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.