Feedback Control Strategies for Diesel Engine Emissions Compliance

Abstract

Modern diesel engines are equipped with aftertreatment systems which are effective at reducing tailpipe hydrocarbon and oxides of nitrogen (NOx) emissions when the system’s catalysts are lit-off, meaning they are warmed-up to temperatures near 200 degrees Celsius. During engine cold-starts, combustion phasing retard is typically used to provide additional heat to the aftertreatment system to achieve faster light-off. Analysis of emissions cycle data has shown that improved heating during cold-starts could achieve further emission reductions, however combustion phasing retard heating strategies can be limited by combustion variability issues. Aftertreatment temperature issues can also occur after the engine is warmed-up, as real-world driving behaviors like extended idling and low-load operation can result in exhaust temperatures that are insufficient for maintaining catalyst light-off, resulting in emission increases. This thesis presents novel control solutions to achieve emissions reductions during cold-starts and real-world driving.

For cold-start emissions, the concept of closed-loop variance control was analyzed and applied to combustion control, which enables more aggressive combustion phasing retard exhaust heating to achieve faster aftertreatment light-off while avoiding excessive combustion variability issues. Diesel combustion variability was characterized experimentally, and the data was used to identify feedback metrics. Conventional linear controls analysis and statistical theory were used to develop a better understanding of variance feedback control, and the understanding was applied to the engine problem. Closed-loop combustion variability control was performed during both steady-state and transient operation and enabled higher exhaust temperatures while avoiding excessive degradation of engine combustion.

For real-world driving emissions, a model predictive control (MPC) framework was developed that uses long horizon engine speed and load preview along with onboard NOx measurements to control the engine for good fuel economy subject to emission constraints. To reduce computational complexity the controller output is a decision variable selecting between two engine calibrations, one with low brake-specific fuel consumption (BSFC) but high brake-specific NOx (or BSNOx), and one with high BSFC, low BSNOx, and increased exhaust heat to aid aftertreatment conversion efficiencies.The onboard NOx measurements are used to inform the optimization problem formulations, which include constraining NOx. based on windowed limits. Software-in-the-Loop (SIL) experimental results show that the controller has the ability to track a windowed emissions target, and appropriately responds to noise factors such as aftertreatment temperatures and emission rate errors.