How does back pressure work?

DocShay

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Hey guys,

Just trying to understand something here about how the pressure comes back into the barrel of the gun after discharging a shot.

Why does this happen? The cartridge ignites...the bullet is fired from the gun...the pressure pushing the bullet comes out behind the bullet - But then..some of it sucks back in? Why would it do that, wouldn't the pressure want to escape and be pushed away from where it was coming from?
 
Hey guys,

Just trying to understand something here about how the pressure comes back into the barrel of the gun after discharging a shot.

Why does this happen? The cartridge ignites...the bullet is fired from the gun...the pressure pushing the bullet comes out behind the bullet - But then..some of it sucks back in? Why would it do that, wouldn't the pressure want to escape and be pushed away from where it was coming from?

From what I understand there is a gas return tube somewhere around 2/3 up the barrel (so as not to siphon off too much pressure). Once the bullet passes the opening in the barrel, the air will fill any space it can (since it is a gas) and some of it goes into the gas return tube. This is engineered so that there will be enough force to recock the rifle by pushing back the bolt so that when you pull the trigger the hammer drops and the bolt goes forward again.


Here is a picture
http://en.wikipedia.org/wiki/File:M16_rifle_Firing_FM_23-9_Fig_2-7.png
 
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From what I understand there is a gas return tube somewhere around 2/3 up the barrel (so as not to siphon off too much pressure). Once the bullet passes the opening in the barrel, the air will fill any space it can (since it is a gas) and some of it goes into the gas return tube. This is engineered so that there will be enough force to recock the rifle by pushing back the bolt so that when you pull the trigger the hammer drops and the bolt goes forward again.

Yeah, I understand that semi-automatic firearms do take advantage of that gas to make it well- semi automatic.

But lets say for a weapon that is not semi-automatic, why does that gas come back into the barrel? Should it not push as far away possible from the barrel as possible?
 
Back pressure is a pretty generic term, For instance, Rice burners will always tell you how much more backpressure the new exhaust they just bought has. Of course, They seem to forget that a 4 stroke engine does very little exhaust scavenging and back pressure on a 4 stroke engine is for the most part useless. As far as back pressure in terms of a firearm goes, I don't believe there is any. The only reason I can think of for there being any back pressure would maybe have to do with the vaccum created in the barrel. Once the bullet leaves the crown there is an opening for air to rush in, Although on a semi-automatic firearm i would imagine the air rushes in through the breech and down the barrel instead of the other way around although i'm no expert. I suppose on a bolt action or any other sealed breech firearm you may have some back pressure come through the barrel but I can't really see it affecting anything other than maybe pulling some powder residue back in with it. I'm sure someone will shoot me to pieces on this but my understanding of gas and thermodynamics is of no use in my profession.
 
http://www.youtube.com/watch?v=WvX6sXs_ZO4&feature=related

This is how an AK-47 system works.

http://www.youtube.com/watch?v=5oHx6qdteFY

A good one with the AR 15


O and about Chimos post, scavenging exhaust gases in a four stroke works unless it has a turbo. Not trying to rip you to pieces.

They build Tri Y and 180 headers to take advantage of the exhaust pulses.


180%20headers.jpg


180 header.


pic.jpg

Tri Y header.


About what DocShay said "But lets say for a weapon that is not semi-automatic, why does that gas come back into the barrel? Should it not push as far away possible from the barrel as possible?"

Do you mean if you have a bolt action and after you fire there is some smoke hanging in the barrel?
 
About what DocShay said "But lets say for a weapon that is not semi-automatic, why does that gas come back into the barrel? Should it not push as far away possible from the barrel as possible?"

Do you mean if you have a bolt action and after you fire there is some smoke hanging in the barrel?

Yeah I suppose that would be a good example - shouldn't all that smoke be blown away instead of just hanging and slowly coming out?
 
Yeah I suppose that would be a good example - shouldn't all that smoke be blown away instead of just hanging and slowly coming out?

I suppose the last little bit never makes it out, but remember that with a bolt action its sealed off pretty good. To push out all of the smoke, you would have to have more air come in. About the vacuum theory. You can use a straw to suck stuff up, but if you plug off the end, you can't suck (vacuum) out anything.

Like are you concerned when you see this or just curious?

I'm no professional, just speculating. Maybe I just made you more confused. :runaway:
 
I never really bought into the whole backpressure craze in cars. I've yet to see any substantial proof other than heresay to it's benefit. All exhaust system's pulse, There is not an infinite amount of combustion's going on within the chambers so therefore the exhaust can only be pushed out in accordance with the engines RPM. I am completely in agreeance that there are "Tuned" exhaust systems that help the exhaust gasses move out of the system in a efficient manner. But what benefit is backpressure? Are you scavenging unburnt gasses? Modern 4 strokes are very efficient and I doubt it's the loss in exhaust flow. Have you ever wondered exactly how much O2 and O2 sensor picks up? Not very much in contrast to the whole grand scheme of things. The best analogy I can give can be likened to that of a water hose. If you have 1 hose with a 12" diameter outlet but very little water pressure, You'll only have a small amount trickle out the end, conversely, If you have the same amount of water pressure coming out of a 1" diameter outlet you will have a very fast stream but you may actually be impeding water flow. There is a good median between the 2, Exhaust systems are no different. The reason you see a whole bunch of fancy twists and turns with say a magnaflow or cherry bomb header is just so that they can increase the length of the overall exhaust system without actually having it stick 4 feet out further from your bumper. Hence a "Tuned" exhaust system. Keep in mind that I don't have much experience building 800HP detroit V8's but I have tinkered quite a bit in my day and I do have a wealth of experience turning 250cc 4 stroke motocross bike's into 40-45HP snarling beasts, So although on a smaller scale, Same theory applies.
 
I never really bought into the whole backpressure craze in cars. I've yet to see any substantial proof other than heresay to it's benefit. All exhaust system's pulse, There is not an infinite amount of combustion's going on within the chambers so therefore the exhaust can only be pushed out in accordance with the engines RPM. I am completely in agreeance that there are "Tuned" exhaust systems that help the exhaust gasses move out of the system in a efficient manner. But what benefit is backpressure? Are you scavenging unburnt gasses? Modern 4 strokes are very efficient and I doubt it's the loss in exhaust flow. Have you ever wondered exactly how much O2 and O2 sensor picks up? Not very much in contrast to the whole grand scheme of things. The best analogy I can give can be likened to that of a water hose. If you have 1 hose with a 12" diameter outlet but very little water pressure, You'll only have a small amount trickle out the end, conversely, If you have the same amount of water pressure coming out of a 1" diameter outlet you will have a very fast stream but you may actually be impeding water flow. There is a good median between the 2, Exhaust systems are no different. The reason you see a whole bunch of fancy twists and turns with say a magnaflow or cherry bomb header is just so that they can increase the length of the overall exhaust system without actually having it stick 4 feet out further from your bumper. Hence a "Tuned" exhaust system. Keep in mind that I don't have much experience building 800HP detroit V8's but I have tinkered quite a bit in my day and I do have a wealth of experience turning 250cc 4 stroke motocross bike's into 40-45HP snarling beasts, So although on a smaller scale, Same theory applies.

They say that wideband O2 sensors are very sensitive. Unless you are extremely rich or lean IIRC. I will take a quote from my Engine Management: Advanced Tuning book, "Many aftermarket EFI systems incorporate more exotic wideband oxygen (WBO2) sensors with an accurate range between 8.0 and 30.0:1 air/fuel (pg 33). It goes on to say you need 0.8-1.3 Bar of exhaust back pressure to work accurately.

I don't think Magnaflow or Cherry Bomb make headers.

Four strokes don't need backpressure, you can run them straight pipe. Back pressure is only used to quiet them down. Two strokes do of course.

Sorry for Hijjacking the thread DocShay.
 
There seems to be a misunderstanding of terminology and use regarding 'backpressure' in this thread.

Check out Gordon P. Blair's work on two stroke engine technology "Basic Design of Two-Stroke Engines" (it is HARDLY basic!!) available through the SAE. Look specifically at pressure wave and particle motion in Chapter 2, Gas Flow Through Two-Stroke Engines, with 'Reflection of Pressure Waves in a Pipe.'

'Backpressure' is actually the arrival of the reflected positive pulse from the end of the exhaust system (or some cross-section change) back to the exhaust port, often tuned to stop the sudden outflow of fresh intake charge from the opposite port, thereby increasing efficiency and power. It has a trapping effect. It is HIGHLY tunable, but dependant on multiple variables. In a two-stroke engine, the often peaky powerband is due to the convergence of all variables to make one particular RPM point the most efficient. This will vary with atmosphere, and a host of other issues as well, so with sensitive instruments, will be seen to not be constant.

'Backpressure' is often also used to refer to 'exhaust restriction' but this ignores the true science of pressure waves as gases exit to atmospheric conditions. It's kind of a simplified model.

'Scavenging' is a similar but different animal. This makes use of the NEGATIVE pressure waves to coax burnt gases from the cylinder, and of course can be tuned as well.

Most people without expensive instrumentation are flying by the seat of the pants when they design these systems by hand. Some people do okay at it. Some.....should just be plumbers....! :D

As to the rifle issue, a complete understanding of combustion is important there too. Research pressure wave analysis of gun shots for internal ballistics. This is the study of what happens in the barrel. External ballistics is the study of what happens when the bullet is in flight, and is what all of us deal with every day to make a good shot.

Have fun fellas! ;)
 
There seems to be a misunderstanding of terminology and use regarding 'backpressure' in this thread.

Check out Gordon P. Blair's work on two stroke engine technology "Basic Design of Two-Stroke Engines" (it is HARDLY basic!!) available through the SAE. Look specifically at pressure wave and particle motion in Chapter 2, Gas Flow Through Two-Stroke Engines, with 'Reflection of Pressure Waves in a Pipe.'

'Backpressure' is actually the arrival of the reflected positive pulse from the end of the exhaust system (or some cross-section change) back to the exhaust port, often tuned to stop the sudden outflow of fresh intake charge from the opposite port, thereby increasing efficiency and power. It has a trapping effect. It is HIGHLY tunable, but dependant on multiple variables. In a two-stroke engine, the often peaky powerband is due to the convergence of all variables to make one particular RPM point the most efficient. This will vary with atmosphere, and a host of other issues as well, so with sensitive instruments, will be seen to not be constant.

'Backpressure' is often also used to refer to 'exhaust restriction' but this ignores the true science of pressure waves as gases exit to atmospheric conditions. It's kind of a simplified model.

'Scavenging' is a similar but different animal. This makes use of the NEGATIVE pressure waves to coax burnt gases from the cylinder, and of course can be tuned as well.

Most people without expensive instrumentation are flying by the seat of the pants when they design these systems by hand. Some people do okay at it. Some.....should just be plumbers....! :D

As to the rifle issue, a complete understanding of combustion is important there too. Research pressure wave analysis of gun shots for internal ballistics. This is the study of what happens in the barrel. External ballistics is the study of what happens when the bullet is in flight, and is what all of us deal with every day to make a good shot.

Have fun fellas! ;)

Through my experience, Backpressure and scavenging, Although different events within themselves are none the less related and contribute to increasing the efficiency of a 2 stroke engine. The positive pulse created is what keeps the denser fuel/air molecules in the cylinder for the next combustion cycle. One could not exist without the other, Although you could throw an expansion chamber on a 2 stroke engine, if your intent is not to scavenge unburnt gasses than it's really just an expensive skidplate. I realise that backpressure can help to quiet down a 4 stroke engine but what I meant was I never bought the backpressure craze in regards to increasing performance. The harley crowd is notorious for perpetuating the back pressure myth, Everytime I talk to an MMI or similar graduate about what benefit the fancy exhaust on their new 1200 or w/e is besides letting everyone else on the road know where they are, All I ever hear is "Back pressure". A straight pipe would work but again it may not be "Tuned". I merely used magnaflow and cherry bomb as examples, AFAIK they are the big players in the aftermarket industry, But again YMMV as I really don't play with anything bigger than 450cc's anymore. In regards to the O2 sensor statement I was moreso referring to how much oxygen is present after combustion within a 4 stroke engine. Modern 4 strokes are very efficient and I can't see the possible benefit to scavenging what little unburnt gasses exist outside the chamber.
 
It's called Physics. Not just a rule. It's a Law. Every action has an equal and opposite reaction.
"...some of it sucks back in?..." Nope. The 'pressure' goes in every direction. The bullet gets pushed out of the cartridge and down the barrel by combustion gases. In a gas operated semi-auto, some of the gases are bled off(not always 2/3's down the barrel) and are sent into a gas tube to operate the action. The rest of the gases go out of the barrel.
In a bolt/lever/single shot action, none of the gases are used, but the forces still exist. Said forces coming back is recoil.
 
Well, Their is always theoretical physics, But that is for another thread. I'm still waiting for someone to shoot holes in my "Vaccum" theory. FWIW, I've never heard of backpressure in regards to firearms. I always thought that the remaining powder was dispersed in every direction past the crown but now that I think about it, it could exist.
 
"...never heard of backpressure in regards to firearms..." Agreed. I interpreted the post as asking about how recoil works.
 
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