Dynamics of Multiple-Injection Fuel Sprays in a Small-bore HSDI Diesel Engine 2000-01-1256
An experimental study was conducted to characterize the dynamics and spray behavior of a wide range of minisac and Valve-Covered-Orifice (VCO) nozzles using a high-pressure diesel common-rail system. The measurements show that the resultant injection-rate is strongly dependent on common-rail pressure, nozzle hole diameter, and nozzle type. For split injection the dwell between injections strongly affects the second injection in regards to the needle lift profile and the injected fuel amount. The minisac nozzle can be used to achieve shorter pilot injections at lower common-rail pressures than the VCO nozzle.
Penetration photographs of spray development in a high pressure, optical spray chamber were obtained and analyzed for each test condition. Spray symmetry and spray structure were found to depend significantly on the nozzle type. The minisac class of diesel nozzle definitely exhibits superior characteristics regarding hole-to-hole spray symmetry, whereas the VCO class of diesel nozzles produce a more asymmetric, but more dispersed spray plume, with potentially better fuel-air mixing performance than the minisac nozzle. The dual-guided VCO class of nozzle does not significantly improve the spray symmetry over that of the single-guided nozzle. However measurement of spray tip penetration length shows that the dual-guided VCO nozzle provides higher spray-tip velocities than is achieved for single-guided tips. The results obtained show that the liquid spray tip penetration mainly depends on nozzle type, needle guiding strategy, injection pressure, and ambient gas pressure.
Close to nozzle exit spray dynamics observation was carried out. The microscopic visualization results provide very interesting and dynamic information on spray structure, showing spray angle variations, injection-to-injection variation, and primary breakup processes not observed using conventional macroscopic visualization techniques. The near-field spray behavior is shown to be highly transient, strongly depend on the injector design, nozzle geometry, needle lift and vibration, and injection pressure which is a function of the injection system.