Abstract
Modern boosted spark-ignition (SI) engines and emerging advanced compression ignition (ACI) engines operate under conditions that deviate substantially from the conditions of conventional autoignition metrics, namely the research and motor octane numbers (RON and MON). The octane index (OI) is an emerging autoignition metric based on RON and MON which was developed to better describe fuel knock resistance over a broader range of engine conditions. Prior research at Oak Ridge National Laboratory (ORNL) identified that OI performs reasonably well under stoichiometric boosted conditions, but inconsistencies exist in the ability of OI to predict autoignition behavior under ACI strategies. Instead, the autoignition behavior under ACI operation was found to correlate more closely to fuel composition, suggesting fuel chemistry differences that are insensitive to the conditions of the RON and MON tests may become the dominant factor under these high efficiency operating conditions. This investigation builds on earlier work to study autoignition behavior over six pressure-temperature (PT) trajectories that correspond to a wide range of operating conditions, including boosted SI operation, partial fuel stratification (PFS), and spark-assisted compression ignition (SACI). A total of 12 different fuels were investigated, including the Co-Optima core fuels and five fuels that represent refinery-relevant blending streams. It was found that, for the ACI operating modes investigated here, the low temperature reactions dominate reactivity, similar to boosted SI operating conditions because their PT trajectories lay close to the RON trajectory. Additionally, the OI metric was found to adequately predict autoignition resistance over the PT domain, for the ACI conditions investigated here, and for fuels from different chemical families. This finding is in contrast with the prior study using a different type of ACI operation with different thermodynamic conditions, specifically a significantly higher temperature at the start of compression, illustrating that fuel response depends highly on the ACI strategy being used.
Highlights
The PT trajectories of the spark-assisted compression ignition (SACI) and partial fuel stratification (PFS) conditions investigated here were found to be in close proximity to the research octane number (RON) trajectory, with some individual cases extended into the beyond-RON domain, due to their high intake boosting requirements
The prior study used a homogeneous Advanced compression ignition (ACI) mode that had a strongly beyond-motoring octane number (MON) trajectory due the high intake temperatures required. These two investigations demonstrate that a wide range of combustion strategies, operating conditions, and PT trajectories are encompassed under the classification of multi-mode ACI operation
The purpose of this study was to further investigate the accuracy of the octane index (OI) metric, and to determine if fuel chemistry differences not captured in the RON and MON tests plays a dominant role in determining fuel autoignition resistance
Summary
Increasing focus on vehicle fuel efficiency over the last decade has driven a major trend in downsizing and boosting of spark-ignition (SI) engines [1]. Adopting these technologies has allowed significant improvements in vehicle fuel efficiency without sacrificing vehicle performance [2,3,4]. While downsizing and boosting alleviates some of the efficiency penalties for SI engines at part load, it does not eliminate them. Advanced compression ignition (ACI) modes such as homogeneous charge compression ignition (HCCI), spark-assisted compression ignition (SACI), and partial fuel stratification (PFS) operate unthrottled and more effectively address part-load inefficiencies [5,6,7,8]
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