| 摘要 |
PROBLEM TO BE SOLVED: To provide an air filter capable of securing a sufficient dust retention amount and capable of maintaining an excellent carbon-dust removing efficiency and a carbon dust retention amount while suppressing the amount of permeating dust due to the suction pulsation of an engine. SOLUTION: The air filter is made of a filtering material comprising a plurality of nonwoven fabric layers which is not impregnated with oil, and the nonwoven fabric layer of the filtering material on the downstream side has a porosity smaller than the nonwoven fabric layer on the upstream side. The filter has a pulsation permeability of 1.2% or lower by a dust permeation test with a predetermined pulsation load applied after being formed. Preferably, the porosity of the nonwoven fabric layer on the downstream side is within the range of 85 to 92%, a mean fineness of fibers constituting the nonwoven fabric layer on the downstream side is 3 dt or less, and a mean fineness of fibers constituting the nonwoven fabric layer on the downstream side is 3 dt or more. COPYRIGHT: (C)2006,JPO&NCIPI
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