NO and $NO2$ Concentration Modeling and Observer-Based Estimation Across a Diesel Engine Aftertreatment System

This paper presents an experimentally validated control-oriented model and an observer for diesel oxidation catalyst (DOC)-diesel particulate filter (DPF) system in the context of exhaust gas NO and $NO2$ concentration estimations. NO and $NO2$ have different reaction characteristics within DPF and selective catalytic reduction (SCR) systems, two most promising diesel engine aftertreatment systems. Although the majority of diesel engine-out $NOx$ emissions is NO, the commonly used DOC located upstream of a DPF and a SCR can convert a considerable amount of NO to $NO2$⁠. Knowledge of the $NO/NO2$ ratio in exhaust gas is thus meaningful for the control and diagnosis of DPF and SCR systems. Existing onboard $NOx$ sensors cannot differentiate NO and $NO2$⁠, and such a sensory deficiency makes separate considerations of NO and $NO2$ in SCR control design challenging. To tackle this problem, a control-oriented dynamic model, which can capture the main NO and $NO2$ dynamics from engine-out, through DOC, and to DPF, was developed. Due to the computational limitation concerns, DOC and DPF are assumed to be standard continuously stirred tank reactors in order to obtain a 0D ordinary differential equation model. Based on the model, an observer, with the measurement from a commercially available $NOx$ sensor, was designed to estimate the NO and $NO2$ concentrations in the exhaust gas along the aftertreatment systems. The stability of the observer was shown through a Lyapunov analysis assisted by insight into the system characteristics. The control-oriented model and the observer were validated with engine experimental data and the measured $NO/NO2$ concentrations by a Horiba gas analyzer. Experimental results show that the model can accurately predict the main engine-out/DOC/DPF $NO/NO2$ dynamics very well in semisteady-state tests. For the proposed observer, the predictions converge to the model values and estimate the NO and $NO2$ concentrations in the aftertreatment system well.