An investigation of mixing stratification of DME and n-Butane in HCCI engines using chemiluminescence imaging

In HCCI engines, high load operation is limited by an excessive pressure rise rate (PRR). Past studies showed that mixing stratification with the fuel that has large heat release in low temperature reaction (LTR), disperses the timing of local ignition and slows the PRR. The purpose of this study was to gain a better understanding of effect of the mixing stratification on HCCI combustion using chemiluminescence imaging. Di-methyl ether (DME) which has large heat release in LTR and n-Butane, smaller heat release in LTR than that of DME, were used. Experiments were conducted on five mixing conditions depending on the combination of the charging methods (homogeneous charge or stratified charge) of DME and n-Butane, using a single-cylinder engine equipped with optical access. Chemiluminescence imaging was used to assess a local combustion phasing. The experimental observation shows that, in the case of fuels stratification, maximum PRR is reduced compared to the case of fuels homogeneity. In the stratified case, the start timing of local chemiluminescence is dispersed and total chemiluminescence duration is longer than that of the homogeneous case.