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.