| With the increasingly serious problems of energy crisis and environment pollution,the demands for energy saving and emission reduction of vehicular engines are gradually raised.In general,the development of engine performance and the optimization and calibration of various parameters is completed under the steady-state condition while the vehicle engine is mostly operating under the transient condition,so the transient performance is the key to determine the engine actual performance.Consequently,the most direct and efficient solution to realize the energy saving and emission reduction is to improve the transient performance of engine.At present,there are still great difficulties in the comprehensive detection analysis and performance optimization under the transient condition.The main difficulties include: 1)Because the transient performance detection requires the high-frequency cycle-to-cycle detection,part of operating parameters such as residual gas fraction,cyclic injection quantity,transient torque,ignition timing,intake and exhaust variable valve timing are hard to measure;2)The changing rules,influence factors of performance parameters and its inner relationship with operating parameters under transient conditions especially the road conditions are not clear and lack of theoretical basis to improve the transient performance.Firstly,the single variable sweeping test was adopted in this paper and the quantitative research on in-cylinder combustion process under steady-state condition was carried out.Subsequently,the quantitative relation between combustion characteristic parameters and operating parameters were established and the predictive model of main combustion characteristic parameters was developed,which provides the basis for in-depth research on engine transient performance.Then based on the online detection method for engine transient performance proposed by the research group as well as transient fuel consumption and torque detection method,the transient working process of engine was continuously detected under NEDC condition,which shows the changing rule of performance,energy flow,combustion and emission of engine under NEDC.By synchronous coupling analysis of engine transient operating condition,it revealed the distribution and changing mechanism of energy flow as well as changing rules and influencing factors of combustion and emission performance.On this basis,the comparative research on the transient and steady-state performance was conducted by working range,which analyzed the deviation levels,changing rules and influencing factors between the transient and steady-state performance.And then the method to improve engine transient performance was proposed.The main research contents and results are as follows:(1)The quantitative research on in-cylinder combustion characteristic parameters under steady-state condition was conducted according to the results of single variable sweeping test.The influencing degrees of excess air coefficient,residual gas fraction,load,intake tumble intensity and other parameters on combustion rate under various phases are systematically analyzed.Then the quantitative relationships of SA-50% combustion duration and 10-90% combustion duration with engine operating parameters are established and their semi-empirical predictive model are developed and experimentally verified,which provides the basis for follow-up comparative analysis with transient performance.(2)The vehicle energy flow test was conducted under NEDC of cold start on the chassis dynamometer.The mathematical models of various types of engine energy flows under NEDC are established and calculated according to the test results,which shows the distribution characteristics and changing rules of main energy flows including effective power,pumping loss,exhaust loss,coolant loss and others under NEDC.Combined with the operating parameters of vehicle and engine under NEDC,the influencing factors of energy flows are analyzed and revealed,which provides the basic data and guidance to improve the energy efficiency and reveals the potential of waste heat recovery under NEDC.(3)The coupling analysis of combustion and emission were carried out under NEDC of cold start.A method for multi-parameter synchronization comparison was proposed.It is adopted to analyze systematically the main influencing factors of combustion and emission under transient conditions and then the influencing degrees of cold start,rapid acceleration and deceleration on combustion and emission are revealed.Moreover,the inner relationship among vehicle condition,engine operating condition,in-cylinder combustion and heat-work characteristic parameters is established,which provided theoretical basis for improvements of combustion and emission under transient conditions.(4)The comparative research on engine thermodynamics,combustion and emission was conducted based on the results under NEDC and steady-state conditions.A comparison method based on the same working range was proposed.The combustion and emission parameters under NEDC of cold start and steady-state bench test are compared and analyzed by working range,which shows the deviation degree and changing rule between transient and steady-state conditions.Also,it reveals that the main reason why the steady-state performance is better than that under transient condition is due to great deviation of main controlling parameters from target values under transient condition.Consequently,it can be concluded that the precise control of transient controlling parameters can directly and effectively improve the transient performance.The above results not only expand the changing rule and influencing mechanism of combustion under steady-state condition,but also make systematic research and in-depth analysis on the changing rule and influencing factors of engine energy flow,combustion and emission under transient condition.It perfects and enriches the research on transient engine performance and the drawn conclusion provides theoretical basis and data support for improvement of transient engine performance. |
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