Vehicle Emission Test Protocols are Overestimating Internal Combustion Vehicle Tailpipe Emissions

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Vehicle Emission Test Protocols are Overestimating Internal Combustion Vehicle Tailpipe Emissions and Misleading Environmental Policy

For over 40-years laboratory emission test protocols, used worldwide, for light- and heavy-duty vehicles (and engines) have excluded the ambient air pollution consumed (and reduced) by the tested vehicle. The current protocol is appropriate for regulatory purposes enforcing vehicle emission standards, though problematic when emission estimates are also used to represent vehicle emissions for; zero emission vehicle comparisons, regulatory agencies emission inventories and Well-to-Wheel analysis, etc.

Vehicle emission test protocols omit the simple fact that all engines consume air in proportion to their fuel economy. This on-road air has entrained air pollution that according to a 2011 Health Effect Institute study (HEI) is, 3-5 times dirtier than official remote ambient monitoring station estimates. Engines air consumption is easily estimated from the laboratory dilution tunnel volume changes or direct vehicle measurement. Localized air pollution is spatially and temporally highly-varied but now due to new lower-cost technology utilized by Aclima and other emission monitoring portable devices, on-road air pollution estimates are now more assessable.

In the past excluding ambient air pollution was done due to findings that its pollution contribution was irrelevant to tailpipe emissions. However, reexamining this with today’s 97-99% lower tailpipe emissions, and new on-road air pollution data, we are finding most of the ignored ambient on-road air pollution is significant and can be equal to or greater than the vehicles tailpipe emissions.

The particulate matter tailpipe emissions from modern heavy duty diesel transit buses were examined from Altoona Bus Research and Testing. On-road atmospheric particulate matter pollution is overlaid showing the particulate matter that would be consumed by the bus driven on Los Angeles roads. The Low to High air pollution consumption is currently omitted from all vehicle emission testing; this mass is reduced 95-99% by the vehicle’s emission controls. At times, especially in low fuel economy operations, (high air consumption) and high ambient air pollution locations, a vehicle’s net emissions would be zero and frequently negative (cleaning-up the air).

When accounting for the omitted air pollution consumed and cleaned we find current emission test protocols overestimate today’s cleanest heavy-duty diesel vehicle emissions. Particulate matter and carbon monoxide emissions are overestimated 100%, hydrocarbons 22% and NOx 1%. Light-duty vehicle emissions also are overestimated due to not accounting for the on-road air pollution consumption. See Table 1. Ten percent of the vehicles, tested on the Highway cycle, were found to be negative NOx emitters after accounting for the moderate on-road air pollution consumption and clean-up. Light-duty vehicle emissions were based from EPA 2013 model-year new vehicle certification useful life emission rates, using the HEI air pollution ranges.

In conclusion, all internal combustion vehicle testing need to account for the air and localized air pollution consumed and cleaned-up to properly represent on-road vehicle emission contributions to the atmosphere.


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  • Name:
    Gary Yowell
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    Lower-cost alternatives to expensive zero emission vehicle policies. I am interested in spreading the word that it is time to reexamine world-wide vehicle emission testing protocols – especially as governmental policies pursue ZEVs for the sake of clean air benefits.
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