Hydraulic circuit layout analysis, diagnostics and control of the injection process in Common Rail diesel fuel injection systems

Current fuel injection systems should provide more and more reliable performance in management and control of the injection event, to improve the efficiency of the fuel combustion process and thus meet both environmental and consumer demands. Small fuel quantities are usually injected before the main injection to reduce the combustion noise. Whereas, small injections following the main shot are employed to optimize the combustion development and to reduce pollutant emissions. Since the number of shots in multiple injections has increased over the years, the injected amount of fuel for each shot has become smaller, given a fix total amount of fuel. The control of injected fuel quantities is therefore a demanding task and any failure causes a worse combustion process and higher pollutant emissions. This study is focused on the modelling of Common Rail (CR) fuel injection systems for diesel applications and the implementation of specific mathematical techniques to examine phenomena pertaining to the fuel injection dynamics. New methodologies for detecting key events of the injection process and to estimate the injected fuel quantity have been developed for control purposes. A 1D numerical model of a solenoid-actuated injector equipped with a pressure-balanced pilot-valve has internally been developed to evaluate the main hydraulic and mechanical quantities. The impact of a pressure-balanced and a standard pilot-valve layout has been investigated, and the performance of a standard CR system has been compared to that of a novel injection system concept, i.e., the Common Feeding (CF). A sensitivity analysis of the main design parameters of the pressure-balanced pilot-valve layout has been carried out, with the purpose of improving the general performance of the system. A dedicated lumped parameter model of the high-pressure hydraulic circuit of the CR system has been used to calculate the natural frequencies and modes of vibration. It has been found that the direction and the magnitude of the fuel flow rates along each pipe of the apparatus can be derived from the first three modes (and the corresponding eigenvectors). The main objective is to identify which components could primarily be stressed for the main modes of vibration. Finally, external forcing terms acting on the system have been investigated to determine possible causes of hydraulic resonance. The identification of specific events that characterize the injection process, such as pilot-valve and injector nozzle opening and closure phases, might play a significant role for diagnostics and control of the system in on-board and real-life applications. To this end, time-frequency analysis techniques have been used to detect key events of the injection process. The method has then been applied to several working conditions to test its robustness. Specifically, the pressure time-history acquired along the rail-to-injector pipe has been transformed from the time domain into the joint time-frequency domain, so that changes within the new signal would highlight the events to be detected. Even though the diagnostics of injection events is an important topic, one of the main issues for injection systems is the absence of a closed-loop real-time control. The control unit should be able to evaluate the amount of fuel injected into the combustion chamber and eventually to correct it, based on the comparison with the target quantity stored in the engine maps. In this perspective, a quadratic correlation between the fuel amount that enters the injector and the injected fuel quantity has been found. The presented algorithm, which is specific for small injections, converts the pressure time-history acquired along the rail-to-injector pipe into an instantaneous fuel flow rate, calculates the fuel amount at the injector inlet by integration and the actual injected fuel quantity by means of the quadratic correlation. The entire process can be executed in real-time.