The influence of pilot injection on high-temperature ignition processes and early flame structure in a high-speed direct injection diesel engine

Simultaneous high-speed natural luminosity and OH* chemiluminescence imaging is used to characterize high-temperature ignition processes in conventional diesel combustion with a pilot-main injection strategy in a single-cylinder, light-duty optical diesel engine. High-speed imaging provides temporally and spatially resolved information in terms of high-temperature ignition processes and flame structure during the combustion. Using these imaging measurements, the high-temperature inflammation and the diffusion flame development processes are analyzed. The chemiluminescence signal shows a hot, reactive mixture, which gradually decreases after the peak release of the pilot combustion and lasts long after the apparent heat release has ended. Therefore, when the reactive pilot mixture exists near the main injection jets, the high-temperature ignition of the main injection is apparently initiated through interactions with the reactive pilot mixture. High-temperature autoignition, another process by which ignition of the main injection occurs, is observed in main injection plumes where the chemiluminescence signal of the reactive pilot mixture becomes very weak or is absent at the start of main injection. As the reaction of the main injection continues, the non-premixed main injection jet structure is developed and the high-temperature reacting region expands throughout the jet.