CNG for Vehicle transportation
Being an engineer who has grown up around and worked in the natural gas industry I have always been curious about the prospect for using natural gas for vehicle transportation. According to the EIA in 2007 the United States produced about 19.2 trillion cubic feet (TCF) of natural gas and imported about 4.6 TCF. Of the 4.6 TCF imported about 80% came from Canada. Since 1970 dry natural gas production has been down but since 1980 and in recent years production has been up. This is because of price regulations in the 1970’s drove down production and an increase in the production of natural gas plant liquids has reduced the amount of dry natural gas that makes it to market (See figure 5.10 and 6.2 in the EIA’s 2007 Annual energy review). The recent increase in production has been due to advancements in recovery technology.
The consumption of natural gas for vehicle fuel use has been increasing steadily since 1992 from 0.02 TCF to 0.26 TCF per year in 2007. Today the price for of natural gas today is $4.46/MMBTU (cnn.com/money). Compare this to $1.21 /gallon of gasoline which corresponds to about $9.68/MMBTU. On the face of it appears that energy from natural gas is about half the price as energy from gasoline. However, this isn’t exactly true because car engines that run on natural gas are not very efficient. To illustrate this I would like to compare the Honda Civic CNG vs. the regular Honda Civic using data from www.fueleconomy.gov. If we compute the amount of MMBTU used per mile driven for both vehicles the gasoline Honda is about 5.7% more fuel efficient. This makes the CNG Civic about 26% more expensive to operate than the gasoline Civic.
In order for CNG to be cost effective as a vehicle fuel the price of natural gas must be cheaper relative to oil than it is today and engine efficiency must be fixed. Engine efficiency can be fixed by designing dedicated natural gas engines. Most engines used in CNG vehicles today are gasoline engines that have simply changed the fuel compression ratio. A true natural gas engine needs an independent design to be more efficient.
Also there currently isn’t enough natural gas production in the USA to fuel every car while retaining it as a power plant and heating fuel. The United States uses 178 billion MMBTU per year for motor gasoline ( EIA annual review Figure 5.11).This would correspond to about 17 TCF of natural gas. That would displace almost all the natural gas produced in the USA. This last summer T. Boone Pickens (2 words) advocated a plan that uses natural gas to fuel 18 wheeler trucks and offset the additional fuel consumption by building wind or nuclear power plants (www.pickensplan.com) . Overall, CNG for widespread use doesn’t seem like a viable alternative to gasoline unless you can replace the amount of natural gas that is diverted from power plants. Otherwise foreign gas doesn’t look much better than foreign oil. However, it could become important in the heavy transport sector.
There are companies that have designed dedicated CNG engines if you interested visit www.westport.com. They say that their design is 25% more efficient than spark plug CNG engines but they don’t tell you any concrete numbers.