Modeling of Aftertreatment Technologies to Meet a Future HD Low-NOx Standard

The low-NOx standard for heavy duty trucks proposed by the California Air Resources Board represents a significant challenge to the engine and aftertreatment system. In this study, exhaust thermal management requirements were quantified using a combination of engine and aftertreatment modeling. First, a 1-D engine model was used to develop a control strategy capable of increasing the exhaust enthalpy and decreasing the engine-out NOx over the initial portion of the cold FTP cycle. The outputs from this model were then used as inputs to a 1-D model of a representative HD aftertreatment system. Several different passive exhaust thermal management technologies were evaluated with this aftertreatment system model, including insulating the downpipe, close-coupling the aftertreatment system, and reducing the thermal inertia of the DOC and DPF; the last option provides the most benefit to early NOx conversion. As an example of active exhaust thermal management, a model of a mini-burner was inserted into the system model; this device is able to raise the system temperature quickly, using less fuel than the engine control strategy. Lastly, early NH₃ dosing and NH₃ preloading on the SCR catalyst were examined; pre-loading would appear to be much more effective.