A study on characteristics and control strategies of cold start operation for improvement of harmful exhaust emissions in SI engines

Emission regulations for automobiles have become more stringent and the improvement of emission during cold start has been a major key issue to meet these regulations. Among many kinds of factors that affect cold start operation, ignition timing is crucial to improve emission characteristics due to the influence on exhaust gas temperature. Recent progress in variable valve timing allows optimized valve event strategies under various ranges of engine operating conditions including cold start. This study investigates effects of ignition and exhaust valve timing on exhaust gas temperature, combustion stability and emission characteristics through cold start bench tests of an SI engine. Experimental results show that exhaust valve timings and ignition timings significantly affect exhaust gas temperature and stability of engine operation under cold start condition. Exhaust valve timing also affects CO and NO_x emission due to changes in residual gas fraction of the combustion chamber. Ignition timing mainly affects exhaust gas temperature and HC emission. A control strategy, advanced exhaust valve timing and retarded ignition, is plausible in order to achieve reduction of exhaust emission while maintaining stability under cold start operation of SI engines.