Hydraulic analysis of a GDI injector operation with close multi-injection strategies

The increase of injection pressure in GDI engines represents a proven development trend in the effort of limiting raw particulate emission, which is one of the most severe concerns for this kind of combustion system. Since increasing injection pressure level tends to cause significant issues in terms of spray penetration control, in parallel with ultra-high injection pressure multi-event strategies are being considered to improve the air-fuel mixture formation management. The adoption of these advanced injection strategies can potentially affect the injection system hydraulic behavior, hence an adequate control of each event fuel quantity is mandatory for a proper engine optimization. When the injector is operated according to a multi-event strategy, complex hydraulic and electro-magnetic phenomena may occur. Pressure waves triggered by each injection can eventually influence the subsequent events, modifying the injected volume compared to the expected, isolated event value. This hydraulic phenomenon is well-known for Diesel applications and is investigated in the present paper in a typical GDI configuration evidencing the effect of the dwell time value between consecutive events both in terms of injected volume and injection rate profile using a proprietary injection analyzer. Further, when consecutive injections are remarkably close, the succeeding injection event is likely to be influenced by a different, electro-magnetic phenomenon in addition to pressure waves propagation in the system pipes. With reduced dwell time strategies, the current profile driving the injector can be modified by a previous close-coupled actuation due to residual magnetization in the solenoid circuit. This phenomenon determines for the succeeding injection event a steeper transient of the solenoidal current, causing a faster injector needle rise and correspondingly a significantly increased injected quantity. In this paper, an innovative methodology is proposed to investigate in multi-injection strategies the effects of close injector actuations, evidencing the influence of hydraulic phenomena and of current profile modifications, respectively. The developed approach, named ETboost, consists in a double injection with a first dummy command. To this end, a short ETboost command is required not causing the injector needle opening and thus no triggering pressure waves that might influence the second injection. Using this analysis approach, the residual magnetization effects on a production GDI injector were experimentally investigated in terms of mean injected volume and instantaneous flow rate. The results obtained with reduced dwell-time were compared with the standard operation mode (single injection), evidencing a significant increment of the injected volume in particular in the ballistic operation where the actual injected quantity can be more than doubled if compared to standard actuation. The dependence of the injected volume on ETboost and dwell time was explored for different operating conditions, evidencing how reducing the dwell time and increasing the ETboost duration the injected volume is enlarged and the shot-to-shot dispersion is significantly reduced.