Study On Air-to-Fuel Ratio Control For All States Of Heavy Duty Diesel Engine Based On EGR Technology

With high thermal efficiency, good economy and reliability, and wide power ranges,diesel engine is widely used in power machinery; but its emissions pollution problems hasbecome a focus of attention. Particularly, it is said that oxides of nitrogen (NOx) andparticulate matter (PM) emitted from heavy duty diesel engines may have significant effectson the environment and human health.With a view to further strict diesel engine emission regulations, in addition to thedecontamination measures combining machine outside to meet emissions regulationsrequirements, EGR system is one of the most useful way to reduce NOx.Study on EGR Technology was mainly divided into two aspects，on the one hand fromthe formation of mixed gas and combustion in-cylinder impact angle, on the other hand frommaking use of control strategies to adjust EGR flow namely precisely controling air-to-fuelratio to control emissions angle. Specifically from EGR on the emissions performanceimpact，EGR technology is a promising way to optimize the NOx, but with penalty inparticulate increase, and CO and HC will also increase. And under the restrictions of theturbo, brake specific fuel consumption (BSFC) will increase. Accurately, compromiseair-to-fuel ratio need to be calibrated, this can make the emissions meet the standards andachieve the target through the actuator.Euro-Ⅲ and further higher emission control regulations require that the diesel engineequipped with exhaust after treatment system must be increased the transient test. Therefore,in transient conditions diesel engine performance variation law and emissions optimizationstrategies has gradually became the focus of the internal combustion engine industry over theworld. The result had been shown that intake system is the main reason for the deteriorationof emissions in transient process. When higher EGR rate was applied to a turbocharged andintercooled diesel engine, in rapid acceleration process spike smoke was dramaticallygenerated due to delay of boost pressure rise.Aiming at the problem that adding sufficient quantities of EGR is difficult after theintercooler of intake and emissions is deteriorated in transient process on some heavy dutydiesel, and optimized control of throttle. To expressly study controlling EGR flow fromair-to-fuel ratio angle for effects of emissions performance of heavy duty diesel engine underfull working conditions, a lot of numerical simulations and experiments were carried out on MITSUBISHI GDI engine and CY6D180high pressure common rail diesel engine in thispaper. The main work and conclusions were summarized:1、air-to-fuel ratio control system design ideas and processes of high-pressurecommon-rail diesel engine under full working conditions were proposed, which requiredtesting bench and simulation platform were built.1) Three steps of air-to-fuel ratio control system design processes of engine under fullworking conditions were proposed, namely, offline simulation phase, semi-physicalsimulation validation phase and calibration and verification the emissions effects on actualengine test bench phase.2) On MITSUBISHI GDI engine and CY6D180diesel engine a real-time test bench wasset up by a A/D data acquisition system with high-response sensors, corresponding EGRsystem was applied, and transient contol and measurement systems were set up by usingeddy current dynamometer.3) Using EGR valve driven by DC motor as the controlled object, closed loop controlsystem of EGR valve was set up.4) Real-time control simulation test system of engine with hardware in the loop was set up,which was composed of engine dynamics modeling software enDYNA, real-time hardwaredSPACE, xPC-Target and throttle and EGR valve.2、The precise CY6D180diesel engine model was set up making use of enDYNAsoftware, steady air-fuel ratio controller chaining the EGR and throttle valve together wasdesigned and control effect was verified by means of offline and semi-physical simulation.1) State parameters and emission levels under various working conditions of CY6D180diesel engine were collected, and according to universal application emissions optimizationprinciple in the industry EGR was introduced.2) The above-mentioned steady data and basic parameters of CY6D180were submitted toenDYNA, and the precise CY6D180diesel engine model was set up and the model accuracywas verified. A coordinated EGRITH controller was designed and controller test andparameter calibration were carried out on offline environment of the PC computer.Semi-physical simulation test was carried out, and control parameters were furtherdebugged.3) Transient performance test aiming at the coordinated EGRITH controller was carried out,the results were shown that ideal inlet flow could was generally tracked but accelerationperformance was limited, which resulting in poor drivability.3、After analysis of delay problem of engine throttle and EGR system, taking analysis of EGR valve close step response driven by DC motor and study on performance andemission characteristics of basic prototype engine under transient operating conditions as areference, engine transient characteristics after the introduction of EGR was studied. Basedon the above bench analysis results, a new strategy of optimizing the engine transientperformance and emissions was proposed.1) The delay includes induction-to-torque delay under steady state of engine, includesdelay between time of air input controlled by air throttle to actual time of this section into thecylinder under transient state of engine, also includes delay of the action of the air throttle.The time-delay which be discussed are all uncertain time-invariant, but each has a constantupper bound. Finally, making use of the exact engine model established by enDYNAsoftware to verify the modeling correctness and validity of control method. Compared withcontrol effects of controller without considering delay and delay-independent controllerwhich be designed though taking into account the time-delay problem，Integrated controleffects of delay-dependent controller designed is the best. The research results provided abasis for the next diesel engine throttle delay control.2) Several types of transient EGR measurement ways and rational arrangement of EGRsystem on actual engine test bench were discussed. After that in the engine transientconditions, impact and response course on engine performance and emissions of close stepresponse of EGR valve driven by DC motor were discussed. The conclusion were drawn thaton samples process of the transient system performance parameters and design of controlsystem, difference in response delay time and response cycle of engine performanceparameters should be taken into account.3) Taking analysis of performance and emission characteristics of basic prototype engineunder transient operating conditions as a reference, transient characteristics of engine afterthe introduction of EGR was studied. The conclusion were drawn that transient emissionresponse of EGR rate are significantly different from the steady, taking optimized EGR ratein the steady conditions as the control objectives of the transient process will result in EGRrate overshooting, air-to-fuel ratio dropping rapidly and soot emission sharply increasing.4) Based on the above bench analysis results, a new strategy of optimizing the enginetransient performance and emissions after the introduction of EGR was proposed. Namely,by closing and opening the EGR and throttle valve closed loop controlling heavy duty dieselengines transient air-to-fuel ratio, the purpose of reducing smoke emissions during transientcould be achieved. By comparing the transient emissions effects with three methods of timedlong off and open valve, the advantages of the closed loop control method had been verified. 4、Based on steady air-fuel ratio control algorithm and transient air-fuel ratio controlstrategy, switching between steady and transient state of the controller was studied, andprecise control algorithm of air-fuel ratio under full working conditions was proposed. Onthe actual engine test bench by comparing the emissions effects with the method of steadyEGR optimize MAP, verification results of the algorithm were shown that applying thecontroller designed in this article engine emission levels has been improved throughout thetest cycle.