Knocking Combustion in a Heavy-Duty Spark-Ignited Engine Fueled by Methanol

Abstract

Challenges regarding greenhouse gas emissions in general, and especially emissions of carbon dioxide, highlight the need to reduce the use of fossilfuels, which requires more efficient combustion engines and a transition to renewable fuels, such as e-methanol. As knocking combustion limits the efficiency of a spark-ignited engine, thereby increasing fuel consumption and the emissions, it is a very relevant research topic of today. The research literature has proposed several explanations for knocking combustion. A generally accepted hypothesis is that knock is predominantly initiated from so-called hot spots, i.e. exothermic centers with a deviation intemperature. Nevertheless, the scientific literature suggests that hot spots may not be present in all engine-fuel configurations. Moreover, some studies indicate that other reactivity spots within the engine, such as fuel-rich spotsand oil spots, can contribute to knock as well.The standard approach to mitigating knock is to retard spark timing when knock is detected in earlier cycles. Therefore, this approach penalizes thecycles that would have experienced normal combustion at optimal spark timing, thereby reducing overall combustion efficiency. Hence, a preferred solution for controlling knock is to predict in-cycle if knocking will occur and adjust spark timing accordingly. However, the research literature presents conflicting results regarding the possibility of predicting knock before spark timing. This thesis evaluates the potential for predicting the conditions that lead to incycle knocking combustion in a heavy-duty spark-ignition engine running onmethanol, as well as assessing strategies for mitigating knock and enhancing engine efficiency.The thesis also investigates other potential root causes of auto-ignition in theengine-fuel configuration, including whether lubricant oil entering the combustion chamber can be a contributing factor. The results indicate that it is not possible to accurately predict prior to spark timing whether a cycle will knock. Knock control after spark timing is unlikely to be effective due to the significant overlap in combustion characteristics between normal and knocking cycles. Lubricant oil, rather than hot spots or fuel-rich spots, was demonstrated to be the most likely cause of knock in the current engine-fuel configuration.

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