A Novel Nozzle Orifice Design of a Heavy-Duty Diesel Engine to Achieve Higher Thermal Efficiency with Significantly Reduced Heat Loss

Recently, it has been widely recognized that increasing compression ratio is one of the essential technologies for improving thermal efficiency of heavy-duty diesel engines. However, while a higher compression ratio contributes to improved thermal efficiency, it also causes increased heat loss, which can surpass the efficiency gains. Therefore, more effective heat loss reduction technologies should be developed. Despite various attempts, including thermal barrier coatings which are well known as the measure for surface temperature swing, no existing technology has successfully achieved a significant reduction in heat loss while simultaneously improving thermal efficiency across a wide operating range. In this study, a novel nozzle orifice design to improve thermal efficiency was investigated. The offset orifice nozzle has holes drilled offset of less than 0.5 mm from the radial center of the nozzle. The experimental results demonstrated that the offset orifice nozzle achieved significant improvements in both heat loss reduction and thermal efficiency, regardless of the compression ratio and operating load conditions. However, the underlying mechanisms and design guidelines for optimization have not been revealed yet. This study investigates the mechanisms behind heat loss reduction and thermal efficiency improvements with offset orifice nozzles by means of a high-compression-ratio’s heavy-duty diesel engine. It is observed by in-cylinder combustion visualization technique that the flame tip length was shortened with significantly wider flame cone angle from very close to the orifice exit even though the fuel injection rate of the offset orifice nozzle was slightly higher than that of the referenced radial orifice nozzle by enlarging orifice diameter. Numerical simulations of the initial spray structure along with the nozzle internal flow were confirmed its wider spray cone angle. The main cause of heat loss reduction is assumed to be the more homogeneous wall surface temperature distribution by the modified spatial flame distribution.