Lean-burn SIDI engine technology offers improved fuel economy; however, the reduction of NOx during lean-operation continues to be a major technical hurdle in the implementation of energy efficient technology. There are several aftertreatment technologies, including the lean NOx trap and active urea SCR, which have been widely considered, but they all suffer from high material cost and require customer intervention to fill the urea solution. Recently reported passive NH₃-SCR system - a simple, low-cost, and urea-free system - has the potential to enable the implementation of lean-burn gasoline engines.Key components in the passive NH₃-SCR aftertreatment system include a close-coupled TWC and underfloor SCR technology. NH₃ is formed on the TWC with short pulses of rich engine operation and the NH₃ is then stored on the underfloor SCR catalysts. On subsequent lean operations during the NEDC transient cycles, the NOx that breaks through the TWC converter is reduced by stored NH₃ on the SCR catalysts. The design of the TWC is crucial for NH₃ generation and critical for HC/CO emission reduction due to the low exhaust temperatures. In summary, the passive NH₃ SCR has been demonstrated as an efficient and low cost lean NOx aftertreatment technology for lean-burn SIDI. These and other results including steady-state results will be presented.