TWC-SCR AFTERTREATMENT SYSTEM EVALUATION FOR A LEAN BURN GASOLINE ENGINE OPERATING IN HCCI, SACI, AND SI COMBUSTION MODES

摘要

Low temperature and dilute Homogenous Charge Compression Ignition (HCCI) and Spark Assisted Compression Ignition (SACI) can improve fuel economy and reduce engine-out NO_x emissions to very low values, often less than 30 ppm. However, these combustion modes are unable to achieve stringent future regulations such as SULEV 30 without the use of lean aftertreatment. Though active selective catalytic reduction (SCR) with urea injection and lean NO_x traps (LNT) have been investigated as options for lean gasoline engines, a passive TWC-SCR system is investigated in this work because it avoids the urea storage and dosing hardware of a urea SCR system, and the high precious metal cost of an LNT. The TWC-SCR concept uses periodic rich operation to produce NH_3 over a TWC to be stored on an SCR catalyst for subsequent NO_x conversion during lean operation. In this work a laboratory study was performed with a modified 2.0 L gasoline engine that was cycled between lean HCCI and rich SACI operation, or between lean and rich SI (spark ignited) combustion, to evaluate NO_x conversion and reduced fuel consumption. Different lambda values during rich operation and different times held in rich operation were investigated. Results are compared to a baseline case in which the engine is always operated at stoichiometric conditions. SCR system simulations are also presented that compare system performance for different levels of stored NH_3. With the configuration used in this study, lean/rich HCCI/SACI operation showed a maximum NO_x conversion efficiency of 10%, while lean/rich SI operation showed a maximum NO_x conversion efficiency of 60%. However, if the low conversion efficiency of lean/rich HCCI/SACI operation could be improved through higher brick temperatures or additional SCR bricks, simulation results indicate TWC-SCR aftertreatment has the potential to provide near-zero SCR-out NO_x concentration and increased system fuel efficiency. In these simulations, fuel efficiency improvement relative to stoichiometric SI were 7 to 15% for lean/rich HCCI/SACI with zero tailpipe NO_x and -1 to5% for lean/rich SI with zero tailpipe NO_x emissions. Although previous work indicated increased time for NH_3 to start forming over the TWC during rich operation, less NH_3 production over the TWC per fuel amount, and increased NH_3 slip over the SCR catalyst for advanced combustion systems, if NO_x conversion efficiency could be enhanced, improvements in fuel economy and low engine-out NO_x from advanced combustion modes would more than make up for these disadvantages.