Experimental Research On Effects Of EGR On Combustion And Emissions Of Gasoline Direct Injection Engine

Energy depletion and environmental pollution are essential issues to impede thedevelopment of human beings. Automobiles do not only consume energy but alsoproduce exhaust pollutions, which is vital to solve the current resource andenvironmental issues. Due to the large population and long mileage of gasolinevehicles, they make great contribution to resource consumption and engine-outpollutions. However, due to the relatively small compression ratio, fuel rich mixtureand high pumping loss of gasoline engine, their fuel economy is quite poor. GDI(gasoline direct injection) engines can work with lean mixture and highercompression ratio, which has been treated as an effective means to improve thethermal efficiency of gasoline engines. Appling the EGR (exhaust gas recirculation)technology on GDI engines can reduce their pumping loss and tuned the physicaland chemical properties of fresh charge, which has the possibility of improving fueleconomy and reducing engine-out emissions at the same time. In-depthunderstanding the effects of EGR on combustion and emission characteristics of GDIengines is crucial to achieve the goal of ultra-high efficiency and clean burningtechnology ininternal combustion engine.Based on the national "863" project-"passenger car used direct injectiongasoline engine development", this paper designed the GDI injector control system,optimized the GDI ECU (electronic control unit) and investigated the effects of EGRon combustion and emissions of GDI engines through experiments. The mainexperimental results are listed as follows:(1) The electronic control unit for GDI engines using Freescale MC9S12XDP512micro-controller can achieve precise control of ignition timing, injection timing andair/fuel ratio of GDI engines, which can meet the experimental requirements. TheGDI injector driving system based on Peak_Hold driving current can accuratelycontrol of the injector driving current waveform, and can drive the injector to respondquickly and to work reliably. (2) The introduction of EGR to GDI engines significantly affected theircombustion process. With the EGR rate increasing, the engine peak pressure andpeak heat release rate decrease, and the burning center delay. Under EGRconditions, ignition timing significantly affected the GDI combustion process. Withignition timing advancing, peak pressure and peak heat release rate increase, andthe peak combustion pressure phase moves forward. In this experiment, injectiontiming has small-extent effects on combustion process, while with the excess aircoefficient increasing from0.8to1.0, the peak pressure and peak heat release rateincrease, and the peak combustion pressure phase gradually moves forward.(3) The effects of EGR on exhaust temperature of GDI engines are quite obvious.With EGR rate increasing, the exhaust gas temperatures decline. Under EGRconditions, ignition timing, injection timing and excess air coefficient can affect theexhaust gas temperature. With ignition timing, injection timing advancing and theexcess air coefficient increasing, the temperature decrease.(4) The introduction of EGR almost does not affect the CO emissions from GDIengines. Under conditions of EGR, injection timing and ignition timing have littleeffects on CO emissions. Increasing the excess air coefficient can reduce the COemissions, and the maximum reducing rate can reach86.4%.(5) EGR has little effects on HC emissions from GDI engines. With increasingEGR rate, HC emissions slightly elevated. Under EGR conditions, ignition timing andinjection timing have little effects on HC emissions. When the ignition timingadvanced, HC emissions decreased slightly. With the injection timing advancing, HCemissions increased at first, reaching the maximum emission amount at around260°CABTDC, and then they decreased. Under experimental conditions, increasingexcess air coefficient can reduce HC emissions, and the maximum reduction rate isup to62%.(6) EGR can significantly inhibit NOxemissions from GDI engines, and thelargest decline rate could reach81.2%. Under EGR conditions, both ignition timingand air-fuel ratio have obvious influence on NOxemissions. When ignition timingand injection timing advance, NOxemissions from GDI engines would increase.Overall, the introduction of EGR has significant effects on the combustion andengine-out emissions of GDI engines. EGR can cause the decrease of peakpressure and peak heat release rate, the postponing of burning center and reducing NOxemissions. EGR rate combined with ignition timing, injection timing and excessair ratio can optimize the combustion conditions of GDI engines, and can effectivelycontrol the combustion phase and engine-out emissions.