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Problems with the proposed CAFE Standards for 2020. Dennis Silverman Physics and Astronomy UC Irvine. Introduction. The Senate bill on the new CAFE standards for 2020 mandates a fleet average of 35 miles per gallon (mpg) which includes both cars and small trucks (SUVs, trucks and vans).

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Problems with the proposed CAFE Standards for 2020

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Problems with the proposed CAFE Standards for 2020

Dennis Silverman

Physics and Astronomy

UC Irvine


  • The Senate bill on the new CAFE standards for 2020 mandates a fleet average of 35 miles per gallon (mpg) which includes both cars and small trucks (SUVs, trucks and vans).

  • We will show here problems with the single fleet average in actually being able to increase the mpgs of small trucks.

The problem of the total fleet average with very efficient vehicles

  • Some of the present hybrids are even now achieving 50 mpg ratings (Toyota Prius and Honda Accord hybrids).

  • For each of those types made by an American automaker, they can preserve the total fleet average by making a 20 mpg SUV or small truck, since ½(50+20)=35 mpg.

  • This SUV allowance is even lower than the present 21.5 mpg group average for small trucks.

  • One way around this problem is to exclude plug-in hybrids and electric cars from the average.

Future plug-in hybrids or electric cars.

  • Plug-in hybrids are projected to run 40 miles on pure battery power from the charge they acquire from utilities.

  • In terms of greenhouse gases, utilities produce the electrical energy more efficiently than the internal combustion engines of present hybrids.

  • Priuses that are outfitted as plug-in hybrids are said to obtain 70-75 mpg equivalent.

  • Plug-in hybrids are estimated to exceed 100 mpg based on greenhouse gas emissions from utilities, while under the 40 mile all electric range.

  • If the electricity is generated by nuclear, hydro, solar or wind power, than the greenhouse gas emissions are essentially negligible, and the effective mpg are well over 100 mpg.

  • Because of this, it is hard to provide a single effective mpg for these vehicles. A national average based on the average greenhouse gases for the nation’s electricity supply could be used.

CAFE ratings and super efficient plug-ins.

  • I haven’t found an explication of how the NHSTA will decide on mpg ratings for plug-ins.

  • Present tests are based on something like the first 12 minutes of driving, or the first 12 miles of a drive.

  • Since those ranges are covered by the electric charge, even natural gas powered utilities should give and effective mpg of 100 mpg.

  • We assume this is included as the vehicle rating, until otherwise specified.

  • The average American commute is under 30 minutes or miles. If recharged at work, the electric nature of the plug-in could completely cover a day’s driving.

The problem with a single fleet average and the super-efficient plug-ins.

  • We use an example where each plug-in hybrid produced in America is rate as 100 mpg.

  • Using the single fleet average, such a hybrid is 65 mpg over the average.

  • The average can then be balanced say by three SUVs or small trucks that get

    (35 - 1/3(65)) = 35 – 22 = 13 mpg.

The reaction of an environmental public

  • By the time 2020 arrives, and without a wise rating or averaging policy by EPA, each environmentally minded citizen will know that his or her buying a super-efficient plug-in hybrid will be nullified by three purchasers of highly inefficient Hummer type SUVs or giant “small” trucks.

  • This inequity should be fixed before the CAFE bill is passed by the full Congress.

  • We discuss such fixes in the following slides.

The fix for SUVs and small trucks

  • It is better if SUVs and small trucks have their own average, whatever it is.

  • Detroit has proposed a 35 mpg average for cars and a 30 mpg average for SUVs and small trucks. That should be a more safeguarded small truck average that is a real requirement on them.

  • However, among the cars, the same unenvironmental countering of plug-in hybrids might occur with cars that are no more efficient than those that get the current 27.5 mpg average.

A truer average based on the fuel consumed

  • Most cars in America drive an average of 15,000 miles per year.

  • The gas that is consumed by a car or small truck is given by

    fuel = miles driven x (gallons/mile)

  • This contains the reciprocal of the miles/gallon ratio currently used.

  • For simplicity, let us take this measure of fuel used in a 100 mile trip, and call it the CAFE Fuel Standard

  • Then the sample plug-in hybrid at 100 mpg will give a fuel standard of unity:

    fuel = 100 miles / (100 mpg) = 1

The New Fuel Standard

  • The typical vehicle at 35 mpg under the new fuel standard will be rated as

    fuel standard = 100 / 35 = 2.86 or approximately 3. (Environmentally it is 3 times as greenhouse gas polluting as the best car available, the plug-in hybrid.)

  • This would be the new fuel standard average for a 100 mile trip that Congress or the states could adopt.

Applying the Fuel Standard to the SUV example

  • The example had a 100 mpg plug-in hybrid balanced by three 13 mpg SUVs under the 35 mpg average.

  • Applying the fuel standard (fs), the 13 mpg SUV will have fs = 100 / 13 = 7.7.

  • Under a fuel standard average, the four vehicles above would average

    ¼ (1 + 3 x (7.7)) = ¼ ( 24.1) = 6.0

    not 3.0.

What is the vehicle mpg that balances a plug-in in the fuel standard?

  • Take m as the mpg of the gas vehicle.

  • For one gas vehicle to balance one plug-in giving an average fuel standard of 3

    fs = ½(1+100/m) = 3 implies m = 20 mpg.

  • For two gas vehicles balancing one plug-in

    fs = 1/3(1+2x100/m) = 3 ; m = 25.

  • For three gas vehicles against one plug-in

    fs = ¼(1+3x100/m) = 3 ; m = 27 mpg.

  • For four gas vehicles balancing one plug-in

    fs = 1/5(1+4x100/m) = 3 ; m = 29 mpg.

How does balancing work out for a major part of the fleet being plug-ins?

  • From the previous slide, if 20% of the fleet are plug-ins, they are balanced 4 to 1, and the average requirement on gas vehicles is 29 mpg.

  • If 25% of the fleet are plug-ins, the average requirement on the gas vehicles is 27 mpg, comparable to the present mpg average on cars.

  • The above 3 gas to one plug-in balancing case used in the example with the present mpg averaging method only required gas vehicles to have a 13 mpg average.

  • So we conclude that the fuel usage standard comes close to what the Congress intends to occur with enhanced CAFÉ standards.

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