Wednesday, July 15, 2015

Beyond Coal: U.S. Energy in Historical Perspective

I just read a fascinating article on the so-called “war on coal” that has shut down a significant fraction of U.S. coal-fired power plants over the last several years. What was almost unthinkable just a few years ago has become a reality, thanks to a confluence of technology (shale gas extraction, wind power, and efficiency), economics (the great recession), government regulations (thanks, Obama!), and environmental activism (the Sierra Club’s “Beyond Coal” campaign, funded by Michael Bloomberg).

The article is accompanied by a graph that shows all the sources of U.S. electricity over the last 30 years, highlighting the dramatic (roughly 20%) decline of coal since 2007—even while coal remains larger than any other electricity source.

I love graphs like that, but I wanted a longer-term perspective and I also wanted to visualize the data a little differently. So I pulled the data from the EIA web site and plotted it up as a stacked area chart, going back to 1950:


The recent decline of coal is all the more striking when juxtaposed with its remarkably steady rise over more than 50 years. Though if you look closely, you’ll see that the rise had already flattened out before 2007.

The advantage of the stacked area chart is that it also shows the total electricity generation at a glance—and the behavior of the total is also striking. After an almost uninterrupted rise from 1950 through 2007 (with just a couple of hiccups due to the oil price spikes of the 70s and early 80s), U.S. electricity generation (and consumption) stopped growing in 2008. Even though our economy has recovered in most respects since 2009, our electricity use hasn’t quite regained its pre-recession peak. I won’t try to predict whether it will do so in the coming years.

Meanwhile, there’s so much more to notice on that graph. Look at the rise and fall of petroleum as an electricity source. Marvel at the rapid rise of nuclear power and how steady it has remained in recent decades. And don’t overlook that expanding sliver of green at the top, which now comes mostly from wind energy (4.5% of total U.S. electricity in 2014).

To get a better view of wind energy and the other minor contributors, here I’ve plotted the same data on a logarithmic scale (with no stacking):


On this graph, a straight, upward-sloping line corresponds to exponential growth (a fixed percentage increase each year). It’s interesting to look at how each electricity source has experienced a period of approximately exponential growth at some time in the past, but these periods always end when that growth runs up against practical limits. The exponential growth of wind has recently slowed, but now solar-generated electricity is in a period of dramatic exponential growth. Let’s hope this period lasts a little longer!

I find it remarkable, though, that the log-scale graph of total U.S. electricity generation is almost entirely concave-down. The very rapid exponential growth of the early 1950s slowed somewhat in the 60s, then slowed a lot more after 1973, then slowed to a crawl after 2000, and has now more or less stopped.

Of course, electricity isn’t the same as energy. For a bigger-picture view we should also include fuels used for heating and transportation and industrial machinery. The energy sources used for all these things, including electricity generation, are called “primary” energy, and EIA actually has estimates of primary energy use, by source, going back to the founding of the American colonies. For the first 200 years the only important source (besides muscle power, which EIA doesn’t count) was wood. I’ve started the following graph in 1850, when coal makes its first appearance:


The units on this graph are quadrillions of British thermal units, or “quads” for short. One quad equals 293 billion kilowatt-hours, but the inherent inefficiency of heat engines means that a quad can generate only about 100 billion kWh of electricity. Roughly, therefore, the current annual total of about 4000 billion kWh on the electricity graphs requires about 40 quads of primary energy. The other 60 or so quads of primary energy go toward transportation, heating, and industry. (To see a careful breakdown of how each of these energy sources is used, look at the latest energy flow chart from Lawrence Livermore National Lab.)

(A couple of technical notes on the primary energy data: First, the numbers from before 1949 are estimated from various sources and are provided by EIA at only 5-year intervals, so there could be important details that are missing. Second, for non-thermal electricity sources like hydro, wind, and photovoltaic solar cells, EIA defines the “primary” energy to be the amount of some other fuel that would produce (on average) the same amount of electricity. This fictitious accounting allows for fair comparisons between thermal and non-thermal electricity sources.)

Looking at the graph above, notice that coal provided more than half of all U.S. energy from about 1885 through 1940. During that era our cities were badly polluted with soot. My own house, built in 1935, was originally heated with coal; the coal room in the basement now stores assorted outdoor equipment and other hardware. Nowadays, coal burning occurs almost exclusively at electric power plants, mostly outside major cities.

Again it’s also useful to plot the same data on a logarithmic scale, with no stacking:


Here you can see the early growth of each major energy source in detail, notice how they were affected by the Great Depression and the 1970s, and mentally extrapolate to the right to envision a variety of possible energy futures. Petroleum remains our largest single energy source, a distinction it has held since 1950. Biomass is making a bit of a comeback, thanks mostly to ethanol added to motor fuels. Wind and solar are tiny in comparison to the fossil fuels, but their extremely rapid growth is encouraging. The recent flattening of total energy use is even more apparent than for electricity alone, extending back to the late 1990s when all forms of energy are included.

For an even bigger picture I should really plot energy use for the entire world, rather than just the United States. One of the best sources of worldwide energy data is the BP Statistical Review of World Energy. The data in the BP Review goes back only to 1989, but at least it gives the big picture since then.

According to the BP Review, Europe’s coal use was on the decline already in 1989, though it has been fairly stable in recent years. Far outweighing the declines in Europe and the U.S., however, has been the phenomenal increase of coal use in China, especially during the 2000s. China now uses approximately half of the world’s coal, and its per-capita use is now about the same as in the U.S. (although its per-capita use of petroleum and natural gas are much less than ours). Even China’s use of coal, however, was fairly stable for the last couple of years and now seems to be decreasing. And it should be pointed out that a significant fraction of energy use in the developing world goes toward manufacturing products for export to wealthier countries. The coal used to make your iPhone is not included in the graphs on this page.

Saturday, June 27, 2015

Air Travel

After carefully tallying up my home energy use and the associated carbon emissions, I realized that for context (and out of curiosity) I should do the same for my personal travel.

For daily commuting and most short errands I pedal a bicycle: no fossil fuels used there, and no more carbon emissions than if I were merely exercising for health and enjoyment.

For most longer trips (and some short ones) I drive, and I’ve kept track of the odometer readings and approximate fuel economy of all 2.5 of the cars I’ve ever owned. But I usually don’t drive alone, and I’ve never kept records of exactly how often I do, so it would be tricky to figure out my personal share of the associated gasoline and CO2. I’ll try to make an estimate anyway, but not today.

I’ve occasionally ridden on buses and trains, but not often enough for either to have made a significant contribution to my energy/carbon footprint.

That leaves air travel, which in many ways is the most interesting. It didn’t take me long to go through old credit card statements and other records, to reconstruct a list of every trip I’ve ever taken by plane. With just a bit of guess-work I count 71 trips over 35 years. Here’s a plot of my air travel history:


I never flew at all as a child; my first four flights were trips home from college (to St. Louis from Minnesota). Then in 1984 I flew to visit graduate schools on both coasts, and chose to attend one in California. That choice left me making regular flights back east to visit family and friends over the next seven years (including one year in my first full-time job). In 1991, after three flights for job interviews, I moved to Iowa—within driving distance of my immediate family but now a long flight from professional collaborators back in California. In 1993 there were more job interviews, plus my longest trip ever, to a conference in Hawaii. But I ended up in Utah, from which I’ve regularly flown to visit family and to attend professional conferences and workshops. Recently, since my dad’s final illness in 2011, my personal air travel has declined.

The mileages in the chart are somewhat uncertain because I don’t remember the locations of all the intermediate stops and transfers. But by my best estimate I’ve flown a little under 200,000 miles, in a little over 200 separate up-and-down flight legs. Over the last ten years I’ve averaged 3800 miles per year, and my lifetime average (since birth) is about the same. But as you can see from the chart, I was averaging twice that amount during grad school and for several years afterwards.

So is 3800 miles/year a lot or a little? The answer is both, depending on the standard of comparison:
  • The total number of passenger-miles for all U.S. domestic flights is about 600 billion per year. If we divide that by the U.S. population (320 million), we get an average of about 1900 miles/year per person. So my 3800 miles/year is about twice the national average. (If you include international flights, then the average American probably flies somewhat more than 1900 miles/year—but nowhere near twice as much.)
  • World-wide, annual air traffic comes to about 4 trillion passenger-miles, or about 550 miles per person. So my 3800 miles/year is nearly seven times the world average.
  • Among my friends, on the other hand, 3800 miles/year seems to be on the low side. Most of my friends are well-educated, upper-middle-class professionals who, like me, travel for professional reasons and to visit families and friends scattered across the U.S. Unlike me, however, most of them also travel overseas occasionally. And many of them just seem to fly more often than I do. My guess is that most of my friends fly about twice as much as I do today, or about as much as I did 20-30 years ago. A few of them fly significantly more than that. One of my acquaintances has accumulated nearly two million frequent-flyer miles on a single airline.
And what about my CO2 emissions from flying? The most helpful resource I’ve found for calculating this is a 2008 report from the Union of Concerned Scientists titled Getting There Greener: The Guide to Your Lower-Carbon Vacation. This report compares the carbon emissions from flying, driving, and riding buses and trains, for trips of different lengths and for different numbers of travelers. Appendix B, in particular, lists average per-passenger emissions for two dozen types of commercial aircraft, broken down into per-flight and per-mile contributions. The numbers include an additional 20% to account for emissions associated with the production and distribution of jet fuel.

Based on these numbers, I calculate that for my typical mode of flying (coach class on a narrow-body jet with an average flight leg of 950 miles), the average emission rate is 0.415 pounds of CO2 per mile. Multiplying by 3800 miles/year, I find that my flying contributes 1600 pounds of CO2 to the atmosphere in an average year. (It was much higher 20-30 years ago, when I was flying twice as much and planes were less efficient—mostly because they tended to be less full.)

As always with such estimates, this result is rather fuzzy because of all the approximations and assumptions that went into it. Even if all of my calculations are perfectly “accurate,” I haven’t included the emissions associated with manufacturing the aircraft, or operating the airports, or ground transportation. Also, aircraft have other climate impacts besides CO2 emissions, and I’ve applied no enhancement factor to account for these effects.

In any case, 1600 pounds per year is a significant contribution to my total carbon footprint—probably about 10% of the total—but not as large as the contributions from driving or food production or heating my home. For the average American, who flies only half as much as I do but uses much more gasoline and electricity, flying is actually a pretty small fraction of the total carbon footprint. And the same is true worldwide, because most people fly so much less than Americans do.

Although air travel may not currently seem like the biggest carbon concern, it will inevitably become a bigger issue in the future. Global passenger air transportation is currently growing at a rate of about 6% per year, five times as fast as the population growth rate. Further efficiency gains in air transportation will be small, and there’s currently no alternative to petroleum-based jet fuel.

The real issue with flying is that it’s so unequal. Rich people tend to fly a great deal, and increasing numbers of the middle class are becoming wealthy enough to fly 10,000 miles a year if they want to. If the world average ever gets close to that level, petroleum prices will soar and the impact on earth’s climate will be catastrophic.

Monday, May 25, 2015

Home Energy Use

Several of my friends have been receiving home energy use reports for the last few months, comparing their electricity and natural gas use to the average of their neighbors. I wasn’t selected to participate in this program/study, but I’m glad it has generated so many discussions about energy conservation. Meanwhile, folks are talking more and more about rooftop solar photovoltaic systems, which are now more or less paying for themselves even in Utah where electricity is cheap.

As a numbers guy, I’ve always paid attention to my own utility bills, trying to understand (at least in broad outline) how much energy I was using and how I could use less. And I’ve saved my utility bills for many years, so I can document exactly what I’ve used.

Here’s a plot of my monthly electricity use for the last 16 and a half years, since I bought my house. The vertical scale is in kilowatt-hours per day, plotted for each billing month, so multiply by 30.4 to get the typical monthly use, or divide by 24 to get the average power in kilowatts:
There’s quite a bit of information in this graph:
  • The three highest spikes are from when I had renters or guests (one to three at a time) living in my basement.
  • Soon after the first of these renters moved in, in September 2001, I bought a new refrigerator for the kitchen and moved the old refrigerator into the basement for the renter to use. The old fridge used about 3 kWh/day and the new one uses only 1 kWh/day (as measured with a handy power meter), so when the renter moved out in early 2002 and I unplugged the old fridge, my household electricity use dropped by about 2 kWh/day from what it had been before. (The new fridge cost $650, but it saves me about $70 a year, so it paid for itself in nine years.)
  • There are some pretty reliable seasonal cycles. I use the most electricity in the winter, thanks to the furnace fan, a space heater, an electric blanket, and having more lights on. I also use somewhat more in July and August than in the spring and fall, because the refrigerator works harder then and I use fans to keep cool.
  • Finally, there’s been a gradual increase in my electricity use over the last 13 years. I need to make some measurements to figure out exactly why, but I suppose I’m using the fans and heaters more as I become old and soft, and my laptop computers have gotten greedier for power over time. Also, since the beginning of 2012 I’ve been spending about half of every work week at home, helping to edit the American Journal of Physics.
At present, my electricity use averages just under 4 kWh/day, or about 160 watts. For comparison, the average U.S. household uses about 30 kWh/day, or 12 kWh/day per person. I use less than average because my house has no air conditioning, and because my refrigerator and lights and computer are all pretty efficient. I do cook with electricity, but I hang my clothes (indoors) to dry. And I don’t indulge in power-hungry extravagances like a second refrigerator or freezer or hot tub.

Still, my home electricity use is far from negligible. It’s pretty close to the household average (counting only electrified households) in China and Mexico; it’s nearly twice the total per-capita use (including all commercial and industrial uses) in India; and it’s a hundred times greater than the total per-capita use in some African countries.

If all my electricity came from coal, the resulting CO2 emissions would be about 3000 pounds per year. The actual carbon footprint is less than this by an amount that’s ambiguous, because of the way electricity from natural gas and renewables is mixed into Utah’s grid. I actually pay Rocky Mountain Power an extra $3.90 per month to participate in their Blue Sky program, nominally buying 200 kWh of wind-generated electricity—enough to cover 170% of what I use. For about $1500, after federal and state tax incentives, I could install enough rooftop solar panels to cover my household use, and thereby reduce each of my monthly bills by about $10. Neither wind nor sunshine, however, is always available at the times when I’m using electricity, so neither provides complete freedom from the fossil fuels that dominate Utah’s electrical grid.

Meanwhile, there’s a carbon-emitting elephant in the room that I haven’t yet mentioned: natural gas, which my house uses for space heating and water heating. Here is a plot of my monthly gas use over the last 16 and a half years:
I’ve again plotted my average daily use for each billing month, in millions of BTUs (the gas company’s billing unit, also called decatherms). Along the right side I’ve multiplied by 300 to convert this unit to approximate kilowatt-hours (the more accurate conversion factor would be 293), to facilitate comparison to my electricity use. Notice the following:
  • Nearly all of my natural gas use is in the winter. Water heating in the summer is small by comparison.
  • My 2001-2002 renter produced a significant spike, as we kept the basement warmer than usual. My other renters/guests don’t show up on this graph because they weren’t around in the winter.
  • In December 2003 my old (from 1980 or so) furnace died, and the house was without heat for a week or two before I had a new one installed. The new one is a “condensing” furnace, rated at 92% efficiency because it sends less heat up the chimney. At the same time, I moved the thermostat from the front room to the back of the house, so I could close off the front room and avoid heating it for most of the winter. These changes reduced my gas use by more than 40%. The new furnace has just about paid for itself in the 11 years since it was installed, so it would have been a good investment even if the old one hadn’t died.
  • Any other changes or trends (such as the new storm windows that I got in 2011) are indiscernible due to the weather-caused variations.
  • Even with the new furnace, my average daily gas use is about 0.08 MBTU, or 23 kWh: six times as much energy as I use from electricity.
I use a lot of natural gas because my house, though small, is 80 years old and poorly insulated. But the factor of 6 is somewhat misleading, because when electricity is generated from fossil fuels (or nuclear fuel, for that matter), only about a third of the energy in the fuel is actually converted to electricity. (The rest is given off as waste heat at the power plants, and the second law of thermodynamics says there’s not much we can do about it.) So instead of a factor of 6, we could say that my natural gas use is only about twice the amount of fuel that I cause to be burned for electricity. Perhaps coincidentally, the amount that I pay for natural gas is also close to twice what I pay for electricity (about $20/month on average vs. $10), if you neglect the flat fees that are charged just for being hooked up to these utilities.

Burning one MBTU of natural gas emits 117 pounds of CO2, so my annual CO2 emissions from burning natural gas come to 3370 pounds—unambiguously more than the emissions from my electricity use. Thus, even if I reduce my electricity-related carbon emissions to zero, I shouldn’t feel too proud of myself unless I also reduce gas use. Unfortunately, I may have no good cost-effective ways to do that. One option might be to turn the thermostat down and rely more heavily on electric heating pads and blankets and space heaters—and then invest in a rooftop solar system that’s big enough to offset the electricity used by these appliances.

Before wrapping up this article, I should mention that my overall carbon footprint includes quite a few other contributions besides home energy use. There are significant emissions from driving, from flying, from growing and transporting the food that I eat, and from making the stuff that I buy. Perhaps I’ll detail my estimates of these in a future article. For now I’ll just say that each of these four is very roughly comparable to the footprint of my electricity or natural gas use; no one of them seems to be so large that it makes my home energy use negligible in comparison.

In any case, the graphs above make it pretty clear that the new furnace and new refrigerator were the “low-hanging fruit” for reducing my utility bills and the associated carbon emissions. I hope others can learn from these examples, even as I ponder which fruit to reach for next.

Saturday, April 18, 2015

How Grad School Made Me Rich

First let me be clear: I did not go to graduate school in order to get rich. I went because I loved physics and wanted to learn more physics and wanted to have a career in physics.

Besides, how could anyone get rich by going to grad school? Even if, as in my case, you have teaching and research assistantships that pay your tuition plus a stipend, that stipend is far less than what a college graduate “should” be earning.

And yet, as a side effect, grad school made me rich. It did so partly by enabling me to get good-paying academic jobs ever since. But far more important was the way grad school taught me how to happily live on a grad student’s stipend.

I’ve come to appreciate this fact so much that I went back through my old check registers and credit card statements, to see in detail how I did it. My average annual stipend while in grad school, from 1984 to 1990, was about $12,000 (less for the first couple of years and more later on, after I got a research assistantship). Meanwhile, my annual expenses averaged only about $10,500, so I actually accumulated a five-figure bank balance over those six years. (The consumer price index has approximately doubled since then, so double these numbers to convert to today’s dollars.)

Here’s a breakdown of where that $10,500 went:


I kept my housing expenses down by living in on-campus apartments, shared with one to three other grad students. The apartments were furnished, and the rent included utilities.

I kept my other expenses down by eating home-cooked meals (my roommates and I usually took turns cooking dinners) and by not owning a car (a choice that put me in the minority among my classmates).

These savings freed up substantial sums to spend on extravagant luxuries: more books than I would ever make time to read; two Macintosh computers that allowed me to work from home much of the time; a $900 Miyata touring bike; all sorts of other “toys” including backpacks, tents, other outdoor equipment, two nice pairs of binoculars, a telescope, and a new camera; and roughly two trips a year back east to visit family and friends. (The “miscellaneous” category in the chart includes small amounts for clothes and household items, but consists mostly of cash expenditures that I didn’t keep track of, probably including some groceries, plus occasional restaurant meals, concerts, movies, and cash spent while traveling.)

The fact is, I could have lived on 30% less if I’d had to. Or I could have blown that discretionary 30%, and more, on rent or cars or eating out, and ended up feeling like I had no spending money at all.

I did enter grad school with several advantages. My student loans from college totaled only $4500, with payments and interest deferred until after I was out of school. My parents never spoiled me with big-ticket gifts or large sums of cash, but they did make sure I got started with enough clothes and kitchen utensils. My health was always very good, and health insurance (my only significant medical expense) was cheap back then.

The moral of the story, for today’s grad students and anyone else who’s interested, is simple: Minimize your major expenses (housing, meals, transportation), try to avoid other expensive habits (smoking, drugs, debts, children), and you can live extremely well on a graduate student’s stipend.

After I got my degree my take-home pay instantly doubled, and it has continued to rise steadily ever since. But my expenses remained flat, because I was already living an extravagant lifestyle and never had the least desire to spend more. My spending shifted away from books and outdoor toys, since my need for those things was pretty much saturated. I spent less on computers as their prices dropped. I bought a used car in 1991 and bought one and a half new cars (a massive extravagance) more recently. I eventually bought a house, and quickly paid off the mortgage, so my biggest housing expenditures are now for major maintenance and upgrades. I still ride around town on my Miyata touring bike, and I still prefer home-cooked meals to eating out.

My current living expenses, as near as I can figure them, look like this:


The total comes to a little over $20,000 per year, or slightly less than what I spent in grad school when you account for inflation. However, the chart doesn’t include health insurance premiums, which are paid by and through my employer. If I didn’t have employer-provided insurance I would probably buy a “bronze” Obamacare plan and end up paying roughly an additional $4000 a year for premiums and deductibles.

The “miscellaneous” category in this chart includes clothes, household items, books, subscriptions, toys, entertainment, and bike accessories. I’ve tried to average big-ticket expenditures, like car purchases and home improvements, over a suitable number of years. And I’ve mostly tried to separate my own expenses from those of my better half, which isn’t too hard since she has her own financial accounts and her own house.

And where does the rest of my income go? Three places: income tax, savings, and donations to a long list of good causes that I’m proud to support. I won’t detail the breakdown among these three categories, but with a little arithmetic you can safely infer that I could have afforded to retire years ago. I’ve become wealthy without ever trying, and, although I know everyone’s situations and priorities are different, I hope my example can help others do the same.

[For more advice on living a happy life on not much money (or “financial freedom through badassity,” as he puts it), I highly recommend Mr. Money Mustache.]

Friday, December 5, 2014

First Step to Mars?

My Facebook and Twitter feeds are currently flooded with news of this morning’s flight of the Orion space capsule, echoing NASA’s claim that this is the “first step on the journey to Mars.”

Baloney. That claim, and the whole propaganda campaign that it’s a part of, constitutes outright fraud.

Sure, there’s a chance that someday a version of the same space capsule will play some role in carrying people to Mars. I’d put the chance below 5%, but who knows? It could happen.

The claim is still fraudulent, because NASA has no plans for most of the remaining steps to Mars:
  • The Orion capsule is far too small for a months-long mission. You can find drawings on the internet of proposed larger modules that Orion could attach to, but they’re just drawings. 
  • The Orion capsule can’t actually land on Mars. In fact, no technology that NASA has ever developed is capable of landing humans on Mars. NASA has some ideas on how to do it, but it’s not clear whether any of these ideas will even work.
  • There is no consensus on what risk level would be acceptable for a human Mars mission. Is NASA willing to send people on a one-way suicide trip? If not, it also needs to develop a system for getting people back off the Martian surface (not easy!). To increase the chance of survival above 50%, even with reasonably reliable spacecraft, NASA will have to deal with the poorly understood hazards of radiation, long-term weightlessness, and human psychology. Matching the 98% success rate of Space Shuttle missions is completely out of the question for the foreseeable future.
Moreover, even if NASA solves all these problems and actually takes all these further steps to Mars, the Orion capsule will not have been the first step. It is merely another incremental advance, adding to the accomplishments of Mariner, Viking, Spirit, Opportunity, Phoenix, Curiosity, ISS, Mir, Salyut, Skylab, Apollo, Soyuz, Gemini, Mercury, and Vostok.

The “first step to Mars” claim is fraudulent not only in its promises, but also in its intent. The reason NASA uses this language is because it knows that an honest one-line explanation of the Orion space capsule (“slightly larger version of Apollo with no definite destination”) wouldn’t grab headlines and generate the public support that it needs to maintain its funding levels.

Even my well-meaning colleagues who are repeating the “first step to Mars” slogan will usually admit, when pressed, that NASA’s robotic science missions are more important than its human space flight efforts. But, these folks argue, NASA has to keep doing human space flight because otherwise the public—and Congress—would lose interest in space, and funding for the science missions would dry up. And, they continue, human space flight gets kids interested in science, which is always a good thing.

I know I’ll be called a cynic for writing this essay, but to me it’s the attitude I’ve just described that seems cynical. Why can’t we trust the public by telling them the straight truth about what NASA is and isn’t doing? Misleading people is not only morally wrong—it’s also a bad strategy over the long term, because people will eventually stop believing what you say. Skepticism toward scientists is already at epidemic levels in the U.S., and NASA’s credibility, in particular, has plummeted during the Space Shuttle era. Making empty promises about future Mars missions will only hurt this credibility further, whatever cheering it might stimulate today.

Wednesday, April 16, 2014

Fuel Economy vs. Power

The recent experience of buying a new car left me angry and bewildered over the meager choices for those of us who care about fuel economy. Here we are in 2014, more than 40 years after the OAPEC oil embargo, and in the U.S. you still can’t buy a liquid-fueled car with an EPA combined rating above 50 miles per gallon. Only a handful of cars exceed 40 mpg, and your selection is pretty limited until you get down to mpg ratings in the low 30s. The most efficient pickups and minivans get 23 and 24 mpg, respectively. (Throughout this article I’m using city/highway “combined” fuel economy values under the current, less generous, EPA rating system.)

To some extent the limitations on fuel economy are due to basic physics: air and rolling resistance, braking losses, and thermodynamic limits on engine efficiency. But the existence of the 50 mpg Prius and of high-efficiency cars sold outside the U.S., not to mention the 47 mpg Geo Metro from a generation ago, raises the question of why more cars aren’t comparably efficient. The short answer is that most American car buyers don’t care. Or rather, they care much more about other factors such as size, price, appearance, and power. The most interesting of these is power.

Each year the EPA publishes a report under the cumbersome title Light-Duty Automotive Technology, Carbon Dioxide Emissions, and Fuel Economy Trends. All 135 pages of the latest Trends report are informative, but I’ll highlight just Figure 2.3, which shows some trends in fleet-wide averages for new vehicles sold in the U.S.:


First look at the line for weight, which decreased sharply by 20% in the late 1970s (as more people bought small cars), then began creeping upward in the late 1980s (as SUVs became popular). By 2004 the average vehicle weight was back to its 1975 value, and it has stayed there since.

Simple physics predicts that fuel economy should increase by about the same percentage that weight decreases, all other things being equal. But all other things have not been equal. Since 1975 we’ve seen steady improvements in engine and drive train efficiency, as well as in aerodynamics. Today’s new vehicles are 80% more efficient than in 1975, with essentially no change in average weight.

My point, though, is that the efficiency gain could have been significantly more than 80%, even with the same weights and the same technologies. Look at the trend in horsepower, which has been rising much faster than weight for the last 30 years. A higher power/weight ratio translates into faster acceleration, but (other factors being equal) lower fuel economy. Let’s quantify these effects.

A thorough statistical analysis of the acceleration performance of U.S. vehicles was published a couple of years ago (in both report and poster formats) by MacKenzie and Heywood of MIT. Their data set of 1500 cars and light trucks came from tests done by Consumer Reports, and was representative of the U.S. auto fleet as a whole. The results are striking:


Whether you look at the median (black curve), the slowest vehicles (red curve), or the fastest vehicles (green curve), 0-60 mph acceleration times are barely over half what they were in the early 1980s. To quote from MacKenzie and Heywood, “Acceleration performance that was typical in the early 1990s would put a vehicle among the slowest on offer today. Even the slowest end of the market (95th percentile) today delivers performance that was reserved for the fastest vehicles (5th percentile) in the mid-1980s.” Some of this increased performance has come from improvements in drive trains and aerodynamics, but most of it is a direct result of higher power/weight ratios. MacKenzie and Heywood found that with other factors held fixed, each 1% increase in power reduces the 0-60 mph acceleration time by about 0.7% for lower-power vehicles and about 0.58% for higher-power vehicles. (I think these values are less than 1% because limited traction prevents a vehicle from using its full power at low speeds.)

But why should engine power affect fuel economy? The answer lies not so much in basic physics as in the practicalities of engine operation. My amateur’s understanding is that running a gasoline engine at less than its full power means filling the cylinders at less than atmospheric pressure. You then get less force during the power stroke, with a proportional reduction in fuel consumption, while there’s no reduction in the friction between the piston and the cylinder wall—and that friction lessens the efficiency. In other words, a smaller engine running closer to full throttle is more efficient than a larger engine that’s being throttled back to produce the same power output.

So much for qualitative understanding. What about some numbers? 

I couldn’t easily find a quantitative analysis of the effect of engine power on fuel economy, so I did a quick empirical analysis of my own. Lacking the time to study every vehicle on the U.S. market, I started with a list of the 30 best-selling models in 2013. I then looked up each of these models in the 2014 EPA database, and picked out those that come with more than one engine option. I further pruned the list down to pairs of vehicles with different engines but the same (or nearly the same) transmission and drive type, and I eliminated duplicate pairs (e.g., same two engine options with different drive trains, or similar vehicles sold under different names). I also eliminated vehicles with turbocharged engines, which are generally more efficient but add a lot of noise to the data. Finally I was left with 14 vehicle pairs (5 cars, 3 SUVs, and 6 pickups) to compare, and I looked up the engine power for each on the manufacturers’ web sites. Here’s the final list:

Vehicle (transmission)   Engine 1   HP   MPG     Engine 2   HP   MPG     ΔHP   ΔMPG
Chevrolet Impala (auto 6) 2.5L 4cyl 195 24.5 3.6L 6cyl 305 21.4 56% −13%
Honda Accord (manual 6) 2.4L 4cyl 185 27.7 3.5L 6cyl 278 21.6 50% −22%
Hyundai Elantra (auto 6) 1.8L 4cyl 145 31.5 2.0L 4cyl 173 27.9 19% −11%
Nissan Altima (auto CVT) 2.5L 4cyl 182 31.2 3.5L 6cyl 270 25.3 48% −19%
Toyota Camry (auto 6) 2.5L 4cyl 178 28.7 3.5L 6cyl 268 24.8 51% −14%
Chevrolet Equinox AWD (auto 6) 2.4L 4cyl 182 23.5 3.6L 6cyl 301 18.9 65% −19%
Jeep Grand Cherokee 4WD (auto 8)    3.6L 6cyl 290 19.5 5.7L 8cyl 360 15.9 24% −18%
Jeep Grand Cherokee 4WD (auto 8) 5.7L 8cyl 360 15.9 6.4L 8cyl 470 14.9 31% −7%
Chevrolet Silverado 2WD (auto 6) 4.3L 6cyl 285 19.8 5.3L 8cyl 355 18.6 25% −6%
Chevrolet Silverado 2WD (auto 6) 5.3L 8cyl 355 18.6 6.2L 8cyl 420 17.0 18% −9%
Ford F150 4WD (auto 6) 3.7L 6cyl 302 17.5 5.0L 8cyl 360 15.9 19% −9%
Ford F150 4WD (auto 6) 5.0L 8cyl 360 15.9 6.2L 8cyl 411 13.4 14% −16%
Ram 1500 2WD (auto 8) 3.6L 6cyl 305 19.7 5.7L 8cyl 395 17.3 30% −12%
Toyota Tacoma 4WD (manual 5/6) 2.7L 4cyl 159 19.2 4.0L 6cyl 236 17.0 48% −11%

The last two columns show the differences in power and fuel economy, respectively, between the first and second engine options. Here’s a plot of these two columns, showing that there’s quite a bit of scatter in the data but the decreasing trend is clear:


On average, the percentage decrease in fuel economy is about 1/3 of the percentage increase in engine power. So, for example, a 30% increase in power typically results in a 10% decrease in fuel economy.

On one hand, these results help explain why consumers are so inclined to choose power over fuel economy: In percentage terms, you typically get about three times the added power for every bit of fuel economy you’re willing to sacrifice! On the other hand, MacKenzie and Heywood’s analysis shows that your 0-60 mph acceleration time drops by only about 2/3 as much as the power gain (in percentage terms), or about twice the percentage that the fuel economy drops. And of course, bigger engines are also more expensive. Given that Americans were happy to buy much less powerful vehicles only a generation ago, it’s hard to believe that most consumers are behaving rationally when they choose more powerful vehicles.

(In some places you’ll read that cars with slower acceleration are less safe—though I’ve never seen any actual evidence for this claim. Leaving aside the likelihood that powerful cars encourage stupid people to drive stupidly, I suppose the argument is that you need fast acceleration to safely merge onto a freeway where traffic is moving rapidly. Yet somehow we still share freeways with heavy trucks and buses and RVs and vehicles towing trailers and quite a few 25-year-old economy cars, all with accelerations much slower than that of any of today’s light-duty vehicles. In practice, slow acceleration just means you sometimes need to wait a little longer before it’s safe to merge. It’s really a question of incremental convenience, not safety.)

Hypothetically, if Americans were willing to go back to the acceleration performance of vehicles made in 1985, we could immediately increase the average fuel economy of new cars by more than 30%. Realistically, that’s not going to happen unless there’s another oil crisis or similar shock to the economy. The most we can probably hope for is that acceleration performance (and vehicle weight) will plateau, so future technological improvements will translate fully into better fuel economy.

Meanwhile, I wish the auto makers would offer just a few more extra-efficient vehicles of various designs, to give consumers more choice. By combining power levels that were typical of the early 1990s with the best current technologies for engines, transmissions, hybrid systems, and aerodynamics, it shouldn’t be hard to produce a 40 mpg small SUV, a 35 mpg minivan, a 30 mpg pickup, and a 60 mpg subcompact. They might not become the instant market leaders, but they would still get plenty of attention, sell to the niche market of sane consumers, and perhaps raise everyone’s expectations for the future.

Wednesday, March 5, 2014

Little Blue and Big Blue

I don’t especially like cars. They’re too big and too fast and too dangerous and too polluting and too isolating and too seductively comfortable and especially too ubiquitous. For everyday commuting and most errands I’ll stick to my trusty bicycle.

Still, I have to admit that cars are useful. I bought my first one in 1991 when I moved to a small town in Iowa, because I knew I would occasionally need a way to escape. And I still have that car: a 1989 Toyota Tercel hatchback, now known affectionately as Little Blue. I’m a bit embarrassed to admit that I’ve grown attached to it.

My parents helped me pick out Little Blue from the classified ads: automatic transmission, 17,460 miles, $6000. A nice practical car for a young single visiting assistant professor, and an easy car to drive and maneuver and park, for someone who didn’t have much experience behind the wheel.

Oh, the places I went in Little Blue. During my two years in Iowa there were monthly supply runs to Iowa City, occasional trips to St. Louis to see the folks, a canoe outing with five students who all had to squeeze in for the return drive after the other car broke down, a big camping vacation to southern Utah after school was out in 1992, and a spring break (1993) hiking trip to Arkansas (anywhere warmer than Iowa!) when, on the way home near Joplin, Missouri, the differential somehow ran dry and ground itself into smithereens.

After the move to Utah there were road trips all around the West, mostly to hike and camp in the mountains. I made some of these trips alone, but more often brought a friend or two. Little Blue still reminds me of companions from long ago, including the greatly missed friend who put that big dent near the left front wheel.

Little Blue has accumulated several bumper stickers over the years: Radio Free Utah, Save Our Canyons, Kill Your Television, Down the Hatch (24 years is too long!), Transit First, Obama ’08, FOrward, and =.  But the rear bumper faces the noon sun from Little Blue’s parking space, so the stickers that haven’t completely disintegrated are well on their way.

Since we bought a Prius at the end of 2004, Little Blue hasn’t gotten much use. I no longer feel very safe in such a small car without airbags, and of course the Prius gets much better fuel economy. (Its nickname is the Patriot Car, since you don’t have to attack Iraq to get enough gasoline to run it.) So Little Blue’s odometer has only gradually crept beyond 100,000, even as the passage of time has taken a toll on more and more of its parts. Still, we do occasionally need a second car around town, and the Patriot Car is pretty lousy on snow and on Utah’s unpaved back roads.

So I’ve just taken the plunge and bought a replacement for Little Blue: a 2014 Subaru XV Crosstrek, known for the time being as Big Blue. It dwarfs Little Blue, even though by today’s standards it’s not especially big. But it’s about the most modest (and most efficient) vehicle you can buy that has high clearance, which I want for those trips into the backcountry. It also has all-wheel drive, so we’ll use it in town when the roads are icy.


I have extremely mixed feelings about buying a new car. It was a stupid decision financially, especially since I don’t plan to drive it more than 5000 miles a year. It would have been far cheaper to fix up Little Blue, or to buy a used Subaru or perhaps a Ford Escape hybrid or some other small SUV. But the Crosstrek, which came out only a year ago, is closer to what I really want than any of those older models, in terms of capability and fuel economy. And I’m not enthusiastic about spending the time to shop for and maintain a used car. At this point in my life, my money is worth less than my time.

It’s fun and informative to compare some of the specifications of Little Blue, the Patriot Car, and Big Blue. Here for each is the curb weight, engine power (including the hybrid drive for the Prius), and city/highway fuel economy under the current EPA rating system:
1989 Tercel: 2085 lbs, 78 hp, 24/29 mpg
2005 Prius: 2921 lbs, 110 hp, 48/45 mpg
2014 Crosstrek: 3175 lbs, 148 hp, 25/33 mpg
Although the power/weight ratio is about the same for the two Toyotas, the Prius accelerates much faster—presumably because of its better transmission (CVT vs. three-speed automatic). In practice, both Little Blue and the Patriot Car have consistently beaten their EPA mpg ratings on the highway, but fallen short of them in the city. Probably the same will hold true for Big Blue, but we’ll see.

I have more to say about power and fuel economy, but that will have to wait for a future blog.