An investigation of primary reference fuel combustion in the modified derived cetane tester
Contemporary liquid combustion research is constantly in pursuit of higher fuel efficiencies and lower overall emissions. This is done in multiple ways, but of extraordinary value are studies furthering the understanding of combustion thermochemistry and the driving chemical reactions of the combustion process. These studies can be experimentally based, or take computational form, utilizing chemical kinetic mechanisms that have been built and refined with decades of research. This work makes use of the derived cetane tester (CID), a device typically used in the petroleum industry, to combust various fuels of interest to chemical kinetic modeling. The CID is modified to investigate the combustion of primary reference fuels (PRF) within the device’s constant volume combustion chamber. This enables the testing of greater temperatures, pressures, and equivalence ratios through the development of a custom external control system. Studies evaluating the combustion of n-heptane and iso-octane are then performed at a variety of conditions. The combustion pressure trace results are used to analyze these fuels’ autoignition delay time, and various chemistry-based combustion phenomena such as low temperature heat release (LTHR or “cool flame”), and negative temperature coefficient (NTC) behaviors. The studies’ results show that the autoignition behavior of n-heptane (and presumably other high-volatility fuels) can be accurately represented by the CID, given sufficient fuel evaporation and mixing time is allowed. The constant-volume combustion chamber’s minimum mixing time to approximate a spatially homogenous reactor (as is assumed in the study’s simulation models) is determined to be 6 ms. The studies based on iso-octane display less agreement with published experimental results and model predictions than n-heptane based experiments. An additional study is undertaken, and its results analyzed to evaluate this divergence, and several postulations are given in explanation. Finally, a study is undertaken to evaluate a process of autoignition delay time scaling according to assumed pressure relationships. Pressure scaling factors are calculated at each test temperature, and are used to scale 4-bar n-heptane results to a 41-bar test condition. The pressure scaling study results show that through this method, low-pressure results can be scaled quite accurately.