The U.S. Environmental Protection Agency has issued guidelines for engines using selective catalyst reduction to meet 2010 federal emissions standards. The reducing agent must be readily available, the agency says, and drivers must be warned automatically if their tanks are low on it.
SCR reduces emissions of the ozone-forming pollutant nitrogen oxide, or NOx, by injecting a reducing agent, usually ammonia or urea, into the exhaust gas upstream of the catalyst. Drivers must replenish the agent periodically, or NOx emissions can increase greatly.
Because of this, the EPA says, engines using SCR must have a way of alerting the driver that the agent is low. They also must have a backup system to prevent the driver from operating the truck despite the warning – for example, a fuel lockout that makes refueling impossible if the urea tank is empty.
Urea also must be readily available at truck stops, dealerships and retail stores, the EPA says.
The agency says it must approve all SCR designs, which must be durable and tamper-resistant, and must provide ways to identify an incorrect reducing agent.