1 August 2000

On July 31, the Environmental Protection Agency (EPA) signed the final rule “Control of Emissions of Air Pollution from 2004 and Later Model Year Heavy-Duty Highway Engines and Vehicles”. Some specific issues of the earlier proposal, such as the supplemental engine tests and standards, have been adopted with delayed timing.

The 2004 heavy-duty emission standards, originally adopted in 1997, introduced the combined NOx+HC standard of 2.4 g/bhp-hr, down from the 1998 4.0 g/bhp-hr NOx and 1.3 g/bhp-hr HC. In October, 1999, the EPA proposed a revision to these standards, including, among others, the following changes:

  • A more stringent combined NOx + HC standard of 1.0 g/bhp-hr for gasoline heavy-duty vehicles.
  • On-board diagnostics (OBD) requirements for vehicles weighing between 8,500 and 14,000 lbs.
  • Introduction of supplemental standards and testing procedures for heavy-duty diesel engines (NTE and steady state test).

The adopted final rule introduced a number of changes to the 10/99 proposal, primarily in regards to lead time and regulatory stability issues. Most importantly, the gasoline engine/vehicle standards will take effect no earlier than in the model year (MY) 2005 and the supplemental testing requirements are delayed until the MY 2007.

The supplemental testing requirements include not-to-exceed (NTE) limits and testing over the European ESC steady-state test. Heavy-duty engines will be required to meet the same emission standards on both the existing FTP transient cycle and on the ESC test. In addition, an NTE limit of 1.25 times the FTP standard will have to be met at any engine operating conditions, steady or transient, within an “NTE zone” in the engine torque-speed map, as defined by the regulation.

Both the NTE limits and the steady-state test are designed to prevent the possibility of the engine control software to advance injection timing during periods of highway cruising, in order to improve fuel economy at the expense of increased NOx emission. Provisions of the consent decrees, signed in 1998 between six major heavy-duty engine manufacturers and the EPA, prevent the engine manufacturers from using such engine control software, which is considered by the EPA an “emission defeat device”. Some signers of the consent decrees are required to meet the NTE limits and to certify engines on both the transient and steady-state cycles, regardless of the provisions of the 2004 emission rule.

Adoption of the supplemental tests in the 2004 rule would extend these requirements for all engine manufacturers and also make them more permanent, as some consent decree provisions may expire with time. The supplemental tests have been delayed until 2007 based on legal regulatory stability reasons. The US Clean Air Act requires that any heavy-duty vehicles or engines standards that are promulgated or revised “shall apply for a period of no less than 3 model years beginning no earlier than the model year commencing 4 years after such revised standard is promulgated”. Despite some controversies whether this stability provision applied to the 2004 standard adopted in 1997, the EPA decided it was appropriate to provide three years of stability between the implementation of the 2004 NOx standard and the supplemental standards.

Regulatory documents from the EPA server