| Fossil oil is a kind of nonrenewable energy. With the rapid development of theworld economic, car ownership expandsra pidly, leading to the increasing pressure onthe requirement of oil. Thus, to some extent, the tension of the oil resources limits thedevelopment of the automotive industry. In China, car ownership expands rapidly yearby year. Meanwhile, we come across the severe environment issues. It is the mainproblem for all the automotive corporations to achieve cleaner and more efficientcombustion. Therefore, reforming, improving and spreading the advanced technologyof internal combustion engine has become the most important event in the automotiveindustry.Compared with traditional gasoline engine, GDI engine has larger improvementon fuel economy.Faced with increasingly stringent emission regulations; higher NO_Xemissions restrict the development of GDI engine. EGR can limit NO_Xemissionseffectively. The advantage of GDI engine fuel economy in low-middle load attractspeople’s attention increasingly.This thesis has taken a four cylinder direct injection spark-ignition engine as theexperimental engine and established the experimental platform. Under the premise ofa combination of various programs, EGR system is established on the experimentalplatform. Low pressure EGR system is installed between turbine and compressor.EGR rate can be adjusted by using shutoff valve and can be calculated with the tracermethod. Shell-and-tube Heat Exchanger is used for cooling exhaust gas. In thisexperiment, boundary conditions, such as ignition timing,engine speed, engineload(injection pulse width), air-fuel ratio and EGR rate, can be adjusted to explore the principle that how EGR affects the performance and emissions under differentconditions. According to the analysis of experiment result, the following conclusioncan be obtained.1. After the EGR is introduced on a GDI engine, the power performance isimproved.Overall, the torque can be improved by6.3%. This is mainly because thereason that EGR increases the intake air quantity, reduceda part of inlet loss. At lowload, EGR can increase engine output torque by6.7%on average; at medium loadit isabout6.0percent. Under different air-fuel ratio EGR enhances torque differently.When the excess air coefficient is1.0, engine torque can be increased by7.8%onaverage; while the power increase is relatively insignificant with the air-fuel ratio of1.1, it is only4.9%.2. To some extent, EGR can improve the fuel economy of GDI engine. It iscorresponding with the power performance. As a whole, it can reduce the fueleconomy by5.0%. At low and medium load, EGR has the same effect on fueleconomy.However, fuel economy varies with air-fuel ratio.When the excess aircoefficient is1.0, enginefuel economy can be reduced by7.8%on average, at theair-fuel ratio of1.1, it is only4.9%.3. Considered from the torque and fuel economy, with the increase of EGRrate,the optimal ignition timing is becoming earlier gradually. It is because thatexhaust gas dilutes the mixture slowed the burning rate, leading to a more advancedignition timing. After matching the best ignition timing, EGR can still improve enginepower performance and fuel economy. They are4.5%and4.2%respectively.4. The introduction of EGR can restrain NO_Xemissions obviously.Under allthetest conditions,the largest EGR rate can reduce NO_Xemissions by82.3%onaverage.Under the operating conditions of2500r/min,7ms injection pulse width, theexcess air ratio of1.0, the most obvious inhibition of NO_Xemissions is up to91.3%.5. HC emissions will increase with EGR rate. However, it does not risesignificantly at the stoichiometric ratio. At each condition, it increased by an average of15.7%; HC emissions increase with the EGR rate significantly when the excess aircoefficient is1.1, the average rise is up to92.8%. The operating condition of lean gas,large EGR rate, early ignition timing will cause significant deteriorative combustion,resulting in a large number of discharged unburned HC.6. EGR can hardly affect the CO emissions on GDI engine. In the experiment,CO emissions only vary from excess air ratio. The leaner, the lower CO emissions.Generally speaking, when EGR system is installed on the GDI engine, powerperformance and fuel economy can be enhanced to some extent. In the case of smallHC emissions increase, we can achieve effective control of NOx emissions. Facingwith increasingly serious energy crisis and environmental problems, EGR technologyhas great application prospectson a GDI engine. |
