Dynamic Scheduling of Internal Exhaust Gas Recirculation Systems

In this paper we analyze the nonlinear dynamic be havior of an internal exhaust gas recirculation system based on the mean value model of an experimental en gine equipped with a camshaft phaser We develop a dynamic camshaft timing schedule that regulates the in ternal exhaust gas recirculation system while maintain ing transient engine torque response similar to an engine with zero exhaust gas recirculation The dynamic sched ule consists of a steady state map of the camshaft timing for optimum exhaust gas recirculation based on throt tle position and engine speed and a rst order lag that regulates the transition of the camshaft timing to the op timum point A scheme for adjusting the time constant of the rst order lag depending on engine speed and throttle position is described Introduction Exhaust gas recirculation EGR was introduced in the early s to reduce the formation of oxides of ni trogen NOx in internal combustion engines The in ert exhaust gases dilute the inducted air fuel charge and lower the combustion temperature which reduces NOx feedgas emissions Conventionally exhaust gas recircula tion is accomplished by controlling the exhaust gas that is supplied from the exhaust manifold to the intake mani fold through a vacuum actuated valve The EGR control algorithm is a simple PI or PID loop that adjusts the valve position to the scheduled steady state value Ex haust gas recirculation alters the breathing process dy namics and consequently the torque response Careful steady state and transient control design is necessary to maintain good engine torque response For this reason EGR is typically turned o or is considerably delayed in transient engine operations engine warm up and idling The advances in real time computing and hardware are making possible the application of fully controlled valve events Optimized valve events can improve the gas exchange process and enable control of internal EGR Variable camshaft timing is an innovative and simple mechanical design approach for controlling EGR By re tarding the camshaft timing combustion products which would otherwise be expelled during the exhaust stroke are retained in the cylinder during the subsequent intake stroke This is a method of phasing the camshaft to con trol residual dilution and achieve the same results as with the conventional external EGR system thus providing an innovative solution to an old problem Achieving exhaust gas recirculation through the ex haust manifold during the intake stroke is a better way of controlling the residual mass fraction during transients because i we eliminate the long transport delay associ ated to the exhaust to intake manifold path and ii we bypass the slow dynamics associated with the intake man ifold lling dynamics Fast transient control of the inter nal exhaust gas recirculation IEGR is only limited by the camshaft phasor dynamics and computational delays At a rst glance this suggests a possibility of better tran sient control of feedgas emissions than can be achieved with the conventional external exhaust gas recirculation EEGR Analysis of the IEGR system in throttled en gines shows however that the IEGR system interacts with the slow intake manifold lling dynamics and can cause in fact unacceptable engine performance In this paper we analyze the nonlinear dynamic behavior of the IEGR system based on the mean value model of an experimental engine equipped with a camshaft phaser We develop a dynamic camshaft tim ing schedule that regulates IEGR while maintaining tran sient engine torque response similar to an engine with zero EGR The torque response of an engine with zero EGR provides the benchmark for the engine performance that we wish to achieve because any level of EGR can cause severe torque hesitation if not well calibrated Modularity of the IEGR control function is another very important requirement for the control design Mod ular IEGR control function will allow its rapid imple mentation in existing real time engine controllers For this reason the IEGR controller architecture that we de velop can be seamlessly added or removed from the pow ertrain controller depending on the platform needs Fig ure shows the developed IEGR control architecture We dynamically schedule the camshaft timing CT that reg ulates IEGR based on the measured throttle angle m and engine speed N Brie y stated here the dynamic schedule consists of i the steady state camshaft timing values CT for all throttle and engine speeds and ii the time constant ct of the rst order di erential equa tion that de nes the transient behavior of IEGR from one steady state point to the next The time constant is ad justed using a nonlinear static feedback of engine speed and throttle position as to meet the torque response re quirements Figure Control structure of the internal EGR system Internal EGR System The mean value model of an experimental engine with variable camshaft timing shows that signi cantNOx reduction can be achieved by allowing the exhaust valve to remain open for a longer period of time during the in take stroke This is achieved by increasing the fraction of exhaust gases that remain into the cylinder and lower the combustion temperature during the subsequent cycle Figure shows the correlation between NOx formation and percentage of external EGR ow in a conventional engine Similarly Figure shows the e ects of camshaft timing CT to the feedgas NOx generation in an IEGR engine Retarding camshaft timing increases the inter nal exhaust gas recirculation IEGR and that results in reduction in feedgas NOx From Figure it is obvious that to reduce feedgas NOx we have to ensure engine operation with maximum CT At the same time CT lowers the volumetric e ciency of the engine as shown in Fig allowing less mass of fresh air into the cylinders The mean mass ow rate of fresh air into the cylinders mcyl is a function of volumetric ef ciency the intake manifold density m the engine The simple relationship increasing camshaft timing CT in creases IEGR is used throughout this paper A mathematical equation describing the relationship between CT and IEGR is not available because it is di cult to measure EGR mass inside the cylinder 0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18 20 22 no [n o] (g /K W −h r) EGR [egr] (%) camset=0,afset=14.15−15,nset=1500,map=0.5−0.6,spark=25−35 Sun Jan 26 20:53:51 EST 1997 Figure E ects of external exhaust gas recirculation EGR to feedgas NOx −10 0 10 20 30 40 50 0 5 10 15 20 25 no [n o] (g /K W −h r) cam [cam] (degrees) egrset=0,afset=14.15−15,nset=1500,map=0.5−0.6,spark=25−35 Sun Jan 26 20:37:14 EST 1997 Figure E ects of camshaft timing CT to feedgas NOx −10 0 10 20 30 40 50 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 V ol E ff [v ol ef f] (u ni tle ss ) cam [cam] (degrees) egrset=0,afset=14.15−15,nset=1500 for different 16bit crank ang−based MAPs Sun Jan 26 19:58:31 EST 1997 map=0.2 map=0.4 map=0.6 map=0.8 map=1 Figure Volumetric e ciency as a function of camshaft tim ing for di erent manifold pressures at RPM displacement Vd and engine speed N