A Comparative Study of Knock Formation in Gasoline and Methanol Combustion Using a Multiple Spark Ignition Approach: An Optical Investigation

Engine knock is a major challenge that limits the achievement of higher engine efficiency by increasing the compression ratio of the engine. To address this issue, using a higher octane number fuel can be a potential solution to reduce or eliminate the propensity for knock and so obtain better engine performance. Methanol, a promising alternative fuel, can be produced from conventional and non-conventional energy resources, which can help reduce pollutant emissions. Methanol has a higher octane number than typically gasolines, which makes it a viable option for reducing knock intensity. This study compared the combustion characteristics of gasoline and methanol fuels in an optical spark-ignition engine using multiple spark plugs. The experiment was carried out on a single-cylinder four-stroke optical engine. The researchers used a customized metal liner with four circumferential spark plugs to generate multiple flame kernels inside the combustion chamber. The results indicated that generating multiple flames inside the cylinder caused higher pressures and temperatures, which led to the production of more knocking cycles with higher knock intensities. Additionally, the study showed that methanol combustion produced significantly lower knock intensity with better engine power output compared to gasoline combustion due to its higher octane rating enabling more advanced ignition and its faster flame propagation. The researchers characterized various knock intensities obtained by multiple ignition sites and exhibited a transition from normal combustion to slight knock, medium knock, and high knock cases. Furthermore, the researchers performed high-speed natural-flame-luminosity (NFL) imaging to capture the multiple flame development and autoignition kernels inside the cylinder for various combustion cases. This study provides a detailed understanding of the combustion characteristics of gasoline and methanol fuels in a spark-ignition engine and can be used to further optimize engine performance by reducing knock intensity and increasing engine efficiency.