Study On Combustion And Emissions Of DI Diesel Engine During Starting With EGR

Due to poor conditions for mixture formation and combustion, there are many problems during diesel engine starting, such as long cranking periods, unstable combustion, and high emissions, etc. However, as emission regulations promoted, there is urgent need to improve emissions of diesel engine during starting process because of the change in test cycles. Furthermore, startibility is also an essential problem for heavy duty and highly supercharged diesel engine due to its low compression ratio. Therefore, it is necessary to make intensive investigation into starting process of diesel engine.EGR (Exhaust Gas Recirculation) is widely used to reduce NOx emission from diesel engines under normal operating conditions, because the inert components of EGR could effectively suppress combustion temperature. However, during starting process of diesel engine, for poor combustion process, the exhaust gas comprises a great deal of unburned hydrocarbon, fuel vapor, and products of partial oxidation reaction, which have positive effects on combustion. If such compositions are reintroduced into cylinder by EGR during starting process, their kinetic effects may shorten ignition delay of mixture, and thus promote ignition. But none of reports about EGR effects on startibility has been found. Therefore, in this thesis, a study has been done to investigate effects of EGR upon combustion and emission performance during starting process of diesel engine, and to validate the measures for improving startibility through controlling EGR system.In the thesis, firstly, a sampling and controlling test system was established based on a single-cylinder DI diesel engine. With such a test system, reaal-time controlling can be made to fuel injection and EGR system during engine starting experiments, and parameters of engine performance during test can be recorded continuously using a computer. Such a test system would provide reliable guarantee in engine starting experiments.In the second, engine starting experiments under different internal and external EGR conditions were conducted to investigate the EGR effects on engine combustion and emissions during cold-starting process. And the results suggest that, during the first firing cycle, initial phase after first firing and the cyclic fuel delivery decreasing phase, engine combustion can be effectively promoted by introduction of EGR. As showed in the experimental results, with EGR, IMEP of the first firing cycle increased by about 32% when compared with the result of normal starting operation. In the initial phase after first firing, IMEP and ignition timing of engine could also obviously improve for chemical kinetic effects and thermal effects of EGR. And during the cyclic fuel delivery decreasing phase, when compared with normal operation, the average IMEP increased about 29% with internal EGR, while about 22% with external EGR.Furthermore, the experimental results also suggested that, to a certain extent, average opacity of diesel engine during cold start decreased when introducing appropriate amount of internal and external EGR. However, for average NOx emissions, it can be obviously improved in the case with external EGR, while internal EGR result in an increase for the thermal effects.Based on the analysis of experimental results with external EGR, a multi-phase controlling strategy was proposed. As mentioned in the thesis, the whole starting process was divided into three phases according to the EGR ratio controlling. During the phase before first firing and the EGR responding-delayed phase, EGR ratio should keep at as high as possible level (in the thesis, EGR ratio about 70% was achieved). During transitional phase, EGR ratio should decrease and keep at about 58%. And finally, EGR ratio should keep at about 37% during the final steady phase.In addition to the experimental analysis, simulation, which combined CFD and single-zone detailed chemical kinetic model, was conducted to further investigate the effects of EGR components on engine combustion. As the calculation results suggested, the products of low-temperature reactions, which presented in EGR, could significantly promote chain-reaction rate of mixture, and thus effectively shorten the ignition delay. When compared with the fuel evaporation which also presented in EGR, the products of low-temperature reactions showed more positive effects on engine combustion.Finally, starting experiment, in which a double-phase EGR controlling strategy was applied, was conducted to validate the measures for improving startibility through controlling EGR system. And the experimental results suggested that it is an effective measure to improve startibility of diesel engines by controlling EGR system to adjust EGR amounts during different phase of cold start.