Numerical investigation and evaluation of applying PPCI combustion in a HSDI diesel engine

In this study, the Partially Premixed Compression Ignition (PPCI) combustion strategy in the high-speed, direct-injection diesel engine was investigated numerically by KIVA-3V code coupled with detailed chemistry, aiming to find the solution to meet the increasingly stringent emission regulations. Using split-injection, the parameters including injection timing, split-proportion, spray angle and injection pressures have been studied for their effects on combustion performance and emissions. The effects of swirl ratio, EGR rate and boost pressure are evaluated for improving the mixing and combustion of PPCI. The Homogeneity Factor (HF) was proposed for evaluating the quality of mixing and for quantitatively investigating the effects of injection parameters and in-cylinder air motion on mixture formation. Relationships between the quality of mixing and combustion performance and emissions were discussed using this factor. The results showed that HF had well revealed overall quality of mixture and the effects of operating parameters explicitly. Different EGR compositions with varied fractions of CO2 or H2O were applied in PPCI combustion in order to evaluate the effects of EGR constituents on the combustion performance and emissions. Moreover, the parametric study was conducted under a sweep of the 2nd injection timing and EGR rate, for the understanding of the effects of CO2 and water vapour in EGR at different operating modes. The speed range and load range for the PPCI diesel combustion using split injection was investigated. The results showed that the high level of EGR rate limited the implementation of PPCI combustion at high engine load, while the engine speed was limited by increased CO emissions. The application of high level cooled EGR had the potential for extending operating limits. The proposed Premixed Rate (PR) has revealed the correlations between the degree of premixed combustion and ignition delay, together with overall equivalence ratio. Good responses in fuel consumption have been shown with increase PR. And the significant reduce in PR indicated low degree of premixed at high engine load.