In-Cylinder Optical Measurement for Analyzing Control Factor of Ignition Phenomena under Diluted Condition

To increase thermal efficiency of internal combustion engines, dilution combustion systems, such as lean burn and exhaust gas recirculation systems, have been developed. These systems require spark-ignition coils generating large discharge current and discharge energy to achieve stable ignition under diluted mixture conditions. Several studies have clarified that larger discharge current increases spark-channel stretch and decreases the possibility of spark channel blow-off and misfire. However, these investigations do not mention the effect of larger discharge current and energy on the initial combustion period. The purpose of this study was to investigate the relation among dilution ratio, initial-combustion period, and coil specifications to clarify the control factor of the dilution limit. Four coils having different current profiles were evaluated under 2000 rpm and 6-bar net-indicated mean effective pressure under a diluted mixture condition through combustion-performance and in-cylinder optical-measurement tests on a single cylinder engine. The combustion-performance test results indicate a correlation between the dilution limit and initial combustion period. The in-cylinder optical-measurement test results indicate that the initial combustion period has a correlation with spark stretch before the 1st restrike and spark-stretch rate. These results also indicate that variation in the initial combustion period depends on the temporal flow velocity change during discharge and long initial-combustion-period cycles are caused at slow velocity during discharge.