Optical study on characteristics of non-reacting and reacting diesel spray with different strategies of split injection

Abstract

Even though studies on split-injection strategies have been published in recent years, there are still many remaining questions about how the first injection affects the mixing and combustion processes of the second one by changing the dwell time between both injection events or by the first injection quantity. In this article, split-injection diesel sprays with different injection strategies are investigated. Visualization of n-dodecane sprays was carried out under both non-reacting and reacting operating conditions in an optically accessible two-stroke engine equipped with a single-hole diesel injector. High-speed Schlieren imaging was applied to visualize the spray geometry development, while diffused background-illumination extinction imaging was applied to quantify the instantaneous soot production (net result of soot formation and oxidation). For non-reacting conditions, it was found that the vapor phase of second injection penetrates faster with a shorter dwell time and independently of the duration of the first injection. This could be explained in terms of one-dimensional spray model results, which provided information on the local mixing and momentum state within the flow. Under reacting conditions, interaction between the second injection and combustion recession of the first injection is observed, resulting in shorter ignition delay and lift-off compared to the first injection. However, soot production behaves differently with different injection strategies. The maximum instantaneous soot mass produced by the second injection increases with a shorter dwell time and with longer first injection duration.

Analysis of the Influence of Diesel Spray Injection on the Ignition and Soot Formation in Multiple Injection Strategy

“…The approximate ignition location results are presented for all conditions tested in which ignition occurred within the field of view (50 mm), and are exposed in Figure 11, showing a similar trend as the ignition delay [36]. This parameter was estimated in order to provide a reference of how a pilot injection might have affected the lift-off length of the main pulse, since it is a determinant factor on the soot formation, due to depending on whether the combustion occurs closer to the nozzle, more soot is expected to form and combustion takes place in richer conditions [7].…”

“…Results show that the main pulse ignites with a similar delay to the single injection, and the SOC is always decoupled for the cases showed. Overall, the ID of the post injection was shortened by the main pulse compared to the single injection case, due to the increase in chamber temperature caused by the combustion of the first pulse and re-entrainment of the hot gases [36], that also, moves the ignition location of the second pulse closer to the nozzle. This effect is more remarked for this strategy than for pilot-main, presumably owing to the longer duration of the first pulse.…”

Flame–spray interaction and combustion features in split-injection spray flames under diesel engine-like conditions

“…The injected fuel mass or injection duration for the first injection is of great importance to improve fuel stratification [7], optimize the combustion phasing [8], and reduce soot emission [50,51]. Therefore, in order to optimize the combustion process, the ignition process for double injections with different FIDs should be further analysed in detail.…”

Optical study on needle lift and its effects on reacting diesel sprays of a single hole solenoid injector

“…Nowadays, the injection timing and fuel quantity distribution can be controlled in a flexible way by means of the high pressure electronically controlled common rail systems. Thanks to that, various injection rate shapes [3][4] and multiple injection strategies [5][6] have been studied to get a better fuel-air mixing and more homogeneous combustion. The effect of nozzle geometry (such as conicity, inlet rounding, nozzle hole numbers) on mixture formation and combustion of the diesel spray has also been an interest field to the research community and the automotive manufacturer [7].…”