No abstract

Heat release analysis (HRA) is commonly used in combustion studies to derive understandings of chemical and physical processes in situations where direct measurement is not practical. In internal combustion engines, it is typically based on crank-angle resolved pressure diagnostics. However, it has not been applied extensively to rapid compression machine datasets. There are various challenges associated with rigorous application of HRA, including a reasonable accounting of physical processes that occur during the test period, such as heat loss. Limitations associated with transducer robustness and data acquisition system fidelity also exist. On the other hand, there is potential to extract a wealth of information from pressure-time histories via HRA, such as quantifying the evolution and trends of preliminary exothermicity, e.g., low-and intermediatetemperature heat release, across a range of thermodynamic conditions; detecting the existence of non-uniform ignition phenomena during a test; and providing additional targets for the evaluation and improvement of chemical kinetic models. This work discusses such opportunities, and some approaches towards resolving various challenges.

No abstract

Of late there has been a resurgence in studies investigating parameters that quantify combustion knock in both standardized platforms and modern spark-ignition engines. However, it is still unclear how metrics such as knock (octane) rating, knock onset and knock intensity are related, and how fuels behave according to these metrics across a range of conditions.As part of an ongoing study, the air supply system of a standard Cooperative Fuel Research (CFR) F1/F2 engine was modified to allow mild levels of intake air boosting while staying true to its intended purpose of being the standard device for ASTM-specified knock rating, or octane number tests. For instance, the carburation system and intake air heating manifold are not altered, but the engine was equipped with cylinder pressure transducers to enable both, logging of the standard knockmeter read-out, as well as state-of-theart indicated data.For this study, the engine was operated using primary reference fuel 90 (PRF90) at 600 rpm, first following the procedures of the ASTM D2699 research octane number test protocol in order to define the geometric compression ratio set point for standard knock number. Thereafter, compression ratio sweeps were conducted at intake temperatures ranging from 30 to 150 °C and intake air boost extending from 0 to 0.3 bar above ambient. The resulting operating map provided a broad envelope of compressed in-cylinder conditions relevant to modern spark ignition engines. Detailed analysis of the indicated data highlighted a poor correlation between established knock intensity metrics and the knockmeter reading, which is used to characterize a fuel's octane number. It was further found that the autoignition characteristics of PRF90 could be perturbed by means of intake air boosting and heating without being captured by the knockmeter reading.

<div class="section abstract"><div class="htmlview paragraph">The knock resistance of fuels has been historically measured using the ASTM RON and MON methods. However, significant discrepancies between the fuel octane number and knock-limited performance in modern spark-ignited (SI) engines have been well-documented. Differences between the operating conditions of the Cooperative Fuels Research (CFR) engine during RON rating and those attained in modern SI engines have been highlighted in the literature. While octane ratings are performed for each fuel on the CFR engine at the lambda that provides the highest knockmeter reading, modern SI engines are generally operated at stoichiometry and knock intensity is based on the high frequency cylinder pressure oscillations associated with knocking combustion. In the present work, an instrumented CFR engine was used to analyze lambda effects on both the conventional knockmeter RON rating method and cylinder pressure transducer based knock intensity. It was found that both knock intensity metrics reduced significantly at stoichiometry for highly iso-paraffinic fuels (such as primary reference fuels), compared to test fuels composed of higher ethanol or aromatic content. This behavior had a significant impact on how the octane ratings of the fuels were perceived at stoichiometric conditions.</div></div>