As automakers begin to develop and certify vehicles that meet the California Air Resources Board LEV II and Environmental Protection Agency Tier II Regulations, the study/usage of the Bag Mini-Diluter (BMD, or Mini-Diluter) sampling system continues to increase. Previous papers have provided an overview of the BMD and compared the measurements from the BMD to the measurements from a traditional constant volume sampler (CVS). These papers have suggested that the BMD approach offers new opportunities to improve the quality of vehicle exhaust measurement at very low levels, which will be crucial for accurate measurements on vehicles meeting LEV II SULEV standards (NMOG = 10 mg/mile, NOx = 20 mg/mile).
This paper continues the effort to study and implement the BMD sampling system as the optimal sampling system for SULEV measurements. Based on the results from previous testing, a number of investigations have been initiated to improve the quality and understanding of BMD measurements. To improve understanding of the Mini-Diluter concept, the critical components are divided into “functionality modules”. The “functionality modules” include; (1) control of the fixed dilution ratio using critical flow venturis, (2) exhaust flow measurement using an Ultrasonic Meter, (3) bag fill and proportionality control using a mass flow controller, and (4) gas analysis and material selection for gas preservation. Each of the “functionality modules” are discussed in detail, including estimated errors due to the current state of performance, recent investigations to improve the understanding of the module, and recent improvements in each module's performance. Suggestions to improve the measurement accuracy of the BMD (these revisions are to be adopted during the redesign of the BMD for implementation into a new SULEV Test Cell) and to identify areas where technical improvements will be beneficial to BMD measurements are presented. The paper concludes by presenting Mini-Diluter test data from a Prototype SULEV Test Cell. This Mini-Diluter data supports the position that the BMD is the most effective sampling system for accurate SULEV measurements, with potential for further improvements in the future.