Optical study of HCCI Combustion using NVO and an SI Stratified Charge

Artikel i vetenskaplig tidskrift, 2007

The effects of using an SI stratified charge in combination with HCCI combustion on combustion phasing, rate of heat release and emissions were investigated in engine experiments to identify ways to extend the operational range of HCCI combustion to lower loads. In the experiments an optical single-cylinder engine equipped with a piezo electric outward-opening injector and operated with negative valve overlap (NVO) and low lift, short duration, camshaft profiles, was used to initiate HCCI combustion by increasing the exhaust gas recirculation (EGR) and thus retaining sufficient thermal energy to reach auto-ignition temperatures. Two series of experiments with full factorial designs were performed, to investigate how the tested parameters (amounts of fuel injected in pilot injections and main injections, stratification injection timing and spark-assistance) influenced the combustion. In the optical study fourth harmonic light (266 nm) from a Nd:YAG laser was used to induce fluorescence (LIF), from 3-pentanone added as a fuel tracer, to analyze the concentration and distribution of fuel vapor within the cylinder. In addition third harmonic light (355 nm) was used to study the concentration and distribution of formaldehyde in the cylinder, and chemiluminescence signals from OH radicals were used to locate the flame front. It was found that the injection and ignition timing of the SI stratified charge were the main parameters influencing the HCCI combustion phasing. The NOX emissions were found to be significantly affected by the use of a SI stratified charge, and its injection timing. The results show that use of an SI stratified charge can extend the operational range of HCCI combustion to lower loads by advancing combustion phasing. In addition, use of NVO in combination with an SI stratified charge provides a useful, flexible means to control HCCI combustion.