Exhaust Recirculation Control for Reduction of NOx from Large Two-Stroke Diesel Engines

Kræn Vodder Nielsen

Research output: Book/ReportPh.D. thesis

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Increased awareness of the detrimental effects on climate, ecosystems and human health have led to numerous restrictions of the emissions from internal combustion engines. Recently the International Maritime Organization has introduced the Tier III standard, which includes a significantly stricter restriction on NOx emissions from large two-stroke diesel engines on vessels operating in certain NOx Emission Control Areas. Exhaust Gas Recirculation (EGR) is one of the three technologies on the market that are able to reduce the NOx emission adequately for Tier III operation. EGR is well known from the automotive industry, but have only recently been introduced commercially to large two-stroke diesel engines. Recirculation of exhaust gas to the cylinders lowers the oxygen availability and increases the heat capacity during combustion, which in turn leads to less formation of NOx. Experience shows, that while large two-stroke engines with EGR perform well in steady state, fast engine load transients cause smoke formation due to the decreased oxygen availability. The aim of this thesis is to design a control system that enables the large two-stroke engines with EGR to meet the emission limits of the Tier III standard, while still maintaining maneuverability performance without smoke formation. The design methods acknowledge that engine specific parameter tuning is a scarce resource in the industry and controller complexity is kept to a minimum. An existing dynamic model of the engine and EGR system is adapted and used for high-fidelity simulation. By isolating the gas composition part of the model and removing non-essential dynamics, a novel nonlinear reduced model of scavenge oxygen fraction is developed. Based on the reduced model, a novel nonlinear joint state and parameter observer for the scavenge oxygen fraction is designed. This observer compensates for a significant delay in the oxygen sensor, and observer errors are proven to converge exponentially. By inverting part of the reduced model and using the parameter observer, a novel scavenge oxygen controller based on nonlinear adaptive feed forward is developed. The controller error is proven to converge exponentially. This controller requires only one tuning parameter in addition to a number of physical parameters of the engine system. It exploits the availability of fuel and EGR flow estimates and the turbocharger speed to provide fast adjustment of EGR flow. In addition to the scavenge oxygen controller, a novel fuel index limiter based on oxygen/fuel-ratio is introduced and investigated. The limiter ensures that the maximal fuel flow set by the engine speed governor does not exceed the amount that can be completely burned, by considering the oxygen contents of the scavenge gas. The reduced model, observer, controller and limiter designs are validated by simulation of the high-fidelity engine model, and by closed loop experiments on an engine at test bed and on a vessel operating at sea. Significant performance improvements promised by the simulations are verified in the experiments. Scavenge oxygen control during transients is improved, when compared to the reference controller. Formation of visible smoke is completely avoided, while acceleration performance is maintained. The contributions of this project enable the EGR technology on large two-stroke diesel engines to reduce NOx emissions by a factor of four without compromising vessel maneuverability. Project partner MAN Diesel & Turbo has applied for a patent covering the EGR controller design in Japan, China and South Korea. The controllers developed in this project are planned to be included as standard in commercially available EGR controller software by 2017. The thesis consists of a summary of the methods developed and validations performed during the project. The results are disseminated in a number of papers submitted to research journals and a conference.
Original languageEnglish
PublisherTechnical University of Denmark, Department of Electrical Engineering
Number of pages142
Publication statusPublished - 2016


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