Friday, August 1, 2008

Just 1%

As some of you may have noticed, gasoline is now over $4.00 per gallon in the United States, and if that hasn't motivated you to drive less, maybe this will. Your car is only 1% efficient.

That's right, out of the $4 it now costs for a gallon of gasoline (if you can still find it for $4), only 4 cents is actually being put to use getting you around. The other $3.96 is being lost to thermal efficiency and moving a 3000+ pound vehicle.

Let that sink in for a moment. As politicians debate an 18.4 cent gas holiday, off-shore drilling, and a 35mpg CAFE standard that won't take effect until 2020, we're ignoring a much bigger problem. That the automobile is possibly the LEAST efficient way to move a single person around.

Getting to 1%

If we say a gallon of gasoline has 100 units of energy (and ignoring the embodied energy required to produce and deliver it to the pumps), burning it in an internal combustion engine converts only 20% into rotational energy (for diesel engines it's roughly 30%). So from the original 100 units, we are already down to 20. Of those 20, another 15-20% is lost to drivetrain friction. That leaves 17 units actually being put to use driving the car. With most vehicles now over 3000 pounds, the individual inside is only about 6% of the total load, and therefore only using 1% of the energy.

That's 1% of the energy delivered at the pump. If you start calculating the energy that goes into producing the car, the gallon of gas, shipping it to market, the roads, parking spaces, garages, etc. you're talking about very small fractions of a percent.

If we are serious about fuel conservation we have to be serious about driving less, in addition to improving fuel efficiency, and converting to alternative energy. The more we continue to encourage driving, the more we are forced to drive. More cars require more roads, more parking, and longer distances to be driven, meaning more cars more parking, and less effective mass transit and other alternatives. Most of our planning policies are based on a cheap oil era, with a near total reliance on automobiles to get us between buildings sited in single use zones. Our planning policies generally discourage mixed use, walkable, or transit oriented developments, except in rare cases, and need to brought into line with the current and future economic realities of energy.


Anonymous said...

The article you liked from listed 12.5% efficiency for the average vehicle. I think you might have performed one too many subtractions :).

That article is a bit dated though. Variable valve timing for instance has been pretty common for the last 10 years. I know Honda and Saturn use it, probably others. In honda's it's known as VTEC.

There's many cars on the market today which are 30% or more efficient.

For instance, my VW has a turbo, direct fuel injection, AMT, etc... I bought it 3 years ago, and by my math it's 40%+ efficient.

incontrovertible said...

Nimic @ The Green Routine, thanks reading the blog and taking the time to comment, however I stand by my numbers. If you look at my link, it presents the thermal efficiency as 18.2%. The 20% I used in the calculation is still accurate as an average (and the high side of the range at that) for the gasoline internal combustion engine used in passenger cars.

You are right that there are many technologies that can improve on the fuel efficiency compared to a traditional naturally aspirated engine. Adding forced induction (turbo or supercharging), direct fuel injection, variable valve timing/lift, variable intake runners, advanced engine management, higher compression ratios, exotic materials, and tighter manufacturing tolerances can all improve engine performance, however I think you are misinterpreting the improvements from the link you provided. Their percentages are representative of fuel economy gains (7% improvement on 30mpg yields 32.1mpg, not a 7% increase in thermal efficiency as your 40+% VW would indicate). Additionally I am a little skeptical of some of their estimates. In regards to what they call ISG (what BMW terms Start-stop) the realized gains I have seen from manufacturers who have used the technology claim only 2-3% fuel efficiency improvements, much less than the 7% quoted. I would also mention that the site omits weight reduction as a proven "technology" for improving fuel economy (and generally improves overall driving dynamics). Personally, I'm still waiting for someone to make a start-stop turbodiesel with a continuously variable transmission (which would help compensate for their narrow powerband).

Additionally, even if your VW did attain a 40% thermal efficiency, the effective efficiency as calculated in the blog would still only be 2%. Don't get me wrong, 40% thermal efficiency, all else being equal, would translate to something like 60+ MPG and would go a long way to reducing our energy diet, especially when coupled with strategies to reduce our driven miles. But it still illustrates the inherent inefficiency of buring gasoline in cars to transport individuals.

Added food for thought, for every gallon of gasoline burned, roughly 19 pounds of CO2 is produced (so CO2 production for a car is directly linked to its fuel economy).

Sorry, that response wound up being a little longer than I planned.

Nimic said...

I appreciate your long response. I obviously didn't do as much research on this topic as you, and your comment drove home the point you were initially trying to make...

That only 1% of the fuel is actually used to move the passenger(s). I was counting the moving of the car in my calculations, and I missed that you were intentionally subtracting that.

I agree that the estimates are likely high. As it stands, I only get about 32mpg on the highway with all of those features - not 60mpg.

They're probably using a Prius or other hybrid to calculate their ISG gains. Probably choosing best in class theoretical numbers to calculate all of the high end gains.

A car is much less effective than walking, biking, a scooter or even mass transit.

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