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A diesel particulate filter (DPF) for city buses was developed that combines a SiC filter and a full-flow type burner for regeneration. Filter crack problems were averted by suppressing the peak temperature inside the filter to under 900°C. This was done by setting the maximum tolerable amount of collected particulate mass before regeneration at 50 g and controlling the burner so as to increase the regeneration gas temperature slowly up to a set value. This DPF was retrofitted to a Tokyo metropolitan bus to conduct a field test. The field test has been under way for half a year without any trouble or deterioration of system performance. THE DIESEL PARTICULATE FILTER (DPF) receives much recognition as an effective tool for reducing diesel particulates. However, its practical applications are still quite limited, because to date its durability remains immature.

We studied the use of SiC porous material for diesel particulate filters. Crystallities of SiC grow into hexagonal plates during sintering,but their sizes have not been controllable yet. We developed a production process that makes the average pore diameter as desired in limiting the pore size distribution to a narrow range. Diesel particulate filters made of SiC greatly reduce pressure loss even when a large quantity of particulates accumulates as compared with conventional wall-flow type filter of cordierite of the same size. This enables particulate filter to be made smaller. The good thermal conductivity of SiC allows fast regeneration without a temperature spike, but its large coefficient of thermal expansion causes heat cracks during rig tests and engine bench tests. The rig tests showed that the heat cracking problem can be solved if the length and diameter are below a certain threshold for each.

The characteristics of a new diesel particulate filter material made of SiC were studied through engine tests in varying material properties, such as average pore diameter, and wall thickness. Compared to a conventional cordierite filter of the same size, particulate trapping efficiency is almost the same, and the pressure loss and the deterioration of fuel consumption can be reduced to about half with the optimum material properties. If the same pressure loss is allowed, the filter size can be reduced by 30%. Its good thermal conductivity prevents local temperature increases, which doubles the permissible amount of trapped particulates. As heat crack problems occurred in integral-type filters due to the high thermal expansion of SiC, a split-type filter having 49 filter segments with a square section was developed.