Abstract
Keywords:
Idle operation, Spark timing, Exhaust valve timing, Exhaust emission, Combustion stability
Abstract
Emission reduction in cold start period of SI engines is crucial to meet stringent emission regulations such as SULEV. It is the starting point of the study that variable valve timing (VVT) technology may be one of the very promising methods to minimize the cold start emissions while maintaining engine performance, since 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 closecoupled catalysts (CCC) by changing exhaust gas temperature. However, even though these factors may be favorable for reduction of emissions, they may deteriorate combustion stability at these operating conditions. It means the two variables should be optimized for the best exhaust emissions and engine stability. This study investigated the effects of valve and spark timings on idle performance such as combustion stability and exhaust emissions experimentally. Experiments showed that valve timings significantly affected engine stability and exhaust emissions, especially CO and NOx, due to changes of residual gas fraction in the combustion chamber. Spark timing also affects HC emissions and exhaust gas temperature, but it has no significant effects on combustion stability. A control strategy of proper valve timing and spark timing is suggested in order to achieve reduction of exhaust emissions and stable operation of the engine in a cold start and idle operation.