Numerical Simulation of a Naturally Aspirated Natural Gas/Diesel RCCI Engine for Investigating the Effects of Injection Timing on the Combustion and Emissions

The start of injection (SOI) timing has a significant effect on increasing the homogeneity of the air–fuel mixture in an reactivity controlled compression ignition (RCCI) engine. In this paper, the impact of the SOI timing from 14 deg to 74 deg before top dead center (bTDC) and different inlet valve closing (IVC) temperatures on natural gas/diesel RCCI performance and emissions have been studied. Also, the simulations carried out by avl fire which is coupled with chemical kinetics. The results showed that in the SOIs of 14 deg, 24 deg, and 34 deg bTDC, the fuel is sprayed into the piston bowl; however, in the SOI of 44 deg bTDC, the fuel collides the bowl rim edge, because of the downward movement of the piston. With the advancement of diesel SOI timing from 14 deg to 74 deg bTDC, two different combustion trends can be observed. However, this advancement leads to a lower CO emission, but it raises the CO₂ emission level. Although the pressure is a primary parameter for NO_x emission, the difference between the trends of NO_x and pressure plots indicates that different factors affect the NO_x production and also increase the IVC temperature, and raises the in-cylinder pressure, heat release rate, NO_x and CO₂ emissions, while it reduces the CO emission.