US EPA Issues New Assigned Emission Deterioration Factors for Small Volume Manufacturers

| USA | Source: Alex Lawson, NGV Global Technical Committee Chair

The United States Environmental Protection Agency (EPA)’s Office of Air and Radiation has issued a letter to Manufacturers providing revised EPA assigned deterioration factors (ADFs) for determining compliance with certain chassis-based exhaust and evaporative emission standards (Ref. CD-12-07, March 20 2012). Assigned emissions deterioration factors (ADFs) are used by small volume manufacturers in lieu of emissions durability testing to calculate the useful life emissions of a vehicle necessary to demonstrate compliance with the EPA’s useful life emissions standards.

The last revision of ADFs was issued in 2005, and involved the use of multiplicative ADFs with values which were challenging for small volume NGV converters to meet Tier 2 standards.  Factors as high as 1.7 – 1.9 times the stabilized low mileage NOx emissions data, for example, were used to calculate useful life emissions performance.  This meant that the calibrator had to achieve emissions performance at about 30% of the standard, which became difficult with increasingly stringent standards.

Experience with Tier 2 vehicles, however, together with an assessment of Tier 2 ADFs, has now led EPA to believe that the use of multiplicative ADFs may no longer be appropriate since the OEMs primarily have used additive DFs since the beginning of emissions compliance under Tier 2 standards.  A new set of ADFs has therefore been issued based on additive factors derived from the EPA Verify database (see endnote).  This is hugely beneficial for OEMs, alternative fuel converters, and independent commercial importers (ICI) who may use the gasoline ADFs shown in the Tables recently released.  For example the new ADFs for NOx range from 0.00 – 0.04 as an additive factor.

For alternative fuel converters and ICIs, these ADFs are applicable to model year 2004 and later Tier 2 certified vehicles.  Earlier model year vehicles may have to use multiplicative factors after consulting with EPA.

EPA is to be congratulated on moving ahead with ADFs reflecting the current state of the industry which will facilitate the introduction of alternative fuel vehicles including NGVs.


“Verify” is EPA’s engine and vehicle compliance information system. It is used to collect emissions and fuel economy compliance information for all types of vehicles (mobile sources of air pollution) including light-duty vehicles and heavy-duty and non-road engines. Engine and vehicle manufacturers report this information to the Verify system.

“Deterioration Factors” indicate how emissions rise with the vehicle miles. EPA defines Deterioration Factor as the relationship between emissions at the end of useful life and emissions at the low-hour test point, expressed in one of the following ways:

(1) For multiplicative deterioration factors, the ratio of emissions at the end of useful life to emissions at the low-hour test point.
(2) For additive deterioration factors, the difference between emissions at the end of useful life and emissions at the low-hour test point.

Link to EPA letter – here.

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