DEFEATING NOX: THE VW SAGA
November 16, 2015 § 2 Comments
NOx (oxides of nitrogen) are a Front of the Box pollutant. The effects are short term and on health, in contrast to CO2 whose effects are more in the long term, on targets such as severe weather and drought, leaving some room for doubt relative to causality. Consequently, NOx emissions from devices such as automobiles could be expected to be a public concern. Yet, much of the attention from the VW emissions cheating episode is directed to the behavior and not the attendant pollution. In fact the reporting has shown that much of the industry has cheated in one way or another. In Europe the emissions testing is done by the companies with no regulatory oversight. The use of non-standard vehicles during the tests is a common practice to which everybody turns a blind eye. For mileage testing cars are routinely stripped of wind drag components such as wing mirrors. The real world kilometers per liter are in the vicinity of 35% worse than in these tests.
VW managed to do something that was shocking even against this backdrop of routine avoidance of emissions regulations. Interestingly evidence is piling up to indicate that the “defeat devices” they use (more on it below) may not have explicitly violated any European strictures. No such doubt exists in the US. The issues that I will address below are: 1. what was the technology for NOx reduction they employed, and 2. how was it circumvented and why.
When a diesel engine is burnt “lean”, it performs the best, especially with respect to fuel economy. This condition is defined as air somewhat in excess of the stoichiometric amount required to combust the fuel. Less unburnt fuel is also good on emissions. However, the excess air causes more production of oxides of nitrogen, NOx. This must be reduced in the exhaust gas stream.
VW use a technology known as the Lean NOx Trap (LNT). There are two steps (there is a preliminary step which we will skip for this discussion for simplicity). In the first NOx is captured on a coating that adsorbs NOx. Adsorption is a surface phenomenon that is easily reversed. When the coating is considered filled up, the second step kicks into gear. This involves removing the NOx to regenerate the coating activity. This is the key step that got VW in trouble. The NOx is reduced to nitrogen and CO2 on a special catalyst by reacting it with some mixture of hydrocarbons, hydrogen and CO. This mixture is created by switching the engine to a “rich” burn mode, away from the lean. The reactant is the fuel from the cylinders that is only partially combusted. Not surprisingly, during that time, engine performance drops for reasons noted above. The gas mileage reduces as does the torque.
VW was attempting to penetrate the US market with diesels. This was part of an overall goal of being the top seller worldwide. The US consumer had been recalcitrant compared to the Europeans. Also, the NOx regulations in the US were stricter. In the US the regulators do spot checks. It appears that the decision was made to “defeat” the device during normal road operation. This was achieved through a reasonably sophisticated algorithm which detected that the vehicle was in a test mode. When in this mode the engine was allowed to run rich for the needed period to perform the function of the LNT. But importantly, in normal driving the vehicle ran lean all the time, giving the needed performance in miles per gallon and torque. In other words it was peppy (high engine torque) with high mileage and was great on emissions. Keep in mind that all diesel are better on mileage than gasoline engines. In part this is because the fuel has about 10% more energy content and in part because diesel engines run on much higher compression ratios. But for decades they have had a reputation for being smoky and smelly. This is no longer the case. Particulate filters take away the smoky aspect. The only remaining concern had been the NOx emissions. VW claimed to have met those while delivering a superior driving experience. There is no dispute that they cheated. The key point is not so much the cheating on the testing, but that buyers expecting to get a low emissions car were not getting one.
What were the alternatives to LNT available to them, and why they chose not to use those, are the US rules too stringent to be achieved by small low cost cars, is diesel simply not viable for these cars, will electric cars and hybrids be advantaged, these are all topics for the next post.
Vikram Rao
Research Triangle Energy Consortium 
Looking forward to your next post on this topic! Is there not a means to treat NOx without running rich?
Yes there is, but not easy to install in small cars. All trucks have it. More on next post later this week.