Robust control applied to minimize NOx emissions

Legislation concerning pollutant emissions of diesel passenger cars is becoming increasingly restrictive, especially for nitrogen oxide (NOx) and particulate matter (PM). This article proposes to apply a CRONE control design methodology on a diesel engine in order to adapt the air-path and fuel-path of the engine and minimize NOx emissions. As the multivariable CRONE control strategies need a nominal transfer function (G₀(s)) and some frequency response of the system (G(s)), several test-bench experiments and a linear identification of the system were performed. The aim of this approach was to find a decoupling and stabilizing controller for the combustion engine that minimized NOx emissions at each operating point considered and during transient of torque/engine speed. The system is a square multivariable system with three inputs: Exhaust gas recirculation valve (EGR), variable geometry turbine (VGT), and start of injection (SOI); and three outputs: mass air flow (MAF), boost pressure (Pboost) and NOx level (NOx). The CRONE control approach developed for multivariable square plants is based on the third generation scalar CRONE methodology. Fractional order transfer functions were used to define all the components of the diagonal open-loop transfer matrix, β₀. Optimisation gave the best fractional open-loop transfer matrix and finally, frequency domain system identification was used to find a robust controller k(s) = G^-1₀(s)β₀(s). Performances of the proposed control structure were tested and validated with a number of experiments on a high dynamic test bed (NEDC driving cycle).