Pneumatic / Combustion Performance Disconnect


Postby SPG » Sun Oct 01, 2006 10:39 am

In our thought experiment we've got going on, I'm wondering how much of a sticking point to getting the most out of a combustion or a hybrid our potato is?

The reason I'm thinking this is because a potato is a relatively low friction bit of ammo, and so as people have pointed out it can be pushed out by the increasing pressure building up in the combustion chamber, before peak pressure has reached. Now as DR has pointed out the obvious way round is a burst disk, but as he's pointed out the very act of rupturing the burst disc takes energy, which is robbed for propelling the spud.

So a couple of thoughts.

First I've got a Baikal MP651 CO2 pistol, it'll fire either BBs or pellets down its rifled barrel, but the speed is lower for BBs than for pellets, and it's pretty obvious looking at the two that the BBs are a looser fit, have less friction and that this is connected to the reduced power.

Second thought, a springer air rifle. When I got my first I thought, like I'm sure a lot of people do that the piston coming down the chamber pushed the air and pellet out the barrel. Of course it's none of that, the piston compresses the air, the rapid compression heats the air which further increases pressure, and all of this takes place before the pellet has even started moving in the barrel at all. In fact such is the pressure in a springer's chamber that dieselling can occur, and that the piston has actually started to travel back along the chamber (compressing the spring) before the pellet has started moving.

So what's the point Alex? Well I'm wondering if the way to improve combustion performance is actually to improve the barrel and projectile design. So first off a decent rifled barrel which will actually "cut" into our projectile - this will increase "stiction" and so prevent the projectile coming out with the combustion half cooked. Secondly a choked breech. Might be as simple as a muzzle knife cut on the inside of the tube - anything so that to push the projectile down the barrel you first have to compress it - again increased pressure is needed to get it moving. Thirdly, perhaps some form of hop-up as used by the airsoft people. And I'm sure there's plenty of other ideas.

In all of them the idea is to give the "burst-disc effect" without actually needing a burst disc. I'ms ure a series of simple experiments could be done, testing various options, by loading them into an un-burstdisced hybrid, connecting that straight to a compressor (no propane involved) and then slowly increasing the chamber pressure to see when the projectile started moving.

Just my tuppenyworth before we get into frantic jet building.
SPG
 
Posts: 983
Joined: Mon Jan 12, 2004 4:51 am
Location: France
 

Postby GalFisk » Mon Oct 02, 2006 10:56 am

Wouldn't any 'burst disk effect' result in the same loss of energy as an actual burst disk? In addition, the projectile will probably have more dynamic friction leading to further energy loss. It will improve performance in high C:B guns though, which I'm pretty sure a spring gun is. A high friction projectile will also promote dieseling, but that's not of any use for combustions.
If you model a combustion gun in GGDT and slowly rise the chamber pressure, at what pressure is the performance equal to that of the actual combustion gun? Is this a useful experiment?
GalFisk
 
Posts: 2153
Joined: Tue Jul 29, 2003 5:27 am
Location: Sweden
 

Postby SPG » Mon Oct 02, 2006 12:09 pm

Galfisk I don't know abou the energy "loss" that goes into bursting a disc, and yes of course there's be losses due to friction in the barrel. I suppose we could build a simple test rig with with a normal barrel, and choked barrel and a rifled barrel to see if there were differences.

Running with the spring airgun idea, I'm not certain on C:B ratios, but thinking for a second. When the gun's cocked we've got a "chamber" at normal atmospheric pressure (14.7psi, but I'm going to round it up to 15 just for ease). As the piston is half way through it's travel the pressure is about 30psi, three quarters of the way through it's at 60psi, and 7/8th of the way through we're at roughly 120psi (which is about what we run a pneumatic on).

Very rough measurements on my air rifle give an 18" x 0.22"ID barrel, and a 6"x1" ID chamber size

So at normal atmospheric pressure

barrel volume = 0.68 ci
chamber volume = 3.93

And at 120psi (actually 117.6 psi)

barrel volume = 0.68 ci
chamber volume = 0.49 ci

So our C:B ratio is 0.72:1.

But of course this doesn't take into account any off the heating effects of compressing air that quickly. And that's unfortunately beyond me.

And then I'm not certain after all that whether we can relate it to combusting propane and air at all, apart from the fact that in both cases we have a hot compressed "propellant" gas.
SPG
 
Posts: 983
Joined: Mon Jan 12, 2004 4:51 am
Location: France
 

Postby jimmy » Mon Oct 02, 2006 1:54 pm

D_Hall: Now I get it. The jet is supposed to act sort of like a very large number of spark gaps. Instead of starting with a single small spherical flame front it is supposed to generate a very large initial flame front due to the (nearly) instantaneous appearance of a burning cylinder of gas the length of the combustion chamber.

A quick question, what is the pressure propagation rate in a cylinder? I know the flame front propagation starts slow and accelerates as the pressure and temperature rises (since the rate of combustion increases with both pressure and temperature). But how fast does the pressure wave actually propagate? I would think that in a chamber without restriction that the pressure wave would propagate at the speed of sound. Is that about right?
<pre>--------------------------------------</pre><b>Efficiency of a typical combustion spudgun:</b>
I've re-calculated the efficiency of the only combustion gun that I have chrono data for.

3"Dx11"L chamber, 2"ID x 30"L barrel (C:B is 0.83:1).
Total chamber volume = 78in<sup>3</sup>=1274cm<sup>3</sup>
4% of the chamber volume as propane is 51cc.
22.4 mole/liter at STP (ignore the correction for STP) gives 2.28mmol propane.
Heat of combustion of propane is 2,219 kJ/mol
(2.28mmol)(2,219KJ/mol)=<u>5.1KJ of energy available from the combusting propane</u>.

Gun fires a 340fps (104m/s) and I shoot half-spuds. Figure volume of spud as a 2"x2" cylinder, gives 6.3in<sup>3</sup> (103ml) spud. Wild ass guess for density of a spud is 1.1g/ml. Gives mass of spud as 113g.

Kinetic energy of spud is (1/2)mv<sup>2</sup>=(1/2)(0.113)(104)<sup>2</sup>=<u>611J energy in spud</u>.

Efficiency of gun is 611J/5.1KJ = 12%.

A "good" heat engine (i.e., a car engine) is 25-30% efficient, so it looks like it might be possible to double the efficiency of a spud gun.
jimmy
 
Posts: 790
Joined: Wed Jul 30, 2003 11:02 am
Location: USA
 

Sponsored

Sponsor
 
 

Postby jimmy » Mon Oct 02, 2006 2:34 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by SPG
...Running with the spring airgun idea, I'm not certain on C:B ratios, but thinking for a second. When the gun's cocked we've got a "chamber" at normal atmospheric pressure (14.7psi, but I'm going to round it up to 15 just for ease). As the piston is half way through it's travel the pressure is about 30psi, three quarters of the way through it's at 60psi, and 7/8th of the way through we're at roughly 120psi (which is about what we run a pneumatic on).

Very rough measurements on my air rifle give an 18" x 0.22"ID barrel, and a 6"x1" ID chamber size

So at normal atmospheric pressure

barrel volume = 0.68 ci
chamber volume = 3.93

And at 120psi (actually 117.6 psi)

barrel volume = 0.68 ci
chamber volume = 0.49 ci

So our C:B ratio is 0.72:1.

But of course this doesn't take into account any off the heating effects of compressing air that quickly. And that's unfortunately beyond me.

And then I'm not certain after all that whether we can relate it to combusting propane and air at all, apart from the fact that in both cases we have a hot compressed "propellant" gas.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
What you are describing sounds like a diesel engine cylinder. Unfortunately, if all you have to work with is PV=nRT you can't solve for the temperature and pressure within the cylinder as a function of the piston position. You know P1,V1,T1 and V2 (n and R are constants) but P2 and T2 are both unknown and interrelated.

To model a diesel cylinder you have to go back to statistical thermodynamics for the auxiliary equations to the ideal gas law.

It has been a couple hundred years since I had statistical thermodynamics but IIRC you need ...

For an isentropic, adiabatic compression:
p2/p1=(v1/v2)<sup>gamma</sup>
where gamma = Cp/Cv and
Cp - Cv = R (R is the gas law constant)
If you treat air as an ideal diatomic molecule then gamma is pretty close to 1.4. The quantity v1/v2 is the "compression ratio" you are working with.

So, at an 8:1 compression ratio (your 7/8 compression is v1/v2=8)
P2/P1=(8)<sup>1.4</sup>=18.4
If P1=15psi then <b>P2=275psi</b> not 120psi !!!

To calculate the change in temperature you use (have to use temperature on an absolute scale like Kelvin, not C or F);
T2/T1=(v1/v2)<sup>(gamma-1)</sup>
T2/T1=8^(1.4-1)=2.3
If T1=295K (71F, 22C) then <b>T2=678K=405C=<u>761F</u></b>
761F is well above the ignition temperature of paper, though there probably isn't enough energy in a small cylinder to actually ignite paper.

(I've been trying to understand, model, and eventually make <a href="http://en.wikipedia.org/wiki/Fire_piston">fire-pistons</a> [:p] )
jimmy
 
Posts: 790
Joined: Wed Jul 30, 2003 11:02 am
Location: USA
 

Postby D_Hall » Tue Oct 03, 2006 12:02 am

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by jimmy
[br]D_Hall: Now I get it. The jet is supposed to act sort of like a very large number of spark gaps. Instead of starting with a single small spherical flame front it is supposed to generate a very large initial flame front due to the (nearly) instantaneous appearance of a burning cylinder of gas the length of the combustion chamber.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Correct.

Although it should also be noted that the jet is sure to induce some turbulence which will in turn bump the flame propagation rate a bit.

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">A quick question, what is the pressure propagation rate in a cylinder? I know the flame front propagation starts slow and accelerates as the pressure and temperature rises (since the rate of combustion increases with both pressure and temperature). But how fast does the pressure wave actually propagate? I would think that in a chamber without restriction that the pressure wave would propagate at the speed of sound. Is that about right?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Yes, the pressure wave propagates at the speed of sound.
D_Hall
 
Posts: 1496
Joined: Sat Aug 09, 2003 6:06 pm
Location: USA
 

Postby man_o_brass » Tue Oct 03, 2006 12:40 pm

Since everyone else is chiming in with their thoughts, here's mine. If you're dealing with a chamber filled with a fixed amount of chemical energy, the whole point of jet ignition and chamber obstructions would be to increase the speed of combustion and thereby boost peak chamber pressures. Since I don't feel like digging out my thermo book to back me up, I'm just going to assume that a larger pressure spike will increase the efficiency of combustion (correct me if I'm wrong, it just makes sense to me). Faster, more efficient combustion would yeild more energy for driving a spud, but in a much shorter time. If the reduction in combustion time was signifigantly greater than the increase in energy, the C:B ratio might need to be adjusted.

At any rate, the most important question on my mind is how much more energy can you get out of a fast, efficient burn than a slower, less efficient one. A .480 Ruger, for instance, uses slow burning powder and a long barrel to achieve high muzzle velocities at low chamber pressures, whereas cartridges like the .454 Casull do just the opposite.
man_o_brass
 
Posts: 218
Joined: Tue Jul 29, 2003 11:54 am
Location: USA
 

Postby boilingleadbath » Tue Oct 03, 2006 3:13 pm

Man o brass, the SAAMI max pressure for those two cartriges is nearly the same, as is the amount of powder used in most of the loads I found.

...not saying the point isn't valid.
I'm sure it's very possible to do this - but to get a higher <i>average pressure</i> through the entire barrel, while keeping the max pressure low, I think you'll be needing to use more powder than you would otherwise.
And that, in our launchers, corresponds to a larger chamber.

(oh, and D Hall... does your book's table tell you how it found the pressures? 'Cause I've been calculating the pressure produced as 86 PSIG when starting at 1 atm)
boilingleadbath
 
Posts: 763
Joined: Mon Mar 14, 2005 5:22 pm
Location: USA
 

Postby Navigator7 » Tue Oct 03, 2006 3:45 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by man_o_brass
[br]Since everyone else is chiming in with their thoughts, here's mine. <hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Ok...
Meat and potatoes guy here....
DR has tried on numerous occasions to convert me to the Dark Side... ;-).... that being combustion.

The idea of hybrids is really cool and all but I want a cannon that fires when I tell it too not when it decides.

So....reading thru the outstanding posts, my hydraulic experience has caused me consider some of DR's points in a different light.

Talking in terms we can all relate...and buy and build.
o PVC pipe....chamber size of choice.
o A Supah Valve.
o A spool valve.
o Barrel of choice

o Build a combustion chamber with a Supah on the end.
o A spool valve is attached to the 1/4" dump on the Supah so that when a predetermined pressure is reached, the spool is moved dumping the Supah.
o A (preferably short) line is run from the chamber to the spool.

The chamber is charged with a fuel/air mix and ignited. This charge will not force the projectile out the barrel cause the Supah is in place.
At ignition, pressure climbs to push the spool to activate the Supah.

Aside from the heat melting components and the fact this is a pretty good bomb.....the key is tuning the combustion to activating the valve at the right time matching the load to the projectile.

So ....a metal chamber and metal Supah.......a hybrid is created without that dang burst disk thingy that gives me the creeps. The Supah could even be used to actuate a chamber sealing valve.

A spring loaded and adjustable waste gate for the time the Supah doesn't fire as planned might be handy.

There is a company out there building QEV valves that operate off the presence of pressure rather than the absence of pressure to activate the valve.
http://www.vibco.com/productcats/productcats.htm

The idea I'm envisioning is a combustion pressure activated hybrid.

Then there this small dingle berry:
I've installed low pressure accumulators on large engines. (Most engine wear occurs at start up)
On start up, engine oil pressure fills the accumulator with oil through a one way electrically controlled switch. Turn the engine off and the oil under pressure is stored in the accumulator.
Turn the engine on and the switch dumps the oil to coat the main bearings and elsewhere with pressurized oil. Before the oil pressure is gone a signal makes it to the starter which turns the engine on, fills the accumulator and starts the process over again.

What about a combustion activated piston/accumulator affair which acts like a over-pressure protection device but also stores a charge of fresh air to refill the combustion chamber with fresh air?
Navigator7
 
Posts: 2635
Joined: Sat Jul 17, 2004 12:00 pm
Location: USA
 

Postby man_o_brass » Wed Oct 04, 2006 2:51 pm

Ok, it wasn't the best analogy, but the gist of it all was this: For a particular fuel, can you get more muzzle velocity out of a slower, longer burn or a faster, quicker burn. Perhaps there is an optimal combustion rate which balances peak pressure with burn time, or maybe it's just "quicker is better." Either way, the gain in muzzle velocity probably wouldn't be proportional to decrease in burn time, and a shorter barrel might be needed for an optimized gun.

The main thing we need right now are some pressure vs. time graphs for different combustion circumstances (normal, jet ignition, ect). With those, someone smarter than me could plug the pressures into something GGDT-ish (surely it factors in temperature drop as the spud travels down the barrel, right?) and get some data.
man_o_brass
 
Posts: 218
Joined: Tue Jul 29, 2003 11:54 am
Location: USA
 
 
PreviousNext

Return to Hybrid Launchers

cron