Improvement of Thermal Efficiency in a Diesel Engine with High-Pressure Split Main Injection

This study aims to utilize high-pressure split-main injection for improving the thermal efficiency of diesel engines. A series of experiments was conducted using a single-cylinder diesel engine under conditions of an engine speed of 2,250 rpm and a gross indicated mean effective pressure of 1.43 MPa. The injection pressure was varied in the range of 160–270 MPa. Split-main injection was applied to reduce cooling loss under the condition of high injection pressure, and the split ratio and the number of injection stages were varied. The dwell of the split main injection was set to near-zero in order to minimize the elongation of the total injection duration. As a result, thermal efficiency was improved owing to the combined increase in injection pressure, advanced injection timing, and split-main injection. According to the analysis of heat balance, a larger amount of the second part of the main injection decreased the cooling loss and increased the exhaust loss. Computational fluid dynamics calculations were performed to reveal the causes of the lower cooling loss; however, the results could not capture the experimental trend when using an ordinary spray cone angle. While using a wider spray angle for the second part of main injection, the calculated trend improved. The total cooling loss depends on the balance between the cooling losses by the first and second main sprays.