the alcohol that was used to operate the simple jet was acting as a barrier to stop the heat getting all the way through to the plastic. For the jam jar jet design to work the propellant must be vaporised to ignite which is most often done by a shaking of the jet which causes the propellant to coat the container, therefore giving the theory some validity.[citation needed]
Pros and cons
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. The smallest ones are only successful when extremely fast-burning fuels are employed (acetylene or hydrogen, for example). 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. With modern high-temperature metals for the main structure, engine weight can be kept extremely low. Without the presence of a mechanical valve, the engines require practically no ongoing maintenance to remain operational.
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. Like valved pulsejets, heat (engines frequently run white hot) and very high operational noise levels (140 decibels is possible)[2] are among the greatest disadvantages of these engines. An ignition system of some sort is required for engine startup. In the smallest sizes, forced air at the intake is also typically needed for startup. There is still much room for improvement in the development of really efficient, fully practical designs for propulsion uses.
One possible solution to the ongoing problem of pulsejet inefficiency would be to have two pulsejets in one, with each blast compressing the mixture of fuel and air in the other, and both ends discharging into a common chamber through which air flows only one way. This could potentially allow much higher compression ratios, better fuel efficiencies, and greater thrust.[3]
A jet engine is a type of reaction engine, discharging a fast-moving jet of heated gas that generates thrust by jet propulsion. While this broad definition may include rocket, water jet, and hybrid propulsion, the term jet engine typically refers to an internal combustion air-breathing jet engine such as a turbojet, turbofan, ramjet, pulse jet, or scramjet. In general, jet engines are internal combustion engines.
A ramjet is a form of airbreathing jet engine that requires forward motion of the engine to provide air for combustion. Ramjets work most efficiently at supersonic speeds around Mach 3 and can operate up to Mach 6.
A pulsejet engine is a type of jet engine in which combustion occurs in pulses. A pulsejet engine can be made with few or no moving parts, and is capable of running statically. The best known example may be the Argus As 109-014 used to propel Nazi Germany's V-1 flying bomb.
The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a combination of the preceding generation engine technology of the turbojet, and a reference to the additional fan stage added. It consists of a gas turbine engine which achieves mechanical energy from combustion, and a ducted fan that uses the mechanical energy from the gas turbine to force air rearwards. Thus, whereas all the air taken in by a turbojet passes through the combustion chamber and turbines, in a turbofan some of that air bypasses these components. A turbofan thus can be thought of as a turbojet being used to drive a ducted fan, with both of these contributing to the thrust.
Air-augmented rockets use the supersonic exhaust of some kind of rocket engine to further compress air collected by ram effect during flight to use as additional working mass, leading to greater effective thrust for any given amount of fuel than either the rocket or a ramjet alone.
The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, and a turbine. The compressed air from the compressor is heated by burning fuel in the combustion chamber and then allowed to expand through the turbine. The turbine exhaust is then expanded in the propelling nozzle where it is accelerated to high speed to provide thrust. Two engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany, developed the concept independently into practical engines during the late 1930s.
A pulse detonation engine (PDE) is a type of propulsion system that uses detonation waves to combust the fuel and oxidizer mixture.
A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accordance with Newton's third law. Most rocket engines use the combustion of reactive chemicals to supply the necessary energy, but non-combusting forms such as cold gas thrusters and nuclear thermal rockets also exist. Vehicles propelled by rocket engines are commonly used by ballistic missiles and rockets. Rocket vehicles carry their own oxidiser, unlike most combustion engines, so rocket engines can be used in a vacuum to propel spacecraft and ballistic missiles.
A combustion chamber is part of an internal combustion engine in which the fuel/air mix is burned. For steam engines, the term has also been used for an extension of the firebox which is used to allow a more complete combustion process.
An afterburner is an additional combustion component used on some jet engines, mostly those on military supersonic aircraft. Its purpose is to increase thrust, usually for supersonic flight, takeoff, and combat. The afterburning process injects additional fuel into a combustor in the jet pipe behind the turbine, "reheating" the exhaust gas. Afterburning significantly increases thrust as an alternative to using a bigger engine with its attendant weight penalty, but at the cost of increased fuel consumption which limits its use to short periods. This aircraft application of "reheat" contrasts with the meaning and implementation of "reheat" applicable to gas turbines driving electrical generators and which reduces fuel consumption.
In automotive engineering, an exhaust manifold collects the exhaust gases from multiple cylinders into one pipe. The word manifold comes from the Old English word manigfeald and refers to the folding together of multiple inputs and outputs.
A jet engine performs by converting fuel into thrust. How well it performs is an indication of what proportion of its fuel goes to waste. It transfers heat from burning fuel to air passing through the engine. In doing so it produces thrust work when propelling a vehicle but a lot of the fuel is wasted and only appears as heat. Propulsion engineers aim to minimize the degradation of fuel energy into unusable thermal energy. Increased emphasis on performance improvements for commercial airliners came in the 1970s from the rising cost of fuel.
The Gluhareff Pressure Jet is a type of jet engine that, like a valveless pulse jet, has no moving parts. It was invented by Eugene Michael Gluhareff, a Russian-American engineer who envisioned it as a power plant for personal helicopters and compact aircraft such as microlights.
The Argus As 014 was a pulsejet engine used on the German V-1 flying bomb of World War II, and the first model of pulsejet engine placed in mass production. License manufacture of the As 014 was carried out in Japan in the latter stages of World War II, as the Kawanishi Maru Ka10 for the Kawanishi Baika kamikaze jet.
This article briefly describes the components and systems found in jet engines.
An airbreathing jet engine is a jet engine in which the exhaust gas which supplies jet propulsion is atmospheric air, which is taken in, compressed, heated, and expanded back to atmospheric pressure through a propelling nozzle. Compression may be provided by a gas turbine, as in the original turbojet and newer turbofan, or arise solely from the ram pressure of the vehicle's velocity, as with the ramjet and pulsejet.
Internal combustion engines come in a wide variety of types, but have certain family resemblances, and thus share many common types of components.
An internal combustion engine is a heat engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, the expansion of the high-temperature and high-pressure gases produced by combustion applies direct force to some component of the engine. The force is typically applied to pistons, turbine blades, a rotor, or a nozzle. This force moves the component over a distance, transforming chemical energy into kinetic energy which is used to propel, move or power whatever the engine is attached to.
Pressure gain combustion (PGC) is the unsteady state process used in gas turbines in which gas expansion caused by heat release is constrained. First developed in the early 20th century as one of the earliest gas turbine designs, the concept was mostly abandoned following the advent of isobaric jet engines in WWII.
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