The working of a pulse-jet engine.
THE RAM JET ENGINE
The air rushing toward the inlet of an engine flying at high speeds is partially compressed by the so-called ram effect. If the air speed is high enough, this compression can be sufficient to operate an engine without either compressor or turbine. The ramjet has been called a flying stovepipe, because it is open at both ends and has only fuel nozzles in the middle. A straight stovepipe would not work however; a ramjet must have a properly shaped inlet-diffusion section to produce low-velocity, high-pressure air at the combustion section, and it must also have a properly shaped exhaust nozzle. Ramjets can operate at speeds above 320 km/h (about 200 mph), although they become practical for military applications only at very high or supersonic speeds. Because the ramjet depends on the compression of the in-rushing air for its operation, a vehicle powered by a ramjet must first be accelerated by other means to a sufficiently high speed.
THE PULSE JET ENGINE
A pulse jet is similar to a ramjet, except that a series of spring-loaded shutter-type valves is located ahead of the combustion section. In a pulse jet, combustion is intermittent or pulsing rather than continuous. Air is admitted through the valves, and combustion is initiated, which increases the pressure, closing the valves to prevent back-flow through the inlet. The hot gases are expelled through the rear nozzle, producing thrust and lowering the pressure to the point that the valves may open and admit fresh air. Then the cycle is repeated. The most widely known pulse jet was the German V-1 missile, or buzz bomb, used near the end of World War II, which fired at a rate of about 40 cycles per sec. The pulsing effect can also be achieved in a valve-less engine, or wave engine, in which the cycling depends on pressure waves traveling back and forth through a properly scaled engine. A pulse-jet engine delivers thrust at zero speed and can be started from rest, but the maximum possible flight speeds are below 960 km/h (600 mph). Poor efficiency, severe vibration, and high noise limited its use to low-cost, pilot-less vehicles. Pulse-jets use the forward speed of the engine and the inlet shape to compress the incoming air, then shutters at the inlet close while fuel is ignited in the combustion chamber and the pressure of the expanding gasses force the jet forward. The shutters then open and the process starts again at a high frequency. This results in the buzzing drone for which the experimental pulse-jet missile is named the "Buzz-Bomb". People have pointed out that pulse-jets can be cooled which would solve the heating problem of the ramjets. They could also, theoretically, generate "donut-on-a-rope" contrails due to the engine's operational design.
The known advantages of the Pulse-jet engine are:-
a) Theoretically the pulse jet engine has a higher fuel efficiency than a normal jet engine that keeps constant pressure. Intermittent rather than constant fuel combustion is another key factor in making the pulse-jet engine more fuel efficient. than ordinary
b) Engines can be produced in many sizes with many different thrust outputs ranging from a few pounds to thousands of pounds.
c) They have a very high thrust-to-weight ratio, which means a lighter engine producing more pounds of thrust than it's weight.
d) They are mechanically very simple and have very little moving parts.