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 CO₂. However, NO_x control is more difficult on a diesel because of the high O₂ 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 N₂ under lean conditions.