Simulation And Optimization Of Working Process For Miller Cycle Natural Gas Engine

As the rapid development of automobile industry, the automobile emission andenvironmental problem have attracted more and more attention, and the development of newenergy vehicles that have higher efficiency has been an imminent event because of thecombustion detonation and low thermal efficiency. In view of the present research status quoof heavy natural gas engine, Miller cycle is put forward to improve thermal efficiency,enhance the compression ratio and reduce the NOx emission, which can meet increasinglystringent emission regulations. In this paper, the working process of Miller cycle natural gasengine is studied by simulation technology.The effect of late intake valve closing, geometry compression ratio and expansion ratioon engine performance is studied by1D simulation, The results show that: As the geometrycompression ratio increases, the torque increases, the specific gas consumption decreases,but the detonation tendency increases; As the late intake valve closing angle increase, enginetorque decreases, the specific gas consumption increases, but the cylinder temperaturedecreases and the detonation is inhibited; As the expansion ratio increases, the thermalefficiency increases, but the torque decreases because the reducing air input by late intakevalve closing. Miller cycle is achieved by increasing the expansion and late intake valveclosing angle, the study indicates that the engine thermal efficiency increases by3%.The combustion simulation platform is built to find out the effect of late intake valveclosing and ignition timing on the working process of natural gas engine. In the chargingstage, there are obvious tumble, and the cylinder swirl rate is larger than cylinder tumble rate.And in the compression process, the intensity of cylinder swirl and tumble decrease; Atignition timing, lower swirl and tumble is helpful to form a stable flame core, and mixtureconcentration field display inverse stratified distribution.As the late intake valve closing angle increases, the maximum combustion pressure, heat release rate and temperature decrease, which reduce the detonation tendency, greatly theformation of NOx decreases by50%. At ignition timing, as the late intake valve closingangle increases, the swirl and tumble rate increase, that is advantageous to combustionstability and the rapid flame propagation, but there is more unburned mixture in the bottomof combustion chamber and cylinder wall.As the ignition timing delays, the combustion duration increases, the maximumcombustion pressure and heat release rate decrease. As the ignition timing advances, themaximum combustion pressure and temperature increase, that may cause the enginedetonation, but reasonable control of ignition timing can male up the reducing powerbecause of late intake valve closing.Although as ignition timing delays, the cylinder temperature decreases, the NOxemission significantly decreases by85%, the engine exhaust temperature is higher due to thelonger combustion duration, and HC emission increases. Therefore, the reasonable control ofignition timing help to higher engine power performance and thermal efficiency, lowerexhaust emission. The advantage of Miller cycle can be much larger in combination with thecalibration of many engine parameters.