Preface

It is widely recognized that Diesel vehicles are sure to significantly increase their worldwide penetration, particularly in countries like the United States where the present market share is not remarkable in comparison to that of vehicles with gasoline engines. This is mainly due to Diesel engines being inherently more thermodynamically efficient than spark-ignition engines, thus offering the prospect of reducing emissions of carbon dioxide. Diesels produce, however, higher emissions of nitrogen oxides (NOx) and particulate matter (PM). The emissions levels which can be achieved depend on both the engine-out emissions and the performance of the emissions control system. For the engine-out emissions, there is a well-known trade-off between PM and NOx. Such a trade-off is not, however, a constant relationship between the two pollutants. Developments in combustion systems, fuel injection equipment, turbocharging, and associated control systems have allowed and continue to allow the trade-off curve to move to lower values of both NOx and PM. The trade-off thus gives engine developers the opportunity to combine the optimization of ‘raw’ emissions and fuel economy with the optimization of the emissions control system. Improvements in combustion and/or alternative fuels can lead to lower NOx emissions, but it is generally recognized by now that, in order to meet the current and forthcoming legislative emissions standards both in Europe and in the USA, the application of after treatment systems is required. Indeed, Diesel particulate filters (DPFs) are needed to achieve the PM emission levels regardless of the system used to reduce NOx. But, most importantly, the significant CO2 reduction (i.e., the improved fuel consumptions), that is also dictated by the upcoming regulations, are forcing a drastic decrease in the average temperature profile of the exhaust gases; in such conditions, the catalytic removal of NOx becomes extremely challenging. In fact, a significant portion of the present test cycles (e.g., the NEDC, New European Driving Cycle, and the WHTC, World Harmonized Transient Cycle) is characterized by very low exhaust temperatures. This makes very difficult the fulfillment of NOx emission limits. Currently, the major deNOx after-treatment technologies under consideration include the so-called Lean-NOx Traps (LNT), which are used with direct injection gasoline and Diesel engines, and the Selective Catalytic Reduction (urea-SCR) process.