Detailed Kinetic Modeling of Low-Temperature Heat Release for PRF Fuels in an HCCI Engine

Now more than ever, the increasing strictness of environmental regulation and the stronger need of higher efficiency standards are pushing for the development of cleaner and energy-efficient powertrains. HCCI engines are suitable candidates to achieve these objectives. Understanding the autoignition process and how it is affected by operating conditions is central to the development of these engines. In addition to experiments, detailed kinetic modeling represents a very effective tool for gaining deeper insight into the fundamentals of HCCI autoignition and combustion. Indeed, modeling activities are today widely used in engine design, allowing a significant reduction in prototype development costs and providing a valuable support to the improvement of control strategies.

Because the amount of low-temperature heat release (LTHR) strongly affects HCCI autoignition, it is important to determine the ability of kinetic models to accurately predict the LTHR over a range of operating conditions. In this work, the combustion behavior of PRF80 (a fuel showing LTHR at the conditions studied) in a 14:1 compression ratio engine operating in HCCI mode is experimentally and computationally analyzed. A new version of the Lawrence Livermore National Laboratory (LLNL) kinetic mechanism is used. This new mechanism contains substantial changes to the low-temperature chemistry that improve its ability to predict the behavior of PRF surrogates. With these improvements, the new mechanism achieves better agreement with rapid compression machine and shock tube data over a wide range of pressures and equivalence ratios. Model results are compared with HCCI engine experiments showing the effects of equivalence the ratio and engine speed on the phasing of both the low - and high-temperature heat release. Particular emphasis is placed on the ability of the model to predict the onset of LTHR at different engine conditions.