Characterization of Mixture Formation in Split-Injection Diesel Sprays via Laser Absorption-Scattering (LAS) Technique

Experimental results of a diesel engine have shown that using split-injection can reduce the NOx and particulate emissions. For understanding the mechanism of emissions reduction, mixture formation in split-injection diesel sprays was characterized in the present paper. A dual-wavelength laser absorption-scattering (LAS) technique was developed by use of the second harmonic (532nm) and the fourth harmonic (266nm) of a pulsed Nd:YAG laser as the incident light and dimethylnaphthalene (DMN) as the test fuel. By applying this technique, imaging was made of DMN sprays injected into a high-temperature and high-pressure constant volume vessel by a single-hole nozzle incorporated in a common rail injection system for D.I. diesel engine. The line-of-sight optical thickness of both fuel vapor and droplets in the sprays was yielded from the sprays images. The quantitative information of the distributions of the vapor concentration and droplets density was further deconvoluted by using the onion-peeling model. As a result, it is clarified that the injection mass ratio and dwell between injections of double-pulse injection schemes have great effect on fuel distributions and air-entrainment inside the sprays. Using split-injection with a small quantity injected in the first pulse and an appropriately chosen dwell between injections is of importance for governing the amount of premixed burn. The subsequent injection of double-pulse injection schemes has a turbulent effect on the fuel-air mixing in the diesel sprays. The turbulent effect of the subsequent injection with a smaller injection ratio, e.g., <50%, can markedly improve the fuel-air mixing by allowing more air to entrain into the spray. Thus, for a D.I. diesel engine, utilizing the turbulent effect of split-injection may enhance the combustion in the later stage and the re-burning of the particulate formed in the earlier combustion stage, reducing particulate emissions.