Volkswagen has admitted to rigging the emissions control systems on 11 million diesel cars over the last seven years, but those only represent a fraction of all the vehicles produced by VW during that time. Why did the carmaker only choose to tinker with its diesel vehicles instead of the larger number of gasoline cars? And how do we know VW didn’t mess with these vehicles?

These are questions that Consumerist reader Dick has been wondering since the VW emissions scandal first broke on Sept. 18.

“In other words, we already know that they cheat,” he says. “Why do we seem to think they cheat only with their diesels?”

First of all, the Environmental Protection Agency, the agency that took action against VW, says it isn’t excluding the possibility that unleaded vehicles could be skirting emission standards.

“Computer controls on modern vehicles can be used for efficient, effective emission control – but they can also be used to only look like effective emission control,” the agency said in a statement to Consumerist. “This is true for both diesel and gas engines. EPA will be reviewing our compliance protocols in light of the VW case.”

The agency, which previously announced it would overhaul emissions tests to catch defeat devices – software used to trick emissions tests as a way to evade standards for certain pollutants – says the new compliance processes will apply to both diesel and unleaded gas engines.

So why has much of the focus surrounding the VW scandal been squarely placed on diesel engine cars?

Jake Fisher, director of auto testing for our colleagues at Consumer Reports, hypothesizes that it could be the fact that diesel vehicles have generally created more pollutants than unleaded gas engines, and controlling those emissions can be costly for automakers.

Historically, that’s often been the trade-off for buying a diesel vehicle: higher fuel efficiency but more pollutants.

Diesel vehicles burn fuel more efficiently because their cycle produces less heat, meaning these vehicles often get better gas mileage, Fisher says.

On average, diesel-fueled cars are 33% more fuel-efficient than gasoline when it comes to mileage, according to the U.S. Department of Energy.

But the on the flip side, Fisher says, these vehicles release more “nasty” pollutants, such as NOx (nitrogen oxides), into the air than their unleaded gasoline counterparts. In fact, the EPA found the affected VW diesel-engine vehicles produce 40-times the allowable EPA standard for NOx.

“It’s more difficult to make diesel vehicles that meet emissions standards than it is for unleaded cars,” he says of aftermarket treatments needed to reduce pollutants. “Not only is it more difficult, but it’s more costly in terms of treatments. So there might be more incentive to defeat the system if you can build a [diesel] car at a lower cost.”

Omitting a system that lowers NOx concentration in diesel exhaust could have saved nearly $300/car, Fisher theorizes, based on his knowledge of the VW case.

Since every other manufacturer has put in that sort of system, he says it’s possible that the VW cars would not have passed emissions tests without defeat devices installed.

Still, Fisher says that while it’s more plausible that diesel vehicles would have more use for a defeat device, it’s possible that some gasoline-powered vehicles contain similar emission-skirting software.

 

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