The Fallout from Volkswagen's "Defeat Device"

• The repercussions from VW's error in judgment seem likely to extend beyond the hit to their reputation and stock price, and the unnecessary extra pollution from these cars.
• This incident will make a useful, fuel-saving alternative to gasoline cars less attractive, at least for now, resulting in higher future oil consumption and CO2 emissions.

I find the revelations concerning Volkswagen's reported efforts to circumvent vehicle emissions rules disturbing, especially as a VW owner and someone who has advocated diesel technology as a tool for reducing oil consumption and greenhouse gas emissions. VW has apparently admitted its colossal error. However, I haven't seen anyone attempt to explore the implicit emissions tradeoffs involved. As bad as this decision was, did it at least, on balance, help the environment?

The details that have emerged so far have focused on a software routine that manipulated diesel engine performance to produce one level of emissions in regulatory testing, presumably on a dynamometer, and different, much less acceptable results in real-world driving. Aside from the obvious questions about ethics and compliance, what did this mean for actual emissions?

For many years regulators have been tightening restrictions on allowable emissions of so-called criteria pollutants from cars. These include oxides of sulfur and nitrogen, particulates, and hydrocarbons, but not CO2. A whole gamut of technology was developed to tackle these pollutants, starting with catalytic converters on cars and deep desulfurization of fuels in refineries. Today's cars are much cleaner than those of a generation ago.

Oxides of nitrogen, referred to as NOx, are combustion byproducts that don't originate in a car's fuel, but from the nitrogen and oxygen in the air in which it is burned. NOx emissions from diesel engines have always been challenging, because they operate at higher temperatures and compression ratios than gasoline engines. Manufacturers that produce diesel vehicles have deployed different technologies to control NOx. As far as I know the VW Group uses at least two, depending on model.

Larger (and more expensive) vehicles appear to use a process called Selective Catalytic Reduction (SCR), in which small amounts of a liquid chemical such as urea chemically react with the NOx. The liquid must be refilled at service intervals. The technical manual for VW's 2-liter diesel engine involved in the current fiasco indicates it uses EGR, or exhaust-gas recirculation, which reduces the oxygen in the engine available to form NOx .

If controlling emissions from diesels is so challenging, why bother with them? Well, a typical diesel car uses up to a third less fuel than a comparably equipped gasoline model. After adjusting for the carbon content of the fuels, the lifecycle CO2 emissions are around 20% lower than for gasoline. Given the shortcomings of similarly priced electric vehicles in range and convenience, diesel provides a useful option. That helps explain why roughly half of European cars today are diesels, in many cases promoted by national fuel- and/or engine-tax policies.

That leads us to the question of whether such a reduction in CO2 might be worthwhile, even if it came at a penalty in NOx emissions, which act locally, rather than globally. To arrive at a ballpark answer let's assume that the 482,000 affected diesel cars couldn't have been sold at all if their engine software didn't fool emissions testers, and that the buyers would have otherwise chosen a comparable gasoline car. For comparison, the EPA rated the 2015 Jetta diesel at 36 miles per gallon (mpg) overall, while the 1.8 L turbo gasoline Jetta gets 30 mpg. At an average of 12,000 miles per year each, the collective annual fuel savings of the cars involved would be 32 million gallons, resulting in avoided CO2 emissions of about 300,000 metric tons per year, or 0.005% of US annual CO2 emissions.

If the tradeoff in extra NOx emissions is based on the reported maximum estimate of 40 times the EPA's allowed level of 0.07 grams per mile, then the affected cars would collectively emit an extra 16,000 metric tons of NOx per year. That's roughly 1% of the annual US NOx emissions tracked under the Clean Air Interstate and Acid Rain Program cap-and-trade markets in 2012. Even recognizing that those programs don't count all US NOx pollution, and that NOx and CO2 are very much apples and oranges in their environmental and health impacts, the relative proportions I calculated don't make this seem like a tradeoff worth making.

Whoever made the decision to circumvent the pollution controls on these cars did enormous damage to VW's brand and reputation. Unfortunately, the response in Europe and Asia suggests that this event has also raised questions about the emissions testing and compliance of the entire car fleet. Resolving them will take time and money, and if they are not seen to be dealt with properly, the impact on the public's trust of these processes on both sides--manufacturers and regulators--could be long-lasting.

Unlike in Europe, diesels made up just under 1% of new cars sold in the US last year. However, the technology was finally shedding the poor reputation that low-quality diesel cars earned in the 1980s, and the "take rate" was growing, along with the number of models offered.  VW's diesels are among the most affordable in the market. The NOx reduction technologies they use have been proven to work, when they are not circumvented, but that is not the message that this debacle will leave with the average consumer. Carmakers will have to work harder to convince buyers that this driver-friendly alternative to gasoline cars is worth a look, and that has implications for future oil consumption and CO2 emissions.