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   Carrying Saleen wheels and Bullitt wheels.

Saturday, June 09, 2007

The Fuel Economy Trade-Off Game








































































































Technology/Technique

Cost

Safety

Convenience

Durability

Fuel Save

Total

Comments

Reduce Mass: downsize

+1

-2

-2

0

+1

-2

Americans like big roomy cars.
Safety suffers due to interface with older cars.

Reduce Mass: materials

-1

-1

0

0

+1

-1

Al, Mg, carbon fiber cost more.

Reduce Engine Output

+1

0

-2

0

+1

0

Americans like powerful cars.

Mild Hybrid Powertrain

-1

0

0

-1

+1

-1

More complexity (batteries,
generator) hurts durability.

Full Hybrid Powertrain

-2

0

-1

-1

+2

-2

Even more complexity.

Gas Turbo Direct Injection

-1

0

0

-1

+1

-1

More complexity.

Flex Fuel (Ethanol)

0

0

-1

0

+1*

0 (-1)

Fuel availability problems; less
gas used, but nearly same carbon output.

Passenger Car Diesel

-1

0

0

-1

+2

0

More complexity due to emissions
regs.

Hydrogen Fuel Cells

-1

0

0

+1

+3

+3

Still a research project.



How do you compare the different ways to increase fuel economy? If you are an engineer, you might make a table which assigns weights to different characteristics and then levels for each one. Combine the numbers, and you have a handy way to compare different choices.

Here is a swag at the fuel economy trade off game, according to my near-expert opinion. The method is to equally weight Cost, Safety, Convenience, Durability, and Fuel Economy. -2 means big decline (more cost, less safety, less convenience, less durability), while a +2 means a big improvement (less cost, more safety, more convenience, etc.). Minus bad, plus good.

So, the way I see it, for a modest fuel savings, the best all around technique is to reduce power, followed by gas turbo direct injection. For large fuel savings, the light diesel seems best. The Holy Grail, as always, is the hydrogen fuel cell.

What is inevitable is that you can't have everything--this is a law of engineering, where physics and economics meet. Want lots of power? Lose weight (and safety). Want safety and fuel economy? Gain cost.

We Americans need to have a serious discussion about what it is we really want, and we need to tell our legislators. What are we willing to give up?

OK, Edmunds just posted a great article which assigns grades to the various fuel saving technologies. You may not believe me, but in fact I was working on the same type of post. I was trying to figure out how to make the table work right.