Filling Up the Tank of a CNG Vehicle

From the Clean Cities Blog:

When pulling up to a compressed natural gas (CNG) fueling station, you may see some distinct similarities to a traditional gasoline station—a nozzle, a dispenser, and maybe even a nearby convenience store selling snacks. At first glance, the act of pumping gas into a natural gas tank is quite similar to that of filling a conventional vehicle with gasoline.

However, there is one very big difference when it comes to the fuel—CNG is a gas, while gasoline is a liquid. This difference means that your tank will fill differently with CNG than it does with gasoline.

For example, when you fill up an empty 20-gallon gasoline tank, you drive away with 20 gallons of liquid fuel no matter what time of year it is or how quickly the pump dispensed your fuel. This is not the case when fueling natural gas vehicles. In contrast, the amount of CNG that ends up in the tank when the dispenser shuts off will vary depending on the outside temperature and the speed at which fuel goes into the tank, among other variables. Lower outside, or ambient, temperatures at the time of fueling combined with a slower fill rate, for example, will result in a higher volume of natural gas in the tank when compared with higher temperatures or a faster fill rate.

It is easy to be confused by the final fill volume in a natural gas tank, because what is happening inside the tank can't be seen and vehicle operators tend to think in terms of the behavior of a liquid fuel. To demonstrate this phenomenon and help drivers and fuel providers understand what is happening, the Alternative Fuel Data Center (AFDC) website has just launched an interactive animation that demonstrates at what temperature and fill speed a driver can safely get the "fullest" fill of compressed natural gas.