Experimental Investigation of the Effect of Needle Opening Pressure (Nop) on Combustion and Emissions Formation in a Heavy-Duty Di Diesel Engine

This paper presents an investigation of the effects of varying needle opening pressure (NOP) (375 to 1750 bar), engine speed (1000 rpm to 1800 rpm), and exhaust gas recirculation (EGR) (0% to 20 %) on the combustion process, exhaust emissions, and fuel consumption at low (25%) and medium (50%) loads in a single-cylinder, heavy-duty DI diesel research engine with a displacement of 2.02 l. The engine was equipped with an advanced two-actuator E3 Electronic Unit Injector (EUI) from Delphi Diesel, with a maximum injection pressure of 2000 bar. In previous versions of the EUI system, the peak injection pressure was a function of the injection duration, cam lift, and cam rate. The advanced EUI system allows electronic control of the needle opening and closing. This facilitates the generation of high injection pressures, independently of load and speed. The results show that increasing NOP was advantageous for reducing soot in the low and medium load regions, but NOx emissions tended to increase. In addition, the premixed combustion phase was increased at low load although the increase in NOP significantly reduced ignition delay. Furthermore, with a retarded SOI the combustion process became mainly premixed and there were barely measurable soot emissions. At medium load and low speed, high NOP with a retarded injection also resulted in a non-measurable amount of soot emissions. Analysis of the Rate of Heat Release (RoHR) curves and flame imaging showed that the combustion was significantly improved and shortened with increased NOP. The luminous flame appeared and disappeared earlier at higher NOP, while the sooting flame temperature was significantly increased, as were the soot formation and oxidation processes. The combination of a high NOP together with a high amount of EGR led to simultaneous reductions in both soot and NOx emissions. Increased engine speed led to lower specific NOx and higher soot emissions