Filling CNG Fuel Tanks
When fueling a vehicle with compressed natural gas (CNG), the amount of fuel in a "full tank" will vary according to a number of factors, including the ambient temperature, the type of fueling equipment, vehicle design, and the type of fuel tank in the vehicle. This is because CNG expands and contracts substantially depending on its temperature. In contrast, liquid fuels consistently hold about the same amount of energy per unit volume across a broad range of conditions.
The amount of CNG that can be stored in a vehicle's tank varies based on many variables, primarily these:
Fueling rate: Slower rates of fueling result in fuller fills. As natural gas enters the tank during the fueling process, the pressure increases, causing the internal temperature of the tank to increase. This is referred to as the heat of recompression. The warmer the gas becomes, the more it expands, thus reducing its energy density. When the tank is filled over several hours in a time-fill application, the heat has time to dissipate from the tank during the fueling process, resulting in relatively modest decreases in energy density. When the tank is filled rapidly, such as in a fast-fill application, the heat does not have time to dissipate, and the energy density is significantly reduced by the time the tank reaches its maximum fill pressure.
Newer fast-fill dispensers have temperature-compensation features that help account for this phenomenon in order to achieve a fuller fill.
Ambient temperature: Lower ambient temperatures usually results in fuller fills. CNG expands as temperature increases, reducing the amount of fuel that can be stored inside the tank at its maximum fill pressure.
Pressure rating: Higher pressure ratings allow for fuller fills. In the United States, the industry standard pressure rating for a CNG vehicle's fueling system is 3,600 psi at 70°F. However, some systems in the United States and many systems in other countries are rated to 3,000 psi. The fuel cylinders are designed to be filled to a maximum of 125% of their rated pressure: A 3,000 psi tank can safely be filled to a maximum of 3,750 psi, and a 3,600 psi tank can safely be filled to a maximum of 4,500 psi. However, most fueling stations do not fill a tank past 4,100 psi. The goal is to fill the tank so it has a settled gas pressure of 3,600 psi at 70°F.
This makes it possible to fill a tank to a higher pressure than its rating to account for hot ambient temperatures and the heat of recompression. As the tank cools after fueling, the pressure will decrease to the rated pressure. A good rule of thumb is that for every 10 degrees Fahrenheit that the outdoor temperature exceeds 70°F, the tank may be filled to an additional 100 psi above its rated pressure.
For every 10 degrees that the outdoor temperature is colder than 70°F, operators should subtract 100 psi from the tank's rated pressure to determine the proper maximum fill pressure. This allows the gas to expand to the rated pressure in the event that the outdoor temperature increases, or the vehicle enters an indoor garage or maintenance facility.
Cylinder type: Different types of cylinders allow heat to dissipate at different rates, so some cylinder types achieve fuller fills than others during fast filling. There are four types of cylinders (Types 1 through 4), which differ in construction, materials (e.g., steel, aluminum, carbon fiber), weight, and price. The less insulating the material, the more readily it will dissipate heat, thus allowing for a fuller fill. For example, steel tanks release heat more readily than carbon fiber tanks do.
Tanks also come in various sizes, and thus various storage capacities.