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Valveless pulsejets - How and why they work -
How?
First fuel and air mix in the combustion chamber, then a heat source is a applied usually by means of a spark plug but ive seen other people using sparklers. This ignites the fuel and air mixture. The ignition of the fuel/air mixture causes the gases inside the combustion chamber to expand and ultimately are forced out both ends of the chamber. This is what produces the thrust. my first design will not produce much thrust as the gas exits in both opposite directions to each other therefore canceling out alot of the thrust.
The way to get around this is to turn one of the ends through 180 degrees so that both ends point in the same direction. Once the fuel/air is ignited it should be possible to turn off the ignition source if your using a spark plug as the the gasses that are racing out towards the end of the pulse jet cause a low pressure area in the combustion area. Eventually this gets bigger and starts to pull back on the exiting gases down the longest end of the pulsejet and also sucks in fresh air for the next cycle.
As the hot gases are pulled back they collide with the new fuel air mixture which compresses it. Once a certain point is reached ignition occurs automatically and the cycle starts again. As there are no valves this in theory could go on forever as there are no valves to get destroyed. Even when the two ends of a valveless pulsejet are pointing in the same direction the thrust is still roughly half of that of a similar sized engine, probably due to the lack of any sort of valving which reduces the amount at which the fuel/air can be mixed prior ignition.
There are designs for valveless pulsejets that seem to solve this problem and for more infomation you can download a file from my downloads page describing the new engine. Another way of getting round the comparatively low thrust levels for a valveless pulsejet is to add a thrust augmentor. This works by mixing cold air with the exhaust gases, this heats up the cold air rapidly and causes it to expand adding to the amount of mass being thrown out of the pulsejet and increasing thrust. This method is good because it requires no extra fuel for the increase in thrust.
Why?
As far as i can tell in any pulsejet engine the air inside the pulsejet acts kind of like an elastic band. When the pressure is high enough the mixture ignites and forces all the air out of the pulsejet. This produces a low pressure area inside the pulsejet and when the pressure falls enogh it is able to suck back and pull in fresh air and fuel.
This process also pulls back some of the heated gases from the exhaust of the last ignition, at the other end fresh air is sucked in. This is why the length of a pulse jet makes so much difference to the opereration of a pulse jet. If the tailpipe is to short the initial blast will blow all the hot gases out of the end of the tailpipe leaving none for the combustion of the next cycle.
If it is too long then the hot gases may cool down to much to initiate combustion on its return. Getting this part right will make it much easier for your engine to run efficiently or at all.
From what ive learnt from other pulsejet builders ive learned the following which is essential to a working valveless pulsejet:
Tailpipe length - 18 - 22 times bigger than the diameter of tailpipe
Intake area - Intake area should be no more than 1/3 of the tailpipe diameter
Fuel injection - Best injected directly into the middle of the combustion area
The above all depends on many things but as a rough guide they should get an engine that runs if you follow them. I obtained the infomation from experiances builders so if there wrong blame them.
How a pulse jet works
The operation of a pulse jet engine is fairly similar to that of a valveless pulsejet apart from the obvious valves on a traditional pulsejet. They go through the same phases of ignition, expantion, air iintake, compression.
Ignition is pretty much the same, the fuel and air are mixed and then a spark from a spark plug or sparkler ignites the initial mixture and the explosion causes the hot gases to expand but here theres a difference.
Instaed of the gases escaping from both ends the increase in pressure causes the valves on the intake to slam shutand this leaves only one other exit for the gases. this produces thrust at only one end of the pulsejet therefore all the thrust is in one direction without having to do any bending of pipes etc. as soon as the inside of the pulsejet reaches a low enough pressure the fuel and air are drawn in again and hot exhaust gases are pulled back and the fuel air mixture is compressed again before ignition occurs.
This is my own opinion but i think that because of the valves the amount by which the fuel and air is compresses is increased to more than what is achieved in a traditional valveless pulsejet resulting in a stronger ignition. The fuel and air in a pulse jet does not actually explode it blah blah. if the fuel could be made to explode then the pulsejet would be even more useful as an engine.
Bruce Simpsons x-jet produces 6lbs of thrust per square ince of tailpipe are compared to the 4.2lbs in an ordinary well constructed pulsejet. Currently NASA are developing a Pulse Detonation engine as they are called for various different tasks.
Below are some useful things you should know when designing your own pulsejet:
Tailpipe length - 8 - 12 times the diameter of tailpipe
Intake area - 40% - 50% of the total tailpipe area.
The wide part seen on most pulsejet is simply to allow enough room for the valves, a completely straight pulsejet is much better if you can design efficient valves.
Petal shaped valves are good for smaller engines upto 20lbsof thrust but will not last long, although they can be changed very easily
V shaped valves are better for larger engines but are harder to design but do last alot longer. once the valve has been built however you will only need to change the actual valve so initial difficulties are outwayed by increase in performance and life. I will soon be adding a page on the different types of valves showing how to construct them.
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