Maximum Fuel Economy

It's looking increasingly likely that the House version of energy legislation will pass without a specific fuel economy provision comparable to the Senate's higher CAFE standard. Thinking about fuel economy triggered some random thoughts concerning the way we're approaching this problem. While it is quite reasonable and pragmatic to ask how much additional fuel economy we need, we should also be asking two other, related questions: How much more fuel economy can automobiles deliver economically on current fuels without drastic redesign, and is fuel economy even the right metric, in a world that is shifting its focus beyond questions of oil price and availability to the larger consequences of energy use, including climate change?

The first question has long vexed automotive engineers, who don't set out to build cars that deliberately waste fuel. Engineering and economic trade-offs determine how close an actual engine comes to achieving its maximum theoretical thermal efficiency, which for internal combustion is somewhere in the neighborhood of 35-40%. The engines in our cars usually achieve less than 25%, sometimes much less. There are all kinds of strategies that can boost the efficiency of a spark-ignition, Otto-cycle engine--the kind in most American cars. Today's MIT Technology Review looks at one of those, HCCI, which changes the way fuel is mixed in the cylinders. It could achieve diesel-like efficiency gains, and it's welcome news that this may be possible using ordinary gasoline, rather than "designer fuels."

If you look at the other places that energy in a car disappears on its way from the gas tank to the wheels, the engine is only the biggest of many source of losses (see slide #6 of this presentation.) Some of these are unavoidable; you can only make a passenger car so aerodynamic, before it loses functionality. However, designers of hybrids such as the Prius didn't just add electricity; they tackled some of these other losses to boost the car's non-hybrid efficiency, too.

When you add up all the possibilities, and then layer on hybridization, turbo-charging, and other proven technologies, doubling the overall efficiency of any car ought to be possible. And by giving up a bit of weight and power, too, we might be able to triple the fuel economy of the least efficient cars on the road. So when our leaders talk about raising average fuel economy of the new car fleet from 25 mpg to 35 mpg, this should be entirely feasible without requiring the more radical--though possibly desirable for other reasons--step of plug-in hybridization, which adds an external electricity source to the car's powertrain. All of these strategies add cost, however, and that's the crux of the whole argument. Saving 137 gallons of gas per year, the typical quantity associated with boosting the average car's fuel economy by 10 mpg, is only worth about $400/year at current fuel prices. That limits the maximum economic investment in efficiency to about $2000/car. Finding the right solution for each model within that constraint will be the trick.

But is miles per gallon even the right metric? Even if we didn't care about greenhouse gas emissions, the inclusion of increasing quantities of ethanol in the US gasoline pool alters the meaning of the "gallon" part of that ratio. This is compounded by the lower energy content of ethanol. A US fleet running entirely on E-10 will inherently need 3% more fuel than one using 100% petroleum gasoline, reducing average fuel economy from 25 mpg to 24.25. Throw some E-85 and plug-in hybrids into the mix, and it gets even more confusing.

Perhaps Europe has the right answer to this. Their "fuel economy" regulations are based not on the usual European metric of liters of fuel per 100 kilometers, but on grams of CO2 emitted per 100 km. While I'm not sure they yet do this on a well-to-wheels basis--which would factor in the upstream emissions associated with producing gasoline, ethanol or diesel fuel--this just looks like a better metric for the 21st century. The easiest way to drive grams/100 km down is still to increase the efficiency of the vehicle itself, but it's not the only way. This perspective helps avoid potential dead ends that appear to reduce oil consumption, but don't actually reduce energy consumption or total emissions by very much. And it takes us back to the underlying question of the real goal that fuel economy regulations are intended to serve: Is it oil security, energy security, or climate change?