Knocking Identification in a Biodiesel–hydrogen Dual-fuel Diesel Engine Based on Combustion Characteristics

Abstract This study investigates knocking phenomena in a four-stroke diesel engine equipped with a dual injection system comprising a biodiesel pilot injection and varying hydrogen flow rates of 2.5–10 liters per minute (LPM). Combustion characteristics were analyzed to evaluate the influence of hydrogen enrichment. The measured parameters include cylinder pressure versus crank angle at 40°, 60°, 80°, 100°, and 120° after top dead center (aTDC) for hydrogen start of injection (SOI), and at 60°, 80°, 100°, 120°, and 140° crank angle (CA) for hydrogen duration of injection (DOI). At a hydrogen flow rate of 2.5 LPM, the peak cylinder pressure (PCP) increased by up to 3.94% compared with single-fuel biodiesel operation under low-load conditions, while knocking intensity remained within the soft range (1.2–2.8 bar) across all SOI–DOI variations. At higher hydrogen flow rates, the knocking severity increased progressively. At 5 LPM, most conditions remained within the soft knocking range (2.0–4.8 bar). At 7.5 LPM, medium knocking (5–6 bar) occurred at DOI 60 with SOI 100 and 120. At 10 LPM, medium knocking (6–7.5 bar) was observed at DOI 60 with SOI 80, 100, and 120. These results indicate that hydrogen enrichment increases the peak cylinder pressure and overall combustion energy; however, it also intensifies knocking, particularly at higher hydrogen flow rates and with later start-of-injection (SOI) timings under DOI 60° conditions.