Relative Knocking Intensity to Discuss Similarities and Differences between Low- and High-Speed Knocking Phenomena

Knocking intensity, the maximum half-amplitude of pressure oscillation, reaches 1 MPa once in thousands of cycles under a certain boosted high-load condition with a spark-ignition timing advanced to excess at the engine speed of 5000 rpm, which is named high-speed super knocking. In the present study, a mass-production turbo-charged direct-injection gasoline engine is operated for the indicated mean effective pressure of 1.7 MPa at the engine speed of 1500 to 5000 rpm. Unburned-zone autoignition timing is estimated using Livengood-Wu integral coupled with a small set of ignition delay time equations developed by one of the authors, which has a difference below 2 degrees from that detected from the differential value of net heat release rate. Whenever autoignition occurs at 15 degrees after TDC, ignition delay time in the unburned zone is the period of about 10 degrees at the autoignition timing, regardless of engine speed. Knocking intensities with a spark-ignition timing advanced to excess, are divided by the minimum one with the spark-ignition timing retarded by 4 degrees, which is named relative knocking intensity. The relative knocking intensities suggest that true heavy knocking occurs occasionally at the engine speed of 1500 rpm, of which the occurrence rate decreases with the increase in engine speed. High-speed super knocking might be a rare occurrence of true heavy knocking.