The Gasoline Gallon Equivalent
(GGE) is a way comparing the price of CNG to a gallon of gasoline. Normally, natural gas is sold in terms of $/MCF (thousands of cubic feet) and the gas meter on your house measures your gas consumption in cubic feet or cubic metres.
According to the US DOE
, gasoline contains 116,090 BTU/gal LHV compared to 84,500 BTU/gal LHV for propane. This means that gasoline has 1.374 times more energy per unit volume than propane. Interestingly, the GGE uses 115,000 BTU/gal for gasoline which would make the comparison appear to be a bit more favorable for CNG.
Therefore, propane must cost 0.728 times the price of gasoline to be equivalent on an energy basis. If gasoline costs $3.50/gallon, propane must cost $2.55/gallon to be equivalent. However, injection systems are more efficient than what the energy comparison would predict. One factor that improves propane economy is the absence of engine knock (due to propane's 104 octane rating), which allows the engine to automatically run more advance. I'm not sure what other companies use for their fuel economy estimates but Technocarb's FAQ (see FAQ #8)
suggests that their propane injection systems get about 90% of the vehicle's gasoline economy. This implies propane would have to cost $3.15/gallon to be equivalent to gasoline.
The main disadvantage with CNG (especially in a large vehicle like a pickup truck) is fuel storage. A large CNG cylinder only holds the equivalent energy of a 2-3 gallons of gasoline. If your driving allows for frequent fuel stops, a CNG system might make sense in your application.
If you only need to drive a within say 6 gallons of gasoline and you can return home to refuel with MyPhill
, this small compressor would take over 14 hours to refill the equivalent of 6 gallons of gasoline.
Last edited by fraso; 04-18-2008 at 07:39 PM.