Although natural gas is an ideal fuel for the trucking industry, the low energy density of CNG makes it impractical for heavy-duty trucks with semi-trailers. The tractors of these types of vehicles have diesel fuel tanks with capacities often over 200 US gallons. Even with the largest CNG cylinders, CNG energy storage is quite limited. For example, a 320 litre / 84.5 gallon WC (water capacity) 3600 psi cylinder only holds the equivalent of 25.7 gallons of diesel fuel. The liquefaction of natural gas greatly increases its energy density so that:
- 1.520 gallons of LNG contains the equivalent energy of about 1 gallon of gasoline (GGE - Gasoline Gallon Equivalent for LNG)
- 1.733 gallons of LNG contains the equivalent energy of about 1 gallon of diesel (DGE - Diesel Gallon Equivalent for LNG)
In other words, a gallon of LNG only contains about 58% of the energy in a gallon of diesel or about 66% of the energy in a gallon of gasoline..
LNG Fuel Tanks
Although, LNG-fueled trucks (like those that use Cummins-Westport engines) are now available from the factory, they have a limited range because the LNG tanks hold less than their diesel equivalents. This implies that they also require that the trucks return to their yards to refuel because LNG is not readily available in North America. The alternative is use a diesel-LNG dual fuel system (like the natural gas EcoDiesel System) that supplements diesel fuel with LNG, and the typical substitution rate is about 30%. That would mean that a truck with 200 gallons of diesel capacity would require an LNG tank holding about 60 DGE (Diesel Equivalent Gallons) or about 104 gallons of LNG. This would extend the range of the truck approximately 40-45%. Alternatively, if a truck had a pair of 120 gallon diesel tanks (26" x 52" each), the replacement of one diesel tank with a 72 gallon (26" x 52") LNG tank would result in slightly reduced range (~70% of 240 gallon range) but with a significantly lower cost per mile. These systems seamlessly revert back to 100% diesel operation when the natural gas tank reaches empty so running out of natural gas never leaves you stranded.
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