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The particulate matter (PM) emitted by a diesel engineis collectedand then burned in a diesel particulate filter (DPF). A major technologicalchallenge in the operation of the ceramic, often cordierite, filteris that a rapid shift to idle may create a local hot region with atemperature much higher than under stationary feed conditions. Thisexcessive transient temperature rise may cause local melting or crackingof the ceramic filter. Almost all previous studies of temperatureexcursions during the DPF regeneration (combustion of the depositedPM) were of cases in which equal exhaust flow rate was fed to allthe parallel inlet channels. The diesel engine exhaust pipe is sometimesconnected to the DPF by a wide-angled cone (diffuser). This leadsto a mal-distribution of the flow rate to the inlet channels and ofthe deposited PM. Simulations revealed that following a rapid shiftto idle the highest regeneration temperature in a DPF fed by a coneexceeded that in one not fed by a cone and it may exceed the cordieriteDPF melting temperature (1200 °C). Moreover, it may generatetransient radial and axial temperature gradients several times higherthan under stationary regeneration that may crack the cordierite DPF.The increase in the temperature gradient is especially large in theaxial direction. One of the surprising findings is that the highesttemperature attained following a step change to idle is not a monotonicfunction of the initial PM loading. [ABSTRACT FROM AUTHOR]

Several experimental studies have been conducted to determinethe NOxreduction by a series of LNT (leanNOxtrap) and SCR (selective catalyticreduction) catalytic bricks. An important goal is to minimize therequired precious metal loading in the LNT while keeping the NOxemission below a specified level. We presenta mathematical model of this system using hydrogen as the reductant.Simulations are used to determine the influence of the architectureof the LNT–SCR bricks, nonuniform precious metal loading inthe LNT bricks, and the cycle time at temperatures in the range of200–350 °C. The simulations lead to the following observations:(a) Low temperature reduction is the limiting step in the optimizationof precious group metal (PGM) loading in LNT. (b) The NOxconversion increases as the number of the sequentialbricks (with total length fixed) increase and reaches an asymptoticlimit. From a practical point of view, there is little incentive inusing more than two sequential pairs. (c) Nonuniform precious metalloading of the LNT bricks results in only a minor improvement in thedeNOxperformance. (d) The cycle timehas a significant impact on the NOxconversion.In the simulated example, the NOxconversionat low temperatures is increased by about 15–20% by reducingthe cycle time by a factor of 2. (e) Even at low temperature operation,diffusional limitations in the washcoat are most likely to be importantin the LNT but not in the SCR operation. The NOxconversion and ammonia selectivity are reduced when washcoatdiffusion is dominant in the LNT. [ABSTRACT FROM AUTHOR]

A diesel particulate filter (DPF) is used to remove particulatematter (PM) from the effluent of a diesel engine. A major technologicalchallenge in the operation of the ceramic cordierite filter is toprevent formation of local high temperatures that can melt the DPFor generate a thermal stress that may cause cracking. Most previousstudies of the temperature rise during the DPF periodic regeneration(combustion of the deposited PM) considered cases in which the inletvelocity to all the parallel channels was uniform. A wide-angled cone(diffuser) is sometimes used to connect the diesel engine exhaustpipe to the DPF leading to a nonuniform velocity to the inlet channels,with the highest attained at theDPF center. We used a PM deposition and regeneration computationalmodel to investigate the impact of the inlet cone on the DPF behaviorunder stationary feed conditions. The cone led to mal-distributionof the deposited PM, with the highest thickness in the DPF center.The highest regeneration temperature when using an inlet cone maybe quite higher than when it is absent. Moreover, the inlet cone cangenerate higher temperature gradients and the resulting thermal stressesmay crack the ceramic support. The largest radial thermal gradientsare encountered close to the wall in the downstream section of DPF,shortly after the temperature inside the filter reaches its peak.The cone leads to a slightly faster regeneration than when it is notused. [ABSTRACT FROM AUTHOR]