Energy & Fuels 2008, 22, 1527–1534
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Study on the Spray Behavior and Diesel Fuel Distribution Characteristics of a Secondary Injector for a Lean NOx Trap Catalyst Jungmo Oh,† Kihyung Lee,*,‡ and Haeyoung Jeong† Graduate School of Hanyang UniVersity, and Department of Mechanical Engineering, Hanyang UniVersity, 1271 Sa 1-dong, Sangrok-gu, Gyeonggi-do, 426-791, Korea ReceiVed June 13, 2007. ReVised Manuscript ReceiVed March 18, 2008
A lean NOx trap (LNT) catalyst has been used to suppress NOx emission from a diesel engine. A need to control the LNT catalyst has helped introduce a hydrocarbon-lean NOx trap (HC-LNT) system. The system has a secondary injector that injects diesel fuel (HC) into an exhaust pipe. A typically high temperature (i.e., 250-350 °C) in the exhaust pipe affects the spray behavior of the secondary injector. Therefore, in this research, the spray behavior of HC injected into a transparent exhaust manifold was investigated. First, the atmospheric injection characteristic of the secondary injector was analyzed. The injector has a solid-cone-shaped structure that can concentrate the spray along the central axis. The atomization process of the injector is dominated by the liquid film breakup, which generates many droplets with broad dispersion angle. Additionally, the injector was attached in the transparent exhaust manifold, and experiments were conducted in various conditions of the exhaust gas. The results show that the spray cone angle becomes broad and the breakup length of the liquid sheet becomes short as the ambient temperature increases. As the air mass flow increases, the fuel is curved downward and the spray cone angle is reduced because the centrifugal force affects the flow field in the curved part. Moreover, the droplets are found to be distributed by evaporation as the distance from the nozzle tip increases.
1. Introduction To cope with the severe exhaust gas emission and fuel economy regulations for passenger vehicles in the near future, the demand for lower emissions and higher thermal efficiency engine technologies need to be increased. In comparison to other engine systems, diesel engines have become more popular because of their excellent thermal efficiency and durability. The diesel engine minimizes greenhouse gases because low fuel consumption leads to lower CO2 emission. However, it is still difficult to control the emissions of nitrogen oxides (NOx) and particulate matter (PM) for diesel engines. Therefore, many researchers focus on the development of after-treatment systems that can satisfy stringent emission regulations.1 The NOx reduction system, an after-treatment system, requires a rich or stoichiometric operation condition periodically to reduce NOx emission. The HC concentration emitted from a diesel engine is insufficient for NOx conversion because the operating region of a diesel engine is a lean mixture condition. Thus, it was proposed that a rich air fuel ratio in a diesel engine could be realized by post-fuel injection or supplemental fuel injection into the exhaust gas.2 In this study, a secondary injection system was designed to inject diesel fuel into the * To whom correspondence should be addressed. Telephone: +82-31400-5251. Fax: +82-31-406-5550. E-mail:
[email protected]. † Graduate School of Hanyang University. ‡ Department of Mechanical Engineering. (1) Shoji, A.; Kamamoshita, S.; Watanabe, T.; Tanaka, T. Development of a simultaneous reduction system of NOx and particulate matter for lightduty truck. SAE Tech. Pap. 2004-01-0579, 2004. (2) >>Storey, J. M. Hydrocarbon species in the exhaust of diesel engines equipped with advanced emissions control devices. Coordinating Research Council, Inc., CRC project number AVFL-10b-2.
exhaust pipe to create stoichiometric conditions in the hydrocarbon-lean NOx trap (HC-LNT) catalyst. The atomization and distribution characteristics of the spray injected from a secondary injector are key technologies for obtaining a high NOx conversion because inhomogeneous droplets of injected diesel fuel cause not only high fuel consumption but also the deterioration of NOx emission. Therefore, it is necessary to clarify the spray structure and uniform fuel-air distribution inside the exhaust pipe to achieve an efficient purification process in the current HC-LNT catalyst.3–7 This paper describes the spray behavior and distribution characteristics of a HC injector for a HC-LNT catalyst system in the flow field. Optical diagnostics were applied to investigate the spray characteristics. The objective of the investigation was to find the optimum position of the HC injector in the exhaust manifold. 2. Experimental Apparatus and Procedure 2.1. Spray Visualization System. To investigate the macro- and microscopic spray characteristics of a secondary injector, the spray visualization system was established, as shown in Figure 1. The spray-developing processes were observed by direct photography (3) Hiroyasu, H.; Arai, M. Structures of fuel sprays in diesel engines. In SAE International Congress and Exposition, Detroit, MI, February 26March 2, 1990, Society of Automotive Engineers, Warrendale, PA; paper 900475. (4) Naber, J. D.; Siebers, D. L. Effects of gas density and vaporization on penetration and dispersion of diesel sprays. In SAE International Congress and Exposition, Detroit, MI, February 27-March 2, 1996, Society of Automotive Engineers, Warrendale, PA; paper 960034. (5) Heywood, J. B. Internal Combustion Engine Fundamentals, International ed.; McGraw-Hill: New York, 1988; pp 497-502 and 522529.
10.1021/ef800045t CCC: $40.75 2008 American Chemical Society Published on Web 04/30/2008
1528 Energy & Fuels, Vol. 22, No. 3, 2008
Oh et al. Table 2. Laser Specifications
Figure 1. Spray visualization system. Table 1. Specifications of the High Speed Camera description model sensor sensitivity pictures per second (pps) exposure time trigger lens mounts sync image
specification Phantom 7.0 (Vision Research) 800 × 600 pixel 24-bit color array 1200 ISO/ASA color full sensor to 4800 pps 512 × 384 to 10 000 pps variable to 2 ms independent of the sample rate (pps) continuously variable pre/post Nikon mount standard TTL pulse
using a high-speed video camera with illumination from a xenon lamp.8–10 In addition, a delay pulse generator synchronized a highspeed video camera with the injector driver. The specifications of the high-speed camera are summarized Table 1. Figure 2 shows a schematic diagram of the visualization system for measuring the spray sectional pattern. A 500 mW Nd-YLF
description
specification
center wavelength CW output power beam diameter beam mode power stability warm-up time operating temperature cooling system
532 nm 1000 mW
