NOx Control Development with Urea SCR on a Diesel Passenger Car 2004-01-1291
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, NOx 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 NOx reduction onboard diesel vehicles are Selective Catalytic Reduction (SCR) using aqueous urea and lean NOx 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 NOx efficiencies have been shown in past work by Ford and others using urea.
Tailpipe NOx 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 NOx) with a green catalyst system. The green system also had results within the Tier 2 SFTP US06 standard (0.14 g/mi NMHC+NOx). 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.