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2 stroke exhaust theory used on a bbmg

A place to ask general spud cannon related questions.
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2 stroke exhaust theory used on a bbmg

Unread postAuthor: jor2daje » Fri Jun 25, 2010 2:58 pm

Hey guys, I havent been working on any guns in a while but recently I did a bit of reading about 2 stroke exhast and how the different expansion chamber sizes and such have a huge effect on how well a bike runs. Ill attach a quick picture I drew up to help me understand what goes on better.

While this is all fascinating for a 2 stroke engineer, the main part that I found could be usefull in spudgunning is the fact that when the high pressure waves goes from a small diameter to a large diameter, it creates a low pressure wave that travels back toward the small diameter.

In a 2 stroke this serves to pull extra exhaust gases out from the cylinder but in a bbmg with a pulsed air supply I think it may help to pull other bbs toward the barrel.

While Im not sure the same effect would be achieved with simple shop air. However I think this could be investigated quite simply with a highspeed camera, and some sort of pulsed air supply. The expiriment would basically be the same as the one used by the original 2 stroke engineers. Simply cut down the barrel by small amounts until you get the best performance.

Just putting the idea out there to see what you intelligent people think ;) and asking (jsr probably has given this quite a bit of thought) what would be a good way to make a high frequency pulse valve. I was thinking maybe something with a poppet but Im not sure.
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2 stroke exhaust.jpg
Its pretty crude and probably not entirely accurate.
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Unread postAuthor: jackssmirkingrevenge » Fri Jun 25, 2010 3:03 pm

Interesting thoughts, however since we're talking about gases that are pressurised due to heating I'm not sure if the concept would translate well in terms of pneumatics...
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Unread postAuthor: Technician1002 » Fri Jun 25, 2010 5:14 pm

In a 2 stroke, the intake is drawn in the crankcase and compressed as the piston comes down. There is a very short time the transfer from the crankcase to the cylinder is open. It is open for less time then the exhaust is open. Getting a good flow in the short time is what the tuned pipe is for. About the time the transfer port between the crank and chamber opens, the exhaust cycle is well underway. The transfer opens, the fuel is blown and drawn into the cylinder and the transfer closes. The back wave does as shown in your diagram and returns fuel mix to the cylinder.

A spudgun does not have the pressurized air fuel mix blowing into the chamber and a short duration transfer port time, unless you are building it as a linear 2 stroke engine. If you are, then don't add projectiles. It changes the entire dynamic.

A blow back breech on a spring with a transfer port and a carburetor to run like a 2 stoke engine is an interesting idea. I'll have to play with it some. Due to the lack of a valve on the exhaust, I don't think the tuned exhaust will be helpful unless this turns into a full 2 stroke piston hybrid. :twisted:
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Unread postAuthor: nibbler125 » Fri Jun 25, 2010 10:11 pm

You could use a burst disc that would provide a large pulse weather it would be enough i dont know but good luck.
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