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Land Rover on Namibian Dunes
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Well-to-Wheel Efficiency - Part 3

Part three of series by AskPablo on the relationship between energy efficiency and vehicle aerodynamics.

By Pablo Päster

In Part I we learned about the energy required to overcome rolling resistance and aerodynamic drag. In Part II we learned about the energy consumed in acceleration. Now it's time to bring it all together. We know how much energy it takes to get a vehicle up to a certain speed and to keep it there. We also know that the fuel we put into the tank contains more energy than we get back out. My car's efficiency came out to be 19.9% (see Part I), but where does the other 80.1% go? And is there anything else to consider?

On the EPA's Fuel Economy site there is a lot of interesting information about vehicle efficiency. On average a vehicle's internal combustion (IC) engine is 37.6% efficient. Most of the energy is lost as waste heat, removed by the radiator or expelled through the exhaust system. Diesel engines are around 30% more efficient. A further 17.2% of fuel energy is spent on standby or idling, leaving 18.2% of the input fuel energy.

Of the remaining 18.2%, 2.2% is lost to accessories such as AC, lights, and radio. Finally, 5.6% is lost in the vehicle's transmission and other drive train components. This means that only 12.6% of the input fuel energy is turned into useful work to propel the vehicle. This number is lower than my 19.9% because my vehicle is several mpg over the national average and because my efficiency calculation did not take into account any idling or increased fuel consumption due to acceleration.

According to the EPA figures, 2.6% of the remaining 12.6% is lost in overcoming drag and 4.2% is lost to overcome rolling resistance. This leaves 5.8% for accelerating up to speed, all of which is lost to braking (except in hybrid vehicles which recapture about 10% of this energy). Now, if you recall from Part I, around 15% of the embodied energy of gasoline is incurred in extracting, transporting, refining, and transporting again. So, of the energy that comes out of the ground in the form of crude oil only about 10.7% actually move your vehicle from Point A to Point B.

All this talk of how inefficient our personal transportation is is pretty discouraging. And knowing that the average vehicle in the US gets just above 20 mpg does not help either. There is another way to look at vehicle efficiency. In the book Natural Capitalism Lovins et al suggest that most of the energy used by vehicles is used to move the vehicle itself, not the occupants. If you have read all three sections of this column you now have the technical knowledge to understand the forces and energy involved. If you think about it though, the goal is not to transport a 3,000 lb steel box everywhere you go, the goal is mobility. It is getting us, our families, and our groceries from one place to another. By looking at the energy used per person-km (number of passengers x distance traveled) we get a much better understanding of the inefficiency of a single-occupant Hummer vs. a fully utilized vanpool.

This weekend's collapse of the 80/580/880 interchange in the San Francisco Bay Area only serves to remind us of how dependent we are on personal automobiles for transportation and how vulnerable our infrastructure is. According to one CNN.com report around 280,000 commuters, including myself, will be affected. I am grateful that I have marginal access to public transportation and I look forward to the day when we live in a society that is interconnected by an efficient and equitable system that provides personal mobility.

In the meantime, what can we do?

Times Article Viewed: 5326
Published: 02-May-2007

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