Research on the Control Strategy of Variable Nozzle Turbocharger for Natural Gas Engine

Journal of Beijing Institute of Technology, 2010, Vol. 19, No. 1Research on the Control Strategy of Variable NozzleTurbocharger for Natural Gas EngineAO Li-jun(郝利君), HUANG Ying(黄英), ZHUANG Fu-jun(张付军),GE Yun-shan(葛蕴珊)( School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China)Abstract: A variable nozzle turbocharger(VNT)was applied to a 2. 2-liter LA natural gas engine, and a VNTcontrol system was designed to operate it. Based on VNT matching test results, a VNT control strategy was studied, in which VNT adjustment is carried out through pre-calibrated VNT handling rod position, combined with aclosed-loop target boost pressure feedback using proportional-integral-derivative( PID)algorithm. Experimentalresults showed that the VNT control system presented in this thesis can lead to optimized performance of VNTease engine volumetric efficiency over a wide speed range, improve engine dynamic characteristics and up-grade economic performanceKey words: natural gas engine; variable nozzle turbocharger(VNT); proportional-integral-derivative( PID)control; control strategy: experimental studyCLC number: TK 432 Document code: A Article ID: 1004-0579(2010)01-0037-05Turbocharging can reclaim part of the energybringing down its economical efficiency under highthe exhaust, and has become one of the most efficient speed condition, running area of low specific fuel con-means to generate higher power in internal combustion sumption is enlarged and engine's acceleration perform-engine with a small swept volume. Nowadays diesel enanceISgine turbocharging has been widely used.VariableIn this thesis a control method of vnt for thenozzle turbocharger( VNT can result in better matc- natural gas engine is studied, and the VNT controlhing with engine by adjusting the turbine nozzle flow strategy is summarized according to VNT matching reg-gines gradually as well(9-m/nt, ed in diesel en-area, hence has been rapidly adopted in diesel en- ulations condensed from engine tests. Test results havegines-5, and is now gettingshown that the VNT control system proposed in thisthesis can help realize optimized control over VNT, in-After a gasoline engine is modified to use natural crease air intake efficiency over a wider revolutiongas as fuel, its maximum power output and maximum speed range, improve dynamic performance and raisetorque are both reduced, and turbocharging is an effec- economical efficiency of the natural gas enginetive means to improve its dynamic performance. In our 1 Schematic of the Natural Gas En-research. a vnt is used to match a 2. 2-liter naturalgas engine. The VNT control system functions togine Control Systemchange the discharge area of turbine nozzle by changingThe natural gas engine was refitted from anthe angles of turbine nozzle vanes so as to control the nal multi-point injection gasoline engine, with aturbocharger,s rotation speed. The natural gas engine' s volume of 2.2 L, compression ratio of 9: 1 and maxi-torque and power at low speed are increased without mum中国煤化工-'n. An electronical-Sponsored by the Ministerial Advanced Research Foundation( C2002AA002)CNMHGography HAo Li-jun( 1968-), Ph. D, associate professor, haolijune bit. edu. enJournal of Beijing Institute of Technology, 2010, Vol. 19, No. 1ly controlled gas injection system is designed. The newECU built from modifying an original gasoline engine e-lectric control system determines natural gas injectionquantity, ignition timing, idle speed, VNt boost pres-sure and other parameters by monitoring the enginethrough sensors on crank position, camshaft positionintake manifold absolute pressure, throttle position, in-take air temperature, boost pressure and VNT handlingrod position, etc. respectively. Schematic of the natual gas engine control system is shown in Fig. 1Fig 2 VNT turbine nozzle vane opening as a function ofIinn senslcamshaft p sition sensornatural gasrule. When kept at a stable speed, the engine, s powerintak- manifold abslute pressure sensoroutput response to added load is increased as the throt-le valve opens wider; however, the trend of the power(Slant temperature seneoridle controlincrease will slow down with increasing opening, whenit is opened to a certain degree, the power output willbowl pressure senorrise no more, hence shows the phenomenon of"powervNT handling rod poition sensstaling". The lower the engine revolution speed is, thesmaller the throttle valve angle at which this effect be-Fig 1 Schematic of the natural gas engine control systemgins to appear, so at lower revolution speed, VNT tur-The natural gas supply system is consisted of com-bine nozzle vanes need to be adjusted earlier so as toessed natural gas cylinders, gas supply cut-off valveraise power output. When the turbocharged natural gashaving passed the pressure regulator, natural gas willnozzle vane opening will be gradually reduced, so as tohave its pressure reduced to 0. 5 MPa and sent to thereduce the discharge area and increase exhaust gas flowgas supply common rail pipe to be dispatched to cylinspeed, therefore make the turbocharger acders by natural gas injectors controlled by ECU, and crease both boost pressure and intake airECU shuts off gas supply via the cut-off valve when the in the end to serve the aim of achieving higher engineenginepower and torque2 VNT Adjustment RegulationsAs a result, the VNT regulating capability is notonly related to throttle valve opening but also to engineThe VNT adjustment range is shown in Fig. 2, in speed. When the natural gas engine runs under light orhich throttle valve opening is used to represent engine medium load, the engine power output control is mainload. Based on test results we have obtained the rela- ly carried out by adjusting the throttle valve, mean-tionship of VNT control range of turbocharged natural while the VNT turbine nozzle vanes are fully open ingas engine with two variable parameters, i. e., engine order to reduce exhaust resistance, decrease pumpingload and revolution speedloss and improve economical efficiency. However,The major function of the throttle valve is to con- when the engine runs under heavy load with the throttletrol the power output of the turbocharged natural gascrease outputengine through different openings, and this makes it torque中国煤化工adually reduce thedifferent from the way with diesel engines, as can be VNTC N M H Higher turbocharge-seen in the VNT turbine nozzle vane opening-adjusting er revolution speed, therefore increase boost pressureHAO Li-jun(*sM#)et ad./ Research on the Control Strategy of Variable Nozzl turbocharger for Natural Gas Engineto improve dynamic performance.3. 2 VNT Control StrategyFig 3 shows the basic map for calculatinghandling rod position control takes en-value of boost pressure as a function of enginegine load and revolution speed as its basic parametersand throttle anglethe engine load is represented by the engine throttlevalve position, and the actual position of the handlingrod is given by the rod position sensor and used asfeedback control signalValues of absolute pressure of the compressed airat compressor outlet are picked up by the boost-pres0.13sure sensor, and used as closed-loop signals for feed324423back to the VNT controlOperations of the VNT vane control system isbased on the position map of the VNT vane handlingFig 3 Boost pressure as a function of engine speedrod and the boost pressure map. Because of the existand throttle angleence of considerablense lag in the engine workingThe VNT vane position set value map and the process, it is difficult to achieve the exact control overboost pressure target value map are the basic control the turbine nozzle vane opening by using the boostparameters used to calculate VnT guide vane angles pressure feedback closed-loop control alone, so in thisbased on engine speed and throttle angle, and the thesis a combined method of VNT control using a feedboost pressure target map is used for the boost pressure back control of VNT vane handling rod position togethPper with a closed-loop feedback control of boost pres3 VNT Control Systemsure. In the closed-loop control part, PId control algoI scheme is shown in3. 1 VNT Control DeviceFig. 5The electronic actuating device of VNT is con-In the Pid controller oftrolled by a step motor, and relevant control schemePID algorithm is adoptedshown in Fig 4. The turbine nozzle discharge area isu(k)=u(k-1)+△a(k),hanged through adjusting the variable nozzle vane an- where u(k), u(k-1)are PID controller's output obgles by moving the handling rod, so that the overall tained at the sampling time instants k and k-1 respecturbine nozzle free cross-section is altered and turbine tively. Au(k)is PID controllers output incrementrevolution speed controlled, resulting in the desired from k-1 to kboost pressure. The closed-loop control for the VNT△a(k)=ge(k)+q;e(k-1)+q2e(k-2).(2handling rod position is thus conducted by checking theposition of the VNT handling rod through the rod posiIn Eq(2),o =Kp(1 +TS/T,+ Tp/Ts),9.=K(1+2T./Ts), 2= KT/Ts, while e(k)tion sensoreck-1)and e(k-2)are PID controllers sampledⅤ T handling mput value deviations resulted at the instants of k, k-1VNT actualand k-2. Also, K is a proportion constant, and TsT, and To repurbine nozzle bladeYHEnd differential time respectively.中国煤化工hat the latest threeerrorCNMH GetFig 4 Actuating mechanism of VNTknown for the incremental PID algorithm. Kp, T, TDJournal of Beying Institute of Technology, 2010, Vol. 19, No. Iand Ts are all fixed values already determined through tents of which depend on throttle angle variation rateand engine speed and are limited by the minimum valWhen a dynamic engine operation occurs, VNT ue map and maximum value map, as illustrated incontrol parameters must accept modifications, the ex- Fig. 6targetvNT handling n VNT handling rod position Sloation S△s"vNt eontrolprIsIon mapcenI rollerstep mVNT handlling rdactual pition VVT hundlingIe mapcontrollerposition sensormonsteractual boost pressure signalrevolution sper siGnaltummI gathrtle valve position igna thistle valve position sensor-Fig 5 Schematic of VNT corinn value mapⅤ NT handling rnd positioncontroller ou put valuedecelaration medification value mapFig 6 VNT control scheme of a dynamic engine operation process4 Experiments Conducted on Natuthrottle condition, we can see that by adopting VNTral Gas Engineturbocharging technology, the torque of relevant engineFig. 7 shows comparison results of dynamic and is greatly increased than that of a naturally aspiratedconomical performances of the naturally aspirated nat-and also the revolution speed at the maximural gas engine and the turbocharged natural gas en- torque is reduced. Intake air mass is increased greatlygine,being displayed as torque, power output and gas after the introduction of the VNT system, thus moreBased on the comparisons of the two engines' gine power output can be imia cylinders and the en-consumption rate respectivelynatural gas can be burned in theturbochargernaturally aspiratedturiN harged一- naturally aspiratednaturally aspiratedengine speed/(10r.min)engine speer(1or中国煤化工 speed(10r."CNMHGmpoTPnFig. 7 Dynamic and economic performance comparison between nature gas engines of naturally aspirated type and turbocharged typeHAO Li-jun(sF4)et al./ Research on the Control Strategy of Variable Nozzle Turbocharger for Natural Gas Enginethe other hand, VNT control system can properly re- Referencesduce the free cross-section of the vnt turbine nozzle at[1] Noboru Hikosaka. A view of the future of automotivelower engine speed, and increase rotating speed of thediesel engines, SAE paper, 972682[R]. 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Fuel efficient natural gas enginehave shown that by using the VNT control system en-gine operations can be optimized, and the engine's dy中国煤化工rton, SAE Paper,PA, USA Society ofnamic and economical performance can also be imCNMHGEdited by Cai Jianying)