A lean burn engine with a new type of three-way catalyst was developed in order to enhance the potential fuel efficiency of lean mixture operation. A combination of engine modifications and catalyst development extended the lean operating region across engine load and speed with no deterioration in NOx emission levels. A swirl generating port with low flow-restriction characteristics was developed in order to enhance the torque range of lean operation and the in-cylinder mixture distribution was optimized to suppress NOx emissions in high load lean operation. The catalyst has a base support material of zeolite coated with active metals including platinum, iridium, and rhodium and can not only oxidize HC and CO but reduce NOx even in lean exhaust gas, in addition to having all conventional three-way catalyst functions such as the simultaneous conversion of HC, CO and NOx in stoichiometric exhaust gas. A vehicle equipped with the engine showed a 11.6% improvement of fuel economy in the Japanese 10-15 mode emission test compared with conventional engines operating at a stoichiometric mixture. (A)
Abstract