NOx technology breakthrough or media stunt to halt diesel’s demise?
Bosch recently announced, with great fanfare, a breakthrough in the automotive world: A new technology that would solve once and for all the nitrogen oxide (NOx) problem of diesel engines. Being the engine geek that I am, I ran to read the technical paper in which Bosch’s engineers describe the new technology in detail.
It did not take a lot of reading to realize this is certainly no breakthrough.
Instead, Bosch’s new technology is nothing but a clever repackaging of decades-old strategies that should have been common place in all Euro 6 diesel cars, but unfortunately, they are not. To better understand Bosch’s claims, and my disappointment, I have to delve into the details of NOx formation and its control.
The formation of NOx takes place inside the engine during the combustion process. As the diesel fuel burns, the temperature inside of the cylinder is high enough to break apart the nitrogen and oxygen molecules from the air into more reactive atoms that eagerly recombine to form NOx. Once the NOx molecules leave the engine, they need to be converted back into harmless nitrogen and water by a catalytic converter, called selective catalytic reduction (SCR), placed in the exhaust system. The SCR is essentially a small chemical plant that requires high temperatures to function properly.
Let us now take a look at what Bosch is proposing. The backbone of Bosch’s “new technology” relies on optimizing the temperature management of the SCR. During the first few seconds of operation, denominated cold start, the SCR is too cold to be able to deal with the NOx emissions coming from the engine. Therefore, the control system needs to warm it up as quickly as possible, while at the same time minimizing the NOx emissions coming from the engine. The most common strategy to rapidly warm up the aftertreatment system is to purposefully make the engine less efficient by delaying the injection, and thus the combustion timing of the fuel. This reduces the peak temperatures inside of the engine, thus reducing the formation of NOx, while at the same time increasing the temperature of the exhaust gases, accelerating the warm-up of the aftertreatment system.
Once the exhaust gases are hot enough to activate the oxidation catalyst—a catalytic converter taking care of the hydrocarbon and carbon monoxide emissions—the temperature management system commands the injection of a little bit of fuel into the exhaust gases before they leave the engine. This bit of fuel burns up in the oxidation catalyst, further increasing the exhaust temperature at the entry of the SCR and accelerating its warm-up.
As clever as this may sound, this strategy has been common practice in the world of engine calibrators for years. In fact, this approach was described in this paper over a decade ago. This approach is only reluctantly applied due to its negative impact on the fuel consumption. In Bosch’s case, the fuel consumption penalty is more than 5% in busy city traffic.
So, no breakthrough so far.
Once the aftertreatment system is warmed up, the next challenge is to keep it hot. In some operating conditions, such as idling, the temperature of the exhaust gas can be lower than that of the SCR. So, the exhaust flow during idling actually cools the SCR down. Bosch’s “novel” solution: shut-down the engine while idling. Apparently, they are not aware that over 70% of new passenger cars in the EU already have stop-start systems.
What Bosch has shown, however, is that low in-use emissions of diesel NOx could have been possible from the start of Euro 6 standards back in 2014. Sadly, most of Euro 6 diesels currently on the road have either sloppy engine calibrations or are purposefully engineered to shut off the emission control system in real operation.
In Bosch’s defense, creating detailed mathematical models of the catalysts in the control units of the engines and working out the right calibration that delivers low in-use NOx emissions is not a straight-forward task. But it can be done, as not only Bosch, but also others have already shown.
Nevertheless, it remains to be seen if the boundaries of the real-driving emissions (RDE) regulation used by Bosch to showcase its findings will be enough to bring down urban NOx levels. As we have argued before, the 5% of driving situations not covered by the RDE boundaries could represent a large share of total (or real-world) NOx emissions.