Design parameters:
1. Oil-gas ratio
Jet engines rely on the combustion of oil and gas to produce reaction force. According to the explosion limit of petroleum products,
The weight ratio of fuel to air is generally 15-20%. In other words, one liter of air needs about one gram of oil.
2. Injection frequency,
The jet frequency of a jet engine is related to the length of the fuselage. Under the same diameter, the longer the fuselage, the lower the frequency.
3. Length-diameter ratio of fuselage diameter to length
Engine length and diameter are important steps in engine design, and the length and specific diameter are generally 10- 17.
4. Calculation formula
The thrust of the engine is determined by many factors, which can be explained by the following formula: m * va = f * t.
V = engine volume (dm^3.)
F = injection frequency. (Hertz)
Va = injection speed. (m/s)
F = thrust (n, Newton)
Fc = Fuel consumption (g/s)
M = air mass kg
T = time seconds.
With a conversion of time of one second, m= the actual amount of oil and gas entering the engine x.
M*v=F*t. m = mass = X%
Actual thrust: f (Newton) = (x * D2 * 3.1415 * l * v2)/(l * 8).
From the above formula, it can be concluded that the larger the nozzle diameter, the greater the thrust of the engine, and the more oil and gas X entering at the same time, the greater the thrust that can be generated.
5. Tail nozzle length
According to the practical experience of foreign enthusiasts, the length of tail injection has little influence on thrust, but it has great influence on the reliability of engine work.
The engine tail nozzle is long, and the working frequency f of the valve plate is low, but more oil and gas are inhaled each time, which increases the work done each time. The long nozzle can make the engine close to the maximum theoretical thrust. At the same time, the suction performance is good and the engine is easy to start.
Short tail injection will increase the jet frequency f of the engine, and at the same time, less oil and gas will be inhaled, so the thrust will not increase. And it will make the engine start difficult and unstable.
(Hint: In order to adjust the engine conveniently, the actual production length is longer than the theoretical design, because the long one can be sawed short. It will be more troublesome to stretch it in a short time, but it should not be too long. If it is too long, the result will be the same.
The calculation formula is:
Y = 0. 152 * X+470 (mm), metric unit.
(or y = 3.88 * x+18,66 (Inc)-English units.
Reference data:
Engine name
Y= total length
X= cross-sectional area of the tail nozzle.
Brownell
490
907
The first letter of the Greek alphabet.
485
53 1
B- 12
600
53 1
Aerojet engine
6 10
1075
Pulse Amplitude Modulation (short for pulse amplitude modulation)
8 10
907
Sov faa
670
1 195
6. Injection speed
Because the jet speedometer of jet engine is a complex process under high temperature and high pressure, it can be calculated by a simplified formula for enthusiasts to use.
va=2*L*f
Take the calculation of p90 as an example:
Injection speed:150 * 2 * 0.86 = 258m/s 。
7. Ventilation hole area of one-way valve
The ventilation area of one-way valve is the most critical part in engine design, because it is related to the oil-gas ratio entering the engine.
computing formula
Y = 0.4922 * x–37 (mm2)
Here (X= cross-sectional area of tail nozzle, Y= area of one-way valve vent hole, which can be reduced by 37 if it is a large engine).
In addition, the design should take into account that the area of ventilation holes will be reduced by 10-20% after installing the valve plate, so a certain margin should be left.
The calculation result is about 50-60% of the cross-sectional area of the tail nozzle, and 55% is acceptable for general design.
(Hint, a slightly larger ventilation area can make the engine easier to ignite).
Reference for foreign engine design;
Engine name
Valve ventilation area y
Cross-sectional area x of tail jet
Brownell
452
907
The first letter of the Greek alphabet.
38 1
53 1
B- 12
22 1
53 1
Aerojet engine
603
1075
Pulse Amplitude Modulation (short for pulse amplitude modulation)
506
907
Sov faa
66 1
1 195
You can also check the manual processing drawing yourself, and the general error is between 5%.
8. Air inlet area
The minimum area of the air inlet located at the front end of the engine cannot be less than the area of the one-way valve ventilation hole.
In order to atomize the fuel, the speed of air increases in the contraction part, so the inlet is designed as a trumpet, also called an air throttle valve.
9. How to design your own engine
First, determine the thrust of the engine,
According to the above formula, the actual oil and gas entry coefficient X=0.75 is calculated and simplified.
The design formula of the relationship between engine thrust and tail jet cross-sectional area is
F (pounds) =4.2 pounds * square inch (nozzle area)
Or:
F (Newton) =2.65 Newton * square centimeter
(One kilogram force =9.8 Newton)
According to foreign design:
If you want to build an engine that produces 25 pounds of thrust, 25/4.2 = 5.95 square inches gives a nozzle diameter of about 2.75 inches.
The area of the valve hole is 5.95*0.6552=3.9 square inches. (The coefficient here is 0.6552, and the designer takes the empirical value for calculation. )
Due to the limitation of the processing shape of the valve, the cross-sectional area of the one-way valve can be 3.9/0.55 = 7. 1 sqr inc, and the area of each hole is 0.39 sqr inc according to the ten holes in the valve. The cross-sectional area of the combustion chamber is almost the same as that of the check valve, and it can be installed in the check valve.
The nozzle length can be simplified as l = 5.95 * 3.88+18.66 = 41.8, and the rest is 50 inches. If the tail of the nozzle adopts an expanded part with the length of 0.2*4 1=8 and the total length is 50, the actual tail nozzle length is 50-8=42.
The minimum air inlet area is the valve hole area, which is 3.9 square inches.
Experimental data of foreign P-90 engine (for reference)
These parameters are as follows
V = 2.9 liters
Fc = 6.7 g/s
F = 150 Hz
Va = 258/s
F = 85 Newton
Chapter II Jet Engine Manufacturing
1. Material selection
Because the engine works at high temperature, low melting point metals such as aluminum cannot be used.
Generally speaking, for enthusiasts, carbon steel and aluminum alloy can be used. Stainless steel pipe is the best material, which is available in hardware stores and various specifications. The material you can also use is the exhaust pipe of motorcycle or car, carbon steel. Aluminum plating on the surface is not easy to rust, but it is slightly heavier because of its thickness. The price is not expensive. You can find one around 40 yuan in the motorcycle repair department. Used old ones are cheaper and can be sold to 10 yuan. Tapered parts can also be processed according to drawings.
Aluminum alloy can only be used as the front intake throttle cover of the engine.
3.
How to make intake check valve
The key to start is the processing of one-way valve. The machining of the valve requires the lathe to process it as a whole. If there is no lathe, another design can be adopted. For example, as can be seen from the blueprint, the required holes can be drilled on the 3- 10 mm thick circular iron plate for replacement, and then the valve plate can be installed.
The plum blossom valve plate is the key of the engine, and it must be made of thin steel plate with strong elasticity and high temperature resistance, and the thickness is about 0. 1-0.3mm, otherwise the engine will not work. The valve plate can be cut into the required shape or electrolyzed. Like the printed circuit board, paint is applied to the board first, and then the desired pattern is drawn after drying. Cut off the paint along the line with a steel needle, then put it in salt water and electrolyze it with 6- 12v DC.
4. Assembly of the engine
The installation of jet engine is relatively simple, just process the parts according to the drawings and install them. When installing the check valve, pay attention to bend the plum blossom valve inward 10 to 30 degrees. Open the valve vent. In addition, pay attention to whether the engine contact is airtight.
Chapter III How to Start the Engine
abstract
Is it difficult to start the pulse engine? Actually, it is not. According to the engine principle, in order to start engine combustion, the following conditions need to be met:
1. Fuel
2. Air
3. Ignition source
fuel
Pulse engine can use a variety of daily fuels, such as household liquefied gas, gasoline, diesel, kerosene, methanol (industrial alcohol) and so on. Generally, gasoline is used as fuel. For ordinary enthusiasts, any brand of automobile gasoline can be used. If the temperature is low, it may make the fuel difficult to volatilize, and it is also possible to add no more than 25% ether components to the engine oil to make the ignition easier. The best fuel is methanol, because combustion produces water, which is volatile and has a wide explosion point.
air
Air cannot be automatically sucked into the combustion chamber before the jet engine is started. At this time, it is necessary to use a small bellows or pump to input air at the entrance of the engine to help the engine input oil-gas mixture. Note that air needs a certain pressure and flow rate to completely atomize fuel into oil and gas.
Ignition method
The best way is to install spark plugs in the combustion chamber of the fuselage. It doesn't matter if there is no spark plug. The wire head can be wrapped in cotton balls dipped in gasoline, and the tail nozzle can also be ignited. Various ignition modes are shown in the figure.
Ignition steps:
1. Connect the oil pipe, and pay attention to the height between the liquid level of the oil tank and the fuel injection port of the engine not exceeding 20mm.
2. Open the spark plug or light a small fuel torch and extend it from the nozzle.
3. Press the bellows by hand, or blow air into the air inlet of the engine with a pump, and pay attention to observation. Only when the check valve is blown open can the oil be sucked and atomized.
Adjust the oil valve needle to control the throttle size, and find the best blowing angle to completely atomize the oil. If the engine still can't ignite, you can disassemble the fuselage and adjust the angle of the valve plate and the tightness of the fixing screw. Then try again until the best working point is found, the jet engine is started, the bellows and ignition source are removed, and the operation continues.
In addition, canned lighter gas can also be blown in from the air inlet and ignited. The steps are the same as above, but you need to adjust the air volume.
Chapter IV Questions and Answers
1. Why doesn't the engine work?
Many engines can't work normally due to the problem of material selection in design and processing. In fact, it can be seen from the combustion conditions that the main reasons are as follows:
1. Air shortage and excess
Because the valve plate is made of different materials, the valve plate is too hard, which will make the outside air unable to be inhaled. Therefore, it is necessary to adjust the clearance of the valve plate in advance and choose the appropriate material to make it. In addition, due to the resistance of the valve plate, the actual air intake is reduced by more than 20%.
2. The design of the air inlet is too large, resulting in too much air, so that the spark in the combustion chamber is blown away and the inhaled oil-gas mixture cannot be ignited.
3. The nozzle is too short, resulting in extremely unstable engine. Because the frequency is too high, the inhaled oil and gas can not be completely mixed, which will lead to engine flameout.
4. The oil atomization is not good, and the overweight oil is not easy to vaporize. Therefore, it is not recommended to use oil heavier than gasoline, such as diesel, as fuel, preferably methanol, because methanol is easy to vaporize and has a wide explosion concentration range.
5. The oil level at the oil inlet is low, so the oil cannot be sucked in due to the low oil level in the oil tank. At this point, the position of the fuel tank should be raised.
2. Why is the working life of the engine valve plate shorter?
Because of the high working temperature and high vibration frequency of the valve plate, the working life of the valve plate has become the weakness of the engine. If the material melts easily, it will be finished in a few minutes at high temperature. Therefore, how to design one-way valve to increase the working life of valve plate has become a research topic for engine manufacturers.
One is to choose high temperature resistant materials, and the other is to adopt valveless design. According to the existing valveless pulse engine design, the fuselage manufacture is more complicated and the thrust is smaller.
Pulsating jet engine is a kind of jet engine, which can be used for unmanned aerial vehicles, missiles or aviation models. The German Nazis used it to propel the V- 1 missile and bombed London in the late World War II. The structure of this engine is shown in the figure. The front of the engine is equipped with a one-way valve, followed by a combustion chamber containing fuel nozzle and spark plug, and finally a specially designed long tail nozzle.