The Tank

When we fill up, natural gas is delivered from the pump into the storage tank on the vehicle.  The pump compresses the gas into the tank to a preset pressure (not volume), usually 3,600 psi.  It is easy to think that when we are filling up an empty 23 gge tank that we should get 23 gge of natural gas into that tank, but that isn't the case. There are a couple of points to understand so that you don't feel disappointed.


The first is that there is no fuel pump in play for a CNG tank.  It is sort of like a balloon.  It is "inflated" by the CNG pump, then slowly "deflated" as the vehicle burns the natural gas.  As the CNG is used, the pressure in the tank drops until the pressure is nearing the point of being unable to force enough natural gas into the fuel system to meet the demands of the vehicle.  This is determined by the vehicle.  If you have a bi-fueled vehicle it automatically switches over to gasoline at this point.  If you have a dedicated CNG vehicle, you automatically switch to Nike or Adidas because you are walking.  Being that the CNG isn't mechanically pumped into the engine like gasoline, there is volume of CNG in the tank that has to be compressed in order to push the CNG out of the tank while driving.  This volume is always present in the tank and takes up the space between 23 gge and what I can actually pump into the tank before it gets to 3,600 psi.

The second is that since we are talking about a gas, and one which is lighter than air, we have to remember that it's volume changes with temperature.  We will bypass the academic explanation which nobody wants to read anyway, and use the balloon example.  You can search "Boyle's Law" if you need more.  We have all seen the experiment where we put a balloon in cold water and it shrinks, warm it up and it expands.  Same thing here.  The volume of gas is subject to its temperature.  Some weird things happen with CNG as it moves from the storage tank through the pump and into the vehicle tank.  When we fill the tank, the pump shuts off at a preset pressure, typically 3600 psi.  The pumps in Bossier display a percentage of fill, while the pumps west of the river display the current psi.  If we leave one of the pressure displaying pumps connected to the tank after the pump stops filling, we can watch the pressure slowly drop.  This is due to the volume of gas changing due to the change in temperature of its new environment (the tank).  Some hardcore CNGers even wait until it equalizes, then refuel again to get the most out of a fill.

So when we fill up, we have to remember that we may pay for GGEs, but we are actually purchasing a volume of a gas which has been compressed to 3,600 psi.  In my case, with a 23 gge tank the most I have put into it so far has been 17 gge.  That may change as it continues to cool.  This summer 12 -14 gge fills were common.

The tank is never truly empty, even when it is "empty" according to the vehicle (unless it is disconnected from the system). This is important to know because we are a bunch of rednecks here is north Louisiana.  Rednecks love to tinker.  We can't help it.  Don't tinker with this system unless you have been trained professionally and are sober.  This isn't like natural gas in your home where you hear the soft little hiss when you turn on a burner.  The pressure of natural gas in the home is highly variable, but we are talking single digits, maybe up to 12 psi.  The pressure of a tank that the vehicle has determined is  "empty" is enough to kill you.  Leave the little crescent wrench in your pocket on this one, trust me!