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

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. 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. This study investigates the mechanism of cooling-loss reduction associated with changes in flame distribution when using an offset orifice nozzle, through in-cylinder combustion observations, two-color method image analysis, and local heat-flux measurements. High-speed combustion visualization was conducted to capture the growth of luminous flames. Radial profiles of the mean and standard deviation were computed at each crank angle to quantify spatial temperature 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 heat flux. These findings confirm the hypothesis that cooling-loss reduction is achieved by suppressing localized hot spots on the piston surface through the altered flame distribution.