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To investigate their ignition delay and combustion behavior, experiments with two biomass pyrolysis oils and No. 2 diesel fuel were performed in a direct injection diesel engine. It was found that while the indicated thermal efficiency of both pyrolysis oils equaled that of the diesel fuel, they exhibited excessive ignition delays and required a moderate degree of combustion air preheating to ignite reliably. Despite the longer ignition delays associated with the pyrolysis oils, the cylinder pressure rise rates were significantly less than with No. 2 diesel fuel. Experimental ignition delay and heat release rates were interpreted using a phenomenological spray combustion model. Using a three parameter fit for vaporization, ignition, and combustion rate, the model showed that the longer ignition delays of the bio-oils result from slow chemistry relative to diesel fuel. The model also showed that the heat release profiles of the bio-oils are consistent with slow combustion chemistry and rapid mixing relative to diesel fuel. As a result, whereas diesel combustion is predominantly mixing limited, pyrolysis oil combustion is predominantly limited by chemistry through much of the process.

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Rapid compression machines (RCMs) are widely-used to acquire experimental insights into fuel autoignition and pollutant formation chemistry, especially at conditions relevant to current and future combustion technologies. RCM studies emphasize important experimental regimes, characterized by lowto intermediate-temperatures (600-1200 K) and moderate to high pressures (5-80 bar). At these conditions, which are directly relevant to modern combustion schemes including low temperature combustion (LTC) for internal combustion engines and dry low emissions (DLE) for gas turbine engines, combustion chemistry exhibits complex and experimentally challenging behaviors such as the chemistry attributed to cool flame behavior and the negative temperature coefficient regime. Challenges for studying this regime include that experimental observations can be more sensitive to coupled physicalchemical processes leading to phenomena such as mixed deflagrative/autoignitive combustion. Experimental strategies which leverage the strengths of RCMs have been developed in recent years to make RCMs particularly well suited for elucidating LTC and DLE chemistry, as well as convolved physicalchemical processes. Specifically, this work presents a review of experimental and computational efforts applying RCMs to study autoignition phenomena, and the insights gained through these efforts. A brief history of RCM development is presented towards the steady improvement in design, characterization, instrumentation and data analysis. Novel experimental approaches and measurement techniques, coordinated with computational methods are described which have expanded the utility of RCMs beyond empirical studies of explosion limits to increasingly detailed understanding of autoignition chemistry and the role of physical-chemical interactions. Fundamental insight into the autoignition chemistry of specific fuels is described, demonstrating the extent of knowledge of low-temperature chemistry derived from RCM studies, from simple hydrocarbons to multi-component blends and full-boiling range fuels. Emerging needs and further opportunities are suggested, including investigations of under-explored fuels and the implementation of increasingly higher fidelity diagnostics.

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