Lean NOx Trap Modeling Based on Novel Measurement Techniques
Shawn Midlam-Mohler, Ohio State University
Lean NOx Traps (LNTs) are a type of storage catalyst under investigation for the aftertreatment of nitrogen oxide emissions (NOx) from lean-burn engine exhaust. The difficulty of NOx reduction in lean exhaust is due the presence of significant concentrations of oxygen, which compete with NO and NO2 for available reductants. One method of overcoming this issue of selectivity is through judicious choice of reductant and catalyst to promote NOx reduction, which is the technique employed with urea-SCR systems. An alternative to this is to chemically store the NOx during oxygen rich periods for later reduction under brief oxygen deficient phases. This is the approach used in a LNT catalyst, which is advantageous since it does not require any specialized reductants other than the on-board fuel.
Method and Approach:
A measurement technique is presented which uses internal catalyst temperature measurements to detect the chemical reactions occurring in the catalyst during the rich reduction phase. The magnitude of the temperature change is shown to correlate with the mass of NOx and O2 stored reduced from the catalyst substrate. This information is available at each temperature measurement location, allowing spatial information to be collected non-intrusively. Furthermore, the technique contains temporal information regarding the reactions. Experimental results as well as a theoretical model explaining the technique are discussed.
Based on the data from the analytical method, a model structure is formulated to take advantage of these newly available measurements. The model is a non-linear, distributed lump model which contains three states per lump. These states correspond to the lumped substrate temperature, lumped NOx mass storage, and lumped O2 storage. The model has two distinct structures, one for the NOx storage phase and one for the oxidizer reduction phase. During the storage phase, the model predicts the outlet concentration of NOx from the catalyst as well as the distribution of stored NOx within the catalyst. The regeneration model predicts the spatial and temporal usage of reductant through the catalyst.
Experimental and simulation results are presented for validation as well as demonstration of the utility of this model and analytical technique. Specifically, the analytical technique is demonstrated experimentally as a stored NOx and oxygen estimator, as a sulfur poisoning detection method, and as feedback signal for a regeneration closed-loop control. Validation data is presented for steady-state engine dynamometer tests and simulated results are shown for the model over a vehicle driving cycle.