Pulse jet
The pulse jet is one of the most simple jet engines there is, that's why I built one. I just had to see if I could do it. It isn't finished yet. I'm about half-way in building the valve assembly and still need to make a fuel supply system. I have great expectations about the power output, after all why go to all the trouble of making a jet engine if you don't build it big.
Big means that the combustion chamber is carbon steel tubing with an outside diameter of 10 cm. The exhaust tube is 7 cm outside diameter. The complete engine is 1,5 meters long.
For those of you who are not familiar with the principles of pulse jets, I will give a brief description.
To start the engine, you need to feed the engine with pressurized air to open the spring loaded valves and achieve effective mixing of air and fuel. A sparkplug is located just aft of the valve assembly, this is needed to start the combustion process but may be turned off later. (It is a good idea to design the sparkplug with shroud to produce a local low pressure area that will make it easier to achieve ignition.) Once the first fuel mixture has been ignited, a shockwave will travel forwards and aft, first closing the fuel valve and air intake valve at the front (Which are both one-way valves) and secondly hitting the constriction in the aft of the combustion chamber. This raises the pressure in the combustion chamber. The shockwave travels down the exhaust tube, since this is a smaller tube, the gases will increase their speed thereby reducing the pressure in the combustion chamber very quickly. As the shockwave reaches the end of the exhaust tube it has reduced the pressure in the combustion chamber to under the atmospheric pressure, causing the valves to open and suck in a new mixture of air and fuel. (This means that the diameter/length ratio of the exhaust tube is very critical to the operation of the engine.) There will still be enough heat/flames in the combustion chamber to ignite the next mixture.
Easy, isn't it?
The only problem is the loud noise these engines make, hence the nickname of the German V1 bombs of WWII, The buzz bombs. These were bombs with 950 Kg of TNT mounted with wings and a pulse jet engine running at 50 ignitions per second and a ingenious mechanical guidance system. Thousands of these where launched from the French coast against London during WWII.
