A New Constant Dilution Ratio Concept for Vehicle and Engine Exhaust Particle Sampling

This paper presents a new concept of a partial flow sampling system (PFSS), involving a two-stage dilutor which operates at underpressure, while exhaust is sampled through a capillary. The sample flowrate is in the order of few cubic centimeters per minute. Due to the low flowrate, no tight fixation is required between the exhaust line and the capillary inlet. The dilutor may sample from an opening in the exhaust line which freely exhausts to ambient pressure. As a result, the PFSS operates at constant pressure conditions even upstream of diesel particle filters (DPF). A straightforward mathematical model is then developed to calculate the dilution ratio (DR), depending on the dilution air flowrate and the dilutor underpressure. The model is validated using CO₂ as a trace gas, and a very good agreement is demonstrated between the calculated and the measured DR values. After validation, the PFSS is combined with electrical aerosol detection devices to measure the exhaust particle concentration of a diesel engine operating at different steady-state modes. Also, the filtration efficiency of two particle filters is examined by respectively providing exhaust samples upstream and downstream of each filter. These applications demonstrate the stability and applicability of the PFSS combined with an aerosol instrument to measure the filtration efficiency of a DPF at low cost. Future applications of such configuration may include particle exhaust concentration measurements and on-board (portable) emission measurements.