Research On General Trends Of Performance As Well As Evaluation Under Transient Conditions Of Vehicle Diesel Engines

Compared to gasoline engine,diesel engine has higher effective thermal efficiency,better torque characteristics and lower specific CO2 emission.Dieselization of passenger car power has become one of the most effective technical approach for energy saving and emission reduction.Conceptual design is the key stage which determines the product development direction as well as the competitiveness of a newly developed diesel engine.Continuous evolution of new technologies has introduced more variables every time,those affects the engine performances in different manner and different extent.On the other hand,many of those variables are associated with each other somehow,and others may even affect engine performance in a conflicting way.The combination of those design and operational variables forms a huge case study matrix,which makes the conceptual study an tremendous effort and a difficult job.In addition,the performance of a diesel engine is usually developed on a steady-state test bench,however,as the mobile power source,a diesel engine in an automotive application would be mostly operated under transient conditions.That is,the instantaneous off-design performance determines its actual performance.Therefore,diagnose of the actual state of the combustion process under the transient conditions,the identification of the influencing parameters,the development of the general trends,etc.,are the few key problems to be solved in order to achieve a intelligent control of the various parameters to meet the energy-saving and emission reduction targets.This thesis focuses on solving the key scientific problems associated to the above aspects.The major results and conclusions of this research are summarized as following:(1)The advanced AVL engine bench test system is used to conduct the detailed experimental research on four advanced diesel engines,and to obtain the more comprehensive engine operating parameter and performance parameter Maps.Through the secondary development and in-depth analysis of the test data,the variation trends and key influencing factors of the performance parameters of different types of diesel engines are summarized.The general trends that the engine performance,e.g.IMEP,raw emissions(NOx and PM),as well as the combustion parameters,i.e.combustion efficiency,combustion duration,combustion quality parameters,vary with engine speed,load,excess air coefficient,EGR and other changes are demonstrated for the steady-state conditions.(2)The key parameters which influence the thermal energy to mechanical work conversion process of a diesel engine are analyzed.The general trend of diesel engine combustion efficiency with excess air coefficient is derived.The concept of Effective Expansion Efficiency(EEE)is also extended to quantify the controlling equation of the indicated thermal efficiency.The prediction results of the indicated efficiency were compared to the experimental values of 4 different diesel engines and a maximum deviation of less than 5% is shown.The control equations of diesel engine power and fuel economy are further refined by establishing quantitative relationship between the indicated thermal efficiency of diesel engine and excess air coefficient,combustion efficiency,adiabatic efficiency and effective expansion efficiency.The similarity law and performance transplantation method are extended to diesel engines of different displacements.(3)Via a combined method of real time signal measurement plus steady state map interpolation,the problems associated to the real-time torque and instant fuel consumption rate measurements are solved.Good agreements are shown between this indirect measurement approach with direct measurement results.This provides a simple but reasonably accurate method for the detection,evaluation and analysis of diesel engine performance under transient operating conditions.(4)Taking a 2.8T turbocharged diesel engine as the research object,both the steady state performance map tests and the instant load-step tests are carried out on the AVL engine test bench.Utilizing the continuous on-line measurement method developed in this study,the engine performance under various transient load-step operating conditions were analyzed and compared to the equivalent steady state conditions.Results show that the engine behaves very similar between the steady state and transient process.The key influencing parameters under transient operating conditions are thus identified.Based on the revealed general trends and the quantification equations of engine performance vs.various design and operation parameters,the guidance for numeric simulation based calibration as well as on-line correction of calibration values of the operating parameters could be developed.