Case History – LNT Catalysis at Ford Motor Company
Christine Lambert, Ford Motor Company
Three-way catalysts (TWCs) are extremely effective at converting emissions such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) at the stoichiometric air/fuel ratio; however, they do not provide suitable NOx control under lean exhaust conditions, which may be employed for potentially higher fuel economy. Lean NOx traps (LNTs) are one form of NOx control that Ford has considered and studied extensively for both gasoline and diesel vehicles. The operational and functional requirements for a particular vehicle in a particular region of the world determine the value of using lean NOx control, and Ford has studied several options over the past twenty or so years. Direct NOx decomposition was found to be impractical under realistic exhaust conditions, and non-thermal plasma was deemed too power intensive. SCR with various HCs was tried, but the operating temperature window was typically too narrow to cover a wide range of driving conditions, and the reactions tended to form undesirable byproducts such as N2O. LNTs were successfully applied to gasoline and diesel passenger cars. Later, when an infrastructure for aqueous urea deliver was implemented, urea SCR was applied to diesel vehicles from passenger car to Class 8 trucks. Since LNTs generate ammonia, they are often combined in diesel exhaust systems with SCR catalysts or SCR coated diesel particulate filters (SDPFs) for operation under a wider set of conditions. For gasoline vehicles, the application has been less prevalent. This presentation explores the aspects and trade-offs of implementing LNTs on vehicles, including composition, durability and controls, and in combination systems with TWC, SCR, and/or DPFs.