In an open letter to manufacturers, the Environmental Protection Agency (EPA) has outlined its intended approach to the certification of diesel vehicles using SCR technology in the USA. This letter follows a guidance document originally circulated in November 2006 that initially met with heavy criticism from engine- and truck-makers. Following a consultation period the document was amended and a final version released 27 May 2007.

The release of this document signals the EPA's grudging acceptance of on-highway SCR technology, currently viewed by the industry as key to meeting the EPA's 2010 limits. The EPA had resisted the use of SCR technology, still remembering an earlier cycle-beating scandal where the majority of OEMs employed defeat-devices to override emissions control systems. Their main concern is the possibility of trucks operating without adequate urea reductant on-board and emitting untreated exhaust gas. Because of this, the EPA ruled in its recent OBD legislation that all vehicles operating with SCR must be equipped with a rugged diagnostic system capable of 'driver inducement’.

The final SCR guidance has taken into account some of the manufacturers concerns, softening wording and reducing the burden on manufactures. It focuses on two main criteria, vehicle compliance and AdBlue availability and accessibility.

Vehicle Compliance

There are five main criteria that all manufacturers must satisfy if they wish to incorporate SCR technology into their emissions control systems.

· A driver warning system must be incorporated along with an OBD system to indicate to the driver a low level of reducing agent. The indicator will need to be a separate visual alarm, increasing in intensity as the tank empties.

· Driver inducement, is a last-resort deterrent designed to enforce a refill of reducing agent by activating a 'limp home mode' or 'no engine restart or after restart countdown'. These measures would be followed by a 'fuel lockout', preventing the driver from refuelling without refilling the reductant tank.

· The SCR system must also be able to identify incorrect reducing agents and high tailpipe NOx emissions associated with the use of an incorrect reductant (e.g. diluted urea solution). If an incorrect reductant is detected, the vehicle's warning system should respond as if the tank was empty.

· The SCR system must be a tamper resistant design. Key components including the warning system, driver inducement system and reductant dosing system must be as tamper resistant as reasonably possible and not easily disabled.

· The last criterion, durable design, ensures that the entire SCR system is able to operate effectively over the useful life of the vehicle - 110,000 miles for light duty, 185,000 for medium duty and 435,000 miles for heavy duty. The EPA will be testing all individual components of the SCR systems for durability.

Reducing agent availability and accessibility

The EPA will review each manufactures plans to ensure their customers have access to urea reductant. Manufacturers must use best efforts to ensure that reductant will be available within a reasonable time at dealerships. The manufactures must also demonstrate that reductant is reasonably available at truck stops, and that they will employ a backup plan to ensure availability, such as overnight delivery to the driver. Previously the EPA specified that this service should not incur any extra charges, however, this clause was heavily contested by manufacturers and subsequently removed.