Impact of engine pressure-temperature trajectory on autoignition for varying fuel properties

The U.S. Department of Energy's Co-Optimization of Engines and Fuels (Co-Optima) initiative is pursuing higher efficiency engines on the basis of fuel properties, and in particular, using fuels that enable multimode combustion strategies that include both boosted spark ignited (SI) and advanced compression ignition (ACI) operation. Despite the fact that autoignition plays a central role in both, a limiting factor in boosted SI and a requirement for ACI, conventional autoignition metrics do not adequately describe fuel behavior under these combustion modes: research octane number (RON), motor octane number (MON), and antiknock index (AKI). The octane index (OI), which is an emerging autoignition metric, has gained significant acceptance as a superior autoignition metric to describe fuel performance under engine operating conditions that are outside of the thermodynamic pressure-temperature (PT) constraints of RON and MON tests. However, there are questions about whether OI is adequate for the unconventional fuels being investigated and developed within the Co-Optima initiative. This experimental investigation studies the performance of 19 different fuels, including several with unconventional chemistries relative to petroleum-derived gasoline, across a range of PT trajectory conditions, including both boosted SI and ACI operating conditions. It is found that the fuels with unconventional chemistry perform in accordance with OI, while the largest deviations in performance can be attributed to high concentrations of aromatics and olefins, which are chemical families ubiquitous to petroleum-derived gasoline.