Numerical investigation of late injection strategy to achieve premixed charge compression ignition mode of operation

Abstract

Premixed charge compression ignition mode of operation simultaneously reduces oxides of nitrogen (NO_x) and particulate matter with improved fuel economy. Late injection is one such strategy to achieve premixed charge compression ignition mode along with the control over the ignition timing. Late injection along with charge dilution retards combustion and prepares homogeneous mixture by providing sufficient ignition delay. The in-cylinder charge distribution dictates mixture formation which affects the performance and emissions characteristics. Experimental results fail to provide an insight of the in-cylinder processes involved. Hence, numerical simulations have been performed to get in-cylinder distributions of scalars, which help in understanding the combustion process. The work consists of late in-cylinder injection of Diesel wherein the heat release rate trace shows only a premixed mode with lower soot and NO_x. The absence of diffusion combustion confirms that the engine operates in a premixed charge compression ignition mode. In this work, effects of various injection parameters and swirl ratio on engine in-cylinder processes, performance and exhaust emissions have been studied using a three-dimensional model with detailed chemistry by using Converge, a computational fluid dynamics tool. Increasing swirl ratio improved air–fuel mixing causing lower soot emission up to a certain limit beyond which further increase in swirl ratio led to higher soot emission due to increased heat loss. Delayed start of injection resulted in delayed combustion leading to lower soot and NO_x emission but partial oxidation resulted in higher carbon monoxide (CO) emissions. Nozzle tilt angle beyond 70° led to the formation of fuel-rich mixture at the center of combustion chamber resulting in higher soot and CO emissions. A narrow spray cone angle improved air–fuel mixing leading to lower soot but produced higher NO_x. Medium swirl with late injection timing and wide nozzle tilt angle along with narrower spray cone angle resulted in lower emissions without deteriorating engine performance.