Effect of composite aftertreatment catalyst on alkane, alkene and monocyclic aromatic emissions from an HCCI/SI gasoline engine

Designing automotive catalysts for effective control of NOx, HC and CO emissions under both lean and stoichiometric engine operation is a challenging task. The present work assesses the performance efficiency of a three-zone prototype catalytic convertor in reducing exhaust emissions from a gasoline engine, operating in Homogeneous Charge Compression Ignition (HCCI) and Spark Ignition (SI) mode under lean and stoichiometric conditions. The performance of the convertor for HC oxidation follows the order: lean HCCI > stoichiometric SI > stoichiometric HCCI. The study mainly focused on the quantitative analysis of C1-C7 hydrocarbon compounds before and after the catalytic convertor. The results show that monocyclic aromatic hydrocarbons such as toluene are present at higher concentrations in the exhaust under HCCI operation than in the SI case. On the other hand, benzene concentrations are higher in the SI exhaust. The most common exhaust products of the two engine operating modes are methane, ethylene, propylene, benzene, and toluene. The prototype catalytic convertor eliminates most of the hydrocarbon species in the exhaust under both combustion modes, especially with a lean mixture. Conversion efficiencies for the different hydrocarbon species over the catalyst were in the order of alkenes > alkanes > aromatics. Hydrogen was added upstream of the catalyst primarily to assess its ability to promote NOx reduction, however it was also found to influence the oxidation characteristics of the catalyst. During H2 addition, the methane concentration was higher downstream of the catalyst.