Pulse electric jet engine. Model aircraft pulse jet engines

05.04.2019

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OPERATING PRINCIPLE

PUVRD has the following main elements: inlet section a - b (Fig. 1) (hereinafter, the inlet part will be called the head /), ending with a valve grill, consisting of disk 6 and valves 7; combustion chamber 2, section c - d; jet nozzle 3, section d - e; exhaust pipe 4, section e - e.
The inlet channel of the head / has confuser a - b and diffuser b - c sections. At the beginning of the diffuser section, a fuel tube 8 with an adjusting needle 5 is installed.

Air, passing through the confuser part, increases its speed, as a result of which the pressure in this area, according to Bernoulli's law, falls. Under the action of reduced pressure, fuel begins to be sucked from tube 8, which is then picked up by an air stream, broken into smaller particles and evaporated by it. The resulting carbureted mixture, passing through the diffuser part of the head, is somewhat compressed due to a decrease in the speed of movement and, in the final mixed form, enters the combustion chamber through the inlets of the valve grill.
Initially, the fuel-air mixture that filled the volume of the combustion chamber is ignited with the help of an electric candle, in extreme cases with the help of an open flame brought to the edge of the exhaust pipe, i.e. to the section c - e. When the engine reaches the operating mode, again the fuel-air mixture entering the combustion chamber is not ignited by foreign source but from hot gases. Thus, an electric spark plug or other source of flame is needed only during the starting period of the engine.

The gases formed during the combustion of the fuel-air mixture sharply increase the pressure in the combustion chamber, and the plate valves of the valve grid close, and the gases rush into the open part of the combustion chamber towards the exhaust pipe. At some point, the pressure and temperature of the gases reach their maximum value. During this period, the speed of the outflow of gases from the jet nozzle and the thrust developed by the engine are also maximum.
Under the influence high blood pressure in the combustion chamber, hot gases move in the form of a gas "piston", which, passing through the jet nozzle, acquires maximum kinetic energy. As the main mass of gases leaves the combustion chamber, the pressure in it
starts to fall. The gas "piston", moving by inertia, creates a vacuum behind it. This rarefaction starts from the valve grid and, as the main mass of gases moves towards the outlet, it extends to the entire length of the engine working pipe, t. to the section e - e. As a result, under the action of more high pressure in the diffuser-non-part of the head, the plate valves open and the combustion chamber is filled with the next portion of the fuel-air mixture.
On the other hand, the rarefaction that has spread to the edge of the exhaust pipe leads to the fact that the speed of part of the gases moving along exhaust pipe towards the outlet, drops to zero, and then gets the opposite value - gases mixed with sucked air begin to move towards the combustion chamber. By this time, the combustion chamber has been filled with another portion of the fuel-air mixture and the gases moving in the opposite direction (pressure wave) are somewhat pressing it and igniting it.

Thus, in the working tube of the engine during its operation, the gas column oscillates: during the period of increased pressure in the combustion chamber, gases move towards the outlet, during the period of reduced pressure - towards the combustion chamber. And the more intense the fluctuations of the gas column in the working tube, the deeper the vacuum in the combustion chamber, the more fuel-air mixture, which, in turn, will lead to an increase in pressure, and, consequently, to an increase in the thrust developed by the engine per cycle.
After the next portion of the top-line-air mixture has ignited, the cycle repeats. On fig. 2 schematically shows the sequence of engine operation for one cycle:
- filling the combustion chamber with a fresh mixture with open valves during the start-up period a;
- the moment of ignition of the mixture b (the gases formed during combustion expand, the pressure in the combustion chamber increases, the valves close and the gases rush through the jet nozzle into the exhaust pipe);
- the combustion products in their bulk in the form of a gas "piston" move towards the outlet and create a vacuum behind them, the valves open and the combustion chamber is filled with a fresh mixture;
- a fresh mixture r continues to enter the combustion chamber (the bulk of the gases - the gas "piston" - left the exhaust pipe, and the rarefaction spread to the edge of the exhaust pipe, through which the suction of part of the residual gases and clean air from the atmosphere begins);
- the filling of the combustion chamber with fresh mixture e ends (the valves close and from the side of the exhaust pipe towards the valve grille a column of residual gases and air moves, compressing the mixture);

- mixture e ignites and burns in the combustion chamber (gases rush through the jet nozzle into the exhaust pipe and the cycle repeats).
Due to the fact that the pressure in the combustion chamber varies from some maximum value, more than atmospheric, to a minimum, less than atmospheric, the rate of gas outflow from the engine is also not constant during the cycle. At the moment of the greatest pressure in the combustion chamber, the speed of the outflow from the jet nozzle is also the greatest. Then, as the main mass of gases leaves the engine, the exhaust velocity drops to zero and then is directed towards the valve lattice. Depending on the change in the speed of the expiration and the mass of gases per cycle, the thrust of the engine also changes.

On fig. 3 shows the nature of the change in pressure p and gas outflow velocity Ce for a cycle in PUVRD with a long exhaust pipe. It can be seen from the figure that the gas outflow rate, with some time shift, changes in accordance with the change in pressure and reaches its maximum approximately at the maximum pressure value. During the period when the pressure in the working pipe is below atmospheric, the exhaust velocity and thrust are negative (section w), since the gases move along the exhaust pipe towards the combustion chamber.

As a result of the fact that the gases, moving through the exhaust pipe, form a vacuum in the combustion chamber, the PUVRD can operate in place in the absence of velocity pressure.

ELEMENTARY THEORY OF MODEL AIRJET

Thrust developed by the engine

Traction developed jet engine(including pulsating) is determined by the second and third laws of mechanics.
The thrust for one cycle of the PUVRD changes from the maximum - positive value to the minimum - negative. Such a change in thrust per cycle is due to the principle of operation of the engine, i.e., the fact that the gas parameters - pressure, exhaust velocity and temperature - are not constant during the cycle. Therefore, proceeding to the definition of the thrust force, we introduce the concept of the average speed of the outflow of gas from the engine. Let us designate this speed Сavr (see Fig. 3).
Let us define the thrust of the engine as a reactive force corresponding to the assumed average exhaust velocity. According to the second law of mechanics, the change in the momentum of any gas flow, including in the engine, is equal to the momentum of the force, i.e., in this case thrust force:
P* = tg - C, cf - taU, (1)
where tg is the mass of fuel combustion products;
mm is the mass of air entering the engine; С,ср is the average velocity of the outflow of combustion products;
V is the flight speed of the model; P is the traction force; I is the time of the force, Formula (1) can be written in another form by dividing its right and left parts by I:
t.. gpp
, (2)
where tg. sec and MB. sec - are the masses of combustion products and air flowing through the engine per second, and, therefore, can be expressed in terms of the corresponding second weight flow rates Cr. sec
II Ov. sec, T.S.
_ ^g. sec _ "r. sec
. sec - ~~a "v- sec - ~~~a
Substituting into formula (2) second massive spending, expressed in terms of second weight flow, we get:
g-ssk v-ssk
*-*
g>-. p. sec
Bracketing - , we get the expression
. sec g. sec
. sec
It is known that for complete combustion 1 kg of hydrocarbon fuel (eg gasoline) requires approximately 15 kg of air. If we now assume that we burned 1 kg of gasoline and 15 kg of air were required for its combustion, then the weight of the combustion products 6G will be equal to: and the ratio is ~ in weight units
IN
will look like:
vg (?t + (?in] + 15
—^ . R
The same value will have the relation ^-1
per-sec
p g sec
Assuming the ratio t^ - equal to unity, we obtain a simpler and fairly accurate formula for determining the thrust force:
I \u003d ^ (C, ep - V). (5)
When the engine is running in place, when V \u003d O, we get
P \u003d ^ C "av- (6)
Formulas (5 and 6) can be written in a more expanded form:
, (T)
where St. c is the weight of air flowing through the engine
for one cycle;
n is the number of cycles per second.
Analyzing formulas (7 and 8), we can conclude that the thrust of the PuVRD depends on:
- on the amount of air passing through the engine per cycle;
- on the average speed of gas outflow from the engine;
- from the number of cycles per second.
The greater the number of engine cycles per second and the more fuel-air mixture passes through it, the greater the thrust developed by the engine.
Basic relative (specific) parameters
PUVRD
Flight performance pulsating jet engines for aircraft models It is most convenient to compare using relative parameters.
The main relative engine parameters are: specific thrust, specific fuel consumption, specific gravity and specific frontal thrust.
The specific thrust Rud is the ratio of the thrust P [kg] developed by the engine to the weight per second air flow through the engine.

Substituting in this formula the value of the thrust force P from formula (5), we obtain
1
When the engine is running in place, i.e. at V = 0, the expression for specific thrust will take a very simple form:
n*sr
* ud - - .
UD ^
Thus, knowing average speed outflow of gas from the engine, we can easily determine the specific thrust of the engine.
Specific consumption fuel С?ud is equal to the ratio of hourly fuel consumption to the thrust developed by the engine
bt G * g H G g 1 aUD - ~ p ~ "|_" / ac-^ [hour -g] *
where 6 beats - specific fuel consumption;
^ "g kg g] 6T - hourly fuel consumption - " - | .
Knowing the second fuel consumption St. sec. you can determine the hourly consumption by the formula
6t = 3600. Sg. sec.
Specific fuel consumption is an important operational characteristic engine, showing its economy. The smaller 6UL, the greater the range and duration of the flight of the model, all other things being equal.
The specific gravity of the engine -, "dp is equal to the ratio of the dry weight of the engine to the maximum thrust developed by the engine in place:
„
Tdv
_^ G«1GO
- p "["g] [g]"
where 7dp is the specific gravity of the engine;
6DP is the dry weight of the engine.
At a given value of thrust, the specific weight of the engine determines the weight of the propulsion system, which, as is known, strongly affects the flight parameters of a flying model and, first of all, its speed, altitude, and payload. The lower the specific gravity of the engine at a given thrust, the more perfect its design, the more weight this engine can lift into the air.
Specific frontal thrust R.™-, is the ratio of the thrust developed by the engine to the area of \u200b\u200bits largest cross-section
where Rlob is the specific frontal thrust;
/""loo - the area of the largest cross-section of the engine.
Specific thrust plays important role when evaluating the aerodynamic qualities of the engine, especially for high-speed flying models. The larger the RLob, the smaller the share of the thrust developed by the engine in flight is spent on overcoming its own resistance.
The PUVRD, which has a small frontal area, is convenient for installation on flying models.
The relative (specific) parameters of the engine change with changes in flight speed and altitude, since the thrust developed by the engine and the total fuel consumption do not retain their value. Therefore, the relative parameters usually refer to the operation of a stationary engine at maximum thrust on the ground.
Changing the thrust of the PUVRD depending on the speed
flight
The thrust of the PUVRD depending on the flight speed can vary in different ways and depends on the method of regulating the supply of fuel to the combustion chamber. The change in the speed characteristics of the engine depends on the law by which the fuel is supplied.
On the known designs of flying models of aircraft with a PuVRD, as a rule, special automatic devices to supply fuel to the combustion chamber depending on the speed and altitude of the flight, and regulate the engines on the ground for maximum thrust or for the most stable and superimposed mode of operation.
On big aircraft with PBRJ, an automatic fuel supply is always installed, which, depending on the speed and flight altitude, maintains a constant quality of the fuel-air mixture entering the combustion chamber, and thereby maintains a stable and most efficient engine operation mode. Below we consider speed characteristics engine in cases where a fuel dispenser is installed and when it is not installed.
For complete combustion of fuel, a strictly defined amount of air is required. For hydrocarbon fuels, for example gasoline and kerosene, the ratio of the weight of air required for complete combustion of fuel to the weight of this fuel is approximately 15. This ratio is usually denoted by the letter /,. Therefore, knowing the weight of the fuel, you can immediately determine the amount of theoretically required air:
6B \u003d / ^ g. (13)
Second costs are exactly the same dependence:
^ i. sec ==<^^г. сек- (103.)
But the engine does not always receive as much air as is needed for complete combustion of the fuel: it can be more or less. The ratio of the amount of air entering the combustion chamber of the engine to the amount of air theoretically necessary for the complete combustion of the fuel is called the excess air coefficient a.
(14) * = ^- (H a)

In the case when more air enters the combustion chamber than theoretically necessary for the combustion of 1 kg of fuel, but there will be more than one and the mixture is called lean. If less air enters the combustion chamber than theoretically necessary, then a will be less than one and the mixture is called rich.
On fig. 4 shows the nature of the change in the thrust of the PUVRD depending on the amount of fuel injected into the combustion chamber. This means that the engine is running on the ground or its blowing speed is constant.
It can be seen from the graph that, with an increase in the amount of fuel entering the combustion chamber, the thrust first grows to a certain limit, and then, having reached a maximum, quickly drops.
This character of the curve is due to the fact that on a very lean mixture (left branch), when the combustion chamber
there is little fuel, the intensity of the engine is weak and the engine thrust is low. With an increase in the flow of fuel into the combustion chamber, the engine begins to work more steadily and intensively, and the thrust begins to grow. With a certain amount of fuel injected into the combustion chamber, i.e., with a certain quality of the mixture, the thrust reaches its maximum value.
With further enrichment of the mixture, the combustion process is disrupted and the engine thrust drops again. The operation of the engine on the right side of the characteristic (to the right of the pH point) is accompanied by abnormal combustion of the mixture, as a result of which spontaneous cessation of operation is possible. Thus, the PUVRD has a certain range of stable operation in terms of mixture quality, and this range is a ~ 0.75–1.05. Therefore, in practice, the PuVRD is a single-mode engine, and its mode is chosen slightly to the left of the maximum thrust (point Рр) in such a way as to guarantee reliable and stable operation both with an increase and a decrease in fuel consumption.
If the curve / (see Fig. 4) was taken at a speed equal to zero on the ground, then at some constant airflow or at some constant flight speed, also near the ground, the thrust change curve, depending on the amount of fuel entering the the combustion chamber will move to the right and up, since with an increase in air consumption, fuel consumption also increases, and, consequently, the maximum thrust will increase - curve //.
On fig. 5 shows the change in thrust of a PUVRD with an automatic fuel feeder depending on the flight speed. This character of the change in thrust is due to the fact that with an increase in flight speed, the weight of air flow through the engine increases due to the velocity pressure, while the automatic fuel feeder begins to increase the amount of fuel injected into the combustion chamber or into the diffuser part of the head, and thereby maintains a constant fuel quality. - air mixture and normal -
Rice. 5. Changing the thrust of a PUVRD with an automatic fuel dispenser depending on the flight speed
now the combustion process.
As a result, with an increase in flight speed, the thrust of the PuVRD
with automatic fuel feeder starts to rise and reaches
its maximum at a certain speed
flight.
With a further increase in flight speed, the engine thrust begins to fall due to a change in the phase of opening and closing of the inlet valves due to the impact of velocity pressure and strong suction of gases from the exhaust pipe, as a result of which their reverse flow towards the combustion chamber is weakened. The cycles become weak in intensity, and at a flight speed of 700-750 km/h the engine can switch to continuous burning of the mixture without pronounced cyclicity. For the same reason, the maximum thrust also decreases on the /// curve (see Fig. 4). Therefore, with increasing flight speed, it is necessary to regulate the supply of fuel to the combustion chamber in such a way as to maintain a constant quality of the mixture. Under this condition, the thrust of the PUVRD in a certain range of flight speeds changes insignificantly.

Comparing the thrust characteristics of an aircraft model PUVRD and a piston motor with a fixed-pitch propeller (see Fig. 5), we can say that the PUVRD thrust in a significant speed range remains practically constant; the thrust of a piston motor with a fixed-pitch propeller begins to fall immediately with an increase in flight speed. The points of intersection of the available thrust curves of the PUVRD and the piston motor with the required thrust curve for the corresponding models with equal aerodynamic qualities determine the maximum flight speeds that these models can achieve in level flight. A model with a PUVRD can reach significantly higher speeds than a model with a piston engine. This determines the advantage of the PUVRD.
In fact, on models with a PUVRD, the flight weight of which is strictly limited by sporting standards, as a rule, they do not install an automatic fuel feeder, since at present there are still no automatic machines that are simple in design, reliable in operation and, most importantly, small in size and weight. Therefore, the simplest fuel systems are used, in which fuel enters the diffuser part of the head due to the vacuum created in it during the passage of air, or is supplied under pressure taken from the combustion chamber and sent to the fuel tank, or using a pumping device. None of the fuel systems used maintains a constant quality of the fuel-air mixture with changes in speed and flight altitude. In Chapter 7, when considering fuel systems, the influence of each of them on the nature of the change in the thrust of the PWRJ depending on the flight speed is indicated; relevant recommendations are also given there.

Determination of the main parameters of the PUVRD

Compare pulse jet engines for aircraft models, engines among themselves and it is most convenient to identify the advantages of one over the other by specific parameters, to determine which it is necessary to know the basic engine data: thrust P, fuel consumption Cg and air consumption C0. As a rule, the main parameters of the PUVRD are determined experimentally, using simple equipment.
Let us now analyze the methods and devices by which these parameters can be determined.
Definition of thrust. On fig. 6 is a schematic diagram of a test bench for determining the thrust of a small-sized PuVRD.
On a box made of 8 m plywood, two metal racks are attached, ending in half rings at the top. On these half rings, the bottoms of the engine mount collar are pivotally suspended: one of them is located at the transition point of the combustion chamber to the jet nozzle, and the other is on the exhaust pipe. lower parts

racks are rigidly riveted to steel axles; the sharp ends of the axes fit into the corresponding conical recesses in the clamping screws. Clamping screws are screwed into fixed steel brackets mounted on the top of the box. Thus, when turning the racks on their axes, the engine maintains a horizontal position. Attached to the front post is one end of a coil spring, the other end of which is connected to a loop on the drawer. The rear pillar has an arrow that moves along the scale.
The scale can be calibrated using a dynamometer by hooking it to a rope loop tied to the fuel pipe in the diffuser. The dynamometer must be located along the axis of the engine.
During engine start, the front strut is held by a special stopper, and only when it is necessary to measure the thrust, the stopper is removed.
1
!
H
~P/77 .../77
Rice. 7. Starting circuit diagram
PUVRD:
B - push-button switch; Tr - step-down transformer;
K \ and L "a - terminals; C - core; II", - primary winding; №r - secondary winding; C\ - capacitor; P - interrupter; Etc -
spring; R - spark gap (electric candle); m - mass
Inside the box are placed an air cylinder with a volume of approximately 4 liters, a starting cartridge and a transformer used to start the engine. Electric current is supplied from the network to a transformer that lowers the voltage to 24 0, and from the transformer to the starting coil. The high voltage conductor from the starting coil is connected through the top bottom of the box to the engine's electric spark plug. The circuit diagram of the ignition is given in fig. 7. When using batteries with a voltage of 12-24 V, the transformer is turned off and the batteries are connected to terminals ^1 and K%.
A simpler diagram of the machine for measuring the thrust of the PuVRD is shown in fig. 8. The machine consists of a base (a board with two iron or duralumin corners), a trolley with mounting clamps for the engine, a dynamometer and a fuel tank. The stand with the fuel tank is shifted from the axis of the engine in such a way that it does not interfere with the movement of the engine during its operation. The trolley wheels have guide grooves with a depth of 3 - 3.5 mm and a width of 1 mm more than the width of the corner rib.

After starting the engine and establishing the mode of its operation, the locking loop is removed from the hook of the cart and the thrust is measured using a dynamometer.
Rice. 8. Scheme of the machine for determining the thrust of the PuVRD:
1 - engine; 2 - fuel tank; 3 - rack; 4 - trolley; 5 — dynamometer; b - locking loop; 7—board; 6" - corners
Determination of fuel consumption. On fig. 9 is a diagram of a fuel tank with which you can easily determine the fuel consumption. A glass tube is fixed to this tank, having two marks, between which
-2
Rice. 9 Scheme of the tank for determining fuel consumption:
/ - fuel tank; 2 — filler neck; 3 - glass tube with control marks a and b; 4 - rubber tubes; 5**fuel tube
mi tank volume accurately measured. It is necessary that before determining the fuel consumption consumed by the engine, the fuel level in the tank was slightly above the upper mark. Before starting the engine, the fuel tank must be fixed on a tripod in a strictly vertical position. As soon as the fuel level in the tank approaches the upper mark, you need to turn on the stopwatch, and then, when the fuel level approaches the lower mark, turn it off. Knowing the volume of the tank between the marks V, the specific gravity of the fuel 7t and the engine operating time ^, one can easily determine the second mass fuel consumption:
*T. sec
(15)
Rice. 10. Scheme of installation for determining the air flow through
engine:
/ - aircraft model PuVRD; 2 - outlet pipe; 3 - receiver; 4 - inlet pipe; 5 - tube for measuring the total pressure; 6 - tube for measuring static pressure; 7 - micromanometer; 8 - rubber
tubes
To more accurately determine fuel consumption, it is recommended to make a supply tank with a diameter of no more than 50 mm, and the distance between the marks should be at least 30-40 mm.
Determination of air flow. On fig. 10 shows a diagram of the installation for determining the air flow. It consists of a receiver (container) with a volume of at least 0.4 l3, an inlet pipe, an outlet pipe and an alcohol micromanometer. The receiver in this installation is necessary in order to dampen the fluctuations in the air flow caused by the periodic intake of the mixture into the combustion chamber, and to create a uniform air flow in the cylindrical inlet pipe. In the inlet pipe, the diameter of which is 20-25 mm and the length is not less than 15 and not more than 20 diameters, the bottom of the tube with a diameter of 1.5-2.0 mm is installed approximately in the middle: one of its open parts is directed strictly against the flow and is designed to measure the total pressure , the other is brazed flush with the inner wall of the static pressure inlet. The outlet ends of the tubes are connected to the micromanometer tubes. which, when air passes through the intake pipe, will show a velocity head.
Due to small pressure drops in the inlet pipe, the alcohol micromanometer is not installed vertically, but at an angle of 30 or 45 °.
It is desirable that the outlet pipe supplying air to the engine under test has a rubber tip for tight connection of the engine head with the edge of the outlet pipe.
To measure the air flow, the engine is started, brought to a stable mode of operation and gradually the inlet part of the head is brought to the outlet pipe of the receiver and pressed against it tightly. After the micromanometer measures the differential pressure N[m], the engine is removed from the outlet pipe of the receiver and stops. Then using the formula:
".-"/"[=].
where Yn is the air velocity in the intake pipe ^]1<р = 0,97 ч- 0, 98 — коэффициент микроманометра;
DR - measured dynamic head ||;
With L! -I
\kg-sec?)
pw is the air density [^4];
Let us determine the air flow velocity Va in the inlet pipe. We find the dynamic pressure AP from the following expression:
7s/15sha, (17)
|/sgt
where Hs is the specific gravity of alcohol -,;
I and" ^
H is the pressure drop on the micromanometer [m]\
a is the tilt angle of the micromanometer. Knowing the air flow velocity Vа [m/s] in the inlet pipe and its cross-sectional area Pa [m2], we determine the second weight air flow.G, = 0.465 ^ , , (19)
where P is the barometer reading, [mm rg. Art.]; T is the absolute temperature, °K.
T \u003d 273 ° + I ° С, where I ° С is the outside air temperature.

Thus, we have determined all the main parameters of the engine - thrust, second fuel consumption, second air consumption - and we know its dry weight and frontal area; now we can easily find the main specific parameters: Ruya, Sud, ^sp. Love-
In addition, knowing the basic parameters of the engine, it is possible to determine the average speed of the outflow of gases from the exhaust pipe and the quality of the mixture entering the combustion chamber.
So, for example, when the engine is running on the ground, the formula for determining thrust is:
r__ c. sec r. ..
~~~G~ SR"
Determining from this formula С,ср, we get:
P Ses — ^------^, [m/s].
^c. sec
We find the quality of the mixture a from formula 14:

All quantities in the expression for a are known.
Determination of pressure in the combustion chamber and frequency of cycles. In the process of experimentation, maximum pressure and maximum vacuum in the combustion chamber, as well as the frequency of cycles, are often determined to identify the best examples of engines.

The frequency of cycles is determined either using a resonant frequency meter, or using a loop oscilloscope with a piezoelectric sensor, which is installed on the wall of the combustion chamber or is substituted for the cut of the exhaust pipe.
Oscillograms taken when measuring the frequency of two different motors are shown in fig. 11. The piezoquartz sensor in this case was connected to the edge of the exhaust pipe. Uniform, one-height curves / represent the countdown. The distance between adjacent peaks corresponds to 1/30 sec. The middle curves 2 show fluctuations in the gas flow. The oscilloscope recorded not only the main cycles - flashes in the combustion chamber (these are the curves with the largest amplitude), but also other less active oscillations that occur during the combustion of the mixture and its ejection from the engine.

The maximum pressure and maximum vacuum in the combustion chamber can be determined with approximate accuracy using mercury piezometers and two simple sensors (Fig. 12), and the sensors have the same design. The difference lies only in their installation on the combustion chamber; one sensor is installed so as to release gas from the combustion chamber, the other to let it in. The first sensor is connected to a piezometer that measures the maximum pressure, the second - to a piezometer that measures vacuum.
Rice. 12. Scheme of the device for determining
maximum and minimum pressures in
engine combustion chamber:
/. 2 - sensors and capture in the combustion chamber; 3. 4 - mercury piezometers 5 - pressure sensor housing; b1—valve (steel plate 0.05—0.00 mm thick)
By the pressure and rarefaction in the combustion chamber and the frequency of cycles, one can judge the intensity of the cycles, the loads experienced by the walls of the combustion chamber and the entire pipe, as well as the lamellar grill valves. At present, in the best samples of the PuVRD, the maximum pressure in the combustion chamber reaches 1.45-1.65 kg / cm2, the minimum pressure (vacuum) is up to 0.8-t-0.70 kg] "cm2, and the frequency is up to 250 and more cycles per second.
Knowing the main parameters of the engine and being able to determine them, experimental aircraft modelers will be able to compare engines, and most importantly, work on better models of PuVRD.

STRUCTURES OF ELEMENTS OF AIR-MODEL PUVRET

Based on the intended purpose of the model, the corresponding engine is selected (or designed).
So, for free-flight models, in which the flight weight can reach 5 kg, engines are made with a significant margin of safety and with a relatively low cycle rate, which helps to increase the life of the valves, and they also install flame-retardant grids behind the valves, which, although they reduce somewhat the maximum possible draft, but protect the valves from exposure to high temperatures and thereby further increase their service life.
Other requirements are imposed on engines installed on high-speed cord models, the flight weight of which should not exceed 1 kg. They are required to achieve the maximum possible thrust, minimum weight and a guaranteed period of continuous operation for 3-5 minutes, i.e., during the time necessary to prepare for the flight and pass the test kilometer base.
The weight of the engine for cord models should not exceed 400 g, since the installation of larger engines makes it difficult to manufacture a model with the required strength and aerodynamic quality, as well as with the necessary fuel supply. Engines of cord models, as a rule, have streamlined external contours, good aerodynamic quality of the internal flow part and a large flow area of the valve grids.
Thus, the design of the PUVRD, the thrust they develop and the required duration of operation are determined mainly by the type of models on which they are installed. The general requirements for a PUVRD are as follows: simplicity and low weight of the structure, reliability in operation and ease of operation, the maximum possible thrust for given dimensions, and the longest duration of continuous operation.

Now consider the design of individual elements of pulsating jet engines.
Input devices (heads)
The inlet device of the PuVRD is designed to ensure the correct supply of air to the valve grill, the conversion of the velocity head into static pressure (high-speed compression) and the preparation of the fuel-air mixture entering the engine combustion chamber. Depending on the method of supplying fuel to the inlet channel of the head - either due to rarefaction or under pressure - its flowing part will have a different
Rice. 13. The shape of the flow part of the feed heads
fuel: a - due to rarefaction; b - under pressure
profile. In the first case, the inner channel has confuser and diffuser sections, and together with the fuel supply pipe and the adjusting needle, it is the simplest carburetor (Fig. 13, a). In the second case, the head has only a diffuser section and a fuel tube with an adjusting screw (Fig. 13.6).
The supply of fuel to the diffuser section of the head is structurally simple and fully ensures high-quality preparation of the fuel-air mixture entering the combustion chamber. This is achieved due to the fact that the flow in the inlet channel is not steady, but fluctuates in accordance with the operation of the valves. When the valves are closed, the airflow rate is 0, and when the valves are fully open, it is the maximum. Fluctuations in speed contribute to the mixing of fuel and air. Further, the fuel-air mixture that enters the combustion chamber ignites from residual gases, the pressure in the working pipe increases, and the valves close under the action of their own elastic forces and under the influence of increased pressure in the combustion chamber.
Two cases are possible here. The first is when, at the moment of closing the valves, gases do not break into the inlet channel and only valves act on the fuel-air mixture, which stop its movement and even, as it were, throw it towards the entrance to the head. The second is when, at the moment of closing the valves, the fuel-air mixture is affected not only by the valves, but also by the mixture that has already entered the combustion chamber, but has not yet ignited, breaking through the valves due to their insufficient rigidity or excessive deviation. In this case, the mixture will be thrown to the head inlet by a much larger amount.
Throwing the mixture away from the valve grill disk towards the inlet can be easily observed in heads with a short internal channel (the length of the channel is approximately equal to the diameter of the head). In front of the inlet in the head, during engine operation, there will always be a fuel-air “cushion” approximately the same as shown in Fig. 13.6. This phenomenon can be tolerated if the “cushion” is small and the engine on the ground works steadily, since in the air with increasing flight speed the velocity pressure increases and the “cushion” disappears.

If, however, not a fuel-air mixture, but hot gases, breaks into the inlet part of the head from the combustion chamber, it is possible to ignite the mixture in the diffuser section and stop the engine. Therefore, it is necessary to stop trying to start and repair the defect in the valve grid, as will be discussed in the next section. For stable and efficient operation of the engine, the length of the inlet channel of the head should be equal to 1.0–1.5 of the outer diameters of the valves, and the ratio of the lengths of the confuser and diffuser sections should be approximately 1: 3.
The profile of the inner channel and the outer contour of the head must be smooth so that there is no separation of the jet from the stacks when the engine is running both in place and in flight. On fig. 13a shows a head whose profile satisfies the flow perfectly. It has a convenient bootable shape, and there will be no separation of the flow from the walls. Consider a number of characteristic head designs PUVRD.
On fig. 14 shows a head with a fairly good aerodynamic quality. Forming confuser*
leg and diffuser sections, as well as the leading edge of the fairing, as can be seen from the figure, are mated smoothly.
The technology of manufacturing individual elements of this head is described in Chapter 5. The advantages of the design of the head include its low weight, the ability to quickly replace the valve grill and the placement of the nozzle in the center of the inlet channel, which contributes to the symmetrical flow of the air flow.
The quality of the mixture is regulated by the selection of the diameter of the orifice of the jet. You can use a jet with a hole larger than the nominal one, and when adjusting its flow area, reduce it by inserting separate veins with a diameter of 0.15-0.25 mm from the electrical wire. The outer ends of the veins are bent to the outer side of the jet (Fig. 15), after which a PVC or rubber tube is put on it. It is possible to adjust the fuel supply using a small homemade screw valve.
The head of one of the domestic RAM-2 engines, which was mass-produced, is shown in fig. 16. The body of this head has an internal channel, a nozzle attachment point, a valve grill, a thread for attachment to the combustion chamber and a seat for the fairing.

The nozzle is equipped with a needle piit to adjust the quality of the mixture.
The disadvantages include poor aerodynamics of the flow path that reduces engine thrust - a sharp transition of the flow from the axial direction to the inlet channels of the valve lattice and the presence of the channels themselves (section b - d), which increase the resistance and worsen the qualitative homogeneous mixing of fuel with air.
The design of the head shown in Fig. 17, special mount with engine combustion chamber. Unlike threaded fasteners, a trough-shaped collar is used here, made on a special mandrel by compression. A special profiled shoulder is made on the front edge of the combustion chamber. The valve grill, inserted into the combustion chamber, rests against the protrusion of this shoulder. Then the body of the inlet device is inserted, which also has a profiled collar, and three nodes - the head body, the valve grill and the combustion chamber are tightly pulled together by a clamp 7 with a screw 8. Mount B is generally easy and reliable in operation.
The space between the inlet shell and fairing is often used as a container for the fuel tank. In these cases, as a rule, the length of the inlet channel is increased in order to accommodate the necessary fuel supply. On fig. 18 and 19 show such heads. The first of them mates well with the combustion chamber; the fuel in it is reliably isolated from hot parts; it is attached to the diffuser body with screws 4. The second head shown in fig. 19, differs in originality of fastening to the combustion chamber. As can be seen from the figure, head 4 - a profiled tank, soldered from tin or foil, has a special annular recess for fixing its position on the flange of the valve grille. The valve grill 5 itself is screwed into the combustion chamber.

The head-tank is connected to the valve grid and the combustion chamber with the help of springs 3, which tighten the ears 2. The connection is not rigid, but this is not required in this case, since the head is not a power body; also no need for special sealing
Rice. 16. RAM-2 engine head:
/ - internal channel; 2 - fairing; 3 - nozzle; 4 - adapter; 5 - needle screw; b - inlet channel of the valve grill; 7 - fitting for
fuel pipe connections
between the bare and valve grille. Therefore, this mount, in combination with the design of the valve grill and combustion chamber, is fully justified. The author of the design of this head is V. Danilenko (Leningrad).
The head shown in fig. 20, designed for engines with thrust up to 3 kg or more. Its design feature is the method of attachment to the combustion chamber, the presence of cooling fins and the fuel supply system. Unlike the previous methods, this head is attached to the combustion chamber with coupling screws. On the combustion chamber, six ears 7 with an M3 internal thread are fixed, into which the coupling screws 5 are screwed, while capturing the power ring of the diffuser with special pads 4 and pressing it to the combustion chamber. The mount, although laborious to manufacture, is advisable to use with large engine dimensions (in this case, the diameter of the combustion chamber is 100 mm).
8
1
Rice. 19. Head attached to the combustion chamber with
springs:
/ - the combustion chamber; 2 - ears; 5—spring; 4 - head; 5 - valve grille; b - shoulder of the valve grille; 7 - filler neck; d-drainage tube
During operation, the engine has a high thermal regime and four cooling fins are provided on the outer part of the diffuser to protect the fairing made of balsa or foam plastic and the fuel system from high temperatures.
The fuel supply is carried out by two jets - the main 11 with a non-adjustable hole and the auxiliary 12 with a needle 13 for fine adjustment.

Valve grill designs

The only moving parts of the engine are the valves that allow the fuel-air mixture to flow in one direction into the combustion chamber. The thrust of the engine, as well as the stability and duration of its continuous operation, depend on the selection of the thickness and shape of the valves, on the quality of manufacture and their adjustment. We have already said that engines installed on cord models require maximum thrust at low weight, and engines installed on free flight models require the greatest duration of continuous operation. Therefore, the valve grids installed on these engines are also structurally different.
Consider briefly the operation of the valve lattice. To do this, let's take the so-called disk valve grill (Fig. 21), which is most widely used, especially on engines for cord models. From any valve grill, including the disk one, they achieve the maximum possible cross-sectional area and a good aerodynamic shape. It can be seen from the figure that most of the disk area is used for inlet windows separated by bridges, on the edges of which valves lie. Practice has shown that the minimum allowable overlap of inlets is shown in fig. 22; a decrease in the area of \u200b\u200bfitting the valves leads to the destruction of the edges of the disk - to indentation and rounding of their valves. Disks, as a rule, are made of duralumin grades D-16T or V-95 with a thickness of 2.5–3.5 mm, or from steel with a thickness of 1.0–1.5 mm. Leading edges are rounded and polished. Particular attention is paid to the accuracy and cleanliness of the processing of the valve contact plane. The required tightness of the valves to the disk plane is achieved only after a short running-in on the engine, when each valve “works out” its own seat for itself.
At the time of the flash of the mixture and the increase in pressure in the combustion chamber, the valves are closed. They fit snugly against the disc and do not allow gases to pass into the diffuser of the head. When the bulk of the gases rush into the exhaust pipe and a vacuum is formed behind the valve grill (from the side of the combustion chamber), the valves will begin to open, while resisting the flow of a fresh fuel-air mixture and thereby creating a certain depth of vacuum in the combustion chamber, which in the subsequent torque will extend to the edge of the exhaust pipe. The resistance created by the valves depends
mainly from their rigidity, which should be such that the maximum flow of the fuel-air mixture is achieved and the timely closing of the inlets at the time of the flash is achieved. The selection of valve stiffness that would satisfy the specified requirements is one of the main and laborious processes of designing and fine-tuning an engine.
Let's assume that we have chosen valves made of very thin steel and there is no limit to their deflection. Then, at the moment the mixture enters the combustion chamber, they will deviate by some maximum possible value (Fig. 23, a), and it can be said with full confidence that the deviation of each valve will have a different value, since it is very difficult to make them exactly the same width , and they can also differ in thickness. This will cause them to close non-simultaneously.

But the main thing is the following. At the end of the filling process in the combustion chamber, there comes a moment when the pressure in it becomes slightly less than or equal to the pressure in the diffuser. It is at this moment that the valves must, mainly under the action of their own elastic forces,
Combustion marque
Rice. 23. Deviation of valves without restrictive
washers
have time to close the inlets so that after the ignition of the fuel-air mixture, the gases cannot break into the diffuser of the head. Valves with low stiffness, deviated by a large amount, will not be able to close the inlets in time and the gases will break into the head diffuser (Fig. 23.6), which will lead to a drop in thrust or to a flash of the mixture in the diffuser and engine stop. In addition, thin valves, deviating by a large amount, experience large dynamic and thermal loads and quickly fail.
If we take valves of increased rigidity, then the phenomenon will be the opposite - the valves will open later and close earlier, which will lead to a decrease in the amount of mixture entering the combustion chamber and a sharp decrease in thrust. Therefore, in order to achieve the fastest possible opening of the valves when filling the combustion chamber with a mixture and their timely closure during a flash, they resort to an artificial change in the bending line of the valves by installing restrictive washers or springs.

As practice has shown, for different engine power, the thickness of the valves is taken 0.06-0.25 mm. Steels for valves are also used carbon U7, U8, U9, U10 and alloyed cold-rolled EI395, EI415, EI437B, EI598, EI 100, EI442.
On fig. 24 shows a valve grill with a restrictive washer / made for the entire length of the valves. Its main purpose is to set the valves to the most advantageous bending profile, in which they pass the maximum possible amount of the fuel-air mixture into the combustion chamber and close the inlets in time. In practice, from
technological considerations - Fig" 24- Valve grid. "- d with restrictive washer on
nii, the profile of the washer is made for the entire length of the valves:
NYAYUT ON THE RADIUS WITH SUCH /-limiting washer; 2-, CALCULATED TO END CLZ-valve; 3 - grill housing
the pans moved away from the plane of contact by 6–10 mm. The beginning of the profile radius must be taken from the beginning of the entrance windows. Disadvantages of this washer: it does not allow to use the fully elastic properties of the valves, creates a significant resistance and has a relatively large weight.
The most widely used valve deflection limiters are made not for the full length of the valves, but for an experimentally selected one. Under the action of pressure forces from the diffuser side and rarefaction from the chamber side, the valve deviates by some amount: without a deflection limiter - by the maximum possible (Fig. 25, a); with a deviation limiter having a diameter A, to another (Fig. 25.6). First, the valve will deviate along the washer profile to a diameter c?b and then - by some amount not limited by the washer. At the moment of closing, the end part of the valve at first, as if starting from the edge of the washers with the elasticity that the valve has on the diameter A\%, receives a certain speed of movement towards the seat, much greater than in the absence of the washer.

If we now increase the diameter of the washer to a diameter d. ^ and leave the height of the washer /11 unchanged, then the elasticity of the valve at the diameter c12 will be greater than at the diameter d\\, since its cross-sectional area has increased, and the area of the end of the valve, on which the pressure acts from the side of the diffuser, has decreased, the end part will deviate already by a smaller value 62 (Fig. 25, c). The "repulsive" ability of the valve will decrease, and the closing speed will also decrease. Therefore, the desired effect of the restrictive washer is reduced.
Rice. 25. Influence of the restrictive washer on the deviation of the valves:
/—disc valve lattice; 2 - valve: 3 - restrictive washer; 4 -
clamp washer
Therefore, we can conclude that for each selected thickness of the valves for given engine dimensions, there is an optimal value for the diameter of the restrictive washer c!0 (or the length of the limiter) and the height /11, at which the valves have the maximum allowable deviation and close in time at the moment of flash. For modern PUVRDs, the dimensions of the valve deflection limiters have the following values: the diameter of the circumference of the limiter washer (or the length of the limiter) is 0.6–0.75 of the outer diameter of the valves (or the length of its working part): the bending radius is 50–75 mm, and the height of the edge washers L| from the plane of contact of the valves is 2-4 mm. The diameter of the clamping plane must be equal to the diameter along the root section of the valves. In practice, it is necessary to have a supply of restrictive washers with a deviation from the nominal dimensions in one direction or another, and when replacing valves, testing the engine, select the most suitable one, at which the engine runs stably and the thrust is greatest.
Spring-type valves (Fig. 26) are used for the same purpose - for the maximum possible opening of the valves in the process of filling the combustion chamber with a fuel-air mixture and their timely closing at the moment of combustion of the mixture. Spring valves contribute to an increase in the depth of rarefaction and the flow of more mixture. For spring valves, the thickness of the sheet steel is taken 0.05–0.10 mm less than for valves with a restrictive washer, and the number of spring sheets, their thickness and diameter are selected experimentally. The shape of the spring leaves usually corresponds to the shape of the main lobe covering the inlet, but their ends must be cut perpendicular to the radius drawn through the middle of the lobe. The number of spring petals is chosen within 3-5 pieces, and their outer diameters (for 5 pieces) are made equal to 0.8-0.85 g / k, 0.75-0.80 s1k. Rice. 26. Valve grid with res-0.70-0.75<*„, 0,65—0,70 ^и, сорными клапанами
0.60-0.65 s?k, whereWhen using spring-type valves, the restrictive washer can be dispensed with, since the most advantageous valve bending line can be obtained by the number and diameter of the spring plates. But sometimes a restrictive washer is still installed on leaf spring valves, mainly to equalize their final deviation.
Valves during operation experience high dynamic and thermal loads. Indeed, normally selected valves, opening to some maximum possible value (6-10 mm from the seat), completely block the inlets when the mixture has already ignited and the pressure in the combustion chamber begins to increase.

Therefore, the valves move towards the seat not only under the action of their own elastic forces, but also under the action of gas pressure, and hit the seat at high speed and with considerable force. The number of strokes is equal to the number of engine cycles.
The temperature effect on the valves occurs due to direct contact with hot gases and radiant heating, and although the valves are washed by a relatively cold fuel-air mixture,
their average temperature remains quite high. The action of dynamic and thermal loads leads to fatigue failure of valves, especially their ends. If the valves are made along the fibers of the tape (along the direction of its rolling), then by the end of the service life, the fibers are separated from each other; on the contrary, in the transverse direction, the end edges are chipped. In either case, this leads to valve failure and engine shutdown. Therefore, the quality of valve processing must be very high.
The highest quality valves are produced by electrospark machining. However, most often the valves are cut with special round emery stones 0.8-1.0 mm thick. To do this, blanks are first cut out of valve steel, laid in a special mandrel, processed according to the outer diameter, and then, in the mandrel, inter-valve grooves are cut with an emery stone. Finally, in the serial production of engines, the valves are cut down with a stamp. But no matter how they are made, edge grinding is required. Burrs on valves are not allowed. Valves should also not have bends and warps.
Sometimes, for some relief of the operating conditions of the valves, the contact plane on the disk is processed along a sphere (Fig. 27). By closing the inlets, the valves receive a slight reverse bend, due to which their impact on the seats is somewhat softened. A loose seal between the valves and the disc at rest facilitates and speeds up starting, as the fuel-air mixture can freely pass between the valve and the disc.

Pulsating jet engines.

Rice. 28. Valve grids with flame arrester dampers
grids
The most effective way to protect valves from the effects of dynamic and thermal loads is to install flame-retardant damping grids. The latter several times increase the life of the valves, but significantly reduce the engine thrust, as they create a large resistance in the flow path of the working pipe. Therefore, they are installed, as a rule, on engines that require a long service life and relatively low thrust.
Grids are placed in the combustion chamber (Fig. 28) behind the valve grill. They are made of sheet heat-resistant steel 0.3-0.8 mm thick, with holes with a diameter of 0.8-1.5 mm (the thicker the mesh material, the larger the diameter of the holes).
At the moment the mixture flashes in the combustion chamber and the pressure builds up, hot gases try to penetrate into the L cavity through the grid holes. The grid breaks the main flame into separate thin streams and extinguishes them.

5. Double-circuit turbojet engine
6. Propeller engine
7. Pulsating jet engine
8. Main characteristics of the WFD

Production of an aircraft model with a PUVRD

The principle of operation and the device of the PUVRD

A pulsating jet engine, as its name implies, operates in a pulsating mode, its thrust does not develop continuously, like in a ramjet or turbojet engine, but in the form of a series of pulses following each other with a frequency of tens of hertz, for large engines, up to 250 Hz for small engines designed for aircraft models.

Structurally, PUVRD is a cylindrical combustion chamber with a long cylindrical nozzle of smaller diameter. The front of the chamber is connected to an inlet diffuser through which air enters the chamber.

An air valve is installed between the diffuser and the combustion chamber, which operates under the influence of the pressure difference in the chamber and at the outlet of the diffuser: when the pressure in the diffuser exceeds the pressure in the chamber, the valve opens and lets air into the chamber; when the pressure ratio is reversed, it closes.

The valve can have a different design: in the Argus As-014 engine of the V-1 rocket, it had the shape and acted like window blinds and consisted of flexible rectangular valve plates made of spring steel riveted onto the frame; in small engines, it looks like a flower-shaped plate with radially arranged valve plates in the form of several thin, elastic metal petals pressed against the valve base in the closed position and unbent from the base under the action of pressure in the diffuser exceeding the pressure in the chamber. The first design is much more advanced has minimal resistance to air flow, but is much more difficult to manufacture.

There are one or more fuel injectors at the front of the chamber which inject fuel into the chamber as long as the boost pressure in the fuel tank exceeds the pressure in the chamber; when the pressure in the chamber exceeds the boost pressure, the check valve in the fuel path shuts off the fuel supply. Primitive low-power designs often operate without fuel injection, like a piston carbureted engine. In this case, an external source of compressed air is usually used to start the engine.

To initiate the combustion process, a spark plug is installed in the chamber, which creates a high-frequency series of electrical discharges, and the fuel mixture ignites as soon as the concentration of fuel in it reaches a certain level sufficient for ignition. When the shell of the combustion chamber warms up sufficiently, electric ignition becomes completely unnecessary: the fuel mixture ignites from the hot walls of the chamber.

During operation, the PUVRD makes a very characteristic crackling or buzzing sound, due precisely to pulsations in its operation.

Scheme of operation of the PUVRD

The cycle of operation of the PUVRD is illustrated in the figure on the right:

1. The air valve is open, air enters the combustion chamber, the nozzle injects fuel, and a fuel mixture is formed in the chamber.
2. The fuel mixture ignites and burns, the pressure in the combustion chamber rises sharply and closes the air valve and check valve in the fuel path. The products of combustion, expanding, flow out of the nozzle, creating jet thrust.
3. The pressure in the chamber equalizes with atmospheric pressure, under the pressure of air in the diffuser, the air valve opens and air begins to flow into the chamber, the fuel valve also opens, the engine goes to phase 1.

The apparent similarity of the PUVRD and ramjet is erroneous. In reality, a PUVRD has profound, fundamental differences from a ramjet or turbojet engine.

Firstly, the presence of an air valve in the PUVRD, the obvious purpose of which is to prevent the reverse movement of the working fluid forward in the direction of the apparatus. In a ramjet, this valve is not needed, since the reverse movement of the working fluid in the engine tract is prevented by a "barrier" of pressure at the inlet to the combustion chamber, created during the compression of the working fluid. In a PUVRD, the initial compression is too low, and the increase in pressure in the combustion chamber necessary to perform work is achieved due to heating of the working fluid in a constant volume limited by the chamber walls, the valve, and the inertia of the gas column in the long engine nozzle. Therefore, from the point of view of thermodynamics of heat engines, a PUJE belongs to a different category than a ramjet or turbojet engine its operation is described by the Humphrey cycle, while the operation of a ramjet and turbojet engine is described by the Brayton cycle.

Secondly, the pulsating, intermittent nature of the operation of the PUWRJ also introduces significant differences in the mechanism of its functioning, in comparison with the WPW of continuous action. To explain the operation of a PUVRD, it is not enough to consider only the gas-dynamic and thermodynamic processes occurring in it. The engine operates in the mode of self-oscillations, which synchronize the operation of all its elements in time. The frequency of these self-oscillations is influenced by the inertial characteristics of all parts of the PUVRD, including the inertia of the gas column in the long nozzle of the engine, and the propagation time of the acoustic wave through it. An increase in the length of the nozzle leads to a decrease in the frequency of pulsations and vice versa. At a certain length of the nozzle, a resonant frequency is reached, at which self-oscillations become stable, and the amplitude of oscillations of each element is maximum. When developing the engine, this length is selected experimentally during testing and debugging.

It is sometimes said that the operation of a PUVRD at zero speed is impossible - this is an erroneous idea, in any case, it cannot be extended to all engines of this type. Most PUVRD can operate "standing still", although the thrust it develops in this mode is minimal.

The operation of the motor in this case is explained as follows. When the pressure in the chamber after the next pulse decreases to atmospheric, the movement of gas in the nozzle by inertia continues, and this leads to a decrease in pressure in the chamber to a level below atmospheric. When the air valve opens to atmospheric pressure, enough vacuum has already been created in the chamber so that the engine can "breathe in fresh air" in the amount necessary to continue the next cycle.

Other pulse jets

Samples of valveless PUVRD.

In the literature, there is a description of engines similar to PuVRD.

Valveless PUJE, otherwise U-shaped PUJE. There are no mechanical air valves in these engines, and so that the reverse movement of the working fluid does not lead to a decrease in thrust, the engine tract is made in the form of the Latin letter "U", the ends of which are turned back in the direction of the apparatus, while the jet flow occurs immediately from both ends tract. The intake of fresh air into the combustion chamber is carried out due to the rarefaction wave that occurs after the pulse and the "ventilating" chamber, and the sophisticated shape of the duct serves to best perform this function. The absence of valves makes it possible to get rid of the characteristic drawback of valve-operated PWR their low durability.

Detonation PUVRD. In these engines, the combustion of the fuel mixture occurs in the detonation mode. The detonation wave propagates in the fuel mixture much faster than the sound wave, therefore, during the chemical reaction of detonation combustion, the volume of the fuel mixture does not have time to increase significantly, and the pressure increases abruptly, thus isochoric heating of the working fluid takes place. After that, the expansion phase of the working fluid in the nozzle begins with the formation of a jet. Detonation PUVRD can be both with valves and without them.

The potential advantage of a detonation PUJE is considered to be higher thermal efficiency than any other type of ramjet. The practical implementation of this engine is in the experimental stage.

Scope of PuVRD

PuVRD is characterized as noisy and uneconomical, but simple and cheap. The high level of noise and vibration results from the very pulsating mode of its operation. The wasteful nature of the use of fuel is evidenced by an extensive torch, "beating" from the nozzle of the PUVRD a consequence of incomplete combustion of the fuel in the chamber.

Comparison of the PUVRD with other aircraft engines makes it possible to quite accurately determine the scope of its applicability.

A PUVRD is many times cheaper to manufacture than a gas turbine or piston ICE, so it outperforms them economically in a single use. During long-term operation of the reusable apparatus, the PuVRD loses economically to the same engines due to wasteful fuel consumption.

In terms of simplicity and cheapness, the ramjet is practically not inferior to the puramjet, but at speeds less than 0.5 M it is inoperable. At higher speeds, a ramjet outperforms a ramjet.

The combination of these circumstances determines the niche in which the PuVRD is used - disposable unmanned aerial vehicles with operating speeds up to 0.5M - flying targets, unmanned reconnaissance aircraft.

Valved, as well as valveless, PUVRD are widespread in amateur aviation and aeromodelling, due to their simplicity and low cost.

The PUVRD scheme is shown in Fig. 3.16.

Fig. 3.16. Scheme of a pulsating jet engine:

diffuser, 2-valve device; 3- nozzles; 4 - combustion chamber; 5 - nozzle; 6- exhaust pipe.

Fuel is injected through nozzles 3, forming a fuel mixture with air compressed in diffuser 1.

The ignition of the fuel mixture is carried out in the combustion chamber 4, from an electric candle. The combustion of the fuel mixture injected in certain quantities lasts hundredths of a second. As soon as the pressure in the combustion chamber becomes greater than the air pressure in front of the valve device, the reed valves close. With a sufficiently large volume of the nozzle 5 and the exhaust pipe 6, installed specifically to increase the volume, a backwater is created for the gases in the combustion chamber. During the combustion of the fuel, the change in the amount of gases in the volume behind the combustion chamber is negligible; therefore, it is believed that combustion proceeds at a constant volume.

After the combustion of a portion of the fuel, the pressure in the combustion chamber decreases so that the valves 2 open and let in a new portion of air from the diffuser.

In Fig.3.17. the ideal thermodynamic cycle of a pulsating WFD is presented.

P
cycle processes:

1-2 - air compression in the diffuser;

2-3 - isochoric heat supply in the combustion chamber;

3-4 - adiabatic expansion of gases in the nozzle;

4-1 - isobaric cooling of combustion products in the atmosphere at with heat removal.

Fig.3.17. PUVRD cycle.

As follows from Fig. 3.17, the cycle of the PUVRD does not differ from the cycle of a gas turbine with an isochoric heat supply. Then, by analogy with (3.8.), we can immediately write the formula for the thermal efficiency of the PuVRD

(3.20.)

The degree of additional pressure increase in the combustion chamber;

- the degree of pressure increase in the diffuser.

Thus, the thermal efficiency of a pulsating ramjet is higher than that of a ramjet due to the higher average integral heat supply temperature.

The complication of the design of the PUVRJ resulted in an increase in its mass compared to the ramjet.

3.5.3. Compressor turbojet engines (trd)

These engines are most widely used in aviation. In the turbojet engine there is a two-stage air compression (in the diffuser and in the compressor) and a two-stage expansion of the combustion products of the fuel mixture (in the gas turbine and in the nozzle).

Schematic diagram of the turbojet engine is shown in Figure 3.18.

Fig.3.18. Schematic diagram of the turbojet engine and the nature of the change in the parameters of the working fluid in the gas-air path:

1-diffuser; 2-axial compressor; 3- combustion chamber; 4- gas turbine; 5- nozzle.

The pressure of the incoming air flow first rises in the diffuser 1, and then in the compressor 2. The compressor is driven by a gas turbine 4. The fuel is supplied to the combustion chamber 3, where it forms a fuel mixture with air and burns at a constant pressure. The combustion products first expand on the blades of the gas turbine 4, and then in the nozzle. The outflow of gases from the nozzle at a higher speed creates a thrust force that propels the aircraft.

The ideal thermodynamic cycle of a turbojet engine is similar to the ramjet cycle, but is supplemented by processes in the compressor and turbine (Fig. 3.19).

Fig.3.19. The ideal turbojet cycle inP- Vdiagram

Cycle processes:

1-2 - adiabatic compression of air in the diffuser;

2-3 - adiabatic compression of air in the compressor;

3-4 - isobaric heat supply from the combustion of the fuel mixture in the combustion chamber;

4-5 - adiabatic expansion of combustion products on the turbine blades;

5-6 - adiabatic expansion of combustion products in the nozzle;

6-1 - cooling of combustion products in the atmosphere at constant pressure with heat transfer.

Thermal efficiency is determined by the formula (3.19):

(3.21.)

- the resulting degree of air pressure increase in the diffuser and compressor.

Due to the higher compression ratio than the ramjet, the turbojet engine has a higher thermal efficiency. Without any starting accelerators, the turbojet engine develops the necessary thrust already at the start.

The valveless PUVRD is an amazing design. It has no moving parts, compressor, turbine, valves. The simplest PUVRD can even do without an ignition system. This engine can run on just about anything: replace a propane tank with a can of gasoline and it will continue to pulsate and produce thrust. Unfortunately, HPJEs have failed in aviation, but recently they have been seriously considered as a source of heat in the production of biofuels. And in this case, the engine runs on graphite dust, that is, on solid fuel.

Finally, the elementary principle of operation of a pulsating engine makes it relatively indifferent to manufacturing precision. Therefore, the manufacture of PuVRD has become a favorite pastime for people who are not indifferent to technical hobbies, including aircraft modelers and novice welders.

Despite all the simplicity, PuVRD is still a jet engine. It is very difficult to assemble it in a home workshop, and there are many nuances and pitfalls in this process. Therefore, we decided to make our master class multi-part: in this article we will talk about the principles of operation of the PuVRD and tell you how to make an engine case. The material in the next issue will be devoted to the ignition system and the starting procedure. Finally, in one of the following issues, we will definitely install our motor on a self-propelled chassis to demonstrate that it is really capable of creating serious traction.

From the Russian idea to the German rocket

It is especially pleasant to assemble a pulsating jet engine, knowing that for the first time the principle of operation of the PuVRD was patented by the Russian inventor Nikolai Teleshov back in 1864. The authorship of the first operating engine is also attributed to a Russian - Vladimir Karavodin. The famous V-1 cruise missile, which was in service with the German army during World War II, is rightfully considered the highest point in the development of the PuVRD.

To make it pleasant and safe to work, we pre-clean the sheet metal from dust and rust with a grinder. The edges of sheets and parts are usually very sharp and full of burrs, so you need to work with metal only with gloves.

Of course, we are talking about valve pulsating engines, the principle of operation of which is clear from the figure. The valve at the inlet to the combustion chamber freely passes air into it. Fuel is supplied to the chamber, a combustible mixture is formed. When the spark plug ignites the mixture, the excess pressure in the combustion chamber closes the valve. The expanding gases are directed into the nozzle, creating jet thrust. The movement of combustion products creates a technical vacuum in the chamber, due to which the valve opens and air is sucked into the chamber.

Unlike a turbojet engine, in a PUVRD the mixture does not burn continuously, but in a pulsed mode. This explains the characteristic low-frequency noise of pulsating motors, which makes them inapplicable in civil aviation. From the point of view of efficiency, PuVRDs also lose to TRDs: despite the impressive thrust-to-weight ratio (after all, PuVRDs have a minimum of parts), the compression ratio in them reaches 1.2:1 at most, so the fuel burns inefficiently.

Before going to the workshop, we drew on paper and cut out templates for the parts in full size. It remains only to circle them with a permanent marker to get the markup for cutting.

But PUVRDs are invaluable as a hobby: after all, they can do without valves at all. In principle, the design of such an engine is a combustion chamber with inlet and outlet pipes connected to it. The inlet pipe is much shorter than the outlet. The valve in such an engine is nothing but the front of chemical transformations.

The combustible mixture in the PuVRD burns out at subsonic speed. Such combustion is called deflagration (in contrast to supersonic combustion - detonation). When the mixture ignites, combustible gases escape from both pipes. That is why both the inlet and outlet pipes are directed in the same direction and together participate in the creation of jet thrust. But due to the difference in lengths, at the moment when the pressure in the inlet pipe drops, exhaust gases are still moving along the outlet pipe. They create a vacuum in the combustion chamber, and air is drawn into it through the inlet pipe. Part of the gases from the outlet pipe is also sent to the combustion chamber under the action of rarefaction. They compress a new portion of the combustible mixture and set fire to it.

When working with electric scissors, the main enemy is vibration. Therefore, the workpiece must be securely fixed with a clamp. If necessary, you can very carefully dampen the vibrations by hand.

The valveless pulsating engine is unpretentious and stable. It does not require an ignition system to maintain operation. Due to rarefaction, it sucks in atmospheric air without requiring additional pressurization. If you build a motor on liquid fuel (for simplicity, we preferred propane gas), then the inlet pipe regularly performs the functions of a carburetor, spraying a mixture of gasoline and air into the combustion chamber. The only moment when an ignition system and forced boost is needed is at start-up.

Chinese design, Russian assembly

There are several common designs for pulse jet engines. In addition to the classic “U-shaped pipe”, which is very difficult to manufacture, there is often a “Chinese engine” with a conical combustion chamber, to which a small inlet pipe is welded at an angle, and a “Russian engine”, which resembles a car muffler in design.

Fixed diameter pipes are easily molded around the pipe. This is mainly done by hand due to the effect of the lever, and the edges of the workpiece are rounded with a mallet. It is better to form the edges so that when joined they form a plane - it is easier to lay the weld.

Before experimenting with your own designs of PUVRD, it is highly recommended to build an engine according to ready-made drawings: after all, the sections and volumes of the combustion chamber, inlet and outlet pipes completely determine the frequency of resonant pulsations. If the proportions are not respected, the engine may not start. Various drawings of PUVRD are available on the Internet. We chose a model called "Giant Chinese Engine", the dimensions of which are given in the sidebar.

Amateur PUVRD are made of sheet metal. It is acceptable to use finished pipes in construction, but it is not recommended for several reasons. Firstly, it is almost impossible to select pipes of exactly the required diameter. It is all the more difficult to find the necessary conical sections.

The bending of the conical sections is entirely manual labor. The key to success is to crimp the narrow end of the cone around the small diameter pipe, giving it more load than the wide end.

Secondly, pipes tend to have thick walls and a corresponding weight. For an engine that must have a good thrust-to-weight ratio, this is unacceptable. Finally, during operation, the engine is red-hot. If pipes and fittings made of different metals with different coefficients of expansion are used in the design, the motor will not last long.

So, we have chosen the path that most fans of PuVRD choose - to make a body from sheet metal. And immediately we faced a dilemma: turn to professionals with special equipment (CNC water-abrasive cutting machines, pipe rolls, special welding) or, armed with the simplest tools and the most common welding machine, go through the difficult path of a novice engine builder from start to finish. end. We preferred the second option.

back to school

The first thing to do is to draw a sweep of future details. To do this, you need to remember school geometry and quite a bit of university drawing. Making reamers of cylindrical pipes is as easy as shelling pears - these are rectangles, one side of which is equal to the length of the pipe, and the second is the diameter multiplied by "pi". Calculating the development of a truncated cone or a truncated cylinder is a slightly more difficult task, for which we had to look into a drawing textbook.

Welding thin sheet metal is a delicate job, especially if you use manual arc welding like we do. Perhaps, welding with a non-consumable tungsten electrode in an argon environment is better suited for this task, but the equipment for it is rare and requires specific skills.

The choice of metal is a very delicate issue. In terms of heat resistance, stainless steel is best for our purposes, but for the first time it is better to use black low-carbon steel: it is easier to form and weld. The minimum thickness of a sheet that can withstand the combustion temperature of the fuel is 0.6 mm. The thinner the steel, the easier it is to form and the more difficult it is to weld. We chose a sheet with a thickness of 1 mm and, it seems, made the right decision.

Even if your welding machine can operate in plasma cutting mode, do not use it to cut reamers: the edges of parts treated in this way do not weld well. Hand shears for metal are also not the best choice, as they bend the edges of the workpieces. The ideal tool is electric scissors that cut millimetric sheet like clockwork.

To bend the sheet into a pipe, there is a special tool - rollers, or a sheet bender. It belongs to professional production equipment and therefore is unlikely to be found in your garage. A vise will help bend a decent pipe.

The process of welding mm metal with a full-size welding machine requires some experience. Slightly holding the electrode in one place, it is easy to burn a hole in the workpiece. When welding, air bubbles can get into the seam, which then leak. Therefore, it makes sense to grind the seam with a grinder to a minimum thickness so that the bubbles do not remain inside the seam, but become visible.

In the next series

Unfortunately, within the framework of one article it is impossible to describe all the nuances of the work. It is generally accepted that these works require professional qualifications, but with due diligence they are all accessible to the amateur. We, journalists, were interested in learning new working specialties for ourselves, and for this we read textbooks, consulted with professionals and made mistakes.

We liked the case that we welded. It is pleasant to look at it, it is pleasant to hold it in hands. So we sincerely advise you to take up such a thing. In the next issue of the magazine, we will tell you how to make an ignition system and run a valveless pulse jet engine.

The reason for writing this article was the huge attention to the small engine, which appeared recently in the range of Parkflyer. But few people thought that this engine has more than 150 years of history:

Many believe that the pulse jet engine (PUVRD) appeared in Germany during the Second World War, and was used on V-1 (V-1) projectiles, but this is not entirely true. Of course, the German cruise missile became the only mass-produced aircraft with a PuVRD, but the engine itself was invented 80 (!) years earlier and not at all in Germany.
Patents for a pulsating jet engine were obtained (independently of each other) in the 60s of the 19th century by Charles de Louvrier (France) and Nikolai Afanasyevich Teleshov (Russia).

A pulsating jet engine (eng. Pulse jet), as its name implies, operates in a pulsating mode, its thrust does not develop continuously, like a ramjet (ramjet engine) or turbojet engine (turbojet engine), but in the form of a series of pulses .

Air, passing through the confuser part, increases its speed, as a result of which the pressure in this area drops. Under the action of reduced pressure, fuel begins to be sucked from tube 8, which is then picked up by an air stream and dispersed by it into smaller particles. The resulting mixture, passing the diffuser part of the head, is somewhat compressed due to a decrease in the speed of movement and, in the final mixed form, enters the combustion chamber through the inlets of the valve grill.
Initially, the fuel-air mixture that filled the volume of the combustion chamber is ignited with a candle, in extreme cases, with an open flame brought to the edge of the exhaust pipe. When the engine enters the operating mode, the fuel-air mixture again entering the combustion chamber is ignited not from an external source, but from hot gases. Thus, a candle is needed only at the stage of starting the engine, as a catalyst.
The gases formed during the combustion of the fuel-air mixture increase sharply, and the plate valves of the lattice close, and the gases rush into the open part of the combustion chamber towards the exhaust pipe. Thus, in the engine pipe, during its operation, the gas column oscillates: during the period of increased pressure in the combustion chamber, gases move towards the outlet, during the period of reduced pressure - towards the combustion chamber. And the more intense the fluctuations of the gas column in the working tube, the more thrust the engine develops in one cycle.

The PUVRD has the following main elements: input section a - in, ending with a valve grill, consisting of a disk 6 and valves 7 ; combustion chamber 2 , plot c - d; jet nozzle 3 , plot d - d, exhaust pipe 4 , plot e - e.
The inlet channel of the head has a confuser a - b and diffuser b - c plots. A fuel pipe is installed at the beginning of the diffuser section. 8 with adjusting needle 5 .

And let's go back to history. German designers, on the eve of World War II, conducting a wide search for alternatives to piston engines, did not ignore this invention, which remained unclaimed for a long time. The most famous aircraft, as I said, was the German V-1 projectile.

The chief designer of the V-1, Robert Lusser, chose the PUVRD for it mainly because of the simplicity of design and, as a result, low labor costs for manufacturing, which was justified in the mass production of disposable projectiles mass-produced in less than a year (from June 1944 to March 1945 ) in quantities over 10,000 units.

In addition to unmanned cruise missiles, in Germany, a manned version of the V-4 (V-4) projectile was also developed. As planned by the engineers, the pilot had to point his disposable pepelats at the target, leave the cockpit and escape using a parachute.

True, whether a person is able to leave the cockpit at a speed of 800 km / h, and even having an engine air intake behind his head, was modestly silent.

The study and creation of PuVRD was carried out not only in Nazi Germany. In 1944, for review, England delivered crumpled pieces of V-1 to the USSR. We, in turn, "blinded from what was", while creating an almost new engine PuVRD D-3, iiii .....
..... and hoisted it on the Pe-2:

But not with the aim of creating the first domestic jet bomber, but for testing the engine itself, which was then used to produce Soviet 10-X cruise missiles:

But the use of pulsating engines in Soviet aviation is not limited to this. In 1946, the idea was realized to equip the fighter with PuVRD-shki:

Yes. Everything is simple. On the La-9 fighter, two pulsating engines were installed under the wing. Of course, in practice, everything turned out to be somewhat more complicated: they changed the fuel supply system on the plane, removed the armored back, and two NS-23 guns, strengthening the airframe design. The increase in speed was 70 km / h. Test pilot I.M. Dziuba noted strong vibrations and noise when the PuVRD was turned on. The suspension of the PuVRD worsened the maneuvering and takeoff and landing characteristics of the aircraft. Starting the engines was unreliable, the flight duration was sharply reduced, and operation became more complicated. The work carried out was beneficial only in the development of ramjet engines intended for installation on cruise missiles.
Of course, these aircraft did not take part in the battles, but they were quite actively used at air parades, where they invariably made a strong impression on the public with their roar. According to eyewitnesses, from three to nine cars with PuVRD participated in different parades.
The culmination of the PuVRD tests was the flight of nine La-9RDs in the summer of 1947 at an air parade in Tushino. The planes were piloted by test pilots of the GK NII VVS V.I. Alekseenko. A.G. Kubyshkin. L.M. Kuvshinov, A.P. Manucharov. V.G.Masich. G.A. Sedov, P.M. Stefanovsky, A.G. Terentiev and V.P. Trofimov.

I must say that the Americans, too, did not lag behind in this direction. They were well aware that jet aviation, even at the stage of infancy, was already superior to its piston counterparts. But there are a lot of piston aircraft. Where to put them?! .... And in 1946, two Ford PJ-31-1 engines were suspended under the wings of one of the most advanced fighters of its time, the Mustang P-51D.

However, the result was, frankly, not very good. With the PUVRD turned on, the speed of the aircraft increased noticeably, but they absorbed the fuel, oh-ho-ho, so it was not possible to fly at good speed for a long time, and when turned off, the jet engines turned the fighter into a heavenly slow-moving fighter. After suffering for a whole year, the Americans, nevertheless, came to the conclusion that it would not be possible to get a cheap fighter capable of at least somehow competing with the newfangled jets.

As a result, they forgot about the PuVRD .....
But not for long! This type of engine performed well as an aircraft model! Why not?! It is cheap to manufacture and maintain, has a simple device and a minimum of settings, does not require expensive fuel, and in general - it is not necessary to buy it - you can build it yourself with a minimum of resources.

This is the smallest PUVRD in the world. Created in 1952
Well, you must admit, who has not dreamed of a jet plane with a hamster pilot and rockets?!))))
Now your dream has become a reality! Yes, and it is not necessary to buy an engine - you can build it:

P.S. This article is based on materials published on the Internet ...
The end.