Measurement of dilution characteristics for tailpipe emissions from vehicle

The rate at which fresh combustion emissions are diluted with ambient air can greatly affect the evolution of the particle size distributions in emissions from sources like motor vehicles. In particular, the generation of ultrafine particles within vehicle emissions is influenced not only by the particle and gaseous concentrations in the exhaust, but also by the rate and extent of dilution. This project experimentally investigated the impacts of different vehicle shapes, vehicle speeds, tailpipe positions, and tailpipe emission velocities on the exhaust dilution behavior, focusing especially on the near-wake region immediately downstream of the tailpipe where the most rapid changes in the dilution ratio occur.; The dilution of emissions downstream of model vehicles (5:1 scale) was studied in a wind tunnel located at the Wind Engineering Research Center at Tamkang University, Taiwan. A light duty truck, a passenger car and a heavy duty tractor head (without the trailer) were used to represent three typical vehicle types. A tracer gas was released at a measured flow rate from the tailpipe, to simulate the injection of exhaust emissions into the surrounding air. To evaluate the spatial distribution of the tailpipe emissions, sixty sampling probes were placed in the test section downstream of the vehicle to sample tracer gas concentrations simultaneously. A large number of probes were placed in the near-vehicle region to investigate the early stages of dilution. A number of vehicle speeds, exhaust emission velocities, and tailpipe positions and orientations were tested.; Commercial computational fluid dynamics software, Fluent V6.0, was also used to run a computer simulation under the same experimental conditions. Pressure coefficient was picked for comparison and showed similar trends in both approaches.; Overall results show what range of dilution ratios can be expected as a function of downstream distance (i.e., time). Data analyses show the relative importance of factors such as vehicle shape and velocity on the measured dilution ratios. The detailed spatial information obtained for the exhaust mixing behavior as a function of downstream distance (time) may prove valuable for studying ultrafine particle formation in vehicle exhaust.