Filling CNG Fuel Tanks
Unlike liquid fuel, which consistently holds about the same volume of fuel across a broad range of conditions, compressed natural gas (CNG) can expand and contract significantly depending on temperature. For example, under industry standard conditions (pressure and temperature), a CNG tank on a vehicle may be able to hold 20 gasoline gallon equivalents, but on a hot day the gas will expand and the tank may only fill to 75% (or less) of its potential. The newer, fast-fill dispensers equipped with a temperature compensation feature help vehicles get the best fill possible.
The amount of CNG that can be stored in a vehicle's tank varies based on the following variables:
Fueling rate: As the rate of fueling increases, the temperature of the fuel also increases—dramatically. As the fuel warms up, it expands and becomes less dense, therefore containing less energy by volume when the fuel system reaches the rated pressure. For this reason, you are usually able to get more CNG into a tank with a time-fill versus a fast-fill application. This is because when gas molecules are compressed they create heat. The faster they are compressed, the more they heat up and expand. So when you compress the gas rapidly by using a fast-fill station, the molecules will heat up and expand more than if filled slowly over time.
Ambient temperature: The outside temperature affects the temperature of the CNG. At higher temperatures, CNG is less dense, and therefore does not contain as much energy per unit volume as it would at a lower temperature. When the CNG is stored in warm ambient temperatures, it expands and becomes less dense, so when the tank reaches the rated pressure, the CNG inside does not contain as much energy as it would at lower temperatures.
Pressure rating: The typical industry standard for CNG fueling system pressure is 3,600 psi. Some systems in the U.S. and many systems overseas are rated at 3,000 psi. These fill pressures are based on a 70ºF ambient temperature. The cylinders are actually designed to hold up to 125% of their operating pressure. So, a 3,000 psi tank can be filled to 3,750 psi and a 3,600 tank can be filled to 4,500 psi. This makes it possible to fill a tank to a higher pressure on hot days when the gas is expanding, as well as compensate for the heat of recompression. A good rule of thumb is that for every 10ºF plus or minus 70ºF, the pressure will increase or decrease by 100 psi.
Cylinder type: There are four types of cylinders (Type 1-4). The type designation is based on the way the cylinder is made and the material it is made out of (aluminum, steel, carbon fiber, etc.). The material used to make the cylinder also affects the temperature in the tank, and thus, affects how “full” you can fill the tank. For example, carbon fiber tanks hold heat better than steel tanks. The temperature inside the tank affects the pressure of the gas, and therefore, how much gas the tank will hold.