Experimental Investigation on Early and Late Intake Valve Closures for Knock Mitigation Through Miller Cycle in a Downsized Turbocharged Engine

The application of Miller cycle through Late Intake Valve Closure (LIVC) or Early Intake Valve Closure (EIVC) for knock mitigation at high load on a turbocharged downsized spark ignition engine was experimentally investigated. By reducing the effective compression ratio due to a shorter compression stroke and hence achieving lower charge temperatures inside the cylinder, significant mitigation of knock tendency could be obtained. As a consequence, the spark advance retard could be substantially decreased and the enrichment of the mixture could significantly be reduced, thus obtaining impressive efficiency improvements. In this research, both EIVC and LIVC strategies have been examined aiming to achieve possible improvements for knock mitigation and after some preliminary investigations confirmed LIVC being more effective than EIVC for this goal, the latter was discarded and the research activities were focused on LIVC only. Significant reductions in fuel consumption for high load engine operating points were achieved, especially at moderately high engine speeds, above 2500 RPM, where the turbocharger group was capable to compensate for the reduction of volumetric efficiency caused by the LIVC by means of an increased boost pressure. However, at lower engine speeds, despite these operating conditions being generally the most critical for knock occurrence, only minor positive effects were observed, since the lack of an adequate boost pressure did not allow further delays of the IVC, thus preventing the full exploitation of the Miller cycle. The highest gains were registered at 3000 RPM, at 18 bar and 20 bar BMEP, where the engine indicated fuel conversion efficiency was improved by about 11 and 20 percent, respectively. It is worth mentioning that, thanks to the Miller cycle exploitation, the engine could be operated under stoichiometric conditions in this region of its operating map, while normally adopted IVC timings typically require significant mixture enrichments.