Spoiler Alert: Downforce Accessories May Concentrate Pollution
By JIM MOTAVALLI
A graphic from Dr. McNabola's study. The uppermost image shows the greatest concentration of tailpipe turbulence at head height.<br />Aonghus McNabola A graphic from Dr. McNabola’s study. The uppermost image shows the greatest concentration of tailpipe turbulence at head height.

Like the tail fins of yore, rear spoilers on the majority of today’s passenger cars are primarily appearance enhancers, not downforce tools, because the benefits of downforce are only apparent at high speeds. A new study suggests that rear spoilers do serve a dynamic purpose — one that might induce coughing fits in transportation-design classrooms.

Aonghus McNabola, a lecturer in the engineering department at Trinity College in Dublin, concluded in a recently published paper that rear spoilers set at certain angles created plumes of tailpipe pollution at the head height of pedestrians, cyclists and motorists who might follow in a car’s wake.

Dr. McNabola’s research was based on modeling, not real-world testing. Five different hypothetical spoiler-setup experiments were conducted, all taking into consideration variable wind and car speeds.

Dr. McNabola’s study found that the spoiler configuration with what the scientist called the greatest angle of attack — 20 percent in the study — “showed the most discouraging results with a significant increase in benzene concentrations at head height of over 40 percent.”

On the bright side, one of the flat rear-spoiler designs tested actually decreased those same concentrations by 18 percent.

“If the spoiler is flat it can have a positive effect,” Dr. McNabola said in a telephone interview. “But at angles greater than zero it leads to an effect that could contribute to the overall exposure of someone cycling, walking or even driving behind the car.” He said that the spoiler’s placement on the trunk does not matter as much as its angle.

“The spoiler won’t change the amount of pollution, just the concentration,” Mr. McNabola said. “Our main conclusion was that automakers can design spoilers to reduce their impact on air pollution.”

Jeff Gearhart, a senior auto researcher at the Ecology Center in Michigan, said that automakers should consider such findings before committing to designs that might concentrate pollution. “Since most of these spoilers are design rather than functional elements, there’s no reason they can’t design them to minimize pollution,” he said in a telephone interview.