3-Dimensional & 2-Dimensional Micro-Orifice Spray Nozzles: Method of Attachment and its effect on Diesel Spray Behavior

The current work discusses our efforts to fabricate 3-D micro-nozzles using a novel, modified MEMS-LIGA process. These nozzles may provide a spray pattern to continue to reduce spray droplet sizes while minimizing interspray drop collisions and optimize the entrainment of air among the array of liquid spray streams. The paper also discusses the efforts made to form a bond between the micro-nozzle tips and the production nozzle, providing an improved attachment scheme by using non-intrusive mechanical clamps and quantifies the effect of the use of such clamps on spray performance. Air-entrainment in the near-nozzle region is critical for obtaining low soot emissions in combusting sprays. The clamps have a finite thickness and hence, may impede air-entrainment in the critical near-nozzle region. The current work examines this hypothesis for non-combusting sprays and if such a difference can be seen in “global” average measurements such as Sauter Mean Diameter (SMD). This work provides baseline test data to be compared with future experiments planned in a hot spray bomb. Three different cross-section clamps are used and the SMD is measured at two axial locations for various nozzle configurations with these three clamp designs. Finally, experimental spray results are presented for various micro-orifice nozzles. These data indicate an improvement in the SMD using these 3D micro-nozzles, but a design methodology to optimize nozzle design is under development. The paper also discusses the efforts to form a permanent bond between the micro-nozzle and production nozzle through use of a laser welding system developed specifically for this purpose. Introduction Diesel engines are an efficient alternative to gasoline engines. Over the last decade, especially in Europe, diesels have made impressive gains in what was considered a traditional gasoline market for midsize automobiles. Inherent issues associated with diesels – NOx, soot emission and noise have precluded its extensive use in North-American automotive market. One of the alternatives being considered that have the advantages of high efficiency and low emission is the use of Homogenous-Charged-Compression-Ignition (HCCI) engines. Extensive work is underway in the area of combustion, engine control experiments and modeling at University of Wisconsin-Madison [1, 2] as well as other engine research centers [3]. Unlike a traditional Spark-Ignition or Compression-Ignition engine, HCCI combustion would ideally take place spontaneously and homogeneously. This could eliminate heterogeneous air/fuel mixture regions, minimizing soot formation. In addition, HCCI is a lean combustion process. These conditions translate into a lower local flame temperature, which lower the amount of Nitrous Oxide (NOx) produced in the process. One of the key design parameters to control both emissions and noise is improved spray atomization, since atomization influences fuel-air mixing and fuel vaporization rates. In traditional Diesel engines this has been successfully achieved by using continually higher injection pressures combined with reductions in nozzle diameter [4]. Traditional fuel injection equipment may be ill-suited to HCCI engine requirements. In HCCI engines, injection occurs before the charge is fully compressed and the low cylinder gas density allows current fuel injection sprays to penetrate through the lower density gas to the walls. The resulting wall impingement could result in poor fuel and air mixing. To alleviate this problem, injectors containing many, smaller injection orifices, could be used, providing high-quality atomization without such unacceptable penetration to the combustion chamber wall. Current manufacturing techniques (e.g., EDM) have inherent limits to reduction in nozzle diameter. The advances in the field of Micro-Electro-MechanicalSystems (MEMS) offer advantages in reproducibly manufacturing micron-scale nozzle diameters. MEMS are a class of mechanical-electrical devices that have length scales in the order of microns (1-100μm). MEMS devices conventionally used silicon as the working material and used modified Integrated Circuit fabrication techniques. Silicon is a very versatile material but quite brittle. On the other hand, more ductile microstructures can be fabricated from metals via the LIGA process, which is based on deep etch X-ray Lithography, electroplating and molding [5]. The name LIGA originates from the German acronym: Lithographie, Galvanoformung and Abformung. The process involves the use of a thick layer of X-Ray photoresist and high-energy X-Ray radiation exposure and development to achieve a three-dimensional resist structure. Subsequent electro-deposition fills the mold with a X-Ray Mask PMMA Substrate preparation