Login    Register
User Information
Username:
Password:
We are a free and open
community, all are welcome.
Click here to Register
Sponsored
Who is online

In total there are 79 users online :: 4 registered, 0 hidden and 75 guests


Most users ever online was 155 on Mon Aug 15, 2016 1:40 am

Registered users: Bing [Bot], Google [Bot], Google Adsense [Bot], Yahoo [Bot] based on users active over the past 5 minutes

The Team
Administrators
Global Moderators
global_moderators.png CS

Pneumatic gun efficiency

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
Sponsored 
  • Author
    Message

Unread postAuthor: btrettel » Thu Dec 25, 2008 4:03 pm

I need to qualify my statement. My modeling has focused on small air guns so it would be premature to state that valves that close early are inefficient for all guns. At least on small air guns (with a reasonably long barrel) the gun will be pulling a vacuum due to the momentum of the projectile unless you have an oversized chamber. I have no idea if this is true on larger guns.

The valve is great if your pressure is limited. But, for small guns you pressure would have to be limited to about 40 psig before you'll see a benefit. At about 30 psi the benefit is very clear.

I think closing the valve earlier than when the projectile leaves might result in better air and energy efficiency so I'm going to investigate that after I switch my current model to optimized air mass rather than energy efficiency. I'm sure I'll focus on energy efficiency in the future, but the work of compressing the gas has already been done for me so there's no reason to. It took me long enough to see that!

Edit: Taking a look at the results at fairly large chambers, to me it seems that the HEAR valve is a good idea primarily if you don't want to bother doing the analysis I'm doing and are limited in pressure. If you have a larger tank and regulate down to a low pressure, the HEAR valve will be remarkably efficient. Just make the air chamber larger than you think you might need and don't worry because the HEAR valve limits the total air that will escape.
  • 0


btrettel
Major
Major
 
Posts: 380
Joined: Sun Feb 03, 2008 4:40 pm
Location: Maryland
Reputation: 0

Unread postAuthor: btrettel » Sun Jan 04, 2009 11:21 am

I've run a good deal more simulations as I improved my computer model, so I have a pretty good idea of what improves efficiency and what doesn't. I'll explain what I do know in brief below. Let me make it clear though that most of what I know comes from my model, which is close to GGDT in the comparisons I've made, but is not as robust as GGDT and may have problems I am not aware of.

Most of what I'll say below is for energy efficiency, not gas efficiency. In general, what helps energy efficiency helps gas efficiency. The exception is pressure, as I'll explain below.

Using higher flow valves, reducing pilot volume, reducing valve opening time, having straight and unobstructed flow paths, etc., are obvious. Note that saturation points exist for most things, so stop at what practically limits you.

Dead space between the projectile and the valve is not worthless. In fact, a certain amount is beneficial to efficiency.

Fixing everything else, what make the most difference in efficiency is gas chamber volume and its corresponding pressure. I can run loops of my simulation to find configurations that get a certain muzzle energy. The highest energy efficiencies are obtained at moderate pressure with a moderate volume. What is moderate obviously depends on the configuration. In terms of air efficiency, the highest pressure you can operate at with the smallest volume is ideal.

Let me reiterate that there is an optimal pressure for every optimal volume. Slight deviation obviously won't make too much of a difference, but if you operate at twice the optimal pressure you are seriously hurting your efficiency. Let me also reiterate that this makes the most difference in general, so efficiency Nazis should pay attention.

At first I thought longer barrels would be more efficient to allow for more expansion, but fixing muzzle energy, changes in efficiency due to barrel length are nearly negligible. In general there's a slight increase with shorter barrels, likely due to friction losses, but I wouldn't worry much about it because you're sacrificing accuracy for barely any efficiency.

The HEAR valve (like on clide's GBSemi) can help efficiency, however, it does not always help efficiency. I can not be more clear about this: do some computer modeling to figure out if it will be beneficial. In some cases the benefit is negligible. In some cases you'll get nearly as much benefit by using a valve without any pilot volume. And if the projectile is pulling a vacuum due to its momentum, the HEAR valve hurts efficiency. However, you don't want that because it means your acceleration goes negative at some point, so you can cut the barrel length. With all this being said, when you design the system around a HEAR valve, it can make a big difference in efficiency.

Also, just as a note, my efficiency is based on the maximum work an isentropic process could do decompressing the stored gas to atmospheric pressure. In my research, I have seen some people use pressure multiplied times volume to calculate the stored energy in the chamber, often simply because it produces the correct units. That is very incorrect and it underestimates the total energy stored for most configurations, increasing the calculated efficiency.

With projectile efficiency, most of the discussion I've seen focused on reducing drag, which is of primary important, but other things can be done. I've written a simple external ballistic model as a companion to my internal ballistic model to study what will affect projectile efficiency. Choosing an optimal mass can and will dramatically cut the required kinetic energy to get a certain range. The only way to find the optimal mass is through empirical testing, simulation, or both. In general it is better to have a projectile that is heavier than it should be than lighter, because lighter projectiles require much more kinetic energy. I would suggest making the projectiles heavier than testing or simulation suggests for this reason.

If you are looking to use the least amount of energy to get a certain range, it might be worthwhile to ignore what is necessarily the most "efficient" and just use what seems to require the least energy. Combining my two models should offer some insight into this.

There is much more I could cover but I'll keep it somewhat brief. I'm open to suggestions, questions, or to investigate other changes. I'm waiting for a McMaster-Carr order to start looking more at empirical data so I might revise this post in the future.

Edit:

My model has been revised substantially to the point where it is within 2% of GGDT for "equivalent" inputs and subsonic velocities. I can now continue to make observations on efficiency.

Previously I had stated that the best energy efficiency is achieved with moderate volume chambers and moderate pressures. This statement may not be accurate. As I've looked more closely into what truly is going on, it seems that the moderate values are determined primarily by barrel length. So in reality these "moderate" figures really just perform optimally near the ideal barrel length for that configuration. Not fixing barrel length, what specific difference varying pressure and volume makes at this time is unknown, at least to me.

I'll be looking deeply into this over the next few months. It's a very complicated subject and luckily for me, I find it interesting.

I don't like talking to myself but I think others might find this information useful or interesting. So I'll keep doing it if I have something to day.
  • 0

All spud gun related projects are currently on hold.

btrettel
Major
Major
 
Posts: 380
Joined: Sun Feb 03, 2008 4:40 pm
Location: Maryland
Reputation: 0

Previous

Return to Pneumatic Cannon Discussion

Who is online

Registered users: Bing [Bot], Google [Bot], Google Adsense [Bot], Yahoo [Bot]

Reputation System ©'