Investigation of the dynamic interaction mechanism of split injections during the ignition process under diesel engine-like cold- and hot-start conditions

Abstract

In a range of operating conditions with variable injection parameters, interactions among split sprays present uncertainties across both temporal and spatial dimensions. Understanding these dynamic interactions is crucial for optimizing injection strategies to improve both the performance and environmental adaptability of diesel engines. In this study, optical diagnostics and numerical simulations were conducted to explore the interaction mechanisms between split injections and their impact on the ignition process. Engine-like cold and hot-start operating conditions were configured, and four injection strategies, including a single injection, were examined. The findings reveal that ambient temperature influences split spray interactions by altering their ignition characteristics, while injection dwell time modifies these interactions by changing the combustion spatial and temporal phase between split sprays. Compared to a single injection, split injection promotes the ignition and combustion of the main spray such as shortening ignition delay, reducing ignition location, and increasing flame area, but this promoting effect weakens under hot-start conditions. When the main spray chases the pilot spray before ignition, it increases the local concentration and reduces the temperature upstream of the pilot spray plume. However, since the effect front fails to reach the expected ignition region before the pilot ignition occurs, the ignition of the pilot fuel pocket remains unaffected. Under cold-start conditions, the main ignition delay shows a non-monotonic trend, reaching its minimum at an injection dwell of 900 μs, where the pilot ignition provides the main spray with elevated local temperatures and an increased hydroxyl mass fraction. Under hot-start conditions, the temporal overlap of ignition in split sprays is enhanced, yet spatial interaction weakens. Despite varying degrees of pilot flame decay before the main spray ignites with increasing injection dwells, the thermal radiation emanating from their combustion elevates the local temperature in the interaction zone beyond 1200 K. Consequently, upon catching up with the pilot injection, the main spray mixture rapidly ignites through the high-temperature reaction pathway, rendering the ignition delay insensitive with injection dwells.