| Increasing the compression ratio is one of effective measures to improve the fuel economy of gasoline engine.The increase of compression ratio will lead to the knocking phenomenon in cylinder.But the EGR can suppress in-cylinder detonation effectively by introducing exhaust gas,which allowing the increase of compression ratio,thus improving efficiency of combustion.In order to get a more particular knowledge of the knocking characteristics in cylinder,a gasoline engine with compression ratio of 10.5:1 is taken as the research object in this paper.A one-dimensional thermodynamic model and a three-dimensional model are established,and the availability of these models is verified by experimental data,which is used in follow-up simulation.The compression ratio of original engine model is changed from 10.5:1 to 12.0:1,13.0:1 and 14.0:1 respectively,and the effect of compression ratio on the knocking characteristics and knocking combustion process in cylinder is investigated at the condition of 2000r/min high load.At the same working condition,the effect of EGR rate and EGR temperature on knocking reaction rate,knocking intensity and knocking combustion is researched according to the redesigned gasoline engine model with high compression ratio.Based on the above rules,the effect of EGR combined with high compression ratio on the fuel economy of gasoline engine at this operating point is evaluated by using one-dimensional thermodynamic model.Through the analysis,the following conclusions are drawn:1.At the condition of 2000r/min high load,the compression ratio increases from 12.0:1 to 14.0:1,the spontaneous combustion occurs 7.98 oCA earlier,the knocking reaction rate increases by 65.9%,the knocking index increases by 0.51 MPa,and the knocking intensity in cylinder gets larger.With the increase of compression ratio,the speed of combustion get faster,the midpoint phase of combustion is advanced by 7.00 oCA,the average temperature in cylinder increases by 74.0K,the spontaneous combustion phenomenon at the end of mixture is violent,the high temperature area increases with the area of spontaneous combustion get larger,and the peak value of pressure difference in cylinder increases by 0.52 MPa with the increase of pressure fluctuation in cylinder.2.Increasing the rate of EGR can suppress in-cylinder knock effectively at 2000r/min high load condition.Based on the model of gasoline engine with high compression ratio(14.0:1),the rate of EGR is increased from 5% to 15%,the appearance of spontaneous combustion is delayed by 9.48 oCA,the knocking reaction rate is reduced by 37.9%,and the knocking intensity index is decreased by 0.46 MPa.When the rate of EGR reach to 20%,there is no knocking phenomenon in cylinder,the Knock Index is close to 0.With the increase of exhaust gas ratio,the suppression of flame is enhanced,the midpoint phase of combustion is delayed by 8.06 oCA,the mean temperature in cylinder decreases obviously which is reduced by 563.9K.The spontaneous combustion at the end of mixture disappears gradually,and the temperature distribution is consistent with the normal combustion situation.The peak value of pressure difference in cylinder is also reduced from 0.74 MPa to much less than 0.1MPa because the knock phenomenon disappears when the rate of EGR is 20%.3.The heating effect of different EGR temperature on the mixture is characterized by inlet temperature with the constant rate of EGR at 10%.In this paper,the inlet temperature is reduced from 350 K to 320 K,the spontaneous combustion is delayed by 4.36 oCA,the knocking reaction rate is reduced by 9.5%,the Knock Index is reduced from 0.56 MPa to 0.28 MPa with the mitigating knocking phenomenon in cylinder.With the decrease of the inlet temperature,the flame propagation is restrained,and the midpoint phase of combustion is delayed from 10.94 oCA to 12.36 oCA after top dead center,the mean temperature only decreases by 56.7K,the spontaneous combustion near the cylinder wall eased slightly,and the variation trend of high temperature region is consistent with the flame density,the peak value of pressure difference decreases from 0.66 MPa to 0.44 MPa due to the alleviation of knock in cylinder.4.According to the influence of EGR on knocking combustion in cylinder,the effect of EGR combined with high compression ratio on fuel economy of gasoline engine is investigated without knock under 2000r/min high load.The results show that when the compression ratio increases to 14.0:1,the rate of EGR reach 19.3%,the indicated thermal efficiency is 39.1%,which is 6.5% higher than original engine,the fuel consumption rate is 224.6g/kWh,which is 114% lower than original engine,and the torque is 77.2N·m,which decreases by 12.0%.The fuel economy has been improved,but power is reduced.At the condition of 2000r/min high load,the knocking intensity in cylinder gets stronger with the increase of compression ratio.Increasing the rate of EGR or decreasing EGR temperature can suppress the knocking phenomenon of gasoline engine with high compression ratio effectively.EGR combined with high compression ratio can improve the fuel economy of gasoline engine,but make it lose power.The above simulation lays the foundation for the optimization of fuel economy of gasoline engine under the full operating conditions. |
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