One of the original goals of the original FANG was to be highly efficient in its final state. To accomplish this I wrote BAGS, a simulation of the internal ballistics of a pneumatic gun. I also wrote a simpler simulation of the external ballistics (ballistic trajectory) of a Nerf dart. This simulations allow me to run loops to look at the performance of a variety of situations.
I also researched efficiency a great deal. There is little information available on the efficiency of a pneumatic gun, and the information that is available generally is misleading or nearly negligible nitpicking. I was surprised that even people who referred to themselves as "efficiency Nazis" only had a rough idea of what is efficient and what is not.
A pneumatic gun is efficient when the user has to exert less energy for the same muzzle velocity. Or, the pneumatic gun could get more shots from a tank of gas. Or both. There are many different types of efficiency.
Most specifically, energy efficiency is simply the ratio of output energy over input energy. For a pneumatic gun, this is the kinetic energy of the projectile at the muzzle divided by the maximum work that can be extracted from the compressed gas. I use the maximum work rather than the energy required to pump the gun because at the simulation phase you generally do not know how efficient your pump is, so this efficiency is only the efficiency of the gun's ability to shoot projectiles.
Typical pneumatic guns might be 10% efficient, if that. Two guns that are promoted as "highly efficient", HEAL and the GBsemi both are about 20% efficient, which is respectable, but nowhere near as high as they could be.
With computer analysis I can theoretically get efficiencies as high as 70%, but doing so would require very high pressures. Efficiencies in the range of 40 to 50% can be achieved for most configurations from the limited modeling I've done.
The primary variables that will make the largest difference in efficiency are gas chamber volume, gas chamber pressure, and barrel length. The relationship between these variables is complicated, however, a simple procedure can find optimal configurations.
Most people are aware that certain guns will have ideal barrel lengths--barrel lengths the efficiency of the system is at a maximum. With normal valves this length occurs where the projectile's acceleration equals zero. With a valve that closes before or when the projectile leaves the barrel, this occurs at different locations, generally, with shorter barrels than normal valves. I will focus only on normal valves here.
Given a desired muzzle velocity and fixing gas chamber volume at a set point, the pressure can be increased until the velocity of a barrel at its ideal length is the desired velocity. The efficiencies of thie configuration is noted The chamber volume then is changed and the loop is run again. This is done repeatedly until a good amount of data is collected.
As this search is an exhaustive one, it can take a long time to run, but the results are very worthwhile.
Generally, this method yields optimal efficiencies at high pressures with low volumes. Without no pilot volume or a negligibly small pilot volume compared against the gas chamber volume, it seems the highest efficiencies occur at the highest pressures. However, with a pilot volume that is comparable to the gas chamber volume, efficiencies have a clear peak due to the effect of the pilot volume.
In general what is energy efficient uses less gas. However, to maximize the number of shots from a gas tank the pressure must be significantly lower than the tank's pressure. Generally the peak efficiency has a plateau so you can make a configuration where all efficiencies are high.
As you can see, there are no general rules to this procedure. The relationships are rather complicated to the point that to say simulation is necessary to produce a highly efficient gun.
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 valve and the projectile is not useless. In fact, a limited amount is beneficial to performance and efficiency.
Intentionally designing the gun so that the barrel is the primary limitation on flow is a good idea. Valves with very high flow coefficients do help performance to a certain extent and you do get diminishing returns. Using a valve that is larger than your barrel would get the maximum achievable flow.
The effects of slower valves (or in more general, slow pressure build up) can be improved to a certain extent through high static friction or some mechanical detent. This approach allows the pressure in the barrel to build more before the dart can move.
With my external ballistics model I can play around with the numbers much like I did for my internal ballistics model. Fixing the range and varying the mass, I can find the required muzzle velocity to get a certain range. From that velocity I can see how much energy is required to get that range. The minimum of this curve is what we are looking for, however, as the internal ballistics change for different projectile configurations too, it is a good idea to feed dart configurations near the optimal into the internal ballistics simulation too just to see if it'll use less energy or gas in total.
I will write more on this subject later.
Eventually I will simulate spring guns to get a good idea of how to improve their efficiency.
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©2007 - 2010 Ben Trettel
Last modified on 2009-02-07 16:00:17.