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June 10, 2016   |   Diesel Technology Forum

Policy Insider

Ensuring that Clean Cars Stay Clean

It seems like a no brainer…if a car is tested in a lab, why can’t it be tested on a track or on real roads to determine if it meets the regulation?

Much has been written in the media this past year about automobile emissions and their performance in real world conditions. Particular emphasis has been focused on diesel vehicles and their real world performance. At first glance cars, SUVs and pickups certified by regulators to meet certain emissions standards should translate to real world conditions. Here in the U.S., these standards do a good job of mimicking conditions experienced on the road and these standards periodically undergo revisions and improvement. These standards apply equally to gasoline and diesel passenger vehicles. It seems like a no brainer…if a car is tested in a lab, why can’t it be tested on a track or on real roads to determine if it meets the regulation?

The problem with instituting real world driving testing as part of emissions compliance is the fact that it is not very scientific. Auto manufacturers are required to develop products that meet a specific standard. To determine if products meet the standard, a scientific procedure has been developed to control as many variables, or conditions, as possible. These lab tests have been established and improved over the years to take into account changes in driving styles – like highspeed driving – and new technologies deployed in cars that impact engine performance – like air conditioning and all-wheel drive.

Unlike lab tests, real world driving for emissions compliance introduces too many uncontrolled variables. Is the vehicle traveling at excessive speed on a long, steep hill on the hottest, most humid day of the year? Is the vehicle traveling on uneven pavement and engaged four-wheel-drive? Is the vehicle travelling under excessive acceleration and braking conditions? It is impossible to replicate with precision real world driving conditions for emissions compliance purposes applicable for all vehicles. For example, it would be highly inaccurate and perhaps unfair, to test one vehicle in spring time with a cautious driver and another vehicle in the heat of summer with a driver with a lead foot.

Yet, lab test procedures have changed with the times and the emissions compliance process includes many requirements to help ensure that emissions from vehicles tested in the lab closely approximate emissions on the road. The more stringent “Tier 3” emissions standards that phase-in for automobiles beginning in 2017, include many changes to testing procedures to better approximate real world driving conditions including a shortened time span between test cycles to take into account different driving styles encountered frequently on the road. Regulations also require that emission control systems operate effectively to meet the standard over the useful life of the vehicle. The “Tier 3” standards have increased the useful life of a typical passenger car from 120,000 miles to 150,000 miles. In order to keep manufacturers honest, the regulations also include provisions to randomly test automobiles driven by the public years after they have rolled off assembly lines. As an insurance policy, manufacturers typically certify engines below the standard so as to provide a bit of insurance to take into expected wear-and-tear on equipment and allow engines to meet the standard over the useful life of the vehicle.

While it may be impossible to certify automobiles to meet strict clean air requirements using real world conditions, the U.S. requirements involve a rigorous process to closely approximate conditions experienced on the road and help ensure that these vehicles meet the standard long after they are purchased.


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Ezra Finkin
Director, Policy

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