Experimental Study On Effects Of EGR On Combustion And Exhaust Emission Of Hydrogen Direct Injection Gasoline Engine

With the rapid development of auto industry, China has been the largest country ofautomotive sales amount, leading the progress of global auto industry. However, thedevelopment of auto industry brings us many problems, such as the energy crisis andenvironmental problem. The energy crisis not only influences our sustainable development,but also threatens China’s national security. And the environmental problem threatenspeople’s health and existence. Energy conservation and emission reduction have been thedevelopment direction of auto industry. As a kind of renewable clean energy, hydrogen hasspecial physicochemical properties and attracts the attention of many researchers. ExhaustGas Recirculation can reduce NOXemission effectively and becomes the standardconfiguration of engine. It has great significance to study the influence of EGR oncombustion and exhaust emissions of hydrogen direct injection gasoline engine, which cansignificantly improve the performance of engine.Based on the project from National Natural Science Foundation “Study onEnergy-saving Mechanism of HEV Equipped with Lean-burn Gasoline Engine withHydrogen Direct Injection System and with Local Hydrogenous Mixture”, test bench and owpressure EGR system are set up. A four-cylinder gasoline engine is remoulded into ahydrogen direct injection gasoline engine. Test equipments are installed, includingdynamometer, AVL five-gas analysis meter, combustion analysis meter, hydrogen massflowmeter and fuel consumption meter. Ignition timing，EGR rate, hydrogen blending ratioand excess air ratio are adjusted by electronic control system. Firstly, the influence ofignition timing on combustion and exhaust emissions of hydrogen direct injection gasolineengine is studied, as well as the influence of hydrogen blending ratio on combustion andexhaust emissions of hydrogen direct injection gasoline engine. Secondly, under differentexcess air ratio, the synergistic effects of EGR and hydrogen on combustion and exhaustemissions of hydrogen direct injection gasoline engine are demonstrated. The main conclusions of this thesis are as follows:（1）With the increase of ignition advance angle, the cylinder peak pressure increases,the cylinder peak pressure phase moves forward, the rate of pressure rise becomes higher,the percentage of the peak heat release rate increases, its corresponding phase moves forward,the heat release process becomes shorter, maximum combustion temperature increases andthe combustion temperature curve moves forward. NOXemission increases sharply with theignition advance angle; HC emissions increases slightly with ignition advance angle;However, CO emission fluctuates within a small range, which can be considered that theignition advance angle has no effect on CO emission. While the mixture is slightly lean, COemission significantly decreases. Under different excess air ratio, with the increase of theignition advance angle, engine torque first increases and then decreases, the torque peakposition with EGR is relatively rearward. Exhaust temperature decreases constantly withignition advance angle.（2）With the improvement of hydrogen blending ratio, The peak value of cylinderpressure first increases sharply and then increases slowly, the cylinder peak pressure phasefirst moves forward sharply and then slightly moves; The percentage of the peak value ofheat release increases continuously, its corresponding phase moves forward. The heat releaseprocess becomes shorter; the combustion temperature is higher because of its fastercombustion rate, yet under the test conditions, the maximum combustion temperaturedecreases when the hydrogen blending ratio is25%and the excess air ratio is1.0. Underequivalent air-fuel ratio, with the increase of hydrogen blending ratio, NOXemission firstincreases and then decreases, but when the mixture is lean, NOXemission increasecontinuously, HC and CO emissions decrease with the increase of hydrogen blending ratio,yet when the excess air ratio is1.2, CO emission is very low and it hardly changes. A smallamount of blending hydrogen can increase engine torque sharply, then engine torqueincreases slightly. Although combustion temperature is higher when hydrogen is added, theincrease of the output torque leads to lower exhaust temperature.（3）Under different excess air ratios, the synergistic effects of EGR and hydrogen oncombustion characteristics of engine are demonstrated. With the increase of EGR rate, the cylinder peak pressure decreases and the cylinder peak pressure phase moves backward.However, when excess air ratio was1.0and the hydrogen blending ratio was low, it showsthat the cylinder peak pressure first increases and then decreases. EGR can reduce the intakeloss. Meanwhile, the period of flame development becomes longer, the phase of50%combustion amount gradually moves afterward and the maximum combustion temperaturedecrease. When the hydrogen blending ratio increases, with the increase of the EGR rate, therange that cylinder peak pressure decreases is gradually shortened, so are the phase ofcylinder peak pressure, the period of flame development and the phase of50%combustionamount.（4）The influences on emission characteristics due to the synergistic effect of EGR andhydrogen are studied with different excess air ratio. The results show that the NOx emissionhas a trend of decrease with the increase of EGR ratio. When gasoline is not mixed withhydrogen, NOx emission stays at a lower level; while mixed with hydrogen, NOx emissionwas sharply increased. However, NOx emission could still be controlled close to the highestlevel of original state with the help of20%EGR. HC emission varies from rising trend tonarrow fluctuation when mixed with hydrogen. When20%EGR is added without hydrogen,HC emission is explosively increased, so is CO emission; However, CO emission showsirregular variation with EGR and stays under0.1%when the excess air ratio keeps1.2aswell as mixed with hydrogen.（5）Under different excess air ratio, the synergistic effects of EGR and hydrogen ontorque and exhaust temperature of engine are demonstrated. Without hydrogen, enginetorque will decrease while EGR rate is increased. When5%hydrogen is added, enginetorque is promoted by13N·m immediately at different EGR rate. When mixed withhydrogen, engine torque increases along with EGR rate. However, under different GER rate,when the hydrogen blending ratio is above5%, engine torque will not continue increasing.Overall, without hydrogen, exhaust temperature increase as EGR rate increases. However,when hydrogen is added, exhaust temperature hardly varies under equivalent air-fuel ratio,and exhaust temperature decreases with the increases of EGR ratio under slightly leanmixture. Overall, EGR can reduce NOXemissions and has significant effects on combustion.Hydrogen can improve the speed of combustion and let gasoline burn completely, yetleading the rapid rise of NOXemissions. Combination of EGR and Hydrogen can exploiteach other’s advantages to the full and compensate for the lack. This mean can not onlyimprove the gasoline engine’s output torque, but also can control NOXemissions at theoriginal level, achieving a better performance of gasoline engine.