Happy wrote:So the problem here is the ignition will take to long. Ok ive come up with a soloution. Would it be possible to place a tight fitting concave piece of copper atached to the piston, that has been covered in iron facing the inside of the barrel. Then have the flint placed around the inside of the barrel. So the piston will essentially have a lip that is placed in the barrel and when vented will strike the flint igniting the mix? I know its fiddly and highly improbable but do you have any ideas of making this work?
The problem in both instances is that, even if you get the mix to ignite, you're not allowing the fuel:air mix to remain compressed in a closed-chamber during ignition so the resulting combustion will not reach its full potential. You seem to know a bit about hybrids but you are a little confused about a few aspects of its operation and design, especially with the piston hybrid.
The whole point of a burst disk or a piston valve in a hybrid cannon is to prevent the combustion gases from escaping down the barrel until they have reached the peak of their combustion. We can tell when they reach their combustion peak because that's when the pressure generated by the combustion is the highest. Thus, we set our valves to open at a pressure near this estimated peak combustion pressure.
In a burst disk hybrid cannon, one may have to do a few tests using the cannon as a pneumatic to see at what pressure a chosen piece of materail used as the burst disk will burst at. If we have a 3/4" disk cut from a soda can in a 3/4" union and that bursts when 300PSI is pumped in to the chamber, we can probably assume that 3 or 4 layers would be suitable for a 10x mix as this will result in a peak pressure of about 1000PSI when ignited and contained momentarily.
The same goes for a piston hybrid cannon except instead of fiddling with the number of burst disks we need, we instead alter the pressure in the pilot area behind the piston. Beforehand, one may calculate the forces generated by each fuel:air mix (2x, 3x, 4x, etc) on the front of the piston so we can then find out how much force we need behind the piston so that the piston wont open until the peak combustion pressure has been reached.
Opening the valve at the moment of ignition, or before ignition, is just wasting potential power that could have been utilised had you constructed a valve that doesn't open until the ignited mixes reach peak pressure. Download
HGDT and alter the pressure at which the burst disk will burst and you can see how drastically the performance will drop when you have the disk bursting at too low of a pressure.
edit:
While there is a fair amount of info in the Wiki, you should really trawl through the countless hybrid cannon topics in the 'Hybrid Showcase' section as members may divulge information about their cannon that may not appear elsewhere on the forum unless someone has asked a specific question about it. Such information may be things like what material a person uses for a particular union size and how many layers of that material they use for each mix or they may have a novel method for their ignition system or other things like that.
Other topics to keep an eye out for are the ones in the 'Hybrid Discussion Section' where people are about to build a hybrid but they have some questions they would first like answered. Before I built my first hybrid I found these topics quite useful because sometimes they asked seemingly simple questions that weren't in the Wiki or had been talked about before but the information too ambiguous. However, hybrids have come a long way the past 5 years so we have a lot more information on this forum than we used to and you may find it easier to obtain this information than I did back then.