Tuesday, June 04, 2013

Limits to efficiency of vehicles

Lost in the (rightful) debate over new fuel economy standards of over fifty miles per gallon--more than any mass produced vehicle currently in production save one version of the Toyota Prius--is the question of exactly how efficient a vehicle can be.

A basic rule of thumb is about 70% wind resistance and 30% rolling resistance for a typical 5 passenger vehicle with a drag coefficient of about .35, weight of about 2 tons, and mileage of about 25mpg.  This includes the mpg-e numbers for actual Carnot efficiency for electric vehicles, too. (shame on the EPA for their fraudulent "mpg-e" numbers, by the way)

Let's work the numbers.  You won't get far with streamlining without comfort or safety hazards, as anyone familiar with the rear headroom and visibility in the Prius or Tesla can attest.  Hence, your wind drag drops only from .7 to about .6.  Weight?  Aluminium has a scrap cost of over $1/lb.  Transmission and engine improvements?  Do you think that adding four speeds on every transmission comes free?

A $1000 budget (500 lbs aluminium to replace 1000 lbs steel, $500 for better transmission/turbocharger/etc) could get you to wind resistance of .6, rolling to .25, and a 10% increase in drivetrain effiency and....only 33mpg.  Our goal of 50mpg or more thus requires smaller and less capable cars, exotic materials (carbon fiber, etc..) and powertrains (hybrid, fuel cell). 

Will it help the environment?  Well, hybrids and such require about 8 tons of carbon dioxide emissions, equivalent to about 85 gallons per year.  Given 12000 miles per year on average, our hypothetical "Obama-mobile" gets a net mpg (including build costs) of only 37mpg, hardly better than our conventional vehicle of 33mpg.  Our average driver will spend $750 annually to save $140 in gas.  Typical government logic, but hardly a cost-benefit analysis that will induce me to buy a Prius.

We're at a point of diminishing returns.  Make sure you vote for people who, unlike our President and too many in Congress, understand this.

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