Browse Publications Technical Papers 2018-01-0210
2018-04-03

Insights into Engine Knock: Comparison of Knock Metrics across Ranges of Intake Temperature and Pressure in the CFR Engine 2018-01-0210

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-the-art 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.
The indicated data were used to conduct standard heat release, as well as power spectral density analyses. This provided insight into the effects of intake manifold conditions on the physical manifestation of in-cylinder knock while highlighting anomalies not captured by the standard CFR knockmeter.

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