Dilution rates for tailpipe emissions: Effects of vehicle shape, tailpipe position, and exhaust velocity

Victor W.-C. Chang, Lynn M. Hildemann, Cheng-Hisn Chang

Research output: Contribution to journalArticleResearchpeer-review

13 Citations (Scopus)


The rate at which motor vehicle exhaust undergoes dilution with ambient air will greatly affect the size distribution characteristics of the particulate emissions. Wind tunnel experiments were conducted to investigate the impacts of vehicle shape, tailpipe orientation, and exhaust exit velocity on the dilution profiles under steady driving conditions for three model vehicles: a light-duty truck, a passenger car, and a heavy-duty tractor head. A three dimensional array of 60 sensors provided simultaneous measurements of dilution ratios for the emissions in the near- and far-wake regions downstream of the vehicle. The processes underlying the observations were investigated via nondimensionalization. Many of the trends seen substantially downstream can be well generalized using a simple nondimensionalization technique; however, this is not true in the near-wake region (within a downstream distance equivalent to a few vehicle heights). In the near-wake region, using the vehicle width and length to normalize for the vehicle shape is not enough to fully account for the variations seen. Including the exhaust flow rate in the nondimensionalization process is effective further downwind but does not adequately capture the complexity in the near-wake region. Tailpipe orientation and location are also shown to be influential factors affecting the near-wake dilution characteristics.

Original languageEnglish
Pages (from-to)715-724
Number of pages10
JournalJournal of the Air and Waste Management Association
Issue number6
Publication statusPublished - 2009
Externally publishedYes


  • Automobiles
  • Dilution
  • Particulate emissions
  • Three dimensional
  • Wakes
  • Wind tunnels, Ambient air
  • Dilution profile
  • Dilution rate
  • Dilution ratio
  • Driving conditions
  • Exhaust flow rates
  • Exhaust velocity
  • Exit velocity
  • Heavy-duty tractors
  • Influential factors
  • Light-duty
  • Motor vehicle
  • Nondimensionalization
  • Simultaneous measurement
  • Tailpipe emission
  • Three dimensional arrays
  • Three models
  • Wake region
  • Wind tunnel experiment, Vehicles, air analysis
  • article
  • car
  • dilution
  • exhaust gas
  • priority journal
  • sensor
  • air
  • air pollutant
  • air pollution
  • chemistry
  • environmental monitoring
  • motor vehicle, Air Movements
  • Air Pollutants
  • Air Pollution
  • Environmental Monitoring
  • Motor Vehicles
  • Vehicle Emissions

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