In-Cycle Predictability and Control of Knock in a PFI HD SI Engine Fueled with Methanol

Knock is an anomalous combustion occurrence limiting the efficiency of the spark-ignited engine, hence increasing fuel consumption and emissions. The global aim to cut the emissions from green-house-gases therefore makes knocking combustion a very appropriate research topic of today. This paper explores the possibility to do in-cycle spark timing control of knock, based upon cycle-to-cycle adaptation of the temperature of a hypothesized hot spot. The potential for post-spark timing control is also examined. Experiments were carried out on a single cylinder port fuel injected spark ignited engine fueled with methanol. Knock was quantified by the Maximum Amplitude of Pressure Oscillations metric and predicted by the Livengood-Wu integral. Normalized distributions, together with different σ confidences, of the in-cylinder state such as gas temperature, in-cylinder pressure and Livengood-Wu integral were computed both pre- and post-spark timing. Type I and Type II errors of the computed metrics revealed that knocking cycles cannot be distinguished from normal cycles, and that hot spots are likely not the root cause of auto-ignition in the current engine. Hence, in-cycle control of knock based upon a hypothesized hot spot temperature would be fruitless. A proven method to mitigate knock in-cycle is the use of water injection. Nevertheless, the post-spark timing analysis showed that this control post-spark timing may be counterproductive. The knocking and normal cycle combustions have a large overlap before the knocking occurs. Therefore, in-cycle regulation through water injection can penalize normal cycles, to a degree that the indicated thermal efficiency would drop more than just retarding the spark timing to 1% knocking (regular knock controller). Lubricant oil, instead of hot spots or fuel-rich spots, was demonstrated to be the most plausible cause of knock in the current engine-fuel configuration.