Last week our humble little university got a visit from this handsome guy, and Paul and I were lucky enough to attend and even get seats in the bandstand (thanks to Paul for the great photo!). The speech was carefully tailored to the crowd of mostly college-aged young voters, focusing largely on the themes of education loans, marriage equality, and renewable energy development. One of the achievements the president highlighted was the aggressive tightening of corporate average fuel economy (CAFE) standards in order to reduce petroleum imports and greenhouse gas emissions in the transport sector; that same day it was announced that the National Highway Traffic Safety Administration (NHTSA) had finalized rules doubling vehicle fuel economy to a fleet-average 54.5 mpg by 2025:
While there had been some pushback on the new regulations from industry, the major automakers endorsed the new rules last year after the administration agreed to include a mid-program review to gauge consumer reaction to the changes and adjust the target if need be. It is expected that the increase in efficiency will be achieved through a combination of decreased vehicle weight, improved aerodynamics, advancements in engine technology, and wider offerings of hybrids, plug-in hybrids, and fully electric vehicles. While the introduction of these advanced technologies will raise the sticker price of new cars, the administration argues that the increase will be more than offset by reduced fuel costs for the consumer. Of course in this election year the standard has become a political issue, with Mitt Romney challenging the new requirements as too extreme.
While there’s always some room for debate in the cost-benefit analysis behind these regulations, it must be pointed out that, despite the incredible advances in power, safety, and comfort of the cars in the US fleet over the last century or so, the amount of fuel consumed to move someone from point A to point B has hardly changed since the time that the roads were dominated by these:
Surely we can do better, if we choose to do so.
UPDATE: The other day on a drive up highway 14 in a friend’s Prius, the topic of calculating fuel economy came up. It’s not a straightforward problem (mileage varies with speed, weather, road grade, number of accessories running, etc.), and doing the tests in a transparent and repeatable yet still realistic manner is actually quite challenging. I did some additional reading, and come across the following interesting points:
- In the US, there are two different agencies that do vehicle mileage testing. The Department of Transportation does testing for compliance with CAFE standards, using a really old protocol that does account for things like air conditioning parasitics. This method apparently overestimates real-world mileage on the order of 30%. In contrast, the estimated mileage displayed on the window at the dealership is determined by EPA using a much more modern test cycle that is considered reasonably reflective of the average driver, if not a slight under-estimate (though obviously everyone drives differently). You can find non-technical overviews on it here and here.
- In order to get estimates of rolling resistance (function of velocity) and aerodynamic drag (function of velocity squared) to program into their dynamometer, EPA subjects the vehicle to what they call a “coastdown test” (not a wind tunnel test). Basically, they drive a constant speed on level ground, throw it in neutral, and then fit measurements of the resulting deceleration versus speed to a second-order polynomial. Pretty simple. Interestingly enough, the only place that I could find a clear description of the process (including equations) was in the Code of Federal Regulations: http://cfr.regstoday.com/40cfr1066.aspx#40_CFR_1066pSUBPART_D. Though here’s the simplified version: http://www.instructables.com/id/Measure-the-drag-coefficient-of-your-car/