Better climate models around the bend?

The EPA’s preferred vehicle-emissions simulator doesn’t handle curves so well, according to a study by scientists at Texas Southern University.

Fossil-fuel-burning cars hurt the planet. You already knew that. But the extent of the damage done depends a lot on how they’re driven. A driver who frequently accelerates and decelerates burns more fuel per mile—and therefore spills more carbon dioxide into the atmosphere—than one who keeps a steady pedal foot. Likewise, city driving tends to produce more emissions than highway driving.

Even the geometry of the road can come into play. As a rule of thumb, vehicle emissions are worse on winding roads than on straight ones, since curves force us to tap our brakes. A new study from Texas Southern University’s Innovative Transport Research Institute suggests, however, that the details of that relationship may be more complex than previously thought.

The TSU team of Fengxiang Qiao, Lei Yu, and graduate student Boya You outfitted a 2004 Subaru Forester with a device designed to record second-by-second data on the car’s location, engine performance, and tailpipe emissions. They then took the car on a four-hour joy ride around Houston, TX, circling twice around I-610, the city’s sprawling 38-mile outer loop, and twice again around a tighter downtown loop. Back in the lab, they analyzed the car’s performance along every turn of the nearly 100-mile trek.

It’s no mystery that curves bring out the worst in our vehicles, environmentally speaking. We tend to over-decelerate on our way into one and then punch the gas coming out of it, burning more fuel than we technically need. To estimate just how much more, you might take the predicted emission rates for straightaways and simply add a term accounting for the extra braking and accelerating. The TSU researchers tried doing just that, using emissions rates predicted by the EPA’s Motor Vehicle Emission Simulator (MOVES).

Qiao and his colleagues found, however, that the MOVES strategy almost always overestimated their car’s actual emissions. Surprisingly, variations in the rates of deceleration and acceleration had hardly any effect on emissions along curving stretches of freeway. More important was the average speed through the curve. And then there was this mystery effect, apparently related to the curvature of the road itself: Gentle bends tended to yield more emissions than tight turns, even after accounting for differences in speed.

Suffice it to say that scientists still have a ways to go to fully unravel the effects of road geometry on vehicle emissions. But this much is clear: Be it a bend or straightaway, you can’t go wrong in an electric car.