A jet pulsating engine is a type of power unit operating on the principle of mixing air and reactive pulsating forces. These motors are easily recognizable by the characteristic strong sound. Among the advantages over analogues is an extremely simplified design and low weight. The remaining features of the units will be considered below.
History of creation
The first development of a pulsating jet engine (ramjet) was officially dated the second half of the XIX century. In the 60s, two inventors, apart from each other, received patents for a new propulsion design. The development of Teleshov N.A. and Charles de Voile for that period was of little interest to anyone. But at the beginning of the 20th century, German engineers paid attention to them, who were looking for a worthy alternative to piston power units.
During the Second World War, German aviation was replenished with an FAU-type aircraft shell, which was equipped with ramjet. Despite the fact that this element was inferior in technical parameters to piston variations, it was popular. This fact is due to the simplicity of design and low cost. In the well-known history, this was the only case when similar engines were used to equip aircraft on a production scale.
Improvement Attempts
After the war ended, a pulsating jet engine remained for some time in military development. It was used as an engine for air-to-ground missiles. Low efficiency, poor starting speed and the need for acceleration at startup are the reasons that have become key in the further reduction of ramjet positions to zero.
The specified type of motor has recently again begun to interest engineers and amateurs. New developments, other improvement schemes, appear. It is possible that the updated modifications will again appear in the equipment of military aircraft. Its practical application today is modeling prototypes of rockets and aircraft using modern structural materials.
Pulsed jet engine
The unit under consideration is a cavity open on both sides. An air intake is mounted at the inlet, behind it is a traction unit with valves. Also, the design includes several combustion chambers, a nozzle for the release of a jet stream. The inlet valve is manufactured in several configurations that are different in design and appearance. One of the options is rectangular plates of the type of blinds that are mounted on the frame, open or close under pressure drops. The second, more compact version - metal "petals" placed in a circle.
A spark plug is provided in the combustion compartment. This element produces a series of discharges, and after reaching the desired concentration of fuel, the charge ignites. Since the engine has a modest size, the steel walls of the unit are intensively heated, able to activate the fuel mixture by analogy with a candle.
Principle of operation
Since the jet pulsating motor operates in cycles, it has several basic clock cycles. Among them:
- Intake process. At this stage, the inlet valve opens, discharged air enters the combustion chamber. Synchronously, through the nozzles, fuel enters, as a result of which a peculiar fuel charge is created.
- The resulting mixture is ignited by the spark of a spark plug, after which the formation of high pressure gases is observed. Under their action, the intake valve is clogged.
- Further, the combustion products are blown through the nozzle, creating a jet thrust. In this case, a vacuum is obtained in the combustion compartment. The procedure is repeated - the inlet valve opens, passing the next portion of air.
Fuel is supplied through nozzles with a non-return valve mechanism. With a decrease in pressure in the combustion chamber, the next dose of fuel enters. After increasing the pressure, the flow stops. It is worth noting that there are no nozzles on low-power aircraft models, and the system works according to the traditional carburetor scheme.
Design Features
The pulsating jet engine, the drawing and operation diagram of which is given below, has an inlet valve in front of the combustion chamber. This is its main difference from the closest "brothers" such as direct-flow and jet engines. The specified part is responsible for preventing the return of combustion products, which determines their direction directly into the nozzle. Competing varieties do not particularly need valves, since air is immediately supplied under pressure with pre-compression. Such a "trifle" is actually a huge plus in the work of the unit under consideration, regarding the improvement of thermodynamic characteristics.
Another difference is the cyclical nature of the work. For example, in a turbojet engine fuel is burned continuously, which guarantees uniform and even traction. In ramjet cycles, oscillations within the structure are provided. To ensure maximum amplitude, vibration synchronization of all parts is required. This moment is achieved by selecting the optimal nozzle length.
A pulsating jet engine can operate at low speeds or in an inactive position in the absence of an oncoming air stream. This advantage over the direct-flow version is highly controversial, since initial acceleration is required to launch a rocket or aircraft under the indicated conditions.
Varieties
In addition to the usual version of a pulsating jet engine with a straight and inlet valve, there are also valveless and detonation varieties.
The first modification is not equipped with an inlet valve. This is due to the vulnerability and rapid wear of the additional part. In this embodiment, the life of the power plant is longer. By design, the unit is a form in the form of the letter U, the ends of which are directed along the stream of jet thrust (back). The channel that is responsible for traction is slightly longer. A short nozzle receives air flow into the combustion compartment. As a result of combustion and expansion of gases, some of them are returned through the specified entrance. Such a device makes it possible to provide improved ventilation of the working chamber. There is no loss of fuel charge through the inlet valve, which creates a slight "increase" in traction.
The detonation ramjet is focused on burning a fuel charge through detonation. That is, with a constant volume, a sharp increase in the pressure of the fuel-air mixture occurs in the combustion compartment. In this case, the volume increases starting from the moment the gases move along the nozzle part. This solution allows to increase the thermal efficiency. Currently, this configuration of motors is not in operation, being at the stage of research and refinement.
pros
The principle of operation of a jet pulsating engine along with the simplicity of design and low cost are the main advantages of the system under consideration. These qualities caused the appearance of these engines on military missiles, flying targets and other objects where it is not longevity that matters, but the fast delivery of the aircraft to the target with the most simplified “engine” configuration. Fans of aircraft modeling appreciate this modification for the same reasons. Compact, cheap and lightweight motors are great for aircraft models. Another plus is the ability to make an elementary pulsating jet engine with your own hands.
Minuses
Among the shortcomings are also many points, namely:
- high degree of noise in working condition;
- excessive fuel consumption;
- the presence of fuel residues after use;
- increased vulnerability of the inlet valve;
- speed limit.
Despite all the minuses, ramjet in its segment remains very popular. Such a motor is indispensable for one-time starts, especially if it is impractical to mount powerful and expensive versions.
DIY detonation pulsating jet engine
First you need to create a drawing with a scan of future details. If you remember the basics of school geometry and have minimal drawing skills, you can get to work. The simplest scheme is cylindrical pipes. Rectangles are drawn, one side of which will be equal to the length, and the second to the diameter (multiplied by 3.14 - the number "pi"). Cone and cylindrical reamers can be completed by finding the necessary recommendations in any drawing guide.
The second important question is the choice of metal. Alternatively, you can use stainless steel or low carbon black steel. Let us dwell on the second option, since it is easier to process and form. The minimum sheet thickness is 0.6 mm. In this case, the size was 1 mm.
Preparatory process
Before you start the construction of a pulsating jet engine with your own hands, it is necessary to clean the sheet metal workpieces from rust and dust. For this, a standard grinding machine is quite suitable. Worry about safety - wear gloves as the edges of the sheets are sharp and riddled with burrs.
Before starting the main work, it is necessary to prepare drawings and cardboard templates of parts in full size. To obtain the exact configuration and size, the contours are outlined with a permanent marker. It is highly discouraged to cut reamers using a welding machine, no matter how modern it is. The fact is that the parts obtained in this way are very poorly welded around the edges. It is advisable to use electric scissors for metal for this purpose, since in the manual version there is a high risk of bending the edges of the workpieces. It is necessary to cut carefully, reliably fixing the processed template with a clamp or other suitable method.
Main stage
When making a jet pulsating engine at home, remember that fixed-diameter pipes are easy to form with a larger counterpart. It is quite possible to carry out the operation with your hands due to the lever principle, after which the edges of the workpiece should be treated with a mallet, bending them to the desired condition. It is desirable that the ends when joined form a plane, which will improve the placement of the weld. It is more difficult to bend sheets into a pipe, a listogib or rollers will be required. This professional tool is not for everyone. Alternatively, yews are allowed.
An important and painstaking moment is the welding of a thin sheet of metal. Special skills will be required here, especially if manual arc welding is used in the process. Beginners are better off not trying to experiment (the slightest overexposure of the electrode at one point leads to the burning of the hole). In addition, bubbles may enter the seam area, which subsequently guarantees leakage. It is best to grind the seam to the minimum thickness, which will allow you to see the “marriage” with the naked eye immediately. The conical segments are bent manually, squeezing the narrow end of the workpiece around a pipe of small diameter, making more effort than a wide part.
Recommendations
Knowing how to make a jet pulsating engine yourself, you can use it on aircraft models or to accelerate a skateboard. Experienced users recommend, to obtain the optimal composition of the fuel mixture, first supply gas to the engine, filling the combustion chamber completely with it. Then activate the ignition spark. Lastly, air is supplied, after reaching the optimal concentration of all components, a start is made.