Numerical Simulation On Combustion Optimization Of Gasoline Compression Ignition

To deal with the energy crisis and environmental problems,some advanced combustion concepts with high efficiency and low emissions have been proposed in recent years.Gasoline compression ignition(GCI)has attracted much attention because it can achieve efficient combustion and low NOx and soot emissions.In GCI engines,the start of ignition is closely related to the evaporation characteristics of the fuel and injection times.Therefore,in this paper,the key parameters,including intake temperature,injection time and injection pressure,were numerically simulated in the commercial software CONVERGE.The optimal fuel and the range of key parameters were determined under the low and medium load to optimize the combustion and emission performance of GCI.The primary reference fuel(PRF)from PRF55 to PRF90 was used to study the effect of fuel reaction activity on GCI under the load of 2 bar and8 bar.Results show that the ignition delay was sensitive to the octane number variation under the condition of low equivalent ratio.The net heat efficiency of the engine was not sensitive to the fuel reaction activity under the medium load,while under the low load,that increased first and then decreased obviously with the increase of fuel octane number.The temperature distribution was similar to NOx concentration distribution.PRF70 had the highest HRRmax and the CO and soot emissions of PRF85and PRF90 were significantly reduced under the load of 2 bar.The temperature of intake valve close(IVC)which changed from 350K to 430 K under the load of 2 bar and changed from 297 K to 377 K under the load of 8 bar,respectively,was also studied to optimize the combustion performance of fuel with different octane number.Results show that the variation of initial temperature affected the ignition delay of fuel when the equivalent ratio was lower than 0.9.Under the low load,the combustion characteristics of PRF90 could be improved by increasing the temperature of IVC.As the temperature of IVC was increased by 40 K,the net thermal efficiency of PRF90 was increased by 4%,and the CO and HC emissions were decreased by 76.78%and 94.58%.However,there is no need to increase the temperature of IVC under the medium load.Increasing the temperature of IVC cannot improve the net thermal efficiency,but worsens the emission performance of the engine.Then,the adjustment strategy of injection time and injection pressure was numerically studied in this paper to optimize the combustion process of GCI.It is found that as the injection time was advanced from-8 to-22°CA ATDC,the impingement position of the spray gradually moved up from the bottom of the piston and the upward velocity of the mixture gradually increased.The engine abtained the maximum net thermal efficiency of 46.51%and 39.61%under the medium and low load,respectively.HC and CO emissions decreased first and then slightly increased under the low load.The injection times were in the range of-15～-13 ~oCA ATDC and-18～-14 ~oCA ATDC under the load of 2 bar and 8 bar,respectively,which could achieve better combustion and emission performance of GCI.Increasing injection pressure shortened the ignition delay and combustion duration of fuel.The net thermal efficiency of PRF70 under different injection pressure was higher than that of other fuel and the soot emissions decreased by 68.48%with the increase of injection pressure under the low load.Soot concentration distribution was determined by equivalent ratio distribution.CO was mainly distributed on the incylinder wall and in the center of the cylinder at the crank angle of exhaust valve open(EVO).Under the medium load,the highest reduction of HC emissions was up to93.48%with the increase of injection pressure and CO emissions increased first and then decreased.Based on the above results,the combustion and emission performance of GCI could be improved by increasing the injection pressure to 80 MPa.