Investigation of Design Parameters in Partially Premixed Compression Ignition Combustion Using Adaptive Injection Strategies

Premixed combustion strategies have been shown to yield very low NOx and soot emissions, while maintaining diesel-like efficiency; however, several issues must be addressed before they can gain widespread acceptance. This paper provides guidelines for solving problems with premixed combustion strategies, viz.: lack of combustion phasing control, excessive pressure rise rate, and spray wall impingement due to early injections. Cooled EGR and a multiple injection concept is used to control combustion phasing and reduce the peak pressure rise rate. To address spray-wall impingement, an Adaptive Injection Strategy (AIS) is employed. This strategy uses two injection pulses at different injection pressures to prepare an optimal in-cylinder mixture. The first injection is early in the cycle and utilizes a low injection pressure to minimize spray-wall impingement and create a well mixed charge. The second injection is near TDC and uses a high injection pressure in order to promote air entrainment and droplet dispersion. This study uses a multi-dimensional CFD code coupled with detailed chemistry, the KIVA-CHEMKIN code, to investigate the effects of several influential design parameters and identify emissions and performance tradeoffs. The combustion process considered is at a light load operating condition (nominal IMEP of 5.5 bar and high speed, 2000 rev/min). The parameters studied were: first and second pulse injection pressure and timing, IVC timing, EGR rate, fuel split, swirl ratio, and spray targeting. The investigation showed that the use of low pressure injections early in the cycle allows improved flexibility in fuel quantity and injection timing. An improved solution was found with near zero NOx and soot, a net ISFC of only 175 g/kW-hr, and a peak pressure rise rate of ∼8 bar/deg.