The U.S. Environmental Protection Agency (EPA) is making minor amendments to the Tier 2 emissions standards to be applied to light-duty diesel vehicles. The changes relax a very limited set of standards for nitrogen oxides (NOx) for only high-altitude and high-speed/high-acceleration conditions.
These alternative compliance options will last for only three model years—2007 through 2009—during which time the agency expects advancements in diesel emissions control technologies to further develop to addresses the covered situations.
EPA designed these temporary alternative compliance options to be environmentally neutral, Manufacturers choosing them will be required to meet more stringent requirements in other aspects of the Tier 2 program including tighter particulate matter standards and a longer regulatory useful life.
The Tier 2/Gasoline Sulfur rule, enacted in 2001, phases in a single set of exhaust emission standards that applies to all light-duty vehicles (LDVs), light-duty trucks (LDTs) and larger passenger vehicles. To enable Tier 2 vehicle emission control technologies to be introduced and to maintain the programs’ effectiveness, EPA also requires reduced gasoline sulfur levels nationwide.
Under the Tier 2 program, manufacturers have the flexibility to certify Tier 2 vehicles to different sets of exhaust emission standards that EPA refers to as “bins.” The Tier 2 program implements a structure that has eight emission standard bins. Manufacturers have to choose the bins so that their corporate sales-weighted average NOx level is no more than 0.07 grams per mile (g/mi).
Because diesel vehicles still face some very specific technological challenges in meeting the full suite of Tier 2 requirements, EPA is providing very limited relief with the just-announced changes.
These two narrow areas of operation are the most challenging for diesel vehicles due to the relatively high engine loads of the high-speed/high-acceleration test cycle (known as the US06 test cycle) and the relative lack of oxygen at high altitudes.
The new technologies that have been applied to bring these vehicles into Tier 2 compliance will require further fine-tuning fully to address emissions under these conditions. EPA is projecting that, with only a few more interim years of refinement, these technologies will be able to achieve full compliance under these narrow conditions, as they have already demonstrated under typical operating conditions.
The new ruling contains two voluntary alternative compliance options for 2007 thru 2009 model year diesel vehicles: the US06 Option and the High-Altitude Option.
In the US06 Option, LDVs could meet a slightly higher 4,000 mile US06 standard (0.25 vs. 0.14 g/mi). As an offset, any such vehicles must meet the 50,000 mile Supplemental Federal Test Procedure (SFTP) composite standard equivalent for a full useful life of 150,000 miles (as opposed to 120,000 miles under the base Tier 2 requirements).
In the High Altitude Option, Bins 7 and 8 vehicles, at high altitude only, are allowed to meet a slightly higher in-use NOx standard of 1.2x the Federal Test Procedure (FTP) standard to which they are certified. In return, any such vehicles must comply with the bin 5 PM standard of 0.01 g/mi (vs. bin 7/8 PM standard of 0.02 g/mi), and certify to FTP standards for all regulated pollutants standards for a useful life of 150,000 miles (vs. 120,000 miles under the base Tier 2 requirements).