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Inspired by this question here regarding the possibility of gasoline firearms.

On a world that is equivalent to the Late Victorian Period technologically where smokeless or even black powder was not discovered, a diesel based firearm system was invented in its stead.

Using a biodiesel as a fuel. The propellant would be sprayed into the "firing chamber" through a mechanical injector which is ignited via a spring powered rod inspired by a fire syringe that heats air through compression to combust a flammable material.

Oxidizer is plain air which does limit the upper velocity as after a certain point adding more fuel would not give any returns.

Due to the lack of any computerization, the mechanical injector has a set amount it sprays so no variation ability to customize velocity is possible.

Overall, the operation of a bolt action rifle (with a dust/water covering) will look similar to this:

  1. Rifle is loaded with ammunition and a container of diesel in the stock
  2. Bolt is pulled back to cock fire rod and introduce air into the chamber
  3. Sliding the bolt forward causes a "metal sleeve" to take up a round and ready it for firing
  4. When the trigger is pulled, the rod moves forward which also causes the mechanical injector to spray into the air
  5. Once the air is air/diesel mix has been heated it ignites, causing the bullet to go forward
  6. Repeat process

My question is:

Would this even work? And if not, why and what modifications can be done, if any, to at least salvage the concept.

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  • $\begingroup$ @Escapeddentalpatient. WHAT?! How did I miss this? $\endgroup$
    – Seraphim
    Feb 16 at 3:43
  • $\begingroup$ Cooking and firework, depends on the size of hydrocarbon chains. $\endgroup$
    – user6760
    Feb 16 at 6:21
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    $\begingroup$ My father invented a novel kind of diesel powered piledriver that looks a bit like what you described. It uses an explosion to push the cylinder up, just like what happens in a motor, but his design makes it look like a bullet coming outta barrel when going up. I'll ask him if something like that can be adapted into a cannon. $\endgroup$ Feb 16 at 22:36
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    $\begingroup$ @Escapeddentalpatient no, he doesn't speak English and I figure the book was only translated to our language much later. Also the thumper doesn't look like the machine he made. And finally, while he had an idea for a kind of piledriver, the principle itself was nothing out of this world. He just made a simple piledriver out of hacking around with motors and pipes. BTW I talked to him and he told me weaponizing that would be both suicidal and inefficient. $\endgroup$ Feb 17 at 16:13
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    $\begingroup$ There were also non-Diesel guns that used liquid explosive from a separate reservoir to the bullet feed. This was not quote so fatal to the user, but they were not a success. The basic cartridge with solid propellant and a percussion explosive was a tough act to beat. $\endgroup$ Feb 17 at 19:19

6 Answers 6

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A Better Design

Overall, the idea of using vaporized gas to fire a bullet is perfectly plausible. I made a sort of version of this when I was a kid. However, the compression/glow plug mechanism required to ignite biodiesel would be too heavy and bulky to make for a good firearm, but this is not really a problem. If you can make biodiesel, then you can presumably make a wide range of other organically derived chemicals like alcohol and hydrogen peroxide too. By adding isopropyl or ethanol alcohol to biodiesel, you can make it combustible without a compression mechanism using a simple sparker. You could also add a liquid oxidizer like Hydrogen Peroxide to help speed up the reaction and ensure a cleaner, more complete burn.

The trick to getting a reasonably powerful shot out of such a system will have to do with the shape of your detonation chamber. Without a lot of compression it means that you need a detonation chamber that works like a shape charge by igniting the vaporized fuel is such a way that the detonation starts off expanding as an increasingly energetic shockwave, and is then constricted, focusing the energy into a small place on the back of your bullet. You do this with a biconical detonation chamber in which you place a sparker at one end and the bullet at the other. Your end result should look something like this.

enter image description here

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  • $\begingroup$ Good thought! There are some issues that gunsmiths would need to work out (such as how to keep the bullet/ball from falling into the chamber), but overall it's a big improvement over the straight chambers I (and probably everybody else) had been envisioning. $\endgroup$ Feb 17 at 1:08
  • $\begingroup$ Interesting design. While I did stipulate a bolt action rifle, I think it may be possible for a blow forward operation to be achievable for the design. Though something tells me that it would be easier to just design a more powerful PCP airgun. $\endgroup$
    – Seraphim
    Feb 19 at 4:41
  • $\begingroup$ @Seraphim The big problem with PCP airguns is that compressed air (and its container) has a lot less energy for its volume, weight, and manufacturing cost than a chemical propellant. Modern PCP guns typically have 1/3rd the muzzle velocity of comparatively expensive and sophisticated firearms. That translates to 9 times less power. While a biodiesel gun may have some disadvantages compared to a nitrocellulose based firearm, it still has enough advantages over a PCP to make since. $\endgroup$
    – Nosajimiki
    Feb 19 at 17:53
  • $\begingroup$ @InHocSigno retaining the slug is not that big of a deal, pellet gun makers already solved this problem a long time ago. You make the slug hollow and the chamber slightly bigger than the barrel or the space behind it. That way it is held in place when loaded, and when you shot, the pellet squashes in just a bit to conform to the barrel. You could even replace the need for an electric spark plug and fuel injector with a single pump system that reloads and injects the fuel, and a spring triggered flint & steel ignition system. That will give you something very much like a bolt action rifle. $\endgroup$
    – Nosajimiki
    Feb 19 at 18:02
  • $\begingroup$ @Seraphim, I've expanded on my diagram to (crudely) show how a lever action loading mechanism could be used to operate the weapon. A lever action is in concept a lot like a bolt action weapon, but makes way more since to be able to apply the torque you would want to mechanically spray your fuel. A bolt action gas repeater could probably be used, if you pair it with an electric fuel injector, but it all depends on how simple of a firearm you are looking for. $\endgroup$
    – Nosajimiki
    Feb 19 at 20:23
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The problem with dieseling as a means of propelling a bullet is that the pre-ignition pressures involved are sufficient to move the bullet down the barrel without the fuel present. Air guns are designed to use mechanical energy only to propel their projectiles, and are not designed to use dieseling as a means of propulsion. The problems with dieseling are that it is difficult to make unintended dieseling add a consistent velocity boost, given that the precise timing of ignition of the fuel in the firing cycle is difficult to predict, and secondly, that air guns are typically not made to withstand the increased temperatures and pressures of dieseling.

That said, dieseling as a deliberately designed means of propelling a bullet is not beyond the bounds of possibility. The main problems would be to add a known quantity of fuel and to prevent the bullet from proceeding down the barrel before the fuel began to combust in order to achieve a predictable muzzle velocity.

Both of these goals could be achieved by use of a cartridge that contains the fuel, the oxidiser, carries the bullet and contains a rupture disc a short distance behind the bullet. The base of the cartridge would be a movable plug that would allow the space in the cartridge to be reduced by the spring-loaded firing bolt moving forward when the trigger was pulled. The abrupt reduction of the cartridge's internal volume would result in the fuel and oxidiser sealed in the cartridge undergoing dieseling. The rupture disk would be made so that if for some reason dieseling did not occur, the disk would not rupture under the pre-ignition air pressure; it would only rupture if dieseling took place.

Such a system would also have the advantage that if a firing bolt was used to make dieseling occur within the cartridge using a built-in pre-sealed single-use plunger, the dieseling of the fuel within would result in the firing bolt being pushed back. With a little more development, if the breech block was not a bolt-action device, locking of the firing bolt into its pre-firing position could unlock the breech bolt and allow residual pressure to propel the entire breech backwards, turning the weapon into a semi-automatic or fully-automatic weapon.

If it was not possible to include a sufficient amount of gaseous oxidiser in the cartridge, it should still be possible to contain a measured amount of fuel behind an easily ruptured membrane, and have the breech penetrate the membrane and inject high pressure air into the cartridge.

The mass of the breech and firing bolt would likely mean a lower rate of fire than modern fulminate-primed solid-propellant weapons with their lighter breeches and bolts, but a firing rate of several rounds per second could likely still be achieved.

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    $\begingroup$ In the late Victorian period, manufacturing precision was borderline for making automatic rifles. Yes, they were available, but it took about another twenty years for them to become reliable. $\endgroup$
    – Mark
    Feb 16 at 21:37
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    $\begingroup$ @Mark, hence the With a little more development, $\endgroup$
    – Monty Wild
    Feb 17 at 6:22
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    $\begingroup$ Going full circle would be turning into automatic weapon an existing diesel engine that was meant to start using a Coffman starter $\endgroup$
    – user35577
    Feb 18 at 0:40
  • $\begingroup$ dieseling is indeed possible with air rifles, and of course there is youtube video of tests over chronographs... $\endgroup$
    – ivanivan
    Feb 18 at 3:11
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Feasible, it's feasible, and pretty unsafe too.

Turning the comment into an answer.

This was tried before in real life:

The EL54 was an early attempt (1954-1981) at achieving magnum power in a spring-piston air rifle. It is an HW 35 with an ether injector attached on the right side of the compression chamber tube. A medical ether ampule was inserted into the device and crushed. Each time the rifle was cocked and loaded, a shot of ether was injected into the compression chamber, where the heat of compression ignited it, raising gas pressure in the compression chamber.

The device was hell on pellets, blowing out their heads and leaving the bodies trapped in the barrel, so round balls were the only recommended ammo. It also blew the leather seals out of the early guns. It was very difficult to get medical ether ampules in the U.S., so the system was never popular. Most of the guns seen today are in new or nearly new condition.

The claim was that the rifle could drive a 15-grain .22 caliber ball to 1,000 f.p.s., but the only publicized test, done by writer W.H.B. Smith in 1957, used a rifle with blown seals, so the results were disappointing. One-thousand feet per second with round balls was smokin’ in the 1950s, but an AirForce Condor would exceed it by at least 150 f.p.s. today.

Forced dieseling is NOT SAFE!

This is for CF-X guy and anyone else who wonders what can be done with a dieseling airgun. It’s all been done before, and the facts are well known. Dieseling destroys spring-piston airguns. They aren’t made to take the pounding of the repeated explosions. The EL54 lasted because people couldn’t get the ether ampules. Yes, there are ways around that, and they have been tried – and they destroy guns, too. Shooters have been injured when their guns blew apart or pieces flew off at high velocity. This is not a road to go down. If you do, a lot is known about what will happen, and none of it is pleasant.

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    $\begingroup$ Upvoting the answer, deleting the comment... $\endgroup$ Feb 16 at 4:37
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    $\begingroup$ I don't think that the point about safety is really relevant at all. It is essentially a warning for people not to use their regular air-guns and try to retrofit them to use diesel in a way they were never designed. It's like saying "filling your pneumatic nailgun with black powder instead of connecting it to a compressor is not safe!" Like duh. This is an anti-idiot warning. In the Asker's scenario, diesel guns are all they've ever known, and there's no law that states we can't simply engineer it to be safe: diesel engines withstand literally millions of explosions without issue or repair. $\endgroup$
    – Dragongeek
    Feb 16 at 10:24
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    $\begingroup$ Indeed. "Wildly unsafe practice that people are doing anyway" is the basis of practically any good adventure story. $\endgroup$ Feb 16 at 20:12
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You could do it but:

To the best of my knowledge in theory its possible but the thing is solid propellants are generally more energy dense than a liquid propellant (fuel) like diesel which are in turn (again generally) more energy dense than a gaseous propellant like say propane.

So if for example you have a firearm with a combustion chamber of volume X your going to be able to pack more energy into it in the form of a solid propellant than you will with a liquid or gaseous propellant. Solid propellant contains its own oxidizer BTW which is a big part of what makes it so energy dense in the first place. No oxygen required!

All this means means even a reasonably good quality (fine grained) black powder is probably going to be a better option. Using diesel your firearms will have a lower muzzle velocity, shorter range and less stopping power than their black powder equivalent unless you scale them up in size significantly. Probably to the point where you end up with a crewed weapon rather than a man portable firearm.

So if your army gets into a fight with a good old bog standard musket equipped army? All things considered your going to be at a considerable disadvantage.

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    $\begingroup$ Diesel engines do not need a spark, neither would this rifle $\endgroup$
    – L.Dutch
    Feb 16 at 4:07
  • $\begingroup$ L.Dutch. Thanks I meant glow plug. It will still need a heat source. for ignition $\endgroup$
    – Mon
    Feb 16 at 4:37
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    $\begingroup$ No, diesel ignites simply because of the heat generated by the compression. $\endgroup$
    – L.Dutch
    Feb 16 at 4:38
  • $\begingroup$ L.Dutch. I'll edit immediately. $\endgroup$
    – Mon
    Feb 16 at 4:39
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    $\begingroup$ You might still want a glow plug or other heater, even if you don't need one. If you tune your rifle to ignite during an average day, it might not have enough compression to ignite if you're ambushed on a cold winter night. $\endgroup$
    – Mark
    Feb 16 at 21:41
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Sure, but it probably doesn't scale.

"Diesel" is a red herring.

Diesel engines are named after the guy who invented them, and their unique feature is that they ignite from compression alone. Diesel fuel is anything you burn in that kind of engine (like peanut oil). A diesel engine is not a great basis for a firearm, and your description doesn't really match a diesel engine either. That's probably why other answers are telling you "no."

But yes, you could make something like you describe work. It would be so finnicky that it's probably not a good idea, and you would likely need machining precise enough that the whole firearm would have to be bespoke, but its doable.

Your basic mechanism is about right. You can have a bolt action drive your mechanism to cycle the action and cock your firing mechanism. Basic firing arrangement is a small combustion chamber sealed by a bullet (Or maybe a sabot? Either way). You can have fuel in the stock; it probably needs to be pressurized. Pulling the trigger opens something roughly like a fuel injector and aerosolizes fuel into the chamber, as the back of the chamber pops forward to compress the mix (And you can totally adjust mix and compression manually, probably by set-screw or possibly by knobs).

After a fraction of a second, something ignites the fuel mix. This will not be compression of the chamber, for all the reasons other answers gave. Instead, you're looking at something like a sparkplug. Firearms have been using sparks forever, so there's not much reason not to do that here too, but you could make a fire syringe work.

Oddly, this design almost demands to be semi-or fully-automatic. You already have a reciprocating piston at the rear of the chamber, which will get kicked back by the combustion. It's a very short step from there to making it re-cock itself, and nearly as short a step to tapping that energy to cycle the next round. Fully auto would need you to tune the cyclic rate, but you would need design work to NOT have at least semi-automatic.

However, getting that to work at all would take some significant tuning and calibration, as well as real precision manufacturing. Clockmaking has already taken off, so the kind of custom, precision manufacture you need is possible, but it's going to need a skilled artisan and everything is going to be custom-fit. Production is maybe scalable with the American Method, but that doesn't really exist at all until after 1900, and doesn't get much foothold in Europe until mid-century. Late Victorian Era, this is probably a bespoke one-off.

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  • $\begingroup$ Wait, so ignition from compression alone would basically be able to be kept going after the first explosion, right? Just transfer the explosive compression partially to the next chamber, with a circular system to vent, reload, aerosolize and ready the chamber.. So all it takes is a vent into the next chamber, before the bullet/piston, who is then hammered down along the pipe, igniting the system. So valves at the endcap, valves at the gasport and voila. Diesel gattling gun at your service.. $\endgroup$
    – Pica
    Feb 21 at 13:35
  • $\begingroup$ You basically need no komplexer mechanism then a one way valve and gun port closure. And a starter, that allows for the first explosion to start the reaction, the bullet itself is the pistoin. Man, this thing could be very pure, very simple. You could even pre-load the next bullet at the back of the chamber, then blowing it forward as the bullet leaves the gun.. $\endgroup$
    – Pica
    Feb 21 at 13:39
  • $\begingroup$ @pica Exactly. Except you probably want a spark or similar ignition source to cut down on the required precision. $\endgroup$
    – fectin
    Feb 22 at 1:20
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Possible? Yes.

You'd need to have some way to make sure the bullet doesn't go flying out the barrel before the diesel ignites (perhaps rifling without a Minié ball?), but you could do it.

Feasible? No.

This system sounds like a huge pain. Here's some problems I can think of off the top of my head:

  • Soldiers would have to haul around large amounts of nasty-smelling liquid with them, which would inevitably spill and get everywhere.
  • All the moving parts would be a nightmare to maintenance (even compared to black powder firearms).
  • The plungers (among other things) would break often. L.Dutch goes into this problem in more detail.
  • You'd have a lot of misfires, even when they are working more or less properly.
  • Barrel velocity would probably be suboptimal.
  • As I mentioned in the section on possibility, the ball would have to fit extremely tightly in order to keep it from going out the end of the barrel before the diesel ignites. As a result, they would take ages to reload. There's a reason armies mostly used unrifled muskets until the invention of the Minié ball (which lets you "cheat" and use a slightly-smaller ball in a rifled barrel).

Any society with good enough machining to make your design is probably better off with making air rifles (or even a better crossbow; that's how much I think this design would suck.)

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    $\begingroup$ You have a very dim view of human ingenuity. $\endgroup$
    – Phil Sweet
    Feb 17 at 15:31
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    $\begingroup$ @PhilSweet As I said in my answer, the system can be made to work. People do something similar all the time (albeit with much lower velocity and pressure) with potato cannons. The problem is that it would be such a pain to get working (and be so plagued by problems) that people are better off using more traditional technologies such as airguns or crossbows for anything that isn't a child's hobby project. $\endgroup$ Feb 17 at 22:59
  • $\begingroup$ @PhilSweet Also, I'd like to note that while I was not aware of it when I wrote my answer, people have attempted to make "serious" firearms very similar to the OP's proposal, with pretty much exactly the issues I mentioned. See L.Dutch's answer for an example. $\endgroup$ Feb 17 at 23:05
  • $\begingroup$ How much diesel fuel would you need to replace the energy in a kilogram of gunpowder? $\endgroup$
    – Phil Sweet
    Feb 17 at 23:46
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    $\begingroup$ Do not worry, it will get a "Forward Assist" to solve all problems - said noone sane ever.. $\endgroup$
    – Pica
    Feb 21 at 13:41

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