Study On Working Process Of RCCI Dual Fuel Engine Based On Thermodynamic State Control

The auto industry has come under increasing pressure amid environmental concerns,the energy crisis and more stringent national emissions regulations.While natural gas,as a potential alternative fuel for diesel engines,can meet stringent emission regulations and alleviate environmental and energy pressures.In this paper,a natural gas/diesel dual fuel（CD-DF）engine test bench was built to carry out preliminary tests on different substitution rates and EGR rates under different working conditions.According to the test results and engine parameters,a one-dimensional thermodynamic simulation model and a three-dimensional CFD coupling model were built.By means of simulation studies different valve strategies,the structure of the combustion chamber,intake components,EGR and fuel injection strategy of CD-DF engine combustion and emission characteristics and the effects law of energy flow.At the same time explore low load condition to improve the dual fuel engine combustion stability,promote brake thermal efficiency and the effective ways to reduce the HC and CO emissions,as well as the technical means to restrain the knock tendency of high load.In view of the low load condition of different hot atmosphere,the negative valve overlap strategy,the internal EGR rate increases with increasing negative valve overlap,delay of CA50,maximum pressure rise rate（MPRR）decreases,the peak cylinder pressure is reduced,brake thermal efficiency increases first and.then decreases and significantly reduces NO_x emissions.When the negative valve overlap angle is 51°CA,the brake thermal efficiency reaches the maximum.When adopting intake valve re-opening strategies,with the advance of the intake valve re-opening timing,internal EGR decreased first and then increased,CA50 advanced first and then delayed,the MPRR,peak pressure in cylinder,the NO_x emissions and brake thermal efficiency increased first and then decreased.When the timing of intake valve re-opening is 140°CA ATDC,NO_xemissions reduction to ChinaⅥemission limits.When adopting the exhaust valve re-opening strategies,with the delay of the exhaust valve re-opening timing,CA50advances,MPRR increases,NO_x emissions decreased first and then increased,and the brake thermal efficiency increased first and then decreased.Among the three valve strategies,the negative valve overlap strategy has the highest brake thermal efficiency when the NO_x emission level is the same.Under the same pilot injection strategy,with the increase of the prechamber volume,peak pressure in cylinder and MPRR is reduce.When the pilot injection quantity is 10%,with increase of prechamber volume,emissions of NO and CO decrease and the emissions of CH₄ increase.When the pilot injection.quantity increases,the emissions of CH₄ and.CO decrease and NO.emissions increase slightly.When the bias prechamber is used,under the condition of other strategies being the same,MPRR,CH₄ and CO emissions reduce,and NO emissions are equivalent.For different intake atmosphere under low load condition,when hydrogen or oxygen is blended,the distribution of OH radical and temperature field in the cylinder change the same trend,and at the same blend ratio,the concentration of OH radical in the blending of hydrogen is greater than that in the blending of oxygen,and the area of high temperature region is also greater than that in the blending of oxygen.When hydrogen is blended,at each EGR rate,when the hydrogen blended ratio is greater than5%,the MPRR is too large and the shock on the engine is too large.Considering the limiting maximum pressure rise rate and the peak cylinder pressure,the hydrogen blended ratio should not be too high.When oxygen is blended,at the same EGR rate,the peak pressure and MPRR increase with the increase of oxygen ratio,but the increase amplitude is less than that of hydrogen.It is concluded that the blending of hydrogen（without carbon fuel）has a more significant effect on improving the combustion in the cylinder than oxygen.For emissions,with the increase of hydrogen blended ratio,CH₄emissions decreases first and then increases,while NO_x emissions increases first and then decreases,both reache the lowest when the hydrogen blended ratio is 5%,CO emissions increases.With the increase of oxygen blended ratio,CH₄ and CO emissions decreases,NO_x emissions increases.Compared with the same blended proportion of hydrogen,CH₄emissions is higher and NO_x emissions is lower when oxygen is blended.Based on blended ratio of 8%,compared with the original timing of the main injection,the maximum pressure rise rate was considered to be limited in the blending of hydrogen,and the MPRR could be significantly reduced by appropriately delay the main injection（-4°CA ATDC）;When oxygen is blended,the NO_x emissions are considered to be limited,and the NO emissions can be significantly reduced by delaying the main injection strategy.Compared with single injection,the timing of main injection should not be more advanced due to the limitation of MPRR after the pilot injection strategy with blended hydrogen;Blending oxygen and using the pilot injection strategy can increase the peak cylinder pressure and further reduce the CH₄ emissions.For high load conditions,under the constant pilot injection strategy and 50%substitution rate,with the increase of the.angle of LIVC,the maximum cylinder.pressure decreases,the MPRR.increases,the CH₄ and CO emissions decrease significantly.Compared with without LIVC strategy and the pilot injection strategy,the MPRR can be significantly reduces under the constant pilot injection strategy and 50%substitution rate when the angle of LIVC is 10°CA,and the MPRR changes slightly with the increase of EGR rate.