Effect of Multi-Injection Strategy on Cavitation Development in Diesel Injector Nozzle Holes

The effect of multiple-injection strategy on nozzle hole cavitation has been investigated both experimentally and numerically. A common-rail Diesel injection system, used by Toyota in passenger car engines, has been employed together with a double-shutter CCD camera in order to visualise cavitation inside a submerged and optically accessible (in one out of the six holes) real-size VCO nozzle. Initially the cavitation development was investigated in single injection events followed by flow images obtained during multiple injections consisting of a pilot and a main injection pulse. In order to identify the effect of pilot injection on cavitation development during the main injection, the dwell time between the injection events was varied between 1.5-5ms for different pilot injection quantities. The extensive test matrix included injection pressures of 400 and 800bar and back pressures ranging from 2.4 up to 41 bar. The results have confirmed that cavitation patterns in the pilot are very similar to those of the main injection, while the effect of the dwell time was present through the variation of the actual injection pressure caused by the pressure wave dynamics within the injection system. The flow inside the VCO nozzle was also simulated using a recently developed cavitation CFD model which takes into account the movement of the needle. Simulations have shown that cavitation inception is very fast and synchronous for both the pilot and the main injection events while its intensity in the sense of spatial extent appears to peak at relatively low needle lifts.