Investigation into Cooling Loss Reduction Associated with Changes in In-Cylinder Flame Distribution by Offset Orifice Nozzle

Recently, an increase in compression ratio has been widely recognized as one of the essential technologies for improving the thermal efficiency of heavy-duty diesel engines. However, a higher compression ratio tends to result in increased cooling loss, which could diminish the thermal efficiency gains. Therefore, novel approaches which can reduce cooling loss while maintaining or improving thermal efficiency are required. In our previous studies, it was found that an offset orifice nozzle, in which the orifices are drilled with a small offset from the radial center of the nozzle, improves thermal efficiency and reduces cooling loss simultaneously. However, key factors contributing to the significant reduction in cooling loss have not been sufficiently clarified. This study aims to investigate the mechanism of cooling loss reduction associated with changes in flame distribution when using an offset orifice nozzle in heavy-duty diesel engines, based on in-cylinder combustion observations and local heat flux measurements. High-speed combustion visualization was conducted to capture the temporal and spatial growth of luminous flames. Two-color method was applied to analyze flame temperature and KL factor distributions. Radial profiles of the mean and standard deviation were computed at each crank angle to quantify spatial non-uniformity. Furthermore, multiple thin-film thermocouples embedded in the piston were employed to measure transient surface temperature and to derive heat flux over the entire cycle. The results indicated that the luminous flame distribution with the offset orifice nozzle was significantly different from that with a conventional nozzle, leading to reduction in the spatial non-uniformity of high-temperature regions in the observed area. The piston surface temperature measured at multiple points suggested reduced spatial non-uniformity in surface temperature, with suppressed instantaneous peak heat flux. These findings confirm the hypothesis; cooling loss reduction is achieved by reducing localized hot spots on the piston surface through the altered flame distribution.