NOx Control Development with Urea SCR on a Diesel Passenger Car

Diesel vehicles have significant advantages over their gasoline counterparts including a more efficient engine, higher fuel economy, and lower emissions of HC, CO, and CO 2 . However, NO x control is more difficult on a diesel because of the high O 2 concentration in the exhaust, making conventional three-way catalysts ineffective. Two current available technologies for continuous NO x reduction onboard diesel vehicles are Selective Catalytic Reduction (SCR) using aqueous urea and lean NO x trap (LNT) catalysts. This paper discusses an application with SCR. SCR with ammonia has been used for many years at stationary sources. Aqueous urea is a convenient way to deliver ammonia onboard a vehicle and high NO x efficiencies have been shown in past work by Ford and others using urea. Tailpipe NO x emissions from a modified European production level 1.8L diesel Ford Focus TDCi were reduced to the range of ULEVII levels (0.05 g/mi NO x ) with a green catalyst system. The green system also had results within the Tier 2 SFTP US06 standard (0.14 g/mi NMHC+NO x ). An oxidation catalyst was used to convert engine-out HC and CO upstream of the urea SCR system. Aqueous urea was added to the exhaust using a Ford-developed air-assisted injection system. A base metal/zeolite SCR catalyst utilized the added reductant to convert NOx to N 2 under lean conditions.