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A transient, one-dimensional lean NOx trap (LNT) model is described and implemented for vehicle systems simulations. The model accounts for conservation of chemical species and thermal energy, and includes the effects of O₂ storage and NOx storage (in the form of nitrites and nitrates). Nitrites and nitrates are formed by diffusion of NO and NO₂, respectively, into sorbent particles, and reaction rates are controlled by chemical kinetics and solid-phase diffusion. The model also accounts for thermal aging and sulfation by means of empirical correlations, which have been derived from laboratory experiments. Example simulation results using the Powertrain Systems Analysis Toolkit (PSAT) are presented.

In this study we identify components of a typical biodiesel fuel and estimate both their individual and mixed thermo-physical and transport properties. We then use the estimated mixture properties in computational simulations to gauge the extent to which combustion is modified when biodiesel is substituted for conventional diesel fuel. Our simulation studies included both conventional diesel combustion (DI) and premixed charge compression ignition (PCCI). Preliminary results indicate that biodiesel ignition is significantly delayed due to slower liquid evaporation, with the effects being more pronounced for DI than PCCI. The lower vapor pressure and higher liquid heat capacity of biodiesel are two key contributors to this slower rate of evaporation. Other physical properties are more similar between the two fuels, and their impacts are not clearly evident in the present study.

Internal combustion engines are operated under conditions of high exhaust gas recirculation (EGR) to reduce NOx emissions and promote enhanced combustion modes such as HCCI. However, high EGR under certain conditions also promotes nonlinear feedback between cycles, leading to the development of combustion instabilities and cyclic variability. We employ a two-zone phenomenological combustion model to simulate the onset of combustion instabilities under highly dilute conditions and to illustrate the impact of these instabilities on emissions and fuel efficiency. The two-zone in-cylinder combustion model is coupled to a WAVE engine-simulation code through a Simulink interface, allowing rapid simulation of several hundred successive engine cycles with many external engine parametric effects included.

We propose a procedure by which a two-zone thermodynamic model combined with a flame propagation sub-model can used for predicting the cycle-to-cycle variations of combustion in a spark ignition (SI) engine operating at very lean and high exhaust gas residual conditions. Under such conditions, the variations have been shown to consist of both deterministic and stochastic components. The deterministic component is inherent to the non-linear nature of the combustion efficiency variation with equivalence ratio (or dilution level) while the stochastic component results primarily from noise associated with the parameters (that are inevitable in a mechanical system) that affect combustion. Since the overall dynamics of the instabilities are driven by the low order deterministic component, if a model can be made to capture this component, the stochastic component is easily modeled by adding noise to the parameters.

We study the NOx storage process in lean NOx traps using bench-flow experiments and simulated diesel exhaust. Given that formulation alone is an inadequate indicator of performance (due to the effects of manufacturing processes) a minimal set of experiments is always needed to compare the performance of LNTs. We define simple performance measures based on such a set of experiments that can be used to compare lean phase operations of various LNTs under various conditions concisely. Though the noble metal sites are essential for storage, the benefits of increasing noble metal loading start to wane beyond a certain limit. Our experiments suggest a possibility that a lean NOx reduction reaction may be occuring in LNTs. If this reaction is confirmed further in future experiments, its products need to be identified. The sorbent shifts the equilibrium between NO and NO2 towards NO.

We consider the effect of intra-channel mass and heat transfer in modeling the performance of diesel oxidation catalysts. Many modeling studies have assumed that the intra-channel flow is laminar and, thus, heat and mass transfer between the bulk gas and wall are appropriately described using correlations for fully-developed laminar flow. However, recent experimental measurements of CO and hydrocarbon oxidation in diesel exhaust reveal that actual mass-transfer rates can deviate significantly from those predicted by such correlations. In particular, it is apparent that there is a significant dependence of the limiting mass-transfer rate on the channel Reynolds number. Other studies in the literature have revealed similar behavior for heat transfer. We speculate that this Reynolds number dependence results from boundary-layer disturbances associated with washcoat surface roughness and/or porosity.