Chapter 7 Current tasks and challenges for exhaust after-treatment research: An industrial viewpoint

Abstract

The market of Diesel vehicles in Western Europe is, presently, still growing. In Asia, this market is still in an embryonic state (but should result as gigantic), while tomorrow the USA could become ‘The Market’. Diesel technology is an important issue for the carmakers, because it emits noticeably less ‘Greenhouse gas’ than its gasoline counterpart. Compliance with the EuroIII standards (2000) forced the fitting of Diesel oxidation catalysts (DOC) in the exhaust line [for the after-treatment of unburnt hydrocarbons (HC) and carbon monoxide (CO)]. Additionally, the exhaust gas recirculation (EGR) was adapted to reduce the engine-out emissions of nitrogen oxides (NOx). Under the EuroIV regulation (2005), the Diesel Particle Filter (DPF) emerged and is necessary for the ‘heavily loaded’ applications. Renault plans to make a CSF offer for all the diesel product range. This system enables to treat nearly 100% of the particle matter (PM, soot). When loaded, the DPF requires a regeneration phase to burn the accumulated soot. This regeneration is mandatory to keep not only the engine performance and reliability, but also the DPF reliability itself. The main drawbacks of this regeneration are the dilution of ‘post injected’ fuel in engine lubricant, leading to reducing the lubricant draining interval; the fuel over-consumption and the risk of forced vehicle immobilizations (for example, filter clogging under city driving conditions). The actual issues of EuroV standards aim at optimizing engine's design to decrease the engine-out NOx emissions in order to avoid the need for ‘expensive’ after-treatments in the exhaust line. Only some ‘heavily loaded’ applications would need such NOx after-treatment. Today, two major technological ways of NOx treatment are identified: the NOx Trap and the selective catalytic reduction with ammonia (SCR-NH3). After an adsorption period (during which the system traps the NOx), the NOx Trap requires a regeneration phase (in fact a rich incursion with HC and CO injections) enabling the reduction of accumulated NOx. As for the DPF, the engine combustion strategies carried on imply an increase of fuel diluted in engine lubricant, but also supplementary unburnt HC emissions, which could jeopardize the compliance with EuroV standards. This after-treatment system is also poisoned by sulfur (present in fuel and in engine lubricant) and must be recurrently desulfated (these desulfations are more severe than regenerations, rising concerns about the engine reliability, the fuel over-consumption and the emitted HC). In spite of these efforts, a part of sulfur adsorption on the NOx Trap active sites (normally dedicated to NOx storage) becomes irreversible and a slow process of system degeneration has to be addressed. The SCR-NH3 is a continuous process for NOx treatment and shows very efficient treatment efficiency. But this technology needs to put in the vehicle an additional tank for the urea storage. Moreover, this technology is constrained from an architectural point of view because two DOCs are necessary before and after the DeNOx catalyst to hydrolyze urea and form ammonia and to prevent the NH3 release in the exhaust line. In the challenge for preservation of the Environment, the European carmakers favor Diesel, which must comply with the most stringent standards constraints. The real key point for the future is, on the one hand, the improvement of DPF regeneration phase and on the other hand, but as first priority, the development of alternative NOx after-treatment technologies reliable on a long period. End of Abstract Nowadays, the Diesel vehicles market in Western Europe keeps growing. Diesel technology is of interest for car manufacturers because the ‘Greenhouse gas’ emissions are noticeably lower with Diesel than with gasoline (do not forget the ACEA1 commitment for 2008: 140g/km of C02 for the new cars sold in European Union). A major issue for the Diesel vehicles is to clean-up HC, CO, NOx and soot particles released by the engine at a minimum cost. The evolution of Diesel standards will be discussed and the constraints and limits of Diesel after-treatment systems put in series or in development will be clarified (in particular, those dealing with NOx conversion and soot particles removal).