Why is the efficency of pcp air rifles so low compared to firearms?

[Xplorer]

Scotchmo

The numbers on how much energy is in pressurised air I got from wikipedia.

How much pressurised air a rifle uses is no state secret. Neither is how much energy comes out of the barrel in form of kinetic energy.

Just devide output energy with input energy to get the efficency.

 

[Blackthunder]

Why is the efficency of pcp air rifles so low compared to firearms?



Depends how you look at it .



Dead = Dead. One hole one target = One hole one target. If you can do that for $0.05 (or less) instead of $1, I will call that efficient.

Who cares about the exact numbers. I don't.

 

[Motorhead]

Very simply a RATIO

Being that with equal weight & style of projectile, and the drag within bore it has being equal ...

Operating at 3000 PSI the amount of energy being absorbed to get said projectile moving and out the barrel is at a MUCH HIGHER loss than the same thing happening with 20,000 psi pushing the same projectile.



RATIO of remaining energy to do the job of creating speed from the projectile is VASTLY different due to PRESSURE with greater losses on LP that with HP



IMO ...

 

[Xplorer]

Motorhead

You meen something like if it takes X fpe to push against the friction of the barrel it takes that energy no matter what energy is delivered to the bullet and that X is a bigger portion of the total energy used the less energy we put into the system?

That would make sense.

 

[Motorhead]

Correct ....

 

[Xplorer]

Motorhead

Would that mean that if you put in the same extra air it would yield more FPE if it is done by raising the pressure than by increasing the plenum size?

I figure since pressure when the bullet is at the end of the barrel would be more or less the same so the losses to friction would not be that different in that portion of the barrel but losses in the beggining would be less with higher pressure than with bigger plenum.

 

[Motorhead]

Were not talking speed creation .. OP asked about efficiency differences & the why for ?



Now on the SPEED question ..... pending enough barrel length for said caliber and weight shot, HIGHER PRESSURE which is potential energy to be exerted upon projectile, higher is its speed potential.

 

[billygee]

Hmm m m !

Something like comparing a balloon filled with air pressure vs a hand grenade ?

 

[ackuric]

Why a 400 hp boat is slower than a 400 hp car ?

I lol'd



Pcp projectiles commonly obtain between 20-30% of the total kinetic energy released from the valve depending on a few set of factors....if powder burners are hitting 30% (I am not going to research this myself) then pcp's are nearly on par with them when operating at an efficient level, albeit with design differences to compensate the lack of burst KE which is far more important than the acclimated KE as the projectile travels down the barrel...

 

[Scotchmo]

Lot's of explanations presented and all mostly true. Some more to think about:

A number of reason for the difference.

Here is one:
A powder burner uses gun powder and high temperature gases which are relatively low density for the energy provided. HPA at room temperature is fairly dense. The kinetic energy of the air needs to be taken into account. A .177 with a 23" barrel might be pushing a 8.44 grain pellet at 12fpe. But if the air in the entire barrel were to be at 1000psi on exit, it would also be pushing about 4gr of air at the same velocity. So you are really accelerating 14.5gr, not 10.5gr. If the efficiency of the pellet FPE were 22%, then the overall efficiency would be 22% x (10.5gr + 4gr)/10.5gr = 30%.

And another reason:
If the air exiting the barrel really were at 1000psi, then there is still quite a bit of energy remaining in the air. The only way to extract all of the energy, is to expand the air out until it reaches atmospheric pressure. The barrel length would need to approach infinity. And that would only work if there were no barrel friction. The peak efficiency happens way before infinity. Nobody even wants a 6 ft long barrel. Basically the same as Motorhead explained - the high pressure powder burner would need less barrel length to get the same efficiency.

 

[Xplorer]

Scotchmo

The barrel has 1 atm at 14.5 psi. To get to that pressure the barrel volume has to be WorkingPressure/14.5×PlenumVolume in volume.

That will happen way befor infinity. (But still too long)

 

[GrumpterPlacard]

If you had a .22 lr with NO BARREL, just the cartridge, how fast would that bullet be going when it fires?

Not very fast at all compared to having the barrel there. The barrel length for a powder burner is typically sized to be long enough to allow all the powder in the cartridge to burn and produce usable gas. Too short and you get more flash than acceleration.

 

[KITROBASKIN]

Efficiency? An air rifle does not use primer, powder or brass case. The energy and resources required to manufacture and assemble those components make the air rifle much more efficient. The previous post comparing the cost of shooting with these two methods is revealing don't you think? Or is this thread a thought experiment comparing the efficiency of what happens after the trigger is pulled?

 

[Xplorer]

KITROBASKIN

No I just wounderd where the energy goes that's all. I am new to this world of air rifles and try to understand and learn as much as I can.

I have got some good answers in this thread about resistence not being proportional to input energy and that made me feel stupid. It's so obvious.

But now I am wiser thanks to my stupid question and all of you and your generosity with knowledge.

 

[KITROBASKIN]

Hey, not a stupid question at all. It is so interesting to read the different perspectives. I was kind of waiting to see if some member would write that the cost of a compressor or other means to compress gas should be included in the mix when compared to the self contained cartridge.

 

[Smudgerii]

A larger airplane wing is more efficient in creating lift than a small one. Why? Because the scale of the wing changed but the size of the air molecules did not and the farther you get away from the air molecule size the more efficient the wing is in creating lift.

I think it is a matter of scale. You are scaling the pressure. What about bullet weight, friction in the barrel, the afore mentioned air molecules in the barrel, inertia, gravity and even time itself? I would think they, and many others, would all play a part.

Your info on the size of an airplane wing is 180 degrees out! The larger a wing is, the plane is usually much slower as a result. Some of this is because the larger wing is necessary to provide the lift for a bigger, heavier plane to boot. I flew and worked in the aircraft industry for over 35 years, and the smaller the wing, the faster the plane. You can go back to military models, the Starfighter was by far the fastest plane of it's era, and had a knife edge stubby little wing that was so sharp it could actually cut you. I worked for Mitsubishi Aircraft Intl. for over 5 years, and our turboprop wings were small, short, and thinner than the competition...and we were the fastest turboprop on the market. Huge flaps were necessary to provide the lift necessary for takeoff and landing at lower speeds. The small wing did not provide good handling characteristics, but it was fast!

Where did he say “quicker”? I’m sure his post says “lift”...