초록
Emission reduction in the cold start period of SI engines is crucial to meet stringent emission regulations such as SULEV Emissoin reduction is the starting point of the study in the which the variable valve timing (VVT) technology may be one promising method to minimize cold start emissions while maintaining engine performance. This is because it is possible to change valve overlap and residual gas fraction during cold start and idle operations. Our previous study showed that spark timing is another important factor for reducing cold-start emissions since it affects warm-up time of close-coupled catalysts (CCC) by changing exhaust gas temperature. However, even though these factors may be favorable for reduction of emissions, they may deteriorate combustion stability in these operating conditions. This means that the two variables should be optimized for best exhaust emissions and engine stability. This study investigated the effects of valve and spark timings in idle performance such as combustion stability and exhaust emissions. Experiments showed that valve timings significantly affected engine stability and exhaust emissions, especially CO and $NO_x$, due to change in residual gas fraction within the combustion chamber. Spark timing also affects HC emissions and exhaust gas temperature. Yet it has no significant effects on combustion stability. A control strategy of proper valve timing and spark timing is suggested in order to achieve a reduction in exhaust emissions and a stable operation of the engine in a cold start and idle operation.
