Wind tunnel measurements of the dilution of tailpipe emissions downstream of a car, a light-duty truck, and a heavy-duty truck tractor head

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

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The particle and gaseous pollutants in vehicle exhaust emissions undergo rapid dilution with ambient air after exiting the tailpipe. The rate and extent of this dilution can greatly affect both the size evolution of primary exhaust particles and the potential for formation of ultrafine particles. Dilution ratios were measured inside of a wind tunnel in the region immediately downstream of the tailpipe using model vehicles (~one-fifth to one-seventh scale models) representing a light-duty truck, a passenger car, and a heavy-duty tractor head (without the trailer). A tracer gas (ethene) was released at a measured flow rate from the tailpipe, and 60 sampling probes placed downstream of the vehicle simultaneously sampled gas tracer concentrations in the near-wake (first few vehicle heights) and far-wake regions (beyond 10 vehicle heights). Tests using different tunnel wind speeds show the range of dilution ratios that can be expected as a function of vehicle type and downstream distance (i.e., time). The vehicle shape quite strongly influences dilution profiles in the near-wake region but is much less important in the far-wake region. The tractor generally produces higher dilution rates than the automobile and light-duty truck under comparable conditions.

Original languageEnglish
Pages (from-to)704-714
Number of pages11
JournalJournal of the Air and Waste Management Association
Issue number6
Publication statusPublished - 2009
Externally publishedYes


  • Aerodynamics
  • Air pollution
  • Automobiles
  • Dilution
  • Ethylene
  • Fog
  • Tractors (agricultural)
  • Tractors (truck)
  • Truck transportation
  • Wakes
  • Wind tunnels, Ambient air
  • Dilution profile
  • Dilution rate
  • Dilution ratio
  • Exhaust particles
  • Gas tracers
  • Gaseous pollutants
  • Heavy duty trucks
  • Heavy-duty tractors
  • In-vehicle
  • Light-duty
  • Sampling probes
  • Scale models
  • Tailpipe emission
  • Tracer gas
  • Ultrafine particle
  • Vehicle types
  • Wake region
  • Wind speed
  • Wind tunnel measurements, Vehicles, ethylene, air analysis
  • article
  • car
  • comparative study
  • dilution
  • exhaust gas
  • flow rate
  • measurement
  • priority journal
  • air
  • air pollutant
  • motor vehicle
  • particle size
  • particulate matter, Air Movements
  • Air Pollutants
  • Motor Vehicles
  • Particle Size
  • Particulate Matter
  • Vehicle Emissions

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