Comparison of Partial and Total Dilution Systems for the Measurement of Polycyclic Aromatic Hydrocarbons and Hydrocarbon Speciation in Diesel Exhaust

Two methods of sampling exhaust emissions are typically used for characterizing emissions from diesel engines: total dilution which uses a constant volume sampling (CVS) system and partial flow dilution which relies on proportionally diluting a small part from the main exhaust stream. The CVS dilutes the entire exhaust flow to a constant volumetric flowrate which allows for proportional sampling of the exhaust species during transient engine operation. For partial dilution sampling during transient engine operation, obtaining a proportional sample is more rigorous and dilution of the extracted sample must be continuously changed throughout the cycle in order for the extracted sample flowrate to be proportional to the continuously changing exhaust flow. Typically, regulated emissions measured using both methods for an engine platform have shown good correlation. The focus for this work was on the experimental investigation of the two methods for the measurement of unregulated species. A Sierra Instruments BG-3 Partial Flow Sampling system (BG-3) and a standard CVS system were used to compare the two methods for polycyclic aromatic hydrocarbons (PAH) and hydrocarbon speciation of compounds from methane to dodecane including a number of aldehydes and ketones in diesel engine exhaust. Engine-out emissions from a 2016 12.4L Navistar heavy-duty diesel engine without aftertreatment were sampled with both dilution systems (raw and dilute), and samples for PAH and hydrocarbon speciation were collected and analyzed using the Federal Test Procedure (FTP). A comparison of these two different dilution systems demonstrated that the partial flow sampling method produced similar results for both the particulate emissions and the hydrocarbon emissions when compared to the CVS.