The effect of mixing stratification using two fuels on reducing pressure rise rate in HCCI engines

Operating range of HCCI engines is limited by an excessive pressure rise rate (PRR) in high load region resulting in engine knock. Dispersing auto-ignition timing at each local area in the combustion chamber and retarding combustion phasing (CA 50) are main methods of decreasing the excessive PRR. Changing mixing ratio of two fuels which have different auto-ignition characteristics leads to the change of combustion phasing. The objective of this study was to investigate the effect of making the variation of mixing ratio in the mixture to change the combustion phasing at each local area in the combustion chamber on reducing PRR. The test fuels were DME and n = Butane which both have LTHR and HTHR during HCCI combustion. In addition to in-cylinder gas pressure measurement, in order to assess the combustion at each local area, chemiluminescence imaging and numerical analysis with multi-zones modeling were used. Maximum PRR changes depending on mixing condition of DME and n-Butane When DME is stratified and n-Butane is distributed uniformly in the combustion chamber (Case 3), maximum PRR becomes lowest. In Case 3, the appearance timing of chemiluminescence at each local area is dispersed and the duration of chemiluminescence is longer than in Case 1 (DME and n-Butane are both distributed uniformly). By numerical analysis, it is found that the temperature distribution and difference of HTHR start temperature makes difference of HTHR start timing in each zone.