LIF imaging of OH during the Negative Valve Overlap of a HCCI Combustion Engine

Future requirements for emission reduction from combustion engines in ground vehicles might be met by using the HCCI combustion concept. In this concept a more or less homogenous air fuel mixture is compressed to auto-ignition. This gives a good fuel economy compared to a normal SI engine and it has a positive impact on exhaust emissions. In the study presented here negative valve overlap (NVO) was used to initiate HCCI combustion by increasing the exhaust gas recirculation (EGR) and thus retaining sufficient thermal energy to reach auto-ignition temperatures, and raising the temperature during the NVO sufficiently to allow reactions to occur when a pilot injection was made prior to the NVO. The focus of the investigation was on the resulting high temperature reactions. Since OH radicals are associated with high temperature reactions, the reactions were followed by monitoring planar laser-induced fluorescence (PLIF) of OH, using a dye laser and an intensified LaVision Dynamight camera. The presence of OH radicals detected during the negative valve overlap indicates that high temperature reactions do occur in the highly diluted environment of the trapped exhaust gases during the NVO. Reactions were identified from 20 CAD prior to TDC (during the NVO) and to around 60 CAD after TDC, with the intensity peak at about TDC