Single-pump hybrid concept
Posted: Thu Dec 31, 2020 7:04 pm
I like @hectmarr 's work on these autonomous hybrids. I'm thinking about a design for a single-pump, high-mix hybrid. It seems to me that going for multiple pumps is a waste, since the resistance of the first few strokes is low, so one's arm is getting a lot of exercise just going back and forth. [ahem] I'm thinking of using a pump like the Crossman multipump BB gun, but scaled up. I wrote a little calculator in an iPython notebook to calculate the required lever force for a particular piston diameter, stroke length, lever arm length, and chamber volume. It also calculates the resulting chamber pressure.
I found a nice solution that I could use: with a chamber volume of 0.5 cubic inches, a piston diameter of 1.25 inches, a lever arm 16 inches long, a connecting rod 6 inches long, attached 4 inches up the lever arm from the pivot point, I get a 20x mix in a single stroke, with about 20lb peak force on the lever. That's less than many upper-end spring air rifles, b/c I'm a weakling, lol. This is assuming isothermal compression -- if the air heats up instead, the peak force would be more like 38 lb, and the chamber pressure would start out at 67 atmospheres and drop to 20 after cooling down. The truth is somewhere in the middle probably. I could probably increase the lever arm a bit, reduce the travel, or increase the chamber volume if that turns out to be too much for enjoyable shooting.
With such a small chamber volume, I need to minimize deadspace as much as possible. At 0.5 cubic inches, that's only 0.4 linear inches of space inside a 1.25 inch diameter cylinder. So, I think I would need to make the piston with the o-ring groove very close to the end, and make the end of the piston fill the bottom end of the cylinder very well, and have a check valve built into the wall or end of the cylinder. I considered simply letting the chamber be the dead space at the end of the piston, but I'm worried about the force of the combustion blowing the piston back.
On the other hand, for a 0.22 inch diameter barrel, that equates to 13 linear inches. I figure that's pretty reasonable for a 26 inch long barrel: the gas volume will expand by 3x.
For fueling, I like the idea of metering the fuel into the pump before compressing, provided it doesn't diesel on me. At a 20x mix, if I assume adiabatic compression, the peak temperature is 1000 K, which is above the ignition temperature for butane (678 K) and propane (743 K) and even hydrogen (809 K) and methane (810 K). I can get the peak temperature down to 748 K if I increase the chamber volume to 1.1 cubic inch for the same pump parameters. That limits me to a 9x mix. I figure the peak temperature will be somewhat less than isothermal, especially if the air is flowing through a checkvalve into the chamber, which should provide some mixing & cooling.
I found a nice solution that I could use: with a chamber volume of 0.5 cubic inches, a piston diameter of 1.25 inches, a lever arm 16 inches long, a connecting rod 6 inches long, attached 4 inches up the lever arm from the pivot point, I get a 20x mix in a single stroke, with about 20lb peak force on the lever. That's less than many upper-end spring air rifles, b/c I'm a weakling, lol. This is assuming isothermal compression -- if the air heats up instead, the peak force would be more like 38 lb, and the chamber pressure would start out at 67 atmospheres and drop to 20 after cooling down. The truth is somewhere in the middle probably. I could probably increase the lever arm a bit, reduce the travel, or increase the chamber volume if that turns out to be too much for enjoyable shooting.
With such a small chamber volume, I need to minimize deadspace as much as possible. At 0.5 cubic inches, that's only 0.4 linear inches of space inside a 1.25 inch diameter cylinder. So, I think I would need to make the piston with the o-ring groove very close to the end, and make the end of the piston fill the bottom end of the cylinder very well, and have a check valve built into the wall or end of the cylinder. I considered simply letting the chamber be the dead space at the end of the piston, but I'm worried about the force of the combustion blowing the piston back.
On the other hand, for a 0.22 inch diameter barrel, that equates to 13 linear inches. I figure that's pretty reasonable for a 26 inch long barrel: the gas volume will expand by 3x.
For fueling, I like the idea of metering the fuel into the pump before compressing, provided it doesn't diesel on me. At a 20x mix, if I assume adiabatic compression, the peak temperature is 1000 K, which is above the ignition temperature for butane (678 K) and propane (743 K) and even hydrogen (809 K) and methane (810 K). I can get the peak temperature down to 748 K if I increase the chamber volume to 1.1 cubic inch for the same pump parameters. That limits me to a 9x mix. I figure the peak temperature will be somewhat less than isothermal, especially if the air is flowing through a checkvalve into the chamber, which should provide some mixing & cooling.