Knock investigation in highly boosted spark-ignited engines

Gilliam III, A. L. Knock Investigation in Highly Boosted Spark-Ignited Engines. University of Wisconsin-Madison, 2018.

Spark-ignited combustion engines continue to be the most popular option for light-duty transportation. Modern technologies and intake pressure boosting have led to increased efficiencies and engine downsizing. The increased temperatures and pressures due to intake boosting can lead to knock, i.e., violent endgas auto-ignition. Heat release from the compressed charge before spark timing can be seen at these elevated temperatures and pressures. The behavior of fuels in a highly boosted spark-ignited engine was studied under conditions of high knock propensity. Six fuels, in total, were tested at 1000rpm and 2000rpm from 40C intake temperature up to the upper range of their operating limit; the fuels chosen for investigation were candidate fuels from the Co-optimization of Fuels and Engines Initiative sponsored by the United States Department of Energy. Pre-spark heat release (PSHR) was found to occur in all fuels; a high ethanol-content fuel had minimal PSHR. It was found that spark timing did not need to be retarded to maintain the degree of knock when pre-spark heat release was present. A three-zone model with detailed chemical kinetics was created to simulate the unburned charge chemistry and investigate pre-spark heat release. A wide distillation fuel surrogate mechanism by Ren et al and a gasoline mechanism developed by the Lawrence Livermore National Laboratory were used for simulation. The three-zone model was capable of predicting heat release before spark and matching hot motored and fired pressures of experimental data. The model showed that a chemical change occurred as a result of heat release in the endgas after spark in many conditions, which was not seen in the experimental heat release. The ignition delay of the actual unburned charge after PSHR was significantly changed due to the changed chemical composition. After PSHR, the iso-octane charge increased in ignition delay. Iso-octane, E30, high aromatic fuel, and high alkylate fuels were simulated with the three-zone model. Further improvements in surrogate fuel composition are needed to evaluate a wider variety of fuels and make further use of the three-zone model.