Tank Considerations for Alternative Fuels
Environmental issues weigh heavily on the trucking industry today, and one of the predominant discussions centers on alternative fuels.
Figure 1. CNG tank fill connections and pressure gauge (Click to expand images to full size)
Over time, and especially the past 15 years, natural gas has seen an enthusiastic reception into some fleets around the globe and helped to ease the rising cost of oil. The components for these fuels have different requirements than components on gas or diesel fueled trucks, one of the most significant being the fuel tank.
Compressed Natural Gas
Compressed natural gas (CNG) holds a much higher pressure than conventional fuel tanks and liquid natural gas (LNG) tanks. In addition, CNG fuel tanks have different connections for filling (See Figure 1), and CNG vehicles can be configured to be filled via fast-fill or time-fill. Fast-fill takes much less time but creates more heat in the tank when filling, while time-fill can take several hours for the tank to fill, but with less heat of recompression. (Read more about compressed natural gas fueling stations.)
Fuel in a CNG tank expands with heat, so to compensate, CNG tanks are designed to hold more pressure than their rating. Because of pressure inside the tank and the risk of leaking gas, CNG tanks are labeled with an end-service life date after which the tank cannot be used. With proper inspection and maintenance, tanks can reach this service date with no issues. It’s a good idea to consult sources like Mitchell 1 TruckSeries for information about CNG tank proper inspection, maintenance, or replacement procedures if your tank has reached the end of its service life
Liquid Natural Gas
Liquid natural gas (LNG) is a great option for the commercial vehicle industry not only because LNG vehicles typically have a greater range than a CNG vehicle, but also because their fuel tank takes up less space. For example, CNG trucks usually store four or more tanks on a vehicle, while LNG trucks operate with just one or two.
Figure 2. LNG tank pressure management system and attachments
In LNG tanks, the natural gas is cryogenically stored. When an engine is not running, the liquid gas converts back into a gas. The tank is designed to allow space for the pressure that builds when the vehicle is in use on a regular schedule. However, if a vehicle is parked for an extended time, any excess pressure in the tank will be vented to the atmosphere. LNG tanks typically have a vaporizer integrated in the tank and a system to manage pressures, filling and venting connections (See Figure 2). Again, proper maintenance and inspections are vital to keeping the LNG tank in proper working order — information that may be found in Mitchell1 TruckSeries.
It seems likely that the industry will see an increasing rate of adoption for these systems in the future, in response to growing awareness and demand for more environmentally-friendly fuels. As a truck service professional, you may choose to integrate one of these systems, taking into account things like space, mileage, and fueling station requirements. Whichever system your requirements allow, it’s good to understand the benefits that alternatives like natural gas have for your fleet operation — and resources available to help you maintain alternative fuel vehicles more efficiently (see Figure 3, below).
Figure 3. Sample of natural gas service information in TruckSeries
