| In most of time, automobile engine operates under the transient conditions, andthere is a comparatively great disparity between its practical thermal efficiency andsteady bench performance level. Therefore, to improve the heat-work conversionefficiency under transient conditions is the most direct and effective approach forautomobile energy saving and environment protection, which has great energy savingpotential and practical use. In the meantime, it plays a profund role in energyconservation, environment protection and achievement of automobile energy savingand emission reduction.However, it is a very difficult and also unsolved issue to diagnose the states andvariation histories of in-cylinder combustion and heat-work conversion processes, aswell as the correction of operational parameters in real-time under engine transientconditions. There are two major obstacles:(1) from the viewpoint of diagnosis ofmultiply parameters, there is no suitable approach to meet the requirements of bothdiagnosis frequency and measurement accuracy;(2) from the viewpoint of correctionof operational parameters, there is no theoretic guidance to adjust or correct theoperational parameters, due to the lack of deep understanding of the influencemechanism and common rules of these operational parameters.To solve these issues, our research group has proposed a new approach todiagnose the multi-parameters of automobile engine in real-time, which is based on ahybrid method of dynamic signal measurements coupled with gasdynamics/thermodynamics process simulation in the engine system. On this basis, thecorresponding application technology (in the form of software) has been developed inthis research, and the feasibility of this approach has been validated on an advancedautomotive engine. As a result, the continuous detection of automotive enginemultiple parameters under transient conditions can be realized. In this research,several key theoretic and common issues are studied and solved to make the methodapplicable:(1) To select the suitable numerical solution method for the detectiontechnology with the diagnostic frequency and measurement accuracy considered,which is used to solve the controlling equations of gas dynamics and thermodynamicsprocesses in the engine system.(2) To reveal the fundamental inner-relationshipsamong various engine parameters under transient conditions; and then to analyze theinfluence mechanism of operation parameters on performance parameters; finally, tosummarize the common rules after analyzing lots of detected data (or test data).(3)To find out the reason for the differences between transient and steady state performances after the comparison of the change rules and influence factors ofperformance parameters under both transient conditions and steady state.(4) Toreaval the working performance differences of each cylinder under transientconditions, and then to explain the influence mechanism.(5) To analyze thein-cylinder working process and rate of heat release under typical operatingconditions and transient conditions; then to give deeper explanation for thisphenomenon (such as combustion deterioration) and also to deeply explicate theeffects of operational parameters on combustion characteristic parameters and heatrelease rules.The research results of this thesis show that:1) Both under the steady-state and transient conditions, ignition advance angleplays a decisive role in the heat-work conversion efficiency of engine. And thedetected data (or test data) show that the ignition advance angle of this engine has asharp decline in emergency deceleration conditions and even turns to negative value,resulting in very low engine indicated thermal efficiency. Thus, to precisely controlor adjust the ignition advance angle under transient conditions is a prerequisite toensure and optimize the engine transient performances.2) The degree of load’s (or IMEP) effect on indicated thermal efficiency ofhigh-pressure cycle under transient conditions is different from that understeady-state conditions. When the engine operates under steady-state conditions, theload has little effect on indicated thermal efficiency of high-pressure cycle; but undertransient conditions, when the IMEP is smaller than4bar, the indicated thermalefficiency of high-pressure cycle increases steeply with the increasing of IMEP.However, when the IMEP is larger than4bar, the indicated thermal efficiency nearlyremains the same.3) Under transient conditions, when the engine operates at low-medium load, thePMEP approximately equals to the difference of intake and exhaust pressure, and thissituation is basically coherent as it is under steady-state conditions; however, whenthe engine operates under high speed and high load, the PMEP is much larger than thedifference of intake and exhaust pressure and presents great fluctuation.4) The change rules and influence factors of RGF are displayed as follows. VVThas some effects on the in-cylinder RGF: with the increase of cycle number, theprofiles of valve overlap angle and RGF are similar in low-medium load area, buttheir variation magnitudes are not equivalent. At idle speed, the valve overlap angle is0oCA, but the RGF is still relatively high; yet no matter how the valve overlap anglechanges under high speed and high load, the RGF is very low and nearly remains constant. Thus, it can be shown that the load (or intake pressure) plays a moresignifficant role in the in-cylinder RGF. Consequently, under transient conditions,RGF is mainly influenced by load (or intake pressure), then followed by VVTadjustment.5) Under transient conditions, the otherness of each cylinder’s workingperformances is rooted in uneven distribution of the intake and exhaust parameters.The fluctuation of intake pressure leads to disproportion of each cylinder’s intake gasmass and instantaneous air-fuel ratio; and the fluctuation of exhaust pressuregenerates the inequality of in-cylinder residual gas mass and RGF. Besides, thedeviation of each cylinder’s volumetric efficiency at high speed and high load is notdominantly aroused by the intake mean pressure, but the pressure fluctuation.6) The indicated thermal efficiency of engine is the results of the integrativeeffects of various operating parameters. Also, the poor indicated thermal efficiencyoften appears in the working cycles with lower (or very low) ignition advance anglebut higher RGF. At this moment, the cycle variation is relatively great, and thecombustion process is very unstable. When the ignition advance angle is very smallor even negative value, the smaller excess air coefficient gives rise to the higherindicated thermal efficiency since it can accelerate the burning velocity.The research results illustrated above not only deepen and expand the changerules and influential mechanism of automobile engine steady-state performances, butalso reveal and dissect the change rules and inner-relationships of various engineparameters under transient conditions. So that the summarized common rules aremeaningful for improving engine transient performances, and provide the theoreticfoundation and data support for on-line identification, evaluation and correction ofengine operational parameters under transient conditions. |
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