3. Spray Collapse in a Multi-hole GDI Injector and Its Effect on In-Cylinder Combustion

Gasoline direct injection (GDI) system has been shown to have several advantages over the more conventional Port Fuel Injection system in a SI engine. However, it has also been reported that GDI engines have higher particulate emissions. One of the possible reasons for this higher particulate emission is collapse of the spray emanating from the injector and wetting on the piston head, particularly when the engine is operating under high-load conditions. In this study, a detailed analysis has been performed to understand spray collapse and its effect on in-cylinder combustion for three different fuels: isooctane, n-butanol, and isobutanol. Initially, spray studies in a constant volume chamber were performed. Parameters like liquid and vapor penetration lengths, droplet size, and velocity distribution were estimated from image analysis of high-speed videography and phase Doppler particle analyzer (PDPA). To mimic in-cylinder conditions, the injector body temperature was raised such that injected fuel was also at elevated temperature. Spray collapse was observed at higher fuel temperature conditions, and this resulted in higher axial liquid penetration and finer droplet size distribution. Similar experiments were then performed in an optically accessible engine, and it was observed that spray collapse at higher fuel temperature leads to wetting of the piston wall. It also leads to formation of pool fire over the piston head which may lead to particulate emission. Effect of engine operating conditions like start of injection on wall wetting and formation of pool fire has been quantified using image analysis of high-speed videography.