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Aviation History
1952
1952 - 0236.PDF
102 FLIGHT Fig. 2 (left). The Escopette's thermodynamic cycle is that discovered by Lenoir. Figs. 3 and 4 (right). The Marconnet unit (upper drawing) of 1909 and the modern Escopette pulsejet; the principle of operation is similar. RECUPERATOR COMBUSTION CHAMBER DUCT TAIL PIPE INJECTOR - ESCOPETTE PULSEJET... there is no actual explosion, but rather a high-speed combustion of the order of 30 m/sec. Thermodynamic Principle.—The thermodynamic cycle is that used by Lenoir in his gas engine which functioned without pre- compression. Ignition occurred during the period of induction. As illustrated in Fig. 2, this cycle comprises an important increase in pressure at constant volume, followed by an adiabatic expansion down to the inlet pressure. -It has been calculated that the maximum pressure attained can reach seven times the initial pressure, P0; the theoretic efficiency under these conditions being 28 per cent. In point of fact, the pressure attained is actually less because of the width of the openings of the combustion chamber and because the fresh charge is incomplete, the combustion being relatively slow. It is of the order of two atmospheres, or even less at smaller throttle-openings. Propulsive Efficiency.—The thermodynamic efficiency of a pulse- jet is extremely low, but it is still able to retain its place as a power unit because its propulsive output is relatively high. This is due to the fact that each discharge is also followed by the induction of a certain volume of air into the exhaust end of the tail-pipe. This air is then expelled at the next successive combustion. Thus the kinetic energy produced is applied to a larger mass of air, which, although having the effect of lessening the exit-velocity nevertheless increases the momentum (and therefore the thrust) of the discharge gases. Details of the Escopette.—The Escopette is a pulsejet of the Marconnet type (Figs. 3 and 4). It has no inlet valves and gas movement is regulated only by varying the dimensions of its duct. It comprises the following parts : a detector, combustion chamber, main body or duct, tail-pipe, recuperator, fuel injector and igniter plug. The detector is the name given to the intake unit, which has the general form of a venturi with a bias towards the passage of fluid in one direction only. Although outwardly it appears to be a simple component, development of the detector has, neverthe less, necessitated numerous tests and constitutes the most original feature of SNECMA pulsejets. Apart from eliminating the need for mechanical valves it also has the following advantages. Having no precise frequency of its own it can be adapted to harmonize with the main apparatus; it has a long life; fuel injection is easily made into the detector inlet opening; and a good combustible mixture is obtained even from continued injection at low pressures. Counteracting these advantages, however, is the fact that the detector is not airtight and at each cycle produces a considerable back-pressure. As this back-pressure exerts itself in a forward direction, a curled "walking-stick" tube called a recuperator and shaped something like the cup of a Pelton turbine, is provided to reverse this movement. Its effect is to increase the output of the power unit. There is actually a gap between the recuperator and the detector to permit the entry of "induction air." The functions of the body and tail-pipe are Fig. 5. Fuel circuit of a typical four-unit installation, showing the positions of controls and valves. similar to those of normal pulsejets. The igniter plug located in the combustion chamber is only used for starting, as in normal running the re-igniting of successive charges is automatic. The Escopette operates on petrol of any octane value, injected at pressures varying from 0.3 to 1.4 atm. At fuel-pressures outside these limits the unit will not function. Thrust varies from 3 to 10 kg at consumption figures which can be from 2 to 5 g/sec, according to the type of injector used. It has been discovered that, if two or three pulsejets are mounted close together, the output of each unit is decreased by ten per cent for the same specific fuel consumption. For starting, the spark is provided by a battery- or magneto- operated igniter plug. An injection of compressed air must be made at a pressure of 3 to 5 atm through a iet (4 to 10 mm in diameter) mounted in the entrance of the detector tube and directed towards the combustion chamber. These "squirts" of air must be frequent, but of short duration. Fuel injections are made at the same time as the air so as to avoid the possibility of a residue of liquid fuel accumulating in the combustion chamber. Should ignition not take place, the fuel must be cut off immediately and pure air injected to blow out surplus fluid. Electric contact is switched off as soon as the pulsejet begins to function. A number of safety precautions have to be observed when these units are fitted to an aircraft. For example, they must be located as far as possible away from any inflammable material, while wing- surfaces near to the body of a pulsejet should be covered with a sheet of polished aluminium to reflect the very considerable heat generated by the combustion chambers. A typical installation for four Type 3340 Escopettes (as fitted in an Emouchet sailplane) is shown in Fig. 5. All the requisite fuel- and air-pressure cocks (for starting), together with the two main throttles (controlling fuel pressure and air pressure feed to fuel tank), are located in the cockpit, but outside assistance is required when ground-starting to direct the jets from a com pressed-air bottle into the intake of the pulsejets. For starting, the air pressure to the fuel tank must be regulated to 1.5 atm and fuel pressure to 0.75 atm by the two throttle controls referred to above. Maximum duration at ground level is 10 min at full throttle, i.e. a fuel pressure of 1 atm. In flight, starting or restarting is easily accomplished at a speed of approximately 75 km/hr; all the pilot needs to do» having set his throttle at the starting position, is to switch on the ignition and open the fuel cocks. 1. Fuel-tank filler cap. 2. Excess-pressure vent. 3. Throttle valve. 4. Relay to fuel pressure gauge. 5. individual taps for fuel regulator valves. 6. Fuel pressure gauge. 7. Air pressure gauge. 8. Master fuel-feed cock. 9. Air control throttle. 10. Relay to air-pressure gauge. 11. Safety valve (air, 4 atm). 12. Emergency pressure re lease. 13. Fuel regulator valve. 14. Fuel filter. 15. Non-return valve. 16. Fuel tank (20 litres). 17. Main fuel cut-off. 18. Fuel-control throttle. 19. Compressed air bottle (1 litre at 80 atm). 20. Main air cock. 21. Air regulator valve. 22. Fuel injector.
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