Correlation of Split-Injection Needle Lift and Spray Structure

While the use of injection strategies utilizing multiple injection events for each engine cycle has become common, there are relatively few studies of the spray structure of split injection events. Optical spray measurements are particularly difficult for split injection events with a short dwell time between injections, since droplets from the first injection will obscure the end of the first and the start of the second injection. The current study uses x-ray radiography to examine the near-nozzle spray structure of split injection events with a short dwell time between the injection events. In addition, x-ray phase-enhanced imaging is used to measure the injector needle lift vs. time for split injections with various dwell timings. Near the minimum dwell time needed to create two separate injection events, the spray behavior is quite sensitive to the dwell time. For dwell times that are too short to create two injection events, the spray appears to be one long, continuous injection event. In this case, the spray mass distribution shows relatively minor disturbances from the steady-state structure between the commanded injection pulses. For slightly longer dwell times, two distinct injection events are seen; in this case, the behavior of the second spray is quite similar to that of the first spray. Needle motion measurements show that the needle begins to lift 150-200 µs before fuel emerges from the injector. The delay between commanded and actual start of injection is significantly shorter for the second injection than the first injection, even though the needle lift behavior for the first and second injections is similar.