Being bored, I decided to play around with an idea of mine.  I have an RWS (Diana) Model 48 air rifle in 0.177" caliber.  For those that aren't familiar, this is a spring-air powerplant which with a single cock, compresses a piston-spring.  When fired, the piston flies forward, compressing air behind a pellet.  Standard 8 grain pellets move at about 1100 FPS (verified with chronograph).



First of all, using these numbers, the average pressure on the pellet is about 700 PSI.  Not bad, a bit more than the compression in a good diesel engine.  Since the spring and piston are grease lubed, a little of this seeps into the compression chamber where it burns.  Most springers will "diesel" if oil is in the compression chamber and most rely on a good bit of this natural dieseling to get their power.  Yes, they SMELL like a diesel engine and are just as dirty.



So my brainstorm is how much could this theoretically boost velocity?



So I warmed up the calculator, knowing that a gallon of diesel fuel has about 135,000 BTUs of energy and the compression chamber of this rifle is about 3.8 cubic inches of volume.  This means there is about 74 milligrams of air, meaning the air could support combustion about 1/14th of that in diesel or 5.2 milligrams of diesel.  Sounds trivial?  Not really.



Each gallon of diesel is about 7.1 pounds per gallon.  Or 3.22 million milligrams so a gallon would fuel about 620,000 firings.  How much energy in 5.2 milligrams?  About 0.218 BTU.  Which is about 170 foot-pounds, interesting  How does that relate to smokeless powder?  That is about 200-220 foot-pounds per grain which means rifles are horribly inefficient.  If the 5.56 uses 26 grains to generate 1200 foot-pounds of energy, that makes a rifle about 23% efficient in converting the chemical energy into bullet energy.  I should be able to do at least that so figure on an additional 40 foot-pounds of energy to that pellet....mind you, without intentional dieseling, it already has 19 foot pounds of muzzle energy.  59 foot-pounds of energy in that little pellet would be amazing.  Over 1800 FPS!!!



This field is ripe for development.  Imagine a rifle which uses a tiny bit of diesel (or other similar fuel) and AIR.  The weight savings would be huge, the ammo cost would be tiny.  And since there is no extraction/ejection/chamber temperature issues, it could be easily done full auto.  And since it has no "fixed ammo", no BATFE.



Definitely fappable.

Where would you hang the chrome chick mudflap?

Neat!

I have always wondered why a gun couldn't be made to use a gas like acetylene as a propellant. Perhaps a piezoelectric ignition...





ETA:



Your idea would be great for a sub gun, but anything requiring accuracy would be harder to achieve because of differing air density at different altitudes and temperatures.



Fixed ammo has it's own oxygen supply, so air density doesn't affect the combustion.

diesel+airsoft type rapidfire goodness+60ish grains of copper coated candy pills= win!

propane injection, i hear its like nitrous for diesels

If the ATF does not regulate it, CARB would

There was a gun many moons ago written up in some airgun digest or a beeman publication that features a fuel source just like you mentioned, I think it was german in origin. I'll look to see if I still have that book,

Is it a rifle that uses an explosive charge to propel the projectile? That answer would be "yes".

But it wouldn't work in space.

Acetylene sucks.  The reason springers can (and do) get a bit of push from diesel is compression of the air at the time of firing.  When combustible mixtures are compressed, they become faster burning and therefore, more efficient.  Acetylene is a very poor fuel as it likes to decompose on its own which limits the pressure it can be stored under.  Most bottled welding gas cylinders for acetylene have it stored dissolved in acetone/dimethylforamide much like CO2 in a bottle of soda.  30 PSI in such cases, 15 PSI without the acetone.  And acetylene will break a diesel engine if as little as 1% of the intake air is acetylene
.  Something all diesel owners should know.  Acetylene is also poor in energy density, thanks to the 1:1 C-H ratio and that nasty triple bond between the carbons.



Ideal diesel fuels have low autoignition temperatures (unlike high octane gasolines which have HIGH autoignition temps) and are fully saturated.  





 

BATFE defines a firearm that uses FIXED ammunition.  Nothing about fire/explosion as muzzle loaders are not firearms under BATFE.  Nothing about smokeless powder since Savage made a muzzle loader which used smokeless.





 

A SCUBA cylinder could provide the air.



 

The injectors for a small diesel engine (500 cc cylinder displacement) are at their lower limit of atomization at 5 mg/stroke.  It would take a much smaller injector to keep the fire in the barrel and not have it backfire into the compression chamber.  The gears in my fervent mind have been turning for a while, I have the backfire problem solved.  I think
.



 

tagged for reading for later...

interesting stuff!

Full of win. You could probably make some money off of this.

This is really interesting.

Diesel and guns, what's not to like.

I like the idea, now just put up the capital and get it to market.

Would the chamber need to be reinforced for the increased pressure?

Well, the concept is sound.  But for full auto, I have a BIG issue with the back work ratio.  Typical springer forces are huge and amount to about 70 foot-pounds of energy in the spring.  Yes, the energy of the system is 70 foot pounds, the energy of the pellet is 19 foot pounds.  A bit better than the 23% efficiency of a typical rifle but there is some dieseling already present.



Yes, even a diesel engine has a back work ratio yet they function perfectly in cycles
.  There is a lot of waste heat in these cycles.



 

Check out the new butane powered nail guns, similar sort of operation with what you are thinking.


BTW I made a mapp gas powered blow gun a couple years ago. It consisted of a pop can taped to a piece of 1/2 inch copper pipe. I used a mapp gas trigger torch to first fill and then ignite the charge. It would push my homemade darts thru 1/2 plywood.

Yes, if combustion were in the compression chamber, it would be very hard on the piston given the typical seals are Nylon.  That would never work because the piston would be a huge parasite to the energy produced and it could possibly self-cock at the very least.  Not good for bullet energy.



I have a good concept for a single shot.  Full auto?  That will take time.



 

Diesel could be regarded as a "propellant" - Which means that you now have a firearm (as defined by BATFE)

Dangerous toys.  But that is an atmospheric pressure gun which only works because of the rapidly burning MAPP.  



 

Not even.  Then muzzle loaders would be firearms under Federal law.  The definition of a firearm per BATFE is the use of fixed ammunition.  Even cap and ball revolvers are not firearms, unless you live in oppressed states, mostly concentrated in God-forsaken Yankee-land
.



 

Couldn't you do what electronics does for airsoft and use that for a full auto rig?

It would be an energy hog.  Typical airsoft ammo is 1/4 gram, about 4 grains.  A high velocity is 400 FPS.  About 1.5 foot-pounds of energy meaning you need about 6 foot-pounds of energy in the air system.  Spitting out 900 rounds per minute, this is about 125 Watts which is easily done by a 12 volt electric motor driven by small rechargeable battery like 8 Sanyo FAUP cells
 

Sign me up for interested!

I'm happy with the power of "Adult Air Rifles" now, I never thought a pellet would drop a cottontail in one shot when I was a kid.

Yeah, the first possum I dropped with my 48 really opened my eyes.  A pellet at 1800 FPS would be devastating.  
 

Will a rifle with diesel be considered gas operated?....

The "diesel" air rifle was the Weirauch Barrakuda (German spelling may be different...) made for a couple years during the 1970's.

It was a plain spring-piston barrel-cock rifle that had a gas-injection setup added onto the outside; it looks pretty clunky really. The fuel source was small medical glass ampules of ether.

As I recall it would shoot .177 pellets at around 1400 FPS max, which was about all that the skirt of airgun pellets could reliably take.

Not many made then, even fewer around now. The glass ether ampules are no longer made at all, AFAIK.... It is considered a MAJOR collector's item. The reasons claimed for stopping production vary, but people who owned and shot them at the time (with ether) say that the pellets simply weren't very accurate at such high speeds.
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Seems like the guts of a manual pump white gas camping stove could be used as an aerosol tank and then be "valved" into the chamber behind the pellet.

Don't assume airsoft is limited to 400fps

I had 26gr projs going over 600fps before my mildly upgraded P90 broke

it would draw blood at 35 yards on bare skin

I'd think with a better starting place - stronger gearbox - you could get to 1K fps pretty easily

Not that hard with the spring sniper rifles from what I read

hmm that makes me wonder

an example Link

Try putting a primer in after the pellet.  

Trying to remember where I have seen a really small mechanical injector that would work. Maybe an old Bosch gas injector. Use a closed type system with a regulator to keep the rail pressure constant.The piston stroke could be used to charge the fuel pressure by pumping it up, similar to a fuel injector testor we use. (think Daisey powerline) All the components could be housed in the stock and forearm.

Or you could get fancy and use a HUEY type system that uses constant hydraulic pressure to charge the fuel pressure via a piston in the injector itself, then a trigger could be rigged to trip the injector piston to fire. You would only need to pump the thing occasionally to keep the hydraulic pressure up, use an accumulator to store the energy.

I used to kill rabbits with a Crossman 760 firing BBs.

Every time you post, you sound like you could be either one of my two good friends who also live in CO.  Both engineers who come up with wacky shit that, after some examination, is never so much wacky as "Holy shit, that could actually work!"

http://www.pyramydair.com/blog/2006/05/weihrauch-barakuda-el54-ether.html

http://en.wikipedia.org/wiki/Tippmann_C-3

All you guys can use your fancy pants diesel powered pellet guns, I'll use a .22.
Or in this case to match "ballistics", a .17 mach 2.  

kalifornia will require you to register it as a light truck

I remember somebody made a air gun that injected a bit of eather (sp) into the chamber to cause dieseling

were supposed to use a special pellet as I recall

Off to the garage to experiment with a cheap chinese air rifle, a can of WD-40 and a chrono.

I'll have to tag this thread. I'm not an air-rifle fan, but the physics behind this interest me.

Keith, thanks for running the numbers.  I have been thinking along those same line for a while, but from what I have read (and you confirmed it also) that the seals will not take the abuse.  Now a just a little bit of diesel in the bottom of the pellet skirt might increase the velocities without damaging the seal.  You could not use anywhere near the amount  that the volume of air will burn, however.

Now what about a one way valve at the exhaust port.  You would lose a bit of air volume, but it would keep the pressure contained in the barrel.  Some calculations would need to be made for the lost air volume and some assumptions would also be required of what the pressure would be when the burning diesel exceeds the pressure in front of the piston.