Study On The Flash Boiling Sprays Of Gasoline Direct Injection Engine Fueled With Ethanol Gasoline Blends

Gasoline direct injection (GDI) engines fueled with ethanol gasoline blends which is a main type of future gasoline engines, have a great advantage in fuel economy and reducing CO2, NOx emissions and will be an important technology support for our energy conservation policy. Since multi-hole injector with high-pressure common rail system being widely used in GDI engines, cavitation flow and flash boiling spray emerge more often during the convention injections. The spray quality of GDI engines determines the combustion process directly and therefore affects the engine performance. Flash boiling spray and cavitation flow will cause inaccurate fuel metering and spray structural changes on the one hand, and promote the atomization by increasing disturbance on the other hand. Comparing with convention injection flash injection, which has smaller droplets and shorter penetration distance, atomize and evaporate faster. In order to use these advantages, more understandings about the occurrence of conditions and the law of development of cavitation and flash boiling atomization is required.The bubble point is selected to predict flash boiling phenomenon of ethanol-gasoline. A new fuel model based on discrete components is created to study the component effects on the bubble point/dew point curves and the evolution of the two-phase region, using Peng-Robinson state equation to calculate the vapor-liquid equilibrium. It shows that the two-phase region is significantly broaden with low ethanol content and increases while ethanol content rises. However two-phase region and the PT curves become narrower when ethanol content increased in high content conditions. The prediction using bubble point curve is more accurate and reasonable, while the lower combustion chamber pressure or high fuel temperature will promote the occurrence of flash boiling.Three-dimensional simulation of internal nozzle flow in GDI multi-hole injector during high-pressure injection conditions is carried out using two-fluid method to analyze the details about cavitation flows in nozzle. The criterion and characteristic pattern of cavitation flow are summarized. The results show that cavitation often occurs at the inlet corner as well as the gap between needle and valve seat. Cavitation bubbles at inlet corner gradually developed towards the downstream, while partial cavitation or super-cavitation flow formed accordingly. Cavitation bubbles around needle valve gap won’t develop towards downstream and will disappear when the needle is fully open or close. Partial cavitation can still be observed even when at maximum needle lift. The internal flow patterns are influenced by nozzle structure and fuel properties. Differences of flow rate and velocity between each hole caused by cavitation will affect the spray atomization process, which must be considered during the design.Experiments on spray characteristics of GDI multi-hole injector fueled with ethanol gasoline are carried out on constant volume chamber (CVC) test rig. Results show that ambient pressure is the greatest impact on spray penetrations followed by injection pressure and ambient temperature. Suater mean diameter (SMD) of droplets is mainly influence by ambient pressure and temperature. Spray penetration increases with inject pressure increases and decreases with ambient pressure arise, but slight changes with ambient temperature. Spray cone angle increases with ambient pressure increases, but no changes with inject pressure or ambient temperature. SMD decreases with inject pressure or ambient temperature increase, while increases with ambient pressure increases.Schlieren shadowgraph method was adopted on CVC test rig. Experiments were carried out to study flash spray development under different fuel temperature and ambient pressure conditions. Results show that penetrations of flashing sprays were shorter and the spray angle was larger than traditional sprays. Bubble dynamic development, which will cause spay ’expansion’ or ’collapse’, were found on spray edge in some conditions. Bubble dynamic makes the spray edge more irregular, but promotes atomization. Under the same environment pressure, penetration first increased then decreased with the increasing of temperature, while the spray angle first increased then decreased slightly, implicating that flash boiling may have transition conditions.A new flash boiling spray model whose atomization criterion based on the void fraction and superheat as well as evaporation model based on the dual-zone method is established to simulate the flashing sprays. The model function is coded and implemented in KIVA program. The new flash boiling spray model predicts spray tip penetration and spray cone angle and its development trend, in good agreement with the experimental results. The model has a good capability in simulating flash sprays at low superheat conditions, which breakup is controlled by void fraction, as well as high superheat transition process. It can also predict flare flashing sprays to some extent at higher superheat conditions.