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.

2 comments:

  1. This is a fabulous analysis! I sat down to do a similar analysis this evening only to find your eloquent article. Thanks for your straight forward analysis of acceleration to mpg!

    ReplyDelete
  2. Thanks--glad you found it helpful and glad you were able to find it!

    ReplyDelete

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