Experimental And Numerical Study On The Exhaust Gas Recirculation-Based Homogeneous Charge Compression Ignition

Homogeneous charge compression ignition (HCCI) offers diesel-like efficiency andultra-low NOx and SOOT emissions which raises world-wide attention. However, there arestill two main obstacles, controlling of auto-ignition and combustion rate and extention ofload range, needed to be resolved for HCCI practical application.In this paper, internal exhaust gas recirculation (EGR) is used to achieve HCCIcombustion with absolutely different valve event, such as negative valve overlap (NVO),late exhaust valve closure (LEVC) and secondary exhaust valve opening (SEVO). Loadranges of different type EGR strategies were obtained by means of experiments, andparameters of HCCI performances and combustion features were mapped on thesedifferent load ranges in favour of comparison between different EGR strategies.Experiment results were analyzed and compared with1-D GT-POWER simulation resultswhich showed great differences caused by in-cylinder details.3-D HCCI model wasestablished by STAR-CD code to explore in-cylinder microphenomenon, like intake flowstructures, combustion details and emission formation. Positive effect of chargestratification on HCCI combustion was validated. Local dilute ratio concept wasintroduced for quantitative analysis of charge stratification. Hybrid fuel injection strategywas introduced to externally control in-cylinder stratification, and in turn to control HCCIauto-ignition and combustion.Experiment results indicated that NVO strategy was able to realize stable HCCIcombustion with lowest EGR rate, on the order of25%, due to the highest residual gastemperature and charge stratification compared with LEVC and SEVO strategy, whileEGR rate and IMEP were almost the same at HCCI low load owing to the fact that chargestratification played less important role in affecting the HCCI auto-ignition than dilution.Thermal efficiency of different EGR strategies showed a little difference influenced by thepumping loss. NVO strategy had the lowest efficiency in previous three strategies becauseof the re-compression of EGR. On the contrary, LEVC strategy showed the highest thermalefficiency of34%approximately. NOx emissions were all ultra-low compared withconventional engine-out emission with only NVO strategy emission a slightly higher in these three strategies. Parameters of HCCI combustion in terms of CA10, heat release rate,combustion duration and pressure rise rate between three strategies indicated thatauto-ignition occurred earlier for NVO and SEVO strategy, and heat release rate washigher for LEVC strategy as well as combustion duration and pressure rise rate.Comparison between experiment results and1-D simulation results revealed that bothHCCI high and low load limit were larger than that of simulational ones for these threeEGR strategies due to the naturally occurred charge stratification which provides local hotspots to trigger HCCI auto-ignition, while1-D model would need more EGR to heat thecharge to auto-ignition condition on the assumption that in-cylinder charge washomogeneous. A higher EGR rate indicated a lower HCCI load. This conclusion wasvalidated by the study of microturbulence and dilute ratio.Incylinder details were explored by coupling3-D CFD and detailed chemical kineticsmechanism. Simulation results showed that intake flow structures were totally different forthese three EGR strategies which in turn influenced the fuel and EGR distribution, chargestratification and finally the HCCI auto-ignition. Charge stratification was validated havingpositive effect on HCCI auto-ignition by comparing the auto-ignition process of mixturewith and without natural charge stratification. Local dilute ratio was introduced to estimatethe degree of charge stratification for different EGR strategies and NVO strategy wasfound to have the highest level of dilute ratio change whilst the other strategies showedlow level at10degree CA BTDC.Temperature and OH radical concentration could be used to feature the HCCIauto-ignition process and they showed almost the same spatial distribution in cylinderwhen auto-ignition occurred. OH radical had higher concentration in the field of highertemperature. According to the simulation results, HCCI combustion was characterized withdistributed multi-points auto-ignition.Hybrid fuel injecton strategy was introduced to creat charge stratification outside ofcylinder. Simulation results confirmed the effect of partial direct injection strategy onHCCI auto-ignition. Increasing DI fraction led to delayed auto-ignition timing, smoothedheat released rate and slightly reduced peak pressure and IMEP. DI timings showed weakinfluence on HCCI auto-ignition and heat release rate with constant DI fraction. Emissions were also influenced by adjusting DI fractions and timings. DI fraction showed moresignificant effect on emission formation and engine-out amount.Local dilute ratio and temperature changed a lot as DI fraction increased with a higherfuel concentration and lower temperature area located in the centre of the cylinder due tothe central fuel direct injection. Owing to the relatively low DI fraction, local dilute ratioand temperature changed a little for different DI timings. To summarize, local dilute ratiowas lower and fresh charge concentration was higher despite of the DI fraction beneath theintake valve where HCCI auto-ignition occurred.