Development of After-treatment Systems and Control Strategy for a Hybrid-dedicated Homogeneous Lean Burn Engine

Lean-burn gasoline engines can improve fuel economy thanks to the higher specific heat ratio of the mixture and the reduced cooling loss during the combustion process. In addition, when combined with hybrid technology, the technology can achieve more synergies by overcoming obstacles such as limited operating conditions. However, the challenge of this technology is to reduce exhaust emissions to a level suggested by future stringent regulations. In lean-burn gasoline engines, nitric oxide (NOx) emissions are difficult to convert using conventional three-way catalysts (TWCs). Therefore, developing an effective NOx after-treatment system has been a key technology in the development of lean-burn engines. In addition, hydrocarbon (HC) emissions are also challenging because they increase under lean combustion conditions, while the lower exhaust temperature under lean conditions makes them hard to oxidize. Therefore, a catalyst technology capable of effectively converting HC, even at a low exhaust temperature, is required. This study proposes a novel after-treatment system consisting of a lean NOx trap (LNT) and passive-type selective catalytic reduction (SCR). This system enables effective NOx conversion at an affordable cost and has benefits in control. In addition, conventional catalyst systems such as a TWC and a fuel-cut NOx trap (FCNT) were also optimized to maximize their conversion performance under lean operations. Various control strategies for optimal operation of the after-treatment system were also studied, along with new catalyst technologies. The systems and strategies were verified through vehicle tests on a chassis dynamometer. The newly designed systems and strategies confirm that a hybrid vehicle equipped with a lean-burn gasoline engine has sufficient potential for future emission regulation compliance, along with a 10% improvement in fuel economy.