Reduction of pressure oscillations during high-load HCCI combustion by secondary air injection: Effects of multiple air injections

Abstract

Secondary-air injection has previously been proposed as a means of mitigating the pressure oscillations that can arise during high-load homogeneous charge compression ignition (HCCI) combustion. The present work clarifies how the suppression effect changes when the air is introduced from multiple locations. HCCI experiments were carried out in a rapid-compression machine using one, two, or four wall-mounted gas injectors, while injection pressure and duration were also varied systematically. Over the entire test matrix, the oscillation intensity decreased monotonically with injector count: at a given injection pressure or duration, more injectors produced weaker pressure swings. When the data were reorganized by the total volume of injected air, multi-point injection still required a smaller air dose to achieve the same level of suppression: two injectors halved the air demand relative to a single injector, and four injectors reduced it further. Infrared thermometry revealed that low-intensity cases featured a complex, interlaced temperature field in which hot kernels emerged simultaneously at several sites and spread throughout the chamber. Such a distributed ignition pattern was realized with shorter pulses and smaller air quantities as the number of injectors increased, explaining the enhanced suppression. The findings deepen the understanding of combustion-oscillation dynamics in HCCI engines and provide quantitative guidance for optimizing secondary-air injection systems.