The pulse jet engine is a kind of power unit that works on the principle of mixing air and pulse jet force. These motors are easily recognizable by their characteristic strong sound. Among the advantages over analogues are an extremely simplified design and low weight. We will consider the remaining features of the aggregates below.
History of Creation
The first developments of the pulse jet engine (ramjet) are officially dated to the second half of the 19th century. In the 60s, two inventors, apart from each other, received patents for a new design of propellers. The developments of Teleshov N. A. and Charles de Voilier for that period were of little interest to anyone. But at the beginning of the 20th century, German engineers drew 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 FAA-type aircraft projectile, whichequipped with a ramjet. Despite the fact that the specified 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 known history, this was the only case when such engines were used to equip aircraft on a serial scale.
Attempts to improve
After the end of the war, the pulse jet engine remained in military development for some time. It was used as a propeller for air-to-ground missiles. Low efficiency, low launch speed and the need for acceleration at launch are the reasons that have become key in the further reduction of the position of the ramjet to zero.
This type of motor has recently begun to interest engineers and amateurs again. There are new developments, other improvement schemes. It is quite possible that the updated modifications will again appear in the equipment of military aviation. Its practical application today is the modeling of rocket and aircraft prototypes using modern structural materials.
Pulsing jet engine device
The considered unit is a cavity open on both sides. An air intake is mounted at the inlet, behind it is a traction unit with valves. The design also includes several combustion chambers, a nozzle for releasing a jet stream. The inlet valve is manufactured in several configurations, different in design and externalmind. One of the options is rectangular louver-type plates that are mounted on a frame, open or close under pressure drops. The second, more compact version - metal "petals" placed in a circle.
There is a spark plug in the combustion chamber. 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 heat up intensively and are able to activate the fuel mixture in the same way as a candle.
Working principle
Because a pulsating jet engine operates in cycles, it has several basic cycles. Among them:
- Intake process. At this stage, the inlet valve opens, discharged air enters the combustion chamber. Simultaneously, through the nozzles, fuel enters, as a result of which a kind of fuel charge is created.
- The resulting mixture is ignited by a spark plug, after which high pressure gases are observed. Under their action, the inlet valve is clogged.
- Further, the products of combustion are blown out through the nozzle, creating jet thrust. This creates a vacuum in the combustion chamber. The procedure is repeated - the inlet valve opens, passing the next portion of air.
Fuel is supplied by injectors with check valve mechanism. When the pressure in the combustion chamber decreases, the next dose of fuel enters. After increasing the pressure, the supply stops. It should be noted that on low-power aircraft models, the nozzlesare absent, and the system works according to the traditional carburetor scheme.
Design features
The pulse jet engine shown below has an intake valve in front of the combustion chamber. This is its main difference from the closest "brothers" such as ramjet and jet motor. This 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 the air is immediately supplied under pressure with pre-compression. Such a "trifle" is actually a huge plus in the operation of the unit in question, regarding the improvement of thermodynamic characteristics.
Another difference is the cyclical nature of work. For example, in a turbojet engine, fuel is burned continuously, which guarantees uniform and even thrust. In a ramjet, the cycles provide oscillations within the structure. To guarantee maximum amplitude, vibration synchronization of all parts is required. This point is achieved by selecting the optimal nozzle length.
The pulse jet engine is capable of operating at low speeds or in an inactive position in the absence of oncoming airflow. This advantage over the straight-through version is highly debatable, as launching a rocket or aircraft under these conditions requires initial acceleration.
Varieties
In addition to the regular version of the pulsejet with a straight and inlet valve, there are also valveless and detonation versions.
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 service life of the power plant is longer. By design, the unit is a shape in the form of the letter U, the ends of which are directed downstream of the jet thrust (backward). The channel that is responsible for traction is a little longer. A short pipe enters the air flow into the combustion chamber. As a result of combustion and expansion of gases, some of them are returned back through the indicated inlet. 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 "gain" in tractive effort.
The detonation-type ramjet is designed to burn a charge of fuel through detonation. That is, at a constant volume, a sharp increase in the pressure of the fuel-air mixture occurs in the combustion chamber. 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 motor configuration is not in operation, being at the stage of research and improvements.
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 in question. Thesequality led to the appearance of these motors on military missiles, flying targets and other objects where not durability is important, but the quick delivery of the aircraft to the target with the most simplified configuration of the “engine”. Fans of aircraft modeling appreciate the modification in question for the same reasons. Compact, cheap and light motors are great for aircraft models. Another plus is the ability to make an elementary pulsating jet engine with your own hands.
Cons
Among the shortcomings there are also many points, namely:
- high degree of noise in operation;
- excessive fuel consumption;
- presence of fuel residues after use;
- increased vulnerability of inlet valve;
- speed limit.
Despite all the disadvantages, the ramjet in its segment remains in high demand. Such a motor is indispensable for one-time launches, especially if it is impractical to mount powerful and expensive versions.
DIY Detonation Pulse Jet Engine
First you need to create a drawing with a development 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"). Conical and cylindrical reamers can be performed by findingnecessary guidance in any drawing manual.
The second important issue is the choice of metal. Alternatively, stainless steel or low carbon black steel can be used. Let's 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 building a pulsating jet engine with your own hands, you need to clean sheet metal blanks from rust and dust. For this, a standard grinder is quite suitable. For your safety, wear gloves as the edges of the sheets are sharp and full of burrs.
Before starting the main work, you need to prepare drawings and cardboard templates of parts in full size. To obtain an accurate configuration and dimensions, the contours are outlined with a permanent marker. It is highly not recommended to cut the reamers with a welding machine, no matter how modern it may be. The fact is that the parts obtained in this way are very poorly welded at the edges. It is advisable to use electric metal shears for this purpose, since in the manual version there is a high risk of bending the edges of the workpieces. You need to cut carefully, securely fixing the processed template with a clamp or other suitable method.
Main Stage
When making a pulse jet engine at home, remember that fixed diameter pipes are easy to form whenhelp of a larger analogue. It is quite realistic to carry out the operation with your hands due to the lever principle, after which the edges of the workpiece are processed with a mallet, bending them to the desired condition. It is desirable that the ends form a plane when joined, which will improve the placement of the weld. It is more difficult to bend sheets into a pipe, you will need a bender or rollers. This professional tool is not for everyone. Yews may be used as an alternative.
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. It is better for beginners not to try to experiment (the slightest overexposure of the electrode at one point leads to burning a hole). In addition, bubbles can get into the seam area, which subsequently guarantees a leak. It is best to grind the seam to a minimum thickness, which will allow you to see the "marriage" with the naked eye immediately. The tapered segments are bent by hand, crimping the narrow end of the workpiece around the small diameter pipe, making more effort than the wide part.
Recommendations
Knowing how to make a pulse jet engine yourself, you can use it on aircraft models or to speed up a skateboard. Experienced users recommend, in order to obtain the optimal composition of the fuel mixture, first supply gas to the engine, filling the combustion chamber completely with it. Then the ignition spark is activated. Air is supplied last, after reachingoptimal concentration of all components - launch is in progress.
