| Alongside increasingly stringent emission regulations and public demand for fueleconomy, the combustion and emission problems inherent to automotive dieselengines during transient operation have become crucial and urgent issues. In thispaper, the transient operation conditions of constant speed and increasing torque werepresented as a typical transient operation conditions based on a heavy-dutyturbocharged automotive diesel engines, after the European Transient Cycle （ETC）was analyzed. Using transient measurement and control system, in addition to CFDnumerical simulation(STAR-CD), the experimental study and numerical simulationhave been carried out, and the details include:1) Dynamic response of key parameters,evaluation indexes and relevant laws of performance deterioration of turbochargeddiesel engines under transient conditions were experimentally researched;2) Theessential cause of transient performance deterioration has been investigated bynumerical simulation, which provide the theoretical foundaction for optimalcontrol strategies;3) According to the study of the laws and causes of performancedeterioration, the optimal control strategies were developed, and the performancedeterioration of diesel engines during transient operation have been effectivelyimproved. Detailed contents are as follow:1. The evaluation indexes, such as “lag coefficient”,“deterioration coefficient”,“transient total”,“transient average” and other relevant characteristic parameters werepresented in this paper, to study and evaluate the engine performance convenientlyand accurately under transient operation conditions. They have been used to assess theautomotive diesel engine performance, such as the delay, deterioration and thecomprehensive performance. The experimental results showed that using theseevaluation methods could evaluate the engine transient performance systematicallyand clearly.2. Using transient measurement and control system, in addition to evaluationindexes, the dynamic response and deterioration laws of key performance parametersin turbocharged diesel engines running under different transient conditions wereinvestigated experimentally.The results show that, the dynamic response speeds of fuel-supply，torque，air-supply and NOx emissions are decreased and corresponding lag coefficients are increased during the same transient operation. The lag coefficients are increased withthe increase of loading rate, and the decrease of initial loading load or constant speed.Compared with the steady-state conditions, the engine demonstrates such phenomenalike air-fuel ratio decreasing, intake air and combustion phase delay, brake specificfuel consumption (BSFC), smoke and CO emissions increasing. Furthermore, thesephenomena are more obvious as the loading rate increases, initial loading load orconstant speed decrease. The main deteriorated areas of BSFC locate in light andheavy load areas. However, main deteriorated areas of the smoke and COemissions locate in heavy load at low to medium speed. Besides, the NOx emissionsreducted compared with the levels under steady-state conditions during the wholetransient process.3、Based on the transient process with different loading rates, the essential causesof performance deterioration in turbocharged diesel engines are investigated byexperimental test and numerical simulation based on three distinct levels ofinvestigation， including dynamic response of key performance parameters,combustion process and in-cylinder micro-investigation. The results show that:As far as parameter response, the main causes of performance deterioration arethe inconsistent response rate of key parameters, such as air supply, fuel supply, andexhaust and cylinder wall temperature, which resulte in mixing and combustiondeterioration.Considering the combustion process, combustion rate decrease and combustionphase delay are the primary factors of combustion deterioration during transientprocess.The micro-investigation suggest that the main cause of combustion deteriorationis the reduction of “driving energy of air-fuel mixing (DEM)”, which decrease thefuel atomization quality, evaporation rate and mixing rate, leading to reduction of thelean mixture and combustion rate.The correlation of soot formation with rich mixture (equivalence ratio, φ>2) isstronger than high-temperature condition, and the increase of soot formation region isresponsible for soot emission deterioration. The key strategy for soot emissionimprovement is to promote air-fuel mixing by increasing the “driving energy ofair-fuel mixing”. The NOx formation region decrease due to lessening oxygenavailability, moreover, the dominant factor of NOx emissions is high-temperaturecondition and lean mixture quantity in low-medium and heavy load, respectively.4、To provide the theoretical foundation for control strategy of diesel performancedeterioration during transient process, the effects of combustion boundaris includinginjection timing, injection pressure, global air-fuel ratio and in-cylinder temperature ininjection timing on combustion process of three typical conditions have been simulated using STAR-CD. The results show that:The delay in combustion phasing can be eliminated by advancing the injectiontiming, which could improve combustion process in medium and small load.Unfortunately, it becomes worse in heavy load. The phase of “driving energy ofair-fuel mixing”, lean mixture and soot formation region can be effectively controlledby adjusting the injection timing. The main objectives are respectively increasingthermal efficiency and decreasing NOx emissions in medium and small load andreducing smoke emissions in heavy load, when using the control strategy of injectiontiming as the leading control measure during transient process.The deteriorations in combustion phasing and air-fuel mixing process areimproved and the “driving energy of air-fuel mixing” and the amount of lean mixtureare increased by rising injection pressure, resulting in smoke emissions decreased,while NOx emissions increased in medium and small load.The “driving energy of air-fuel mixing”, the amount of lean mixture andcombustion rate increase as the global air-fuel ratio increase under transient operation,resulting in smoke emission and residual fuel decrease, especially in heavy load.However, the NOx emissions will soar.The combustion phasing in small load can be advanced and the soot emissionsfirst decrease and then increase as the in-cylinder temperature rise，whereas, thecombustion phasing delays in medium and heavy load and the NOx emissionsincrease in medium and small load. To improve the effects of fuel evaporation andair-fuel mixing in small load, the in-cylinder temperature should be appropriately rosein early stage of transient process.5、Based on the deterioration characteristics of turbocharged diesel engine undertransient conditions, the control strategies of injection timing, injection pressure and“non-uniform loading” were developed. The results show that:The deterioration in combustion of the engine can be corrected by adjusting theinjection timing during transient operations. The delay in combustion phasing can becontrolled by advancing the fuel injection timing, resulting in effectively increasingthermal efficiency. Using a suitable CA50(0<△CA50<5°CA) can get the optimaltrade-off of thermal efficiency, smoke and NOx emissions during transient operationwhen the dominant control measure of combustion process is injection timing.The deterioration in combustion process can also be improved by increasinginjection pressure during transient operation, resulting in decreasing smoke emissionsand inceasing NOx emissions. The optimal trade-off can be acquired with a suitableCA50phasing (2<△CA50<4°CA) when the dominant control measure of thecombustion process is injection pressure. The thermal efficiency and smoke emissioncan be improved at the same time by suitable “section-injection pressure” strategy, when the injection pressure decreased or remained in early stage of transient process,and then increase later.The larger the “early loading rate” and “change point load”, the better the torquedynamic response. The peak values of smoke and CO emissions and the “transientaverage” of BSFC, smoke and CO emissions can be decreased by the “non-uniform”loading strategies, with appropriate “change point load” and “dead time” duringtransient operation. However, the combustion deterioration will cause smokeemissions and BSFC increase under the loading process with an overlarge “changepoint load”.6、Finally, based on the advantages of various kinds of control strategies as above,the performance deterioration of the turbocharged diesel engine during transientoperation was comprehensively improved. Based on the trade-off consisting of torquedynamic response, peak values of smoke and CO emissions and “transient average” inBSFC and brake specific emissions, the “strategy-3”(early loading rate is675N·m/s,change point load is50%, dead time is1.2s, injection timing advance2°CA and“section-injection pressure” strategy first decreased5MPa and then increased20MPa)is the better cooperation control strategy. This strategy can improve torque responseby59.35%in50%load, drop the peak levels in smoke and CO emissions and“transient average” of BSFC by35.94%,10.05%and3.41%, respectively, comparedwith the uniform loading strategy. However, the “transient total” of NOx emissionsincrease by31.14%compared with that of uniform loading process, but it is stillbelow the steady-state value with this strategy. |
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