Optimization of fuel/air mixture formation for stoichiometric diesel combustion using a 2-spray-angle group-hole nozzle Sung Wook Park 1 , Rolf D. Reitz * Engine Research Center, 1500 Engineering Drive, University of Wisconsin-Madison, Madison, WI 53706, USA article info Article history: Received 9 June 2008 Received in revised form 10 October 2008 Accepted 18 October 2008 Available online 11 November 2008 Keywords: Stoichiometric diesel combustion 2-Spray-angle group-hole nozzle Fuel/air mixture formation abstract This paper describes a numerical study of fuel/air mixing processes for stoichiometric diesel combustion. In order to overcome the deterioration of combustion efficiency that accompanies stoichiometric diesel combustion due to poor mixing and lack of available oxygen, a new nozzle layout, namely a 2-spray-angle group-hole nozzle, which consists a grouped upper spray plume (squish spray) and a lower spray plume (bowl spray) was investigated. The KIVA code with updated physical and chemistry models, including the KH-RT breakup model, 2-step phenomenological soot model, reduced n-heptane and GRI NOx mecha- nisms was used for the calculations. An optimized 2-spray-angle group-hole nozzle with 170° squish spray angle and 80° bowl spray angle showed significantly improved fuel consumption (178 g/kW h À1 ) compared to the baseline nozzle layout (213 g/kW h À1 ) and the 2-spray-angle nozzle without hole- grouping (193 g/kW h À1 ). Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction In order to meet stringent future emission regulations, espe- cially to reduce particulate matter (PM) and NOx, various kinds of after-treatment devices such as DOC (diesel oxidation catalyst), DPF (diesel particulate filter), and SCR (selective catalytic reduc- tion) are being proposed for use in diesel automobiles. In addition, efforts to improve diesel combustion efficiency are of much inter- est. In the case of PM, previous studies showed that it can be re- duced significantly through the use of a DOC and DPF [1–4] and these devices are in current use for diesel passenger cars in Europe. On the other hand, after-treatment devices applicable for NOx reduction (e.g. SCR and LNT (Lean NOx Trap)) still present obstacles that must be solved that are related to regeneration strategies. Stoichiometric diesel combustion would allow the application of a three-way catalyst, which is well developed technology for gasoline spark-ignition engines. This concept has attracted consid- erable attention because it can treat NOx without the use of addi- tives (e.g. Urea for SCR) and without the need for complex traps that require regeneration. However, there are two barriers that must be overcome for the application of the three-way catalyst to diesel engines; to control the exhaust stream characteristics appropriately for three-way catalysts and to maintain combustion efficiency to be comparable to conventional diesel combustion. The combustion efficiency decrease that can accompany stoichiometric diesel combustion is the most difficult obstacle to be solved. As de- scribed by Lee et al. [5,6], stoichiometric diesel combustion causes around 7% sacrifice in fuel consumption. These studies used high EGR (exhaust gas recirculation) levels in order to control combus- tion phasing and NOx was significantly reduced compared to the best fuel consumption case of standard diesel combustion (i.e. 0.8 equivalence ratio). The study showed that the combustion effi- ciency starts to decrease as the equivalence ratio is increased be- yond about 0.85 due to the lack of oxygen, and the increased carbon monoxide emission is mainly responsible for the energy loss. They also showed that the combustion efficiency of stoichi- ometric combustion efficiency can be increased up to 94% by oper- ating at PCCI (premixed charge compression ignition) injection timings (i.e. 35° BTDC SOI). To improve these performance characteristics, fuel/air mixing strategies for stoichiometric diesel combustion need to be explored for improving oxygen utilization, and to reduce CO emissions. It is believed that this could be achieved by ensuring that the sprays for enhanced stoichiometric diesel combustion should have smaller drop sizes for creation of a more homogeneous mixture, while maintaining adequate spray penetration to utilize oxygen through- out the combustion chamber. From this point of view, the group-hole nozzle suggested by Denso [7] is an attractive method applicable to stoichiometric die- sel combustion. The group-hole nozzle concept is to reduce the injector nozzle hole diameters without sacrificing spray penetra- tion by closely locating two-holes, namely a group with very small inter-hole spacing (around 0.2 mm). With the small spacing, the 0016-2361/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2008.10.028 * Corresponding author. Tel.: +1 608 262 0145; fax: +1 608 262 6707. E-mail addresses: swpark2@wisc.edu (S.W. Park), reitz@engr.wisc.edu (R.D. Reitz). 1 Tel.: +1 608 265 8608; fax: +1 608 262 6707. Fuel 88 (2009) 843–852 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel