Planar laser-induced fluorescence (LIF) imaging of formaldehyde (CH2O) and OH has been performed to investigate the homogeneous charge, compression ignition (HCCI) combustion process inside the piston bowl of an optically-accessible, direct injection Diesel-fueled HCCI engine. In particular, the effects of charge dilution and the adoption of single and split injection strategies on the two-stage HCCI combustion have been studied.
Results obtained show that the level of exhaust gas recirculation (EGR) significantly affects the pre-combustion or so called cool flame phase during which formaldehyde is detected. The cool flame phasing as indicated by the formation of this intermediate species is unaffected by the EGR level however, auto-ignition timing which marks the start of main combustion is inevitably advanced following a reduction in EGR and this ultimately determines the formaldehyde lifetime and consequently the degree of homogeneity attained. The LIF data also revealed a link between the combustion structure, in terms of CH2O and OH homogeneity and the resulting engine-out NOx emissions.
For a given EGR rate, the adoption of a 50/50 split injection strategy with a first injection at 325 CAD followed by a second injection at 345 CAD significantly prolonged the CH2O lifetime prior to auto-ignition. The resulting peak heat release was lower whilst the combustion duration was increased. Furthermore, low concentrations of CH2O were detected at the end of main combustion, suggesting that a proportion of unburned fuel escapes the primary combustion process. Single-shot formaldehyde LIF and direct chemiluminescence imaging studies were also performed using a high speed camera which allowed the acquisition of up to 16 images within the same engine cycle. The LIF images confirm that a certain degree of local charge stratification is present, as indicated by the presence of discrete zones locally rich in formaldehyde. The results show a clear spatial correlation between the formaldehyde fluorescence signal and the ‘hot’ auto ignition sites which are detected several crank angle degrees later at the start of main combustion.