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Professor Quester Answers
Questions on Transportation Energy

Dear Dr. Q:
I was wondering if someone there could help me..... I'm trying to figure out how many lbs. or tons of carbon are used to burn a gallon of unleaded gasoline in the average American car. I recently heard of a 25 year rainforest forestation plan that would save a total of 5000 tons on carbon. I'm just trying to figure out what that means to the average American consumer.   (Pete A. Munoz, East Lansing, MI)

The Professor Answers:
You would not burn carbon, but you would burn gasoline yielding energy and exhaust emissions...most of which is in the form of carbon dioxide (CO2).

I looked at the US EPA Web site and found the following conversion.

Suppose a state had 1990 gasoline consumption of 200 trillion Btu (such data are reported in U.S. DOE, 1993). Using the State Workbook methodology, 1990 CO2  emissions from gasoline consumption would be calculated as follows:

CO2  Emissions = Consumption x Carbon Content Coefficient x Percent Oxidized 44/12
CO2  Emissions = 200,000,000 million Btu x 41.8 lbs C/million Btu x 99% x 44/12
CO2  Emissions = 15.2 million tons CO2

So, based on this conversion, 200 trillion Btu (British thermal units) of gasoline produces 15.2 million tons CO2, or 30.4 billion pounds. There are 125,000 Btu in a gallon of gasoline. So, 200 trillion Btu equals 1.6 trillion gallons of gasoline. That's 1,500,000,000 gallons producing 30,400,000,000 pounds. So, one gallon of gasoline produces 19 pounds of CO2. So, if the rainforest saved 6,000 tons of carbon, that would be equivalent to not burning 631,579 gallons of gasoline.

Dear Professor Quester:
My son is currently researching electric vehicles for a school project and this question arose - Why don't electric vehicles take advantage of the wind that is created by the vehicle in motion? Isn't there a 55 MPH wind to be harnessed? Is it not possible to design and build a means to capture that wind? Wouldn't the additional energy developed out-weigh the associated drag the design would create? It seems to me to be an interesting design problem, one that would greatly increase the distance between stopping for battery recharging. Is this possible? I know it sounds like something for nothing, but that IS what solar and wind energy IS, right?

Thank you for helping.   (Bruce and Matthew, 11th grade, Delaware Valley Friends School, near Philadelphia, Penn.)

The Professor Answers:
Let me see if I can answer this one. This question has been debated in the electric vehicle (EV) discussion groups, and I think I understand the basics of the arguments.

It has to do with air resistance of the blades and how large the blades would have to be to generate any energy. The Law of Conservation of Energy basically says "there's no such thing as a free lunch" when it comes to energy "production" (actually changing energy from one form into another because energy can't be created or destroyed).

To change the air rushing through and turning a turbine blade on an EV into any usable amount of energy would require a fairly large propeller. This would increase wind resistance of the vehicle, which means the vehicle would have to expend more energy to overcome that wind resistance. Also, the added weight of the turbine blades and generator system would be extra weight the vehicle would have to overcome in moving the vehicle forward.

So, while it sounds like a good idea, the bottom line is that it would not be practical and would probably not produce enough energy to compensate for the energy used with the propeller on the car. The best thing to do is make the vehicle as aerodynamic as possible and as light as possible to cut the amount of energy consumed.

If the turbine would work, then all we need to do is get a vehicle up to a certain speed, then let the turbine take over. The turbine could then keep the vehicle going, literally forever...or until we ran out of highway or into a traffic jam. But, alas, this doesn't work because of the Law of Conservation of Energy -- this would be a perpetual motion machine, which, though many have claimed to have perfected one, so far no one has been able to build one and proveit.

As to solar and wind energy ... it is not something for nothing. The energy produced is actually a change in form from light and mechanical energy, respectively, into electrical energy. There are great losses of efficiency when doing this. The best wind turbines are only about 30 percent efficient and solar cells are about 10 to 15 percent efficient in converting light and mechanical energy, respectively, into electricity. Because there is no cost associated with the source of energy -- the "fuel" for a wind turbine and a solar cell, it seems like we're getting something for nothing.

We hope that answers your question. And thanks for writing.

If YOU have a question about energy, send your question by e-mail to "Professor Quester."
Ask your parents or teacher first before sending an e-mail. Please tell us your grade level, the name of your school and your city. We will usually respond within four or five days.

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Page updated: May 17, 2002
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