The US Environmental Protection Agency (EPA) is proposing stringent new emission standards for diesel engines used in a wide range of nonroad applications. The proposed standards are for mobile nonroad diesel engines of all sizes, except those used in locomotives and underground mining equipment, and also except engines over 50 hp used in marine vessels. The proposed standards would reduce emissions from a typical nonroad diesel engine by up to two thirds. By meeting these new proposed standards, manufacturers of new nonroad engines and equipment would achieve large reductions in the emissions that cause ground-level ozone (especially oxides of nitrogen, or NOx) and particulate matter air pollution problems in many parts of the USA. For perspective, a NOx reduction of the scale that this rule would achieve is equivalent to taking more than 2 million heavy-duty trucks off the road.

The proposal envisions a 3-tiered progression to low emission standards. Each tier involves a phase in (by horsepower rating) over several years. Tier 1 standards were set in 1994 for engines over 50 hp and are phasing in from 1996 to 2000. The proposal would set Tier 1 standards for currently unregulated engines under 50 hp, phasing in from 1999 to 2000. It would also phase in more stringent Tier 2 standards for all engine sizes from 2001 to 2006, and yet more stringent Tier 3 standards from 2006 to 2008.

The proposal reflects the Nonroad Engine Statement of Principles (SOP) signed last fall by EPA, California, and the Diesel Engine Industry. All of the SOP provisions are being proposed for public comment. The nationwide program initiated by the proposal will help hold down equipment costs by avoiding the need for manufacturers to meet separate standards in California, as is currently the case. What's more, the European Commission is proposing new standards that are harmonized with EPA's proposed standards, thus allowing manufacturers to design a single product for both markets.

A summary of proposed standards is available in the Emission Standards section under USA Off-Road Engines.