Parameters Selection Of Two Stage Turbocharging System And Its Effect On Combustion Process

Due to the increasingly strict emissions regulations and shortage of petroleum,low temperature combustion mode has drawn much attention from scientists as apromising high efficiency and clean combustion mode. In order to improve theefficiency and reduce emissions of two stage turbocharged diesel engines, bothoptimizing process and control method of two stage turbocharging system parametersare investigated during full range working conditions of diesel engine by applyingboth simulation and experimental study in this dissertation whose core content is twostage turbocharging system.The computation and optimization of two stage turbocharging system parametersmainly include two parts:(1) In the absence of turbocharger MAP, key parameterssuch as the intake air flow rate, boosting pressure and turbine‘s flow section area ofthe two stage turbines are estimated by applying the JTK model. Meanwhile, bydeducing the compressor and turbine energy equilibrium equation, the method tooptimize key parameters is developed. Furthermore, the principle to choose optimizedcompression ratio and expansion ratio is proposed, which is based on the identicalcompression ratio between two stages and turbines energy consumption lowestprinciple. Based on low speed full load condition, two stage turbocharger flow areageometry is optimized. The turbochargers are selected based on this principle.(2)Based on the selected turbocharging system, a combustion system thermodynamicmodel is set up. In order to improve the thermal efficiency of different operatingconditions of diesel engine, the impact of injection timing, amount of fuel injected,EGR ratio, EGR system, intake valve closing timing (IVCT), and the opening ofbypass valve and backpressure valve on the two stage turbocharging system areanalyzed. The optimization method for two stage turbocharging system is proposed,which can provide foundation for further work.The difference between the turbine bypass valve―release‖method andcompressor bypass valve―release‖method when the engine is running at high speedis compared in this dissertation. Simulation and experimental results show that whenintake pressure keeps constant, the turbine bypass valve―release‖method can achievehigher thermal efficiency. This is because lower backpressure and smallerintake/exhaust pressure difference is achieved. The key to achieve high efficiency and clean combustion during full range ofengine operating conditions is to coordinate and optimize the injection strategy,boosting, IVCA, EGR and other cylinder parameters. The experiments show that airintake flow and backpressure can be reduced with the same injection strategy andEGR ratio by applying IVCA. Both NOx and soot emissions can be reduced with animproved thermal efficiency by optimize the matching between IVCA and two stageturbocharging system. The impact of LP-EGR and HP-EGR on the operating behaviorof boosting system is also compared. With the same EGR ratio, similar NOxemissions are achieved. However, soot emissions are different during different enginespeeds. High efficiency and clean combustion can be achieved by choosing the properEGR loop and boosting system.