Experimental and predictive analysis of knock inducing factors for HCNG-fueled spark ignition engines

Hydrogen and its derived fuels offer significant potential in the transportation sector due to their superior performance and lower emissions. However, knock remains a major challenge in hydrogen-enriched fuels, limiting engine efficiency and durability. This study aims to identify the key factors influencing knock in a hydrogen-enriched compressed natural gas (HCNG) fueled spark-ignition (SI) engine under varying operating conditions. Experiments were conducted by altering engine load (25 %–100 %), hydrogen enrichment (0 %–40 %), exhaust gas recirculation (EGR) (0 %–29 %), spark timing (14° CA bTDC to 35° CA bTDC), and engine speed (700 rpm–1700 rpm). The effects on combustion characteristics, including burn duration, knock ratio (KR), coefficient of variation of indicated mean effective pressure COV % (imep), in-cylinder heat transfer rate, indicated mean effective pressure (imep), in-cylinder pressure, and exhaust temperature, were analyzed. Results indicate that increasing engine load from 25 % to 100 % led to a 75.5 % rise in KR and a 77.7 % increase in heat transfer rate. Advancing spark timing from 47° CA bTDC to 55° CA bTDC resulted in a 49.4 % rise in KR and a 3.5 % increase in exhaust temperature. Conversely, EGR application reduced KR by 33.2 % at 1700 rpm. To predict KR, three machine learning algorithms—neural network fitting tool, support vector regression and linear interactions—were applied, with bayesian regularization achieving the lowest mean squared error. These findings provide valuable insights for optimizing electronic control unit (ECU) calibration and advancing HCNG engine development.