Powerplant idea.

[pistolero]

      PROPOSED HYBRID PCP/PISTON POWERPLANT
 Look at your typical PCP, Disco/Marauder, what have you. Imagine at the rear of the reservoir, instead of routing the compressed air to directly drive the back of the pellet you insert a small piston system. A dumbbell shaped piston, a face with oring seals on each end. Or a simple cylindrical shape. It will need to be cocked with a lever like a spring gun. Instead of the trigger releasing a striker against a valve, it trips the cocked piston just like a springer. I'm thinking of the piston traveling "backwards" towards the shooter. This is turning the rear section of the reservoir into a compression chamber that works just like the typical springer.
 
Potential advantages.
1. Virtually unlimited shot count, compared to using up the air in the reservoir and losing pressure with every shot. Instead of double digit shot counts, this should allow shooting all day with little or no change in power. No need to recharge every 15, 20, or 30 shots as required with a pure PCP powerplant.
2. Much smoother shot cycle than a traditional springer. We are using a piston, but without the torque, weight, and vibration of a steel spring with tophat hurtling down the compression chamber to a sudden stop. It should not be a problem to design the piston itself for this application significantly lighter than the typical heavy steel tubes used to contain a large, heavy, powerful spring.
3. Easy tuning. As opposed to tweaking hammer weights, spring power, valve details, etc. This system could be easily user tunable to a high degree simply by varying the pressure in the reservoir.

  Potential Disadvantages.
1. Probably a slightly rougher shot cycle than pure PCP. With the advantages over traditional springers, this should be minimal, and easily much smoother and softer than typical springers.
2. Still requires PCP charging system (pump or tank) to vary air pressure and/or recharge to counter any leakage.
3. Likely less powerful than a pure PCP system. 

 My opinion is the shot count advantage alone would be a huge selling point to shooters utilizing hand pumps especially, but also any who just like to shoot a lot at one time. Head out with a rifle and pellets with no need to haul a tank or pump and shoot all weekend. Self-contained unit without the drawbacks of a traditional springer.
 I don’t know how to calculate what kind of power this would potentially make compared to the typical PCP powerplant. Can 2,000 to 3,000 psi pushing a piston to compress air into an orifice make similar power as piping the compressed air directly to the pellet? Something must be lost somewhere. Would it even need that much pressure?
 I would think it should easily exceed the power of a springer of equal swept volume.
 Has this been attempted already? What am I missing? Gas pistons are considered an improvement over the coiled spring, not by all but I think it's a good idea waiting for the hardware to catch up. This could be even more so over the gas piston. Instead of just replacing the spring with the strut, we are using a comparatively huge volume of air for a spring and (I think) removing a lot of the mass and harsh cycle in that system.


John Moore, 17 OCT 2013       

[Dick Tracey]

If I understand your thought process, you might look at a Gamo 126.  It works on the differential of pressure and it was a bit complicated.  Being a 10 meter target rifle it was very low velocity, but it did work.

DT

[CarsonRatSniper]

Years ago I kicked around using opposing magnets as a powerplant to replace the mainspring and gas piston. I just never went anywhere with it.
The only way to tell for sure is build a prototype and see what happens.

[rsterne]

Right off the bat, I would suggest that the cocking force may be unmanagable.... A reservoir the diameter of a Disco (3/4" ID) at only 1000 psi would produce a force of 440 lbs. on the piston (double the pressure doubles that).... You would need enough stroke on the piston to move enough air to accelerate the pellet.... Your cocking lever would have to be capable of moving the piston against that force....

What you are proposing is not a lot different than a gas-ram springer.... you're just trying to reduce the moving piston mass....

Bob

[PakProtector]

What you are proposing is not a lot different than a gas-ram springer.... you're just trying to reduce the moving piston mass....

Bob

The gas ram springers still depend on piston kinetic energy transfer to store energy in the working  gas to drive the pellet. The force at the end of the stroke is lowest, and not sufficient to oppose the pressure generated on discharge.
cheers,
Douglas

[rsterne]

As I understand the concept, the air in the PCP reservoir is being used as a spring, driving a double-ended piston that is cocked against that "spring" by a lever.... On the other side of the piston is air at atmospheric pressure (until the trigger is pulled) which is then compressed adiabatically (ie with a large temperature rise from the rapid compression).... and the pellet is driven by that hot, high pressure pulse of air.... just like in a gas-ram springer....

The difference would therefore be that the piston would be much lighter.... The dimensions chosen for the diameter and length of the reservoir and the cocking stroke woudl govern the pressures and the cocking force....

Bob

[pistolero]

As I understand the concept, the air in the PCP reservoir is being used as a spring, driving a double-ended piston that is cocked against that "spring" by a lever.... On the other side of the piston is air at atmospheric pressure (until the trigger is pulled) which is then compressed adiabatically (ie with a large temperature rise from the rapid compression).... and the pellet is driven by that hot, high pressure pulse of air.... just like in a gas-ram springer....

The difference would therefore be that the piston would be much lighter.... The dimensions chosen for the diameter and length of the reservoir and the cocking stroke woudl govern the pressures and the cocking force....

Bob

Pretty much. What I'm thinking, and what I believe Pak is trying to say is, the sheer volume of gas compared to the tiny caspsule in a gas ram will "push" much more evenly through the cycle. It should have nearly as much power at the end of the stroke as the beginning. Which does bring up the issue with cocking effort.   
 More efficient with less mass moving = power without the SPRANNNGGG.

[pistolero]

The only way to tell for sure is build a prototype and see what happens.

Yes. I'd be all over it except I don't have the base gun or the "stuff" for fabbing it up. Been out of work. The Research and Development Department here at my place is woefully underfunded at the moment.

[HYspd]

it would be a bigger gas ram...that won't change the shot cycle...

but playing with the diameter of the bore and length of the stroke will..

I would think the biggest engineering compromise would be in the piston length...

being essentially a spring piston design you'd need to cock it like a spring piston gun..

so the piston has to be long enough to be relatively stable when under the extremely offset forcing of cocking..

desired volume of pressurized gas behind the piston, the piston length and the stroke length set the tube length..

a hogged out piston open end to the pressurized gas reduces the tube length needed for a given desired volume...

that's the basic mechanical constraints...

I wonder how a 3" long 2" diameter piston with a 1" stroke driven by 10 cubic inches at 4500 psi would do...

probably jump like a startled cat



I wonder how it would work as a Bullpup?

[rsterne]

2" diameter piston.... at 4500 psi

and have a cocking force of over 7 TONS....

Bob

[pistolero]

2" diameter piston.... at 4500 psi

and have a cocking force of over 7 TONS....

Bob

You would just need a very long lever, or install an electric winch to pull the piston back. 

[HYspd]

7 TONS?....(throws 5 hours of machine work in the trash)....that's a catapult not an airgun....


hhhhummmm (digs parts out of trash).... I have friend that........7 tons (giggle)

[rsterne]

You would just need a very long lever

Let's see, a 1" stroke and 7 tons of force.... a guy weighing 200 lbs would need a 6 foot lever.... Actually, two of them, as you have to be able to hold onto the gun to cock it.... plus a cradle to keep the gun from falling over while you stand on the end of the lever.... oh, and don't forget a ladder.... 

or install an electric winch

One like they install on a 3/4 ton pickup might be big enough.... What do they weigh, with 12V battery?.... I'll never complain about the weight of a SCUBA tank again!....

Bob

[Cal]

The total cocking effort should be very much the same as any springer for any given output (fpe)

Linkage geometry would need be considered.

cheers

Cal

[HYspd]

You would just need a very long lever

Let's see, a 1" stroke and 7 tons of force.... a guy weighing 200 lbs would need a 6 foot lever.... Actually, two of them, as you have to be able to hold onto the gun to cock it.... plus a cradle to keep the gun from falling over while you stand on the end of the lever.... oh, and don't forget a ladder.... 

or install an electric winch

One like they install on a 3/4 ton pickup might be big enough.... What do they weigh, with 12V battery?.... I'll never complain about the weight of a SCUBA tank again!....

Bob

LMBO