NSK Advanced: From the nitrogen storage catalytic converter
to NSK Advanced

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By nature, the exhaust gas in diesel engines contains excess oxygen. Experts refer to this as lean operation. In such an oxidizing environment, it’s very difficult to chemically reduce undesired nitrogen oxides (NOx) by turning them into harmless nitrogen (N2). An elegant solution is offered here by an improved nitrogen storage catalytic converter known as NSK Advanced. It became possible to utilize this system after combustion specialists succeeded in running a diesel engine in rich operation.

Exhaust treatment specialists at Daimler Research began developing the basic aspects of this technology as early as 1996. The key to this approach is the intelligent combination of two catalytic converters — the NSK and the SCR — to create a self-sufficient exhaust treatment system. The nitrogen oxides are broken down in both the NSK and the SCR units. Michel Weibel, head of the Catalytic Systems team, explains how the two catalytic reactions are linked in an efficient and synergetic way: “The basic idea is to create an exact amount of ammonia on board the vehicle — specifically, in the NSK unit. This ammonia passes into the downstream SCR catalytic converter, where it further reduces the molecules of nitrogen oxides that have passed through the NSK unit into nitrogen. That’s why NSK Advanced works very efficiently even without AdBlue additive.”

All of this requires a sophisticated valve timing system. In the system now ready for series production, the diesel engine almost always runs in lean operating mode, whereby most of the nitrogen oxides are stored in the NSK as nitrates and the rest make their way to the SCR unit, where NOx is broken down into harmless nitrogen (N2) with the help of stored ammonia (NH3).

Following this lean phase, the control system puts the diesel engine into rich operation for a few seconds (i.e. with an excess of fuel in the mix). At this point, reducing agents such as carbon monoxide (CO), hydrogen (H2), and hydrocarbons (HC) enter the exhaust flow. These break down some of the nitrates stored in the NSK into atmospheric nitrogen (N2). A specially adapted operating strategy ensures that the remaining nitrates are converted into ammonia (NH3), which then passes through the particulate filter that follows and finally ends up serving as the ammonia reserve in the SCR catalytic converter for use in the next lean operation cycle.

It was only after examining each catalytic converter component in detail that the researchers began to sufficiently understand the chemical processes in the units. “The variety of the parameter dependencies and the complexity of the catalytic reactions presented a huge challenge,” says Weibel, thinking back on the years of hard work that have gone into the system.