Valveless Pulse Jet - Basic Characteristics

Basic Characteristics

A pulsejet engine is an air-breathing reaction engine employing an ongoing sequence of discrete combustion events rather than a constant level of combustion. This clearly distinguishes it from other reaction engine types such as rockets, turbojets and ramjets, which are all constant combustion devices. All other reaction engines are driven by maintaining high internal pressure; pulsejets are driven by an alternation between high and low pressure. This alternation is not maintained by any mechanical contrivance, but rather by the natural acoustic resonance of the rigid tubular engine structure. The valveless pulsejet is, mechanically speaking, the simplest form of pulsejet, and is, in fact, the simplest known air-breathing propulsion device that can operate "statically", i.e. without forward motion.

The combustion events driving a pulsejet are often informally called "explosions"; however, the preferred term is "deflagrations". They are not the violent, very high energy detonations employed in "Pulse Detonation Engines (PDEs)"; rather, deflagration within the combustion zone of a pulsejet is characterized by a sudden rise in temperature and pressure followed by a rapid subsonic expansion in gas volume. It is this expansion that performs the main work of moving air rearward through the device as well as setting up conditions in the main tube for the cycle to continue.

All pulsejet engines work by alternately accelerating a contained mass of air rearward and then breathing in a fresh mass of air to replace it. The energy to accelerate the air mass is provided by the deflagration of fuel mixed thoroughly into the newly acquired fresh air mass. This cycle is repeated many times per second. During the brief mass acceleration phase of each cycle, the engine’s physical action is more or less identical to that of other reaction engines — that is, gas mass is accelerated rearward, accompanied by an application of force forward into the body of the engine. These small "pulses" of force, rapidly repeated over time, comprise the measurable thrust force of the engine.

Some basic differences between valved and valveless pulsejets are:

• Valveless pulsejet engines have no mechanical valve, eliminating the only internal "moving part" of the conventional pulsejet;
• In valveless engines, the intake section has an important role to play throughout the entire pulsejet cycle;
• Valveless engines produce thrust forces in two distinct but synchronized mass acceleration events per cycle, rather than just one.