Hydraulic characterization of a second-generation common rail injector operating under solo and split injection strategies

In modern compression ignition engines, complex fuel injection strategies are adopted in order to enable a clean and efficient combustion process and an effective combustion noise control algorithm. Multi-injection strategies inject fuel into the combustion chamber several times (e.g. pilot, main, and post injections) during each combustion cycle, while split-injection approach further divides the main injection into different shots, with very short dwell time. Split injection may help to enhance air entrainment into the spray core where the fuel droplets are highly dense and mixing quality is poor. These advanced injection techniques lead to complex hydraulic behaviors including injection instability and eventually affecting fuel metering accuracy, hence detailed investigations are required. Understanding hydraulic characteristics especially during peculiar events like start/end of injection and accurately quantifying the actual injection volume, injection rate, and pressure variations in different locations of the injection system in each single activation of a complex strategy are key targets. In this work, the hydraulic behavior of a second generation common-rail solenoid injector operating under split-injection strategy has been experimentally investigated in terms of injection rate and injected volume. An extensive experiment has been conducted in this study using a state-of-the-art injection system operating on a hydraulic test bench equipped with a Zeuch-method type injection analyzer. It is found that although the standard of deviation of injection rates and injected volume is quite small for isolated injection events, the shot-to-shot deviation for split-injection mode can be significantly higher depending mainly on dwell time, fuel quantity ratio between the two shots and injection pressure level, as an effect of both pressure perturbations in the feeding line and in the injector caused by close actuations, eventually joined to inertial phenomena of the injector needle. The present paper reports an analysis methodology for the quantitative evaluation of systematic inter-cycle deviations, in the effort towards a deeper exploitation of the potential benefits offered by advanced injection strategies.