Does the Gas Port Control Pressure?

[MistWolf]

I'm breaking this off from another thread because Bluedrevolver deserves to have his question answered without the distraction of a spitting contest between myself and Varminterror.

I'm going to start this with this quote from SomeGuy. Not to pick on him, but because he raises questions that should be addressed.

The pressure will never equalize because the dwell is too short to fill the carrier and the gas system is not sealed so its leaking out from both ends of the gas tube and bolt. The pressure reached in the carrier will be a fraction of port pressure so restricting the gas port will change the pressure reached in the carrier. Your right that it does not “regulate” pressure, but it absolutely will change the pressure in the carrier.

You almost have it right. Let me start by asking a couple of questions.
1) If the gas port in the barrel controls the gas pressure in the expansion chamber, how does the gas port know when the right pressure has been reached?

2) What must reprogrammed start the expansion chamber moving at a different pressure level? (Note: Regardless of the pressure of the gas entering the expansion chamber, it always starts moving when operating pressure is reached. Not before. Not after.)

I'm going to leave this until we get a few answers. This is to encourage critical thinking to work out the problem.

 

[Vern Humphrey]

Are you talking about the M14?

In the M14, the gas port vents INSIDE the piston (the expansion chamber) and drives the piston back, taking the gas ports out of alignment. This system COMPENSATES for pressure -- low pressure moves the piston more slowly and allows more gas into the interior. High pressure moves the piston faster and cuts off the gas flow more quickly.

 

[MistWolf]

Heh! I see I didn't make it clear that i'm talking about the AR.

If you start a thread about the M14 gas system, I'd be grateful. I'd love to learn more about the technical details of the M14.

 

[Coal Dragger]

I'm breaking this off from another thread because Bluedrevolver deserves to have his question answered without the distraction of a spitting contest between myself and Varminterror.

I'm going to start this with this quote from SomeGuy. Not to pick on him, but because he raises questions that should be addressed.


You almost have it right. Let me start by asking a couple of questions.
1) If the gas port in the barrel controls the gas pressure in the expansion chamber, how does the gas port know when the right pressure has been reached?

2) What must reprogrammed start the expansion chamber moving at a different pressure level? (Note: Regardless of the pressure of the gas entering the expansion chamber, it always starts moving when operating pressure is reached. Not before. Not after.)

I'm going to leave this until we get a few answers. This is to encourage critical thinking to work out the problem.

Port pressure is always higher the closer the port is to the chamber. Period. Move the port farther down the barrel and the pressure drops.



The port diameter will have to be adjusted to control the volume of gas allowed into the system before the bullet leaves the muzzle given a port location and the average pressure of gas the design is expected to see.

Generally speaking the further down the port is moved the larger the diameter of the port can be without over gassing the system.

 

[Varminterror]

The port does not regulate pressure, or control it in terms of dynamic control, or reactionary control.

The analogy of the equilibration of pressure on both sides of the restriction is not apt, because we are dealing with FLOW. Transient state flow at that.

So consider this - I have an 80gal air bubble reclaimed from a burned out shop air compressor. We use it to fill tractor tires around the farm. I fill it to 125psi, rated for 175psi working. When I hook up to a car tire - with an internal capacity of about 10gal - why doesn’t the new 90gal system rupture my tire? If I allowed it to equilibrate, a closed system, the car tire and tank should reach ~111psi, nearly triple the recommended rolling pressure for the tire. Why doesn’t it rupture? Because the flow from the tank to the tire is restricted, and I break the connection before equilibrium is reached.

Now consider the AR - it’s even more complex than the tire case I described above. The AR is also a transient flow system, but the flow has two “exits,” both of which are dynamic in themselves (the bullet traveling in the bore, expanding the in-bore volume, and the opening bolt/carrier piston). Once the bullet leaves the muzzle and the bolt gas rings breech the carrier vents, the system then becomes open and equilized state is atmospheric.

Realize for a moment, orifice plate flow meters, gas letdown stations, and any number of other systems operating every second of every day rely upon mechanical energy loss due to orifice restrictions.

Also consider this: if the pressure were NOT dependent upon orifice size, an adjustable gas block could not solve an overgassed AR problem. If orifice size didn’t matter, it would ONLY be gas port length which matters. Can’t get volumetric flow without pressure, and if you’re affecting mass transfer - volumetric flow - you’re affecting pressure.

When you inhibit mass flow - aka shrink the door the gas can pass through - you inhibit volume flow. When you inhibit volumetric flow, you inhibit the rate of pressure equilibration.

Viscous losses and turbulent expansion losses in mechanical energy are critical. You CAN make a hose long enough you cannot blow through it. An orifice restriction is like shrinking all of the losses occuring in that hose down to happen in a very, very short distance.

So consider my air bubble again. Say I have that 80 gal at 125psi and I have that normal 3/8” air hose mounted, but on the same valve, I have a 2” nozzle open to the atmosphere. I press the head against my valve stem, the air pressure has a choice to flow into the tire through the 3/8” hose and the head and the valve stem, or vent to the atmosphere. Do you think my tire will explode this time if I leave the head connected and allow the system to equilibrate?

As I said in the other thread, all of this is freshman level engineering school stuff. Get your mind out of an equilibrium state closed system, and the AR becomes really interesting.

 

[Llama Bob]

The size of the port absolutely impacts pressure at the bolt carrier. Not only can you see this with an adjustable gas port, but it's a general principle of flowing gasses/fluids. The smaller the constriction, the more pressure drop there is.

 

[MistWolf]

So consider this - I have an 80gal air bubble reclaimed from a burned out shop air compressor. We use it to fill tractor tires around the farm. I fill it to 125psi, rated for 175psi working. When I hook up to a car tire - with an internal capacity of about 10gal - why doesn’t the new 90gal system rupture my tire? If I allowed it to equilibrate, a closed system, the car tire and tank should reach ~111psi, nearly triple the recommended rolling pressure for the tire. Why doesn’t it rupture? Because the flow from the tank to the tire is restricted, and I break the connection before equilibrium is reached.

What would happen if you DON'T break the connection? Pressure in the tire would build until either pressure equalized, or the tire would explode.

Does the gas port/adjustable gas block break the connection in the AR gas system?

 

[Varminterror]

The bullet leaving the bore is the “break.”

 

[Skylerbone]

Does the gas port/adjustable gas block break the connection in the AR gas system?

Unless my thinking is off, there are only three things that can break that loop; the bolt unlocking, the bullet exiting, or, an unintentional rupturing of the barrel. There can be leaks but if the rifle is otherwise in good repair you’re down to one end or the other uncorking.

Regarding your first question, the gas port doesn’t know. Once the pressure spikes enough to overcome the resistance and weight of the spring and carrier it moves, no different than a weight lifter bench pressing (he must exert increasing force until the bar moves). Port size and location was engineered into the original system based on the ammo.

If I understand your second question, nothing will start it moving before it reaches operating pressure. Still, there are factors. Dwell time means there is only so much time allotted to pressurize the system before force ceases to be exerted.

Let’s go back to that weight lifter. We’ve probably all seen someone max out and go to failure before. They begin the lift but cannot complete it before their energy give out. The next guy might use the same amount of weight, press it up smoothly and return it to position. Whether you give him more time exerting the same initial force as the first lifter or whether he is stronger and exerts more force in less time, he overcomes the same load. These two represent undergassed and correctly gassed.

Either I understand some of this or I’m about to feel incredibly stupid. Always willing to learn though.

 

[MistWolf]

The bullet leaving the bore is the “break.”

That's one. The second, as Skylerbone pointed out, is the carrier. When the gas key separates from the gas tube and the exhaust ports open to vent the expansion chamber, excess pressure is dumped. The carrier is a type of poppet valve.

The third is the extraction of the empty case, but by the time that happens, the system is done with the gas and is simply venting.

We can clearly see that breaks in the connection are needed to control the pressure. If the bullet did not break the connection, the barrel would burst. If the carrier didn't break the connection when it moves, pressure in the expansion chamber would be excessive.

We have established that the gas port/AGB does not know when the desired pressure in the expansion chamber has been reached. Therefore, it cannot control the pressure in the expansion chamber.

What gas port/AGB controls is flow and flow determines the time it takes to bring the expansion chamber to operating pressure.

 

[Varminterror]

Extraction happens after the system is already at zero state. The gas key is disengaged from the gas tube by that point, venting into the action, around the bolt, and out the port, and the bullet is gone, again, driving to zero gauge via the muzzle.

 

[Varminterror]

I feel you’ve created a straw man at this point. The original contention in the other thread was your assertion such the gas block does not represent an abatement of pressure. Now you’re changing your objective for the easy win to say the gas block doesn’t control pressure....

Correct, as I stated in that thread, the gas block, gas port, and any other fixed constriction in the gas system cannot CONTROL pressure. But any constriction does represent a loss in mechanical energy, by restricting mass flow - to which you agreed above. A restriction in mass flow in a transient, open, flow system is a restriction of pressure, plain and simple. You can’t have one without the other. The pressure does not have to equilibrate to the bore pressure in the dynamic, real world case, just because folks don’t understand the concept.

 

[Varminterror]

Here’s a thought, another common analogy which illustrates the impact of an orifice on downstream pressure in a FLOW system - how do you whistle? The eddy shedding on the backside of a restrictive orifice (your lips) creates a resonant pressure wave in a flow system, which yields a sound. The mechanical energy of the flow of gas from your lungs is reduced, losing energy to produce the sound wave. Blowing out of your mouth in different positions changes the frequency of the eddy shedding, changes the pitch of the whistle. The tighter the lip, the higher the pitch, meaning the faster the wave oscillation, meaning the sharper the pressure drop on the backside of the orifice. The looser the lip, the lesser the frequency, the lesser the pressure drop. Open the orifice wide, and no pressure restriction exists, and no sound is produced...

 

[someguy2800]

I'm breaking this off from another thread because Bluedrevolver deserves to have his question answered without the distraction of a spitting contest between myself and Varminterror.

I'm going to start this with this quote from SomeGuy. Not to pick on him, but because he raises questions that should be addressed.


You almost have it right. Let me start by asking a couple of questions.
1) If the gas port in the barrel controls the gas pressure in the expansion chamber, how does the gas port know when the right pressure has been reached?

2) What must reprogrammed start the expansion chamber moving at a different pressure level? (Note: Regardless of the pressure of the gas entering the expansion chamber, it always starts moving when operating pressure is reached. Not before. Not after.)

I'm going to leave this until we get a few answers. This is to encourage critical thinking to work out the problem.

They are mechanical devices. They are not “programmed” and they don’t “know” what pressure is. When there is enough force on the carrier to overcome the spring force and friction holding it in place it will move.

 

[243winxb]

An adjustable gas block controls flow/volume, while the pressure remains the same? Same, meaning rise, peak, fall, port pressure curve. PSI?

The location of the port controls when the flow/volume reaches its destination ?

Would this be correct?

 

[someguy2800]

That's one. The second, as Skylerbone pointed out, is the carrier. When the gas key separates from the gas tube and the exhaust ports open to vent the expansion chamber, excess pressure is dumped. The carrier is a type of poppet valve.

The third is the extraction of the empty case, but by the time that happens, the system is done with the gas and is simply venting.

We can clearly see that breaks in the connection are needed to control the pressure. If the bullet did not break the connection, the barrel would burst. If the carrier didn't break the connection when it moves, pressure in the expansion chamber would be excessive.

We have established that the gas port/AGB does not know when the desired pressure in the expansion chamber has been reached. Therefore, it cannot control the pressure in the expansion chamber.

What gas port/AGB controls is flow and flow determines the time it takes to bring the expansion chamber to operating pressure.

The bullet is about 100 yards down range before the key and exhaust ports open. Volume is proportional to pressure. Reduced volume into a certain space results in reduced pressure.

 

[luger fan]

Someone has WAY TOO MUCH time on their hands to worry about this.

 

[Varminterror]

An adjustable gas block controls flow/volume, while the pressure remains the same? Same, meaning rise, peak, fall, port pressure curve. PSI?

The location of the port controls when the flow/volume reaches its destination ?

Would this be correct?

No. You cannot affect flow without affecting velocity, and you cannot affect velocity without affecting pressure.

Such is how airplanes fly.

 

[Varminterror]

Someone has WAY TOO MUCH time on their hands to worry about this.

Some of us do this for a living. It’s about as common as estimating a 20% tip on your meal at a restaurant.

 

[Skylerbone]

Someone has WAY TOO MUCH time on their hands to worry about this.

And that someone should be everyone who owns an AR. That or “he” will be posting a “what’s wrong with my new AR” question.

I should have clarified in my previous post that under normal conditions the bullet IS the break in the system as its movement is far faster than the bolt’s. The only condition under which it would not be is if you have a squib round or previous obstruction preventing exit. Then, and only then, could any other part (burst barrel/burst gas tube, carrier) become the means to atmospheric equilibrium. I was typing when @Varminterror pointed this out. I’m often typing when I should be reading.

 

[MistWolf]

I feel you’ve created a straw man at this point.

If I'm guilty of anything, it's of creating a moving target which wasn't intentional. I haven't had to break this subject down into explainable chunks for a long time and have been struggling to clarify my explanation as I go along. While we are having a disagreement, I'm also trying to understand your side of the discussion.

Correct, as I stated in that thread, the gas block, gas port, and any other fixed constriction in the gas system cannot CONTROL pressure. But any constriction does represent a loss in mechanical energy, by restricting mass flow - to which you agreed above. A restriction in mass flow in a transient, open, flow system is a restriction of pressure, plain and simple. You can’t have one without the other. The pressure does not have to equilibrate to the bore pressure in the dynamic, real world case, just because folks don’t understand the concept.

I agree with this with the clarification that the restriction causes a drop in pressure at the point of restriction. If the downstream side of the restriction were a pressure vessel, pressure would continue to rise until pressures equalized.

In the AR system, pressure builds until the carrier reaches its operating pressure at which time it opens and vents excess pressure. If the carrier didn't vent pressure, pressure in the expansion chamber would build until it was equal to the pressure in the bore and the flow would stop. As the bore continued to vent through the muzzle, the pressure in the bore would drop to less than what was in the expansion chamber and gas tube. Flow would reverse and push gas back through the port to be vented through the muzzle.

The gas port is used to restrict flow to the expansion chamber to keep it pressure from building inside too quickly. It affects the pressure curve- mass and time.

 

[MistWolf]

They are mechanical devices. They are not “programmed” and they don’t “know” what pressure is. When there is enough force on the carrier to overcome the spring force and friction holding it in place it will move.

Mechanical devices are programmed. The diameter of the gas port is chosen because flows a certain amount of fluid over a certain length of time under a certain level of pressure. Gas port diameter is an important part of the AR's programming. So is spring rate of the action spring and the mass of the carrier & buffer

 

[MistWolf]

An adjustable gas block controls flow/volume, while the pressure remains the same? Same, meaning rise, peak, fall, port pressure curve. PSI?

The location of the port controls when the flow/volume reaches its destination ?

Would this be correct?

If I understand your questions correctly, yes, but only at the point it creates a restriction. Keep in mind, the pressure in an AR gas system constantly changes as it rises and falls. However, using the same ammo, the pressure curve is fairly consistent from shot to shot.

 

[MistWolf]

Someone has WAY TOO MUCH time on their hands to worry about this.

Some of us do this for a living. It’s about as common as estimating a 20% tip on your meal at a restaurant.

...and far more important

 

[mcb]

I might have some answers. The best solution/answer would be to instrument an AR and measure chamber pressure, barrel pressure at the gas port and then pressure in the bolt carrier while firing the rifle. Then vary the parameter(s) you guys are arguing about and get some more measurements. Not a trivial experiment to setup but thankfully you all paid your taxes and Uncle same did that experiment for you. The website dtic.mil is a wonderful resource for nearly all technical data from the military research branches that Uncle Sam has deemed acceptable for the proles. Back in 1971 the Internal Ballistics Lab at Aberdeen did such an experiment on an M16A1.

http://www.dtic.mil/dtic/tr/fulltext/u2/731218.pdf

Now this paper gets right down into the weeds as far as the thermal and gas dynamics goes. Its gritty, I spent a week about three years ago grinding through this paper and several others by the author, M.W. Werner and his colleagues trying to creating my own modern version of his simulation in Python (using Sci-Py and Num-Py) but I think most of the questions here in this thread can be answered in a few graphs of the real world data taken as part of the research they did to create a computer model of the M16 gas system.

Page 20 has a nice graph showing the pressure vs time graph of the chamber, gas port, and bolt carrier pressure all overlaid on the same graphe for the standard M16A1

Now how does port diameter effect that carrier pressure?

Page 29 lower graph has the pressure measured in the bolt carrier at 5 different gas port diameters.

There is a whole bunch more data and modeling that goes on in that paper. They look at a few other variable but those two graphs I think answer most of the questions in this thread. Feel free to read the whole paper it's pretty good. I wish the images had scanned better but the report is type written from 1971 and the images are no doubt photo's glued into the report.

ETA a few minor typo's