A LIF-study of OH in the Negative Valve Overlap of a Spark-assisted HCCI Combustion Engine

Journal article, 2008

Future requirements for emission reduction from combustion engines in ground vehicles might be met by using the HCCI combustion concept. In this study, negative valve overlap (NVO) and low lift, short duration, camshaft profiles, were used to initiate HCCI combustion by increasing the internal exhaust gas recirculation (EGR) and thus retaining sufficient thermal energy for chemical reactions to occur when a pilot injection was introduced prior to TDC, during the NVO. One of the crucial parameters to control in HCCI combustion is the combustion phasing and one way of doing this is to vary the relative ratio of fuel injected in pilot and main injections. The combustion phasing is also influenced by the total amount of fuel supplied to the engine, the combustion phasing is thus affected when the load is changed. This study focuses on the reactions that occur in the highly diluted environment during the NVO when load and pilot to main ratio are changed. To monitor these reactions, planar laser-induced fluorescence (PLIF) from OH radicals was analyzed in a series of experiments with an optical single-cylinder engine, since these radicals are known to be associated with high temperature reactions. A series of experiments was also performed using a multi-cylinder engine with varied NVO timings, which showed that the combustion phasing was influenced by both the ratio between the pilot and main injection amounts and the total amount of fuel. Data acquired from corresponding optical analysis showed the occurrence of OH radicals (and thus high temperature reactions) during the NVO in all tested operating conditions. The results also indicate that the extent of the high temperature reactions was influenced by both varied parameters, since decreasing the relative amount of the pilot injection and/or increasing the total amount of fuel led to larger amounts of OH radicals.