What I learned about PCP Energy consistency from a 10 FPE Leshiy

But why a 10 FPE Leshiy to begin with?

The Edgun Leshiy (first generation, second revision) is an interesting air rifle. It is a compact hunting rifle that weighs about the same as a heavy air pistol! The original intent of the air rifle is power in a small package. However, this airgun is also highly (re)configurable. it is an excellent platform for all kinds of experimentation.

Energy Consistency contributes to accuracy

Energy consistency alone does not guarantee accuracy. However, an airgun that lacks consistency is unlikely to be accurate.

Energy consistency refers to the amount of energy released when an airgun fires. There are many factors affecting this consistency:

• Regulated or not

• Air pressure at exhaust valve

• Plenum size

• Exhaust valve spring tension

• Transfer port cross section and volume

• Hammer spring tension

• Projectile mass

• Projectile head size

Regulator

A regulator, nowadays found in many PCP (precharged pneumatic) airguns, regulates a higher reservoir air pressure to a lower and "constant" plenum air pressure. The first thing to understand about regulators is that they are not perfect. In other words, plenum air pressure does vary as a function of reservoir air pressure. Depending on the quality of the regulator, this can be a small amount.

This variation is influenced by the ratio between the regulator inlet cross section area to the cross section area of the piston pushing back from regulated air in the plenum. However, this variation also depends on the spring tension that helps the regulated air pressure plenum to push back to the reservoir to close the valve letting reservoir air into the plenum.

The bottom line is that an increase of reservoir pressure does increase plenum air pressure a little. It is helpful to quantify this amount to tune an airgun for consistency.

For the purpose of this article, let's say the range of plenum pressure is 75 to 78 bars over the fill pressure range of the reservoir.

Plenum volume

The volume plenum is actually an important factor even though this is not adjustable. Because the inlet from reservoir to plenum is small relative to the volume of air used to push a pellet, even though some reservoir air may get behind a pellet, the effect is negligible.

This means plenum volume, combined with plenum regulated pressure, specify the absolute amount of energy available to fire a pellet.

How do you want to be served?

With the amount of energy associated with the pressurized air in the plenum, how does it get distributed behind a pellet over time? What is the best way?

Self regulating HST

If everything else is consistent, most tuners tune for a low spread of muzzle velocity by varying hammer spring tension (HST). The relationship between HST and air pressure is, to an extent, self regulating. With a high pressure, the same amount of energy from the hammer cannot open the exhaust valve as much, therefore limiting the amount of pressurized air propelling the pellet. As plenum pressure decreases (slightly), the same amount of hammer (kinetic) energy opens the valve more, therefore allow more air through to propel the pellet. This is why the relationship between plenum pressure and HST is somewhat self regulating.

However, this approach also means there is leftover pressurized air in the plenum when the exhaust valve closes.

Dump it all

The second approach is to increase HST until there is no or little regulated air is left in the plenum. In other words, use the least HST to "dump" everything in the plenum. This approach can potentially vary muzzle velocity because plenum pressure changes slightly as reservoir pressure changes. However, note that there is still a self regulating effect in play here. The manner plenum regulated air is dumped changes depending on plenum air pressure.

A lower plenum air pressure allows the exhaust valve to open a little faster, which allows the full flow of plenum air into the barrel and start to accelerate the pellet faster and earlier. In other words, the opening act of the exhaust valve also has a self-regulatory property.

But why is dumping all the plenum air a good thing?

The exhaust valve closes due to two forces. One force comes from the exhaust valve spring. This force is constant. The other force comes from the flow of air through the opening of the exhaust valve. This portion depends on a few factors.

One extreme condition is when the projectile is stuck, and as a result there is minimum air flow after plenum air fills the volume up to the skirt of the projectile. In this case, the exhaust valve opens for a longer amount of time. However, in the case that there is no projectile, then the air flow through the exhaust value is high, helping to close the exhaust valve early.

Of course, with a proper projectile, the profile of exhaust valve opening over time is somewhere between the two extreme cases. But this means that how readily a projectile moves in the barrel can influence the amount of energy released by the airgun. This readiness partially depends on the amount of friction (initially static, then kinetic) between the projectile and barrel. This friction also depends on the head size of a pellet, which introduces variance to this equation more so than the variance of mass.

The variance due to how readily a projectile moves initially in the barrel is more pronounced for lighter pellets with a reduced friction coefficient. The author has observed this with lead-free pellets which are both light and have a small friction coefficient.

This interaction of projectile and dwell time (the profile of how the exhaust valve opens over time) can be somewhat reduced when the main reason for the exhaust value to open and stay that way is hammer energy, and the fact that the hammer energy alone is enough to sustain the exhaust valve to open long enough to empty all the regulated air. This approach also ensure the exhaust valve is "fully open" quickly, dumping regulated air at a high rate when it matters the most (the volume between the exhaust valve and the projectile is the least).

Which way is right?

There is no right or wrong way, it depends on many factors (again) and how various factors relate to each other.

In the case of a low-energy regulator set up (small plenum, strong exhaust valve string combined with a low plenum pressure), dumping all the regulated air seems to improve muzzle energy consistency. This is especially the case with pellet sizing to help reduce head size variation.

However, in the case of a heavier lead projectile (much more initial resistance to accelerate), large plenum volume and a higher plenum pressure, there may be enough consistency on the part of the projectile that balancing between HST and plenum pressure for the self-regulating effect may work better.

For PCPs that are not regulated

If a PCP is not regulated, there is no plenum to speak of. This means dumping plenum air is not an option. In this case, it is best to use heavier pellets and size the pellets for consistency. To a certain degree, varying the exhaust valve spring strength can still help shift the coefficient of how projectile movement affects consistency.

Imagine two scenarios. The first one with a weak exhaust valve spring where the closing of the valve is mostly dependent on air flow through the valve. The second one with a strong exhaust valve spring where the spring contributes most of the force to close the exhaust valve.

The first scenario is more sensitive to projectile movement characteristics than the second one. However, in order to use a stronger exhaust valve spring, more hammer energy is needed to open the valve, so HST needs to be increased. Note that the HST vs. pressure self-regulatory effect is still present because pressure affects how readily the exhaust valve opens as well as the extent.