Study On The Regulated Two Stage Turbocharging System For Diesel Engine

Increasing the break mean effective pressure of diesel engine through improving the boost pressure is one of the effective ways of reducing fuel consumption (aka. carbon emission reducing). Meanwhile, the higher boost pressure guarantees enough fresh charge for the techniques of Exhaust Gas Recirculation (EGR), Miller Cycle and Homogeneous Charge Compression Ignition (HCCI) which are valid to fulfil the increasing strict regulation of harmful emission. Thus the boost pressure of diesel engine need to be improved further, however the problem of matching the wide speed range diesel engine and the high pressure turbocharging system has to be resolved firstly. The regulating two stage (R2S) turbocharging system, that combines the advantages of two stage turbocharging and regulating turbocharging, can provide high boost pressure level over a wide range of engine speed. For the purpose of knowing a potential turbocharging technology, a comprehensive study will be carried out on regulating two stage turbocharging system in this thesis.First, theoretical study on the regulated two stage turbocharging system is conducted based on the equivalent turbocharger concept. Expressions for the equivalent turbine flow area and the equivalent turbocharger efficiency of the R2S system were derived. The equivalent turbocharger efficiency expression gives a criterion for matching of the R2S system based on the economy principle which is making more exhaust energy conversion at the stage with higher turbocharger efficiency. The equivalent turbine flow area can be used to analyze the regulating ability of the R2S system. Based on the regulating ability requirement, a new method for match of R2S system and diesel engine has been developed which clearly reveals the correlation between the total pressure ratio, the distribution of pressure ratio between the two stages, the turbine flow area ratio of low pressure stage to high pressure stage and the system regulating ability.For Further Study, key components of the R2S system are designed, furthermore the prototype engine and test bench are established. Firstly, a proof experiment is conducted to verify the new match method of R2S system, which turns out that the method is appropriate. After that, comprehensive testes focus on the steady performance are executed within the whole operation range of the engine, and the open time of the compressor bypass valve which is parallel located with the high pressure stage compressor and the regulating characteristics of the turbine bypass valve which is parallel located with the high pressure stage turbine are analyzed. On the basis of the above analysis, two regulating MAP of R2S system whose objective is maximum of economy or dynamic respectively are proposed. Combined with the further load transit and starting tests, conclusions can be made that the regulating MAP of economy is superior to the regulating MAP of dynamic with regard to the steady performance in open loop control mode. However concerning the transit performance, the regulating MAP of dynamic can achieve more rapid respond and lower smoke than the regulating MAP of economy during the load transit test of 900 r/min and 1100r/min which are both in the low-middle engine speed range. But the result from the load transit test of 1400 r/min which is in the middle-high engine speed range reveal that superiority of the regulating MAP of dynamic isn't obvious any more because of the bypass process of the high pressure turbocharger.To make up the shortage of using regulating MAP in open loop mode directly, close loop boost pressure control strategy is studied numerically on a mean value model at the last chapter. System identification from turbine bypass open command to boost pressure is conducted under different operation points, based on which a control system consist of a gain-scheduling PI feedback controller and a feedforward module is designed. Simulation results from load transit process at 1400 r/min reveal that, control system consist of PI feedback controller and the regulating MAP of dynamic achieve the fastest respond process, even the control system consist of PI feedback controller and the regulating MAP of economy is faster than open loop using of the regulating MAP of dynamic, which means that the close loop boost pressure control strategy can realize the economy and dynamic simultaneously.