Abstract
Multiple injection strategies have increased their capabilities along with the evolution of injection system technologies up to the point that nowadays it is possible to inject eight different pulses, permitting to improve the engine performance, and consequently, emissions. The present work was realized for two simplified strategies: a pilot-main and a main-post, in order to analyze the influence of an auxiliary pulse on the main and otherwise, in reactive conditions for two pilot/post quantities and four hydraulic dwell times. The study was carried out by employing two optical techniques: diffused back-illumination with flame bandpass chemiluminescence for measuring soot, represented by soot-maps distribution, and single-pass schlieren for ignition delay (ID). Furthermore, a novel methodology for decoupling the start of combustion (SOC) of the second pulse was developed and successfully validated. From the ignition delay results, it was found for all test points that the pilot injection enhanced conditions, which promote a faster ignition of the main pulse, also at higher chamber temperatures, all conditions presented a separate combustion event for each injection. In emission terms, soot increased in the pilot-main strategies compared to its single injection case, as well as, in conditions that promote faster-premixed combustion.
Highlights
Over the years, researchers have studied the fundamental processes involved in diesel direct injection and how they impact crucial variables affecting the engine performance and especially pollutant formation, since regulations on engine emissions have considerably become more rigorous.One key factor for controlling pollutant emissions is diesel combustion process, which in turn, is a complex phenomenon affected by numerous mechanisms [1], as it is the diesel spray mixing, which has been recognized as a critical factor in combustion control [2]
Regarding the decoupling of the schlieren contour, given that the second injection occurs in conditions where density gradients were induced by the previous one, the same dynamic background subtraction strategy was applied to the second pulse, as the non-spray pixels of the composed background contain the image with the first injection, so it is substracted, and the remnant pixels, identified as the spray in the previous frame, were filled with the original background before injection
The effect of multiple injections parameters as hydraulic dwell time (DT) and fuel quantities distribution on the main variables of combustion given in terms of the ignition delay, approximate ignition location, and soot distribution maps are shown
Summary
Researchers have studied the fundamental processes involved in diesel direct injection and how they impact crucial variables affecting the engine performance and especially pollutant formation, since regulations on engine emissions have considerably become more rigorous.One key factor for controlling pollutant emissions is diesel combustion process, which in turn, is a complex phenomenon affected by numerous mechanisms [1], as it is the diesel spray mixing, which has been recognized as a critical factor in combustion control [2]. In an effort for reducing emissions from the injection-combustion field, several techniques have been developed One such strategy is multiple injection, which has been proven being useful for lowering fuel consumption and contaminants [3]. Contrarily to a single injection, they permit the using of up to eight injection pulses, meaning eight different quantities that can be injected in one cycle [4]. Regarding the decoupling of the schlieren contour, given that the second injection occurs in conditions where density gradients were induced by the previous one, the same dynamic background subtraction strategy was applied to the second pulse, as the non-spray pixels of the composed background contain the image with the first injection, so it is substracted, and the remnant pixels, identified as the spray in the previous frame, were filled with the original background before injection. This procedure is more detailed in [14]
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