Application of ignition assistant plug to a compression ignition engine fuelled with a blend of jet fuel and synthetic fuel

• IAP protrusion is applied to improve low reactivity fuel combustion. • Effects of IAP and injector tip protrusion are studied in optical CI engine using FIV. • Longer IAP protrusion leads to enhanced flame development and combustion stability. • Optimal combustion is found at optimised spray targeting and thermal distribution. This study applies an ignition assistant plug (IAP) as a new thermal device to enhance ignition and combustion of a lower ignition quality fuel blend of 60 % jet fuel and 40 % synthetic fuel by volume in a small-bore optical compression-ignition engine. To bridge the missing knowledge about combined effects of IAP protrusion and spray targeting on flame developments of low reactivity fuel, both the protrusion depths and fuel injector tip position are systematically varied. High-speed flame imaging is performed for each of nine combinations of IAP and injector tip protrusion depths and further processed using a flame image velocimetry (FIV) algorithm to obtain flow vectors within the flame – i.e. quantitative representation of flame vector magnitude and turbulence distribution. IAP protrusion was varied between 5 and 8 mm in the axial distance from the cylinder head while the injector tip protrusion was also varied between 1.5 and 4.5 mm below the cylinder head. High-speed movies were recorded for 100 cycles for each of nine protrusion combinations as well as for IAP activated and deactivated conditions. With this large sample size, the cyclic variation and combustion stability were analysed using the spatially filtered bulk flow magnitude. With IAP activation, the added thermal energy caused advanced combustion phasing, which was more significant for a fuel jet located closer to IAP. Although the overall flow structures and flow magnitude distribution were similar with the IAP turned-off condition, reduced cyclic variations were achieved due to IAP activation indicating enhanced combustion stability as a major benefit. IAP protrusion variation was found to have minimal influence on the in-flame flow development pattern, bulk flow and turbulence magnitude. However, more advanced combustion phasing and reduced cyclic variations of the bulk flow magnitude was observed for increased IAP protrusion due to better thermal distribution deeper into the piston bowl. Among all protrusion conditions, the baseline injector and long plug protrusion achieved the most effective combustion phasing control and the best combustion stability. This is attributed to optimised in-flame flow fields and better thermal distribution within the piston-bowl, which provides useful information for optimised IAP and injector design and applications.