Dividing Flow-Weighted Sampling Approach in Partial Flow Dilution System for Particulate Emission Measurement in Internal Combustion Engine Exhaust

Light-duty vehicle emission measurement test protocols defined in the Code of Federal Regulation (40 CFR Part 1066) allow sampling particulate matter (PM) of all phases of Federal Test Procedure (FTP-75) on a single PM sampling filter by means of flow-weighted sampling in order to increase PM mass loaded on the filter. A technical challenge is imposed especially for partial flow dilution systems (PFDS) to maintain a precise dilution ratio (DR) over such a wide sample flow range due to the subtraction flow determination method of dilution air and diluted exhaust flows, because the flow difference is critical at high DR conditions. In this study, an improved flow weighting concept is applied to a PFDS by installing a bypass line with a flow controller in parallel with the PM sampling filter in order to improve DR accuracy during flow-weighted sampling. The diluted exhaust flow of the PFDS is kept constant and the flow through the PM sampling filter is adjusted by dividing the total diluted exhaust flow between the PM sampling filter and the bypass line. The flowmeter on the bypass line is calibrated based on the diluted exhaust flowmeter in order to improve the division ratio accuracy between the sample and the bypass flows, which controls the weighting factor. DR accuracy of this sampling concept is theoretically estimated and discussed. Improved accuracy under low exhaust flow and high DR conditions, as well as proportional loading on the PM sampling filter to the weighting factors, has been experimentally confirmed. Feasibility of the sampling technique is demonstrated by performing vehicle emission testing on a chassis dynamometer. Measurement results of flow-weighted single-filter sampling method by the suggested sampling technique show higher sensitivity due to increased particulate loading and reduced number of filters than the conventional multiple-filter sampling method.