A comprehensive analysis of multiple injection strategies for improving diesel combustion process under cold-start conditions

Cold start in diesel engines is an issue of increasing importance due to its high fuel consumption and emissions. In cold-start operations, improving the combustion process is directly related to improving the thermal efficiency and emissions. In this study, a comprehensive analysis of multiple injection strategies was conducted under cold-start conditions to identify an optimal injection strategy that improves diesel combustion process at various ambient temperatures. The cold-start conditions were reproduced in a constant volume combustion chamber (CVCC). While all of the sprays in a single injection strategy were successfully ignited at 293 K, only the sprays near a glow plug showed combustion reactions at 253 K. The low air and diesel temperature at 253 K inhibited the diesel vaporization, which led to the poor combustion development. Pilot injections were implemented to improve the diesel vaporization at 253 K. The pilot injections had a small amount of heat required to vaporize diesel droplets, which reduced the temperature drop in the combustion chamber. The pilot injections also released small amounts of heat prior to the following main injection, thus aiding the vaporization of the main injection. The triple and quadruple injections improved the combustion process significantly at 253 K. However, the double and triple injections showed superior heat release rate and in-chamber pressure rise at 293 K. Based on the results at 253 K and 293 K, the triple injection strategy were determined to be the optimal injection strategy under cold-start conditions.