Homogeneous charge compression ignition (HCCI) offers great potential for improved fuel economy and dramatically reduced NOx emissions, compared to typical spark ignition (SI) combustion. However, the benefits of HCCI are limited to low and medium loads by the simultaneous occurrence of combustion instability and knock at a maximum load that is too low for conventional SI combustion. To provide smooth operation in the intermediate range between HCCI and SI requires alternative combustion strategies. One such strategy is spark-assisted compression ignition (SACI), which uses a spark plug to initiate a flame that consumes a portion of the mixture, followed by autoignition of the remaining charge. This moderates the rapid heat release and allows higher loads to be achieved without exceeding knock and stability limits. In a recent study, we have explored this region and have found that spark assist at first dramatically reduces knock as load is raised above the HCCI limit; however, with further load increase, knock returns but in a form that resembles spark ignited knock rather than the HCCI knock. This study investigates in detail the knocking conditions observed in that work. The objectives of this study are twofold: first, to explore the differences between the two forms of knock and second, to apply and compare a number of commonly used metrics for knock and noise over the range of HCCI, SACI, and SI combustion. Experimental data were acquired on a single-cylinder DI research engine equipped with a fully flexible valve actuation (FFVA) system and fueled by research-grade gasoline. Cycle to cycle results based on filtered pressure traces are shown and compared with a number of knock measures including a widely used correlation for ringing intensity for HCCI combustion. Although based on a limited set of data, the results identify important qualitative features of the two forms of knock and point out significant differences among the knock metrics. The results suggest that further investigations are needed to fully understand both the knocking phenomenon and how best to quantify it.

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