Pulse Jet Engine - Operation

Operation

Pulsejet engines are characterized by simplicity, low cost of construction, and high noise levels. Pulsejet fuel efficiency is a topic for hot debate, as efficiency is a relative term. While the thrust-to-weight ratio is excellent, thrust specific fuel consumption is generally very poor. The pulsejet uses the Lenoir cycle which lacking an external compressive driver such as the Otto cycle's piston, or the Brayton cycle's compression turbine, drives compression with acoustic resonance in a tube. This limits the maximum (pre-combustion) pressure ratio, to perhaps 1.2 to 1.

The high noise levels usually make them impractical for other than military and other similarly restricted applications. However, pulsejets are used on a large scale as industrial drying systems, and there has been a new surge to study and apply these engines to applications such as high-output heating, biomass conversion, and alternative energy systems, as pulsejets can run on almost anything that burns, including particulate fuels such as sawdust or coal powder.

Pulsejets have been used to power experimental helicopters, the engines being attached to the ends of the rotor blades. As an aircraft propulsion system, pulsejets have the advantage over turbine engines of not producing torque upon the fuselage. A helicopter may be built without a tail rotor and its associated transmission and drive shaft, simplifying the aircraft (though it is still necessary to rotate the fuselage relative to the rotors in order to keep it pointing in one direction). This concept had been considered as early as 1945. The Hiller rotor-tip helicopter, known better as the Hiller Powerblade, was the world's first hot-cycle pressure-jet rotor in 1949, however the Hiller YH-32 Hornet was ram jet and not pulsejet powered. Rotor-tip propulsion is estimated to reduce the cost of production of rotary-wing craft to 1/10 of conventional powered rotary-wing aircraft. Pulsejets have also been used in both control-line and radio-controlled model aircraft. The speed record for control-line model aircraft is greater than 200 miles per hour (323 km/h) although the long control lines create 70% of the drag of the system.

A free-flying radio-controlled pulsejet is limited by the engine's intake design. At around 450 km/h (280 mph) most valved engines' valve systems stop fully closing owing to ram air pressure, which results in performance loss. One company, Beck Technologies, has produced a valved pulsejet design with variable intake geometry, allowing the intake to open and close to control ram airflow, and letting the engine produce full power at any speed. Valveless designs are not as negatively affected by ram air pressure as other designs, as they were never intended to stop the flow out of the intake, and can significantly increase in power at speed.

Another feature of pulsejet engines is that their thrust can be increased by a specially shaped duct placed behind the engine. The duct acts as an annular wing, which evens out the pulsating thrust, by harnessing aerodynamic forces in the pulsejet exhaust. The duct, typically called an augmenter, can significantly increase the thrust of a pulsejet with no additional fuel consumption. Gains of 100% increases in thrust are possible, resulting in a much higher fuel efficiency. However, the larger the augmenter duct, the more drag it will produce, and it may only be effective at certain speed ranges. Most vehicles will be drag-limited at a much lower speed than the speed at which a small to moderate-size augmenter will stop producing positive thrust increase.