| Under the increasing pressure of energy shortage and serious environmental pollution,natural gas is widely used because of its abundant reserves and environmental protection of combustion products.Aiming at the knocking problem of a diesel-ignited natural gas engine,this paper uses experiments and simulation calculations to study the influence of different factors on the knocking characteristics of a diesel-ignited natural gas engine.1.To select the shape of the combustion chamber for the subsequent engine bench test,based on the original natural gas engine model,by optimizing the depth of the piston pit and the shape of the combustion chamber,a flat bottom type combustion chamber and a ω type combustion chamber with a compression ratio of 14 are designed to replace the original compression ratio 10.5 Combustion chamber model to study the effect of different combustion chambers on the engine under different operating conditions.(1)When the rotation speed is 1600 r / min,the equivalence ratio of 1 and the initial pressure is 110 k Pa,the optimal injection advance angle of the flat-bottom model is 35 °CA BTDC,which is 15 ° CA earlier than the ω model;after optimization,the dynamic and economic performance of the flat-bottom combustion chamber model is slightly higher than the ω-type combustion chamber;the peak pressure and peak pressure rise rate of the flat-bottom model are higher than the ω-type combustion chamber model.(2)When the rotation speed is 1600 r / min,the equivalence ratio of 1 and the initial pressure is 160 k Pa,the NOx emissions,average cylinder pressure peak and pressure increase rate of the flat-bottom model are higher than the ω model,and the peak pressure increase rate is 28% higher than that of the ω model;the knocking trend and intensity of the ω-type model are smaller,which is more in line with actual work requirements.So this article selects the ω-type combustion chamber for subsequent bench tests.(3)After comparative analysis under(1)and(2)operating conditions,it is found that the sawtooth wave of the engine cylinder pressure fluctuation curve appears on both sides of the peak pressure of the cylinder;the traditional formula for calculating the instantaneous heat release rate is not suitable for the calculation of explosion due to reasons such as cylinder pressure fluctuation.Instantaneous heat release rate during earthquake.2.In order to study the influence of three factors such as the advance angle of pilot diesel injection,load and rotation speed on engine knock characteristics,a ω-type combustion chamber pilot diesel natural gas engine was used for subsequent bench test research.(1)Set the fuel injection advance angle to 44 ° CA,48 ° CA and 52 ° CA BTDC.When the speed is 1600 r / min,the air-fuel ratio is 1.7,and the load is 75%,the earlier the pilot diesel fuel injection,the longer the combustion duration,the greater the knock intensity.After the cumulative heat release curve of the three operating conditions reaches the peak,the in-cylinder combustion deterioration has a negative growth due to the increase in knock intensity.After the negative growth,the curve begins to rise and eventually stabilizes.(2)Set the speed to 1400,1600 and 1800 r / min.When the intake air temperature is308 K,the injection advance angle decreases with the increase of load under the same rotation speed condition,and gradually approaches the top dead center;under the same load condition,the higher the rotation speed,the greater the injection advance angle.Because of the difference in conclusions,three loads of 20%,50% and 100% were selected to explore the detonation characteristics based on the above conditions.At 20%load,the knocking of the three operating conditions is not obvious,and the peak pressure of the cylinder decreases with the increase of the speed.At 50% load,the rotation speed increases,the peak cylinder pressure decreases,and the knock intensity increases;the rotation speed has less effect on the starting point of combustion and the combustion rate.At 100% load,the change trend of knock intensity remains unchanged;the peak value of cylinder pressure at 1800 r / min is significantly higher than other cases and the knock intensity increases by 40% compared to 1400 r / min;the cumulative heat release curve shows a negative increase after the peak value because of combustion deterioration.(3)Set four loads of 30%,50%,60% and 80%.When the speed is 1800 r / min and the amount of diesel fuel ignited is 5mg,the load increases,the replacement rate increases,the excess air coefficient decreases first and then increases,and the cumulative heat release and combustion duration gradually increase;when it is less than50%,the knock in the cylinder is not obvious.At 60% and 80% load,the peak value of cylinder pressure increases significantly,MAPO(maximum amplitude of pressure fluctuation)reaches 0.484 MPa and 0.778 MPa,80% load increases about 16 times compared to 30% load. |
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