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GM's MPG Wonder

American Government Special Collections Reference Desk

Topics:  Chevrolet Volt

GM's MPG Wonder

Klaus H. Hemsath
September 11, 2009

Klaus H Hemsath

The announcement of General Motors, that the new VOLT will be capable of achieving 230 MPG or miles per gallon, has many people wondering what this figure means.

The driving public needs to know fuel consumption of cars to foretell driving costs when selecting a particular brand or model. The availability of hybrids is complicating matters further. And how does one compare costs of gasoline to the costs of electricity?

Comparing the cost of driving a certain distance is really the final objective of potential buyers. Therefore, it makes the most sense to introduce a figure that compares energy costs in dollars for driving 100 miles.

A hybrid is able to consume electricity exclusively when driving short distances in the city. When driving longer distances, hybrids and gasoline powered cars must still use gasoline. Regrettably, recharging of hybrids on the road will face three annoying complications: - Electric energy at recharging stations will be at least 50% more expensive than at home, costing about 17 cents at the pump versus 11 cents at home, - Recharging will take a long time, - For at least the next ten years it will be difficult to find recharging stations.

Energy consumption is measured at the meter installed by the utility supplying the electricity. Each gallon of gasoline contains a certain amount of energy. Energy content of gasoline can be measured readily in kWh, the same unit that is used for measuring electric energy. The cost of gasoline at the pump is either t $2.60/gallon or 7.1 cent per kWh. Prices for electricity have been rather steady. Prices of gasoline are much more volatile.

GM has announced that the VOLT will be able to store 16 kWh in its battery and will be able to drive 40 miles on this energy. Recharging the battery is an energy conversion process and is about 90% efficient. The cost of driving the VOLT 100 miles in the city when using electricity will be $2.96 when paying 11 cent per kWh at the receptacle.

The VOLT also has a gasoline powered electricity generator on board. The exact technical details are not yet known. We will assume temporarily that the Volt will consume 45 miles per gallon when driving 55 miles per hour on the highway. We will further assume that GM has developed a very fuel efficient engine that has an energy conversion efficiency of 37%. From these data one derives that driving the VOLT on the highway will cost $5.77 per 100 miles. Driving on gasoline only in the city will be roughly 25% more expensive. Therefore, city driving on gasoline will increase cost to $7.21 per 100 miles. There will also be a huge difference in driving experience. Driving on electricity the VOLT can accelerate much, much faster compared to using gasoline.

Now all needed data are available to make a cost comparison. Arriving at a mileage of 230 MPG is still difficult and will require the services of qualified lawyers and advertising professionals.

Driving the Volt exclusively on batteries in the city will result in costs of $2.96/100 miles. Driving on gasoline in the city will increase costs to $7.08/100 miles. Driving on the highway on gasoline at 55 mph will be less expensive at $5.66/100 miles. Driving on the highway with electricity only costs $4.57/100 miles but will not be feasible for many years.

One word of caution. Several assumptions had to be made to arrive at these figures. It is hoped that GM will correct these preliminary figures in due time.

Some readers may ask why electric cars are promoted that heavily by car companies and governments. Using electricity instead of foreign, imported petroleum can save billions of dollars annually in a country's balance of payments. Major savings will strengthen the international exchange rate of the dollar.

There is also a major, negative impact of using electricity in cars. Electric power plants will have to burn more coal. Burning more coal will increase air pollution in the US and will add millions of tons of greenhouse gases to the Earth's atmosphere.

The ability of the US to replace coal fired power plants is very limited. Therefore, the US will increase its greenhouse gas emissions when converting a large percentage of automobiles to hybrids or all electric operation. This fact has been sadly neglected by our media when generating the hype about using electricity instead of gasoline for powering our irreplaceable, combustion engine equipped cars.

By the way, if one does not count electric energy in the mileage computation, one can arrive at any value between 0 MPG and one million MPG! Happy automobile shopping.

Dr. Hemsath's books, Climate Change-Gold Rush or Disaster? and Clean Energy For Centuries, offer a comprehensive plan for saving Earth from overheating. He is now writing a follow-on book, Petroleum Substitutes From Biomass. For fifty years he has worked on advanced energy technologies as scientist, engineer, inventor, Corporate R&D Executive, CEO, entrepreneur, and author. He holds more than 60 US Patents. Go to http://www.thermalexpert.com

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