IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 1, JANUARY/FEBRUARY 2011 331 Electrostatic Charging and Precipitation of Diesel Soot Hideaki Hayashi, Yasuhiro Takasaki, Kazuki Kawahara, Kazunori Takashima, and Akira Mizuno, Fellow, IEEE Abstract—Exhaust treatment systems consisting of new catalyst technologies and particulate filters are indispensable to meet in- creasingly stringent global regulations on limits particulate matter and nitrogen oxide (NO X ) emissions from heavy-duty to light- duty diesel vehicles. Diesel particulate filter (DPF) has been estab- lished as a key technology in reducing diesel particulate emission. However, technological improvements to pressure drop, dura- bility, and insufficient collection efficiency for nanoparticles are still required. Electrostatic precipitator (ESP) is another leading technology used in exhaust treatment, but it is currently limited to applications for stationary sources. In this paper, we have proven that concurrent use of ESP and DPF shows synergetic effects with very high collection efficiency and slower increase of the pressure drop. The number concentration of particles observed downstream of the combined system was 98% less compared with that of DPF only. At the same time, it was confirmed that increase in the pressure drop of DPF was slower. In this paper, the filter that was exposed to exhaust gas was observed by using scanning electron microscope. In addition, potential increase by charged particles on the surface of the filter was measured with a suction-type Faraday cage. The influence of the diesel particulate exerting on DPF by such an experiment was able to be clarified. Index Terms—Diesel engine, electrostatic precipitator, plasma. I. I NTRODUCTION T HE DEMAND for more efficient automobiles has in- creased due to the sharp increase in oil prices. At the same time, strong economic growth in several countries has caused serious air pollution due to exhaust from vehicles. Suspended particulate matter (SPM) and NO X are main pollutants. SPM is a particle smaller than 10 μm suspended in the atmosphere for a long time [1]–[4]. Diesel particulate filter (DPF) has been widely used [5] to prevent emission of SPM. Although this method has a high particulate matter (PM) collection efficiency, it has still many problems, such as high pressure drop and Manuscript received January 29, 2010; revised April 4, 2010 and April 14, 2010; accepted April 14, 2010. Date of publication November 9, 2010; date of current version January 19, 2011. Paper 2010-EPC-002.R2, presented at the 2009 Joint Conference on Electrostatics, Boston, MA, June 16–18, and approved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Electrostatic Processes Committee of the IEEE Industry Applications Society. H. Hayashi, K. Takashima, and A. Mizuno are with Toyohashi University of Technology, Toyohashi 441-8580, Japan (e-mail: mizuno@eco.tut.ac.jp). Y. Takasaki is with Hokuriku Electric Power Company, Toyama 930-8686, Japan. K. Kawahara is with Toyota Boshoku Corporation, Kariya 448-8651, Japan. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIA.2010.2090846 Fig. 1. Experimental setup for the combined ESP and DPF system. durability during the regeneration of collected soot by combus- tion. Electrostatic precipitator (ESP) is another technology for removing particles, and it has been used satisfactorily for the cleaning of flue gas from large-capacity factories, combustion furnaces, and thermal power plants. Using ESP, SPMs agglom- erate to large particles [6]. However, a problem with ESP is an abnormal dust reentrainment. The agglomerated particles repeat jumping and emitting from ESP. To combine ESP and a DPF, collection efficiency could be improved, and the increase in the pressure drop can be slower due to agglomeration. In this paper, the characteristics of an ESP–DPF combined system are reported. In addition, the situation of collected particles in a mock filter was observed by using scanning electron micro- scope (SEM) to evaluate the influence of the particles electrified by ESP, due to the difficulty confirming collected particles on the surface of DPF. Moreover, charge on the particles was observed in the Faraday cage. II. EXPERIMENTAL SETUP Fig. 1 shows the schematic diagram of the experimental setup. The exhaust gas from a diesel engine generator (Fuji Heavy Industries Ltd., SGD3000S-III) of 3-kW capacity was used with a high load of 2.6 kW in order to reduce the ratio of soluble organic fraction (SOF). SOF consists of unburned fuel and lubricant. The mass concentration of particles mea- sured with the quartz fiber filter was about 20 mg/m 3 in the exhaust gas. Exhaust gas of 250 L/min was passed through the precipitator. The gas from the combined ESP and DPF system was analyzed with an engine exhaust particle sizer (EEPS) (TSI Ltd, 3090). EEPS is a measurement device for engine exhaust, monitoring in real time (every 0.1 s) the 0093-9994/$26.00 © 2011 IEEE