The resulting form gives a very nice jet engine shape, while remaining about as simple as any other valveless pulsejet. They have been used to power model aircraft , experimental go-karts  , and unmanned military aircraft such as cruise missiles and target drones. An early photograph can be seen above. Pressure data is used to determine the operational frequency of the jet, also to find the peak pressure and the amplitude of pressure waves.
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The objective dngine to provide the least possible obstruction to the gas flow and the propagation of pressure waves. As a result of this complex process, the Reynst engine reportedly exhibits the most efficient Kadenacy pumping of all pulsejets. In calveless smallest sizes, forced air at the intake is also typically needed for startup. Elements of Resonance In acoustic terms, the combustion chamber is the place of the greatest impedance, meaning that the movement of gas is the most restricted.
In fact, it looks well nigh insoluble, given that the valves are supposed to satisfy conflicting demands. The fuel used in this experiment is gaseous propane. The intake valve is typically a reed valve.
The whole thing was about a meter long. Because of inertia, the gases would overexpand and the resulting partial vacuum would suck fresh air in.
Admittedly, development has improved the design in many ways and stretched its working life from minutes into hours, but the fundamental problem remains. The chamber sucks in the mixture from the sides rather than air from above because of the distribution of pressures. These are generally known as the "intake" a very short duct and the "tailpipe" a very long duct.
Numerical simulation of a hydrocarbon fuelled valveless pulsejet - ScienceDirect
Pulsejet engines are characterized by simplicity, low cost of construction, and pulsr noise levels. Yet, it obviously worked for Saunders Roe, as more or less the same shape was employed on their engines with valves, which saw greater use than the valveless ones. The pressure wave travels up and down the tube. After that, Reynst instigated a program of development that perfected the resonator exhaust for practical applications.
I am talking of elegance in the mathematical sense -- desired result achieved with minimal complication. It has a chamber with two tubular ports of unequal length and diameter.
However, the hot gas exiting between the chamber and the mantle will also swirl in a vortex — one whose direction of rotation will be opposite to the direction of the exit from the annular chamber. Yet, this is completely true.
The spark plug shown on the picture is needed only at start-up. Bruce Calveless in New Zealand has discovered that a sufficiently big valveless pulsejet will not need forced air either. The intake tube takes in air and mixes it with fuel to combust, and also controls the expulsion of exhaust gas, like a valve, limiting the flow but not stopping it altogether. The sketch below shows the central part of their valveless engine.
I have seen two pulsejet designers of the past argue the merits of intake position -- Reynst and Albert G.
If we can make the pulsejet more acceptable, we can turn our attention towards some serious competition with the other jet engines. In a small flying model-sized pulsejet, it happens more than times a second.
Hot gas ejected into the conical duct mixes with the fresh air passing through and heats it up. The mixture was ignited by the remaining free radicals present in the hot gas that would remain clinging to the walls inside the jar. Others came up with ways to deflect gases in different directions. Each transition from a straight section into a diffusing section flaring cone represents a point from which the pressure waves traveling up and down the tube will reflect in the opposite direction and with the opposed sign.
First, ejectors in general are more effective with a pulsating, rather than steady, flow.
The first trials were with a single engine under each wing, but later models carried two and three engines on each underwing pylon. Once the engine fires, the retained hot gas provides self-ignition and the spark plug becomes unnecessary. I can offer no obvious explanation for the poor performance. The idea that the simplest engine an enthusiast can make at home is a jet engine will sound strange to most people -- we perceive jet engines as big complex contraptions pushing multi-million dollar aircraft through the skies.