PARTIAL FLOW SYSTEM WITH DOUBLE DILUTION FOR PM SAMPLING UNDER TRANSIENT TEST-CYCLES When a motor vehicle is driven under simulated road conditions, carbon particles and high boiling-point hydrocarbons are emitted from the tailpipe, generating particulate matter (PM) after being diluted with ambient air. In order to measure PM emissions under simulated driving conditions in the laboratory, a full flow dilution constant volume sampling (CVS) tunnel is traditionally used, where the total engine exhaust is diluted. The PM emissions measurement regulations require gravimetric determination of PM collected onto filter media under specific condition. There have been discussions about whether current sampling and measurement practices are sufficiently accurate in quantifying PM at the upcoming 1mg/mi PM emissions standards of CARB LEV III. The flow rate and size of the CVS system is determined on the exhaust flow rate of an engine in order to provide sufficient dilution; therefore a CVS may be impractical in case of heavy duty engines due to the high flow rate required. An alternative sampling technique to CVS such as a partial flow dilution system (PFDS) was developed previously to address this issue. PFDS have been widely applied for heavy duty and non-road engine certifications. However CFR Part 1065 requires specific dilution factor, filter face velocity, and temperatures for accurate PM sampling under transient test condition. Because of flow rate and dilution factor limitations of a partial flow sampling system, it is difficult to maintain CFR Part 1065/1066 requirements with a single dilution tunnel, especially for heavy-duty engines. A new partial-flow dilution system (MDLT-ONE) with double dilution has been developed in this study for transient type engine testing requirements. The flow rate of diluted gas can be varied from 25L/min up to 80L/min in the system. Two identical dilution tunnel configurations are employed in both 1st stage and 2nd stage dilution. The first stage tunnel extracts a proportional sample from second stage which maintains constant dilution ratio. The PM dilution system is configured to have an overall residence time of 1.0 to 5.5s, as measured from the location of initial dilution air introduction to the location where PM is collected on the sample media. The system meets the CFR 40 Part 1065 & 1066 requirements. Raw exhaust is introduced into the tunnel by directing it downstream along the centerline of the tunnel. Dilution air is added tangentially to create turbulence. The minimum overall ratio of diluted exhaust to raw exhaust is within the range of 5:1 to 20:1. The system can operate at different control modes for dilution; Proportional sampling mode (Constant split ratio), Fixed dilution ratio mode (Constant dilution ratio), and Fixed flow rate mode (Manual control). MDLT-ONE heated filter holder has one bypass filter and four sample filter holders, sufficient to collect phase-by-phase PM on a single filter. Moreover, all three phases of the FTP light-duty chassis dynamometer test cycle can also be sampled on a single particulate sample filter by means of flow weighted mean function defined by CFR 1066. The MDLT-ONE with double dilution developed in this study meets the sampling requirements of transient test cycle, and provides good correlation with the full flow CVS tunnel for a wide range of engines. The system is able to show correlation with the CVS within 5% over transient engine operation.