In a guidance sent to engine makers last week, the Environmental Protection Agency has mapped out how it expects suppliers to prove they can overcome concerns regarding availability of a "reducing agent" for SCR systems, as well as keeping drivers from operating trucks with empty reducing agent tanks.

"Most SCR system designs rely on storing reducing agent in a tank located on the vehicle and on the vehicle operator taking responsibility for refilling the tank," states the EPA guidance, obtained by TodaysTrucking.com. "This means that the vehicle operator has to be made aware that reducing agent needs to replaced. Without a mechanism to inform the vehicle operator, and without a readily available source of replacement, there is a high likelihood that the (emission standard will) exceeded on vehicles in use."

To prove their SCR systems will not run out of reducing agent while on the road, the EPA is requiring engine makers to work with truck OEMs on the following:

Driver Warning Systems: Visual and audible alarms informing the vehicle operator that reducing agent level is low and must soon be replenished should be a priority for suppliers, says EPA. "The warning system would need to escalate in intensity as the reducing agent level approaches empty, culminating in driver notification that cannot be defeated or ignored, and cannot be turned off without replenishment of the reducing agent."

Driver Inducement: While in-cab warning systems should prove effective, EPA wants suppliers to have back-up systems in place in case a driver still attempts to operate the vehicle without proper urea replenishment.

A “No-Engine Restart after Restart Countdown” approach, for example, allows a limited number of restarts once the reducing agent reaches a certain minimal level of miles before the vehicle is unable to restart. A “No-Start after Refueling” system has the vehicle unable to start after refueling below a certain level. Lastly, a “Fuel-Lockout” approach would 'lock out" the fuel filler system, preventing the user from being able to refuel after the reducing agent range drops below a certain level.

Another way to make sure drivers are adding urea when needed is to "have vehicle performance degraded in a manner that would be safe but would be onerous enough to discourage the user from operating the vehicle until the reducing agent tank was refilled."

Most importantly, the system must be able to identify and appropriately respond to a situation when the storage tank is filled with a fluid other than the manufacturer-specified reducing agent, or when diluted with water. NOx sensors or urea sensors, therefore, would also need to be included.

Meanwhile, a long-held concern by some in the trucking industry is that North America doesn't have the proper urea-fuelling infrastructure in place to meet SCR's replenishing demands.

To make sure urea or reducing agent is available to all truckers on the road, "it is important to ensure that SCR vehicle operators have opportunities to go beyond a specific OEM network," says EPA.

The enviro-agency suggests manufacturers supply their dealers with the reducing agent. Also, either individually or as part of a collective effort, OEMs should supply truckstops and similar facilities with reductant.

Furthermore, manufacturers should provide "a back-up plan," such as a toll-free phone number, that customers can call and have reducing agent delivered overnight if they are unable to obtain it from a dealership or other convenient source.

Besides, Freightliner and Volvo -- whose head offices or parent companies are in Europe -- other North American manufacturers such as International, Paccar, and Cummins have not yet thrown their support behind SCR.

Caterpillar, which went with its own ACERT technology instead of EGR in 2002, has expressed several times in the past that SCR is not a viable solution for its 2010 emission goals.