Evaluation of various rich combustion strategies for HD-DI diesel engines
by Dimitrios T. Hountalas, Herwig Ofner, George Maragiannis, Dimitrios C. Rakopoulos
International Journal of Vehicle Design (IJVD), Vol. 45, No. 1/2, 2007

Abstract: The diesel engine, because of its superior fuel economy compared with spark ignition engines, has attracted considerable attention. Significant improvement in their performance and engine emissions has been achieved. The use of various advanced internal measures has resulted in significant reduction of pollutant emissions inside the combustion chamber. Nevertheless, considering the increasingly restrictive 'engine-out' emission standards to be adopted, it appears that the use of air throttling systems is possibly inevitable in order to meet the future limits. This is because NO_x and soot emissions are usually affected in the opposite way from the various internal measures. However, the use of conventional air throttling systems in DI diesel engines is not possible owing to the lean nature of diesel combustion, which does not permit the use of catalysts for controlling NO_x emissions. Currently, alternative 172 D.T. Hountalas et al. solutions are proposed focusing on the use of a selective catalytic reduction system, requiring a carefully controlled injection of urea or NO_x traps (adsorbers) When the storage capacity of the trap is reached, during normal lean operation, the engine must switch to rich operation in order to produce the necessary reducing agents, that is, CO and H₂, which release the stored NO_x to react with CO in a catalyst to finally form N₂. In this work, an extended computational investigation has been conducted to examine various strategies for achieving rich combustion in diesel engines. Rich diesel combustion has recently attracted attention for the regeneration of NO_x traps. Rich combustion can be achieved using either internal or external measures, (fuel injection at the exhaust manifold). Rich operation is expected to have a negative impact on engine performance and emissions resulting in an increase of brake specific fuel consumption (bsfc), soot and exhaust gas temperature. For this reason, in this work, various strategies are examined for achieving rich combustion using internal measures, namely intake air throttling or/and waste gate operation, EGR and late post injection. The investigation was conducted on a heavy-duty DI diesel engine, using a phenomenological simulation code that is based on a multizone combustion model. Information is provided concerning the impact of the various rich strategies examined on engine performance and emissions. This enables us to estimate the most appropriate one. Using this information, we established a combination of strategies to achieve the lowest possible λ value with acceptable overall fuel and soot penalties.

Online publication date: Fri, 11-May-2007

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