Use of time

One of the problems of maintaining a maximum speed of 55 m.p.h. is that it seems to waste a lot of time. Of course, use of the mobile phone, listening to books on tape, etc. can make this time less unproductive than it might seem at first. But it's a constant challenge to find ways to be productive during commutes, whether on a professional basis (phone conversations with business associates, etc.) or a personal basis (books on tape).



But what about the time itself? How much is actually lost? I drive about 60 miles per day on average, I estimate that 36 of those miles are spent driving at 55 m.p.h. when I could drive 70 m.p.h. That's faster than the speed limit but probably about what would be considered "normal." At 55 m.p.h. I spend 39 minutes, 16 seconds driving more slowly than "normal." If I were driving at 70, I would spend 30 minutes, 51 seconds driving the 36 miles. So I seemingly lose 8 minutes, 25 seconds per day.



However, I also only have to refuel about every 8 days instead of every 5.3 days (on average). I estimate that pulling off the road, fueling and getting back on the road takes about 8 minutes, so I save an average of 31 seconds per day stopping for fuel less often. Thus, the grand total loss is 7 minutes 54 seconds per day. In the course of a year, I lose 33 hours and 11 minutes behind the wheel (assuming 252 such days per year).



Looking at this time another way, my company values the 7 minutes and 54 seconds at about $12.66. An interesting comparison is that, on a given day, I save about 1.4 gallons of fuel compared to what I would have consumed using my pre-experiment driving style, which, at current Southern California prices, is worth about $4.57.



Or, yet another way is that I try to sleep 7 hours per night. Thus, the extra time spent driving amounts to 0.77% of my waking hours.



Hmmm....

Ween us from imported oil? Maybe not...

One of the things I've done in my quest to get the maximum out of a tank of gas is to forgo the drive-through window. In so doing, I began to wonder how much gasoline is burned in the United States idling at drive-through windows. I decided to try to figure it out - maybe outlawing drive-through windows (something a person with a libertarian bent such as myself would never advocate) would end our reliance on imported oil.

I've read that Enrico Fermi felt his students should be able to come up with an estimate for almost anything. The classic example is to estimate how many piano tuners there are in New York City. There's even a competition to come up with order of magnitude estimates ("Fermi Answers") to obscure questions. Though I've never entered any sort of formal competition of this nature, the idea fascinates me. Anyway, such skills helped me with my answer to the query mentioned above.

Though "google is my friend," I wasn't able to find every piece of data I needed in my fairly brief search. I couldn't find statistics on how many fast food drive-through window visits occur in the U.S. in a day (or a year). Using estimates based on how many drive-through windows I think there might be, how often the people I know use them, what I've seen at the restaurants, etc. I decided maybe 1.5 million such visits occur each day. I estimated that an average visit is four minutes, and an average car burns 0.25 gallons per hour at idle (mine burns 0.38). I subtracted a minute because that was my guess as to the idling equivalent of pulling into a parking space, stopping and then starting the car after going into the restaurant.

Using these figures, I estimate that 18,750 gallons of fuel are wasted in drive-through window visits per day in the U.S., or about 6.84 million gallons per year. I was able to find a very informative site from a consulting geoscientist. I learned that a barrel of oil produces 19.5 gallons of fuel, so 6.84 million gallons require 350,000 barrels of oil.

I also learned that we use 21.93 million barrels PER DAY of which 13.21 million barrels are imported. So we import 4.82 billion barrels of oil per year. Thus, elimination of all fast food drive-through window stops could eliminate maybe .007% (that is, 7 one-thousandths of 1%) of our oil imports.

Well, ya gotta start somewhere...

To floor it or not to floor it

I'm trying to understand the effects of rate of acceleration on minimizing fuel consumption over a given distance. We've all read and heard that "jackrabbit starts" waste fuel. How can this be shown? Well, fuel contains energy in hydrogen and carbon chemical bonds. We oxidize fuel to release this energy. The energy does two things in our vehicle - it adds kinetic energy, thus changing the potential energy of the chemical bonds to the kinetic energy of the moving vehicle, and it does the work of moving the car against the sum of the forces acting to resist motion. These include engine and driveline friction, pumping fluids, aerodynamic drag, tire rolling resistance, etc. For the purists, it also can increase our potential energy by utilizing chemical potential energy to raise our position in the Earth's gravitational field (i.e., take us up hills). But since, on average, our vehicle stays at one elevation (that is, at the elevation of wherever it lives) we can ignore this.

It's easy to show that the more slowly we accelerate, the farther we go in the process of adding a given amount of kinetic energy to the car (that is, getting up to a given speed since kinetic energy is one half the product of the mass of the vehicle and its contents times the square of the velocity or speed). Thus, if we accelerate from 0 to 60 in twice the time, that is, accelerate at half the rate, we will go twice as far to add the same amount of kinetic energy, which comes from burning the fuel. So, the same amount of fuel will take us twice as far while getting us up to the same speed. It will take longer but be more fuel efficient. As I said, this is quite easy to show mathematically.

So it seems like a no-brainer. But... It's also true that there is a speed at which an automobile gets the best fuel economy. Because it burns fuel while idling (my Grand Cherokee burns 0.38 gal/hour as nearly as I can determine), the car gets 0 m.p.g. standing still. As we start moving and achieve higher speeds the fuel economy, that is, the m.p.g., increases. Above some speed, different for each vehicle, the aerodynamic drag, which increases approximately with the square (at least according to my calculations - others say cube) of speed begins to take its toll and the efficiency decreases. So, the vehicle is more efficient (gets better gas mileage) the faster you go, up to some speed at which efficiency begins to decrease. I'm still working on the mathematics of this but it seems intuitively reasonable and agrees with the nuggets I've found on the web (see here for example) and some "back of the envelope" calculations.

So let's say, for conversation's sake, that for my Grand Cherokee, the most efficient speed is 50 m.p.h. I think this isn't too far off, based on my experiments and a website I found (but since lost). And let's say I'm going to take a 20 mile trip. To make it easy, the trip is on a level road, no stops. I could, in principle, accelerate so slowly that I don't get to 50 m.p.h. before reaching my destination. Or, I could floor it and reach 50 m.p.h. as quickly as possible and drive the greatest possible portion of my trip at the most efficient speed. So flooring it gives me the maximum number of miles at the most efficient speed. On the other hand I could, in principle, accelerate so slowly that I only get to, say, 5 m.p.h. before reaching my destination and drive the entire trip at a very inefficient speed. This seems to imply that quick acceleration is most efficient. So which is best?

I'm still working on it, so more to follow.....

The Goal

I'd like to welcome myself to the world of blogging. I have a hard time with the term "blogos.....," maybe in time I'll come to accept it.

The intent of the blog, in the beginning, will be to document my efforts to minimize my use of energy, particularly auto gas. I drive a 2000 Jeep Grand Cherokee Limited with a 235 horsepower V8.

Until August of 2005, I drove it like I used to drive my 1970 Roadrunner in high school, that is, as fast as traffic would allow (and sometimes faster). As for acceleration, I used to say "I don't need an accelerator pedal, I just need a switch." It was full throttle all the time. The Jeep is equipped with a display of "average mileage" and "instant mileage." As nearly as I can tell, the average represents mileage over the last several tanks - maybe 4000 or 5000 miles. It can't be from 0 on the odometer - it changes too quickly for that. Or so I think.

In any case, at the start of my experiment, the average m.p.g. was 14.9, reflecting my extreme driving habits (according to the E.P.A. the car is rated for 15 m.p.g. city, 20 m.p.g. highway).

In August of last year, as gas prices in the Los Angeles area approached $3.00 per gallon, I started toward the opposite extreme of driving, incorporating snail-like acceleration, coasting down hills in neutral, shutting the engine off at long stops, cruise-controlled 55 m.p.h. and attempts to look ahead and put the car in neutral to coast to a stop for traffic, etc. The brake pedal became my enemy, I pictured burning gasoline to heat chunks of metal. I filled my tires to 2 p.s.i. over the maximum rating (don't try this at home).

My current average is 22.5 m.p.g., a 51% increase. I estimate that I have saved approximately 350 gallons of fuel. Obviously, someone starting from a more normal driving technique could not expect as great an improvement. But even for someone getting average mileage consistent with the EPA estimate, say 18 m.p.g., which many do not achieve, a 25% improvement would be possible. Again, most would not take it to the other extreme as I have, but a 15% improvement would seem to be easily achievable.