Closed-Loop Ignition Timing Control for SI Engines Using Ionization Current Feedback Guoming G

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416 IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 15, NO. 3, MAY 2007 Closed-Loop Ignition Timing Control for SI Engines Using Ionization Current Feedback Guoming G. Zhu, Ibrahim Haskara, and Jim Winkelman, Fellow, IEEE

Abstract—Minimal advance for best torque (MBT) timing for an internal combustion (IC) spark ignition (SI) engine is the minimum advance of spark timing for the best torque or, in other words, for the best fuel economy. But MBT timing is often limited by engine knock in the advanced direction and spark timing is also constrained by partial burn and misfire in the retard direction. It is preferred to operate IC engines at MBT timing when it is not knock limited and at borderline knock limit when it is knock limited. During cold start conditions it is desired to operate IC engines at its maximum retard limit subject to combustion stability constraints to reduce catalyst light-off time. Traditionally, both MBT timing and retard spark limit are open-loop feedforward controls whose values are experimentally determined by conducting spark sweeps at different speed and load points, and at different environmental conditions. The borderline knock limit is controlled by a dual-rate count-up/count-down closed-loop control utilizing information from engine knock sensor signals. A closed-loop control architecture for spark timing is proposed in this paper. Using in-cylinder ionization signals both borderline knock and retard Fig. 1. Feasible region of spark timing. spark limits are regulated using closed-loop stochastic limit controls. MBT timing is also controlled closed-loop using an MBT criterion derived from in-cylinder ionization signals. The proposed control strategy and architecture was experimentally validated on due to the requirement to avoid engine knock. In order to obtain a 3.0-L V6 engine for steady state and slow transient conditions. maximum brake torque it is required to operate the engine at Index Terms—Automotive controls, closed-loop systems, en- the knock limit (borderline knock limit) of the feasible region. gines, ionization, microcontrollers, stochastic systems. Under different operating conditions, in order to reduce cold start hydrocarbon (HC) emissions it is desired to locate the spark timing at the retard limit of the feasible region for fast I. INTRODUCTION catalyst light-off. This is due to the desire to maintain a certain NTERNAL combustion (IC) engines are optimized to meet level of combustion stability. I exhaust emission requirements with the best fuel economy. Fig. 1 shows a typical spark timing feasible region as a func- Spark timing is used as one of the optimization parameters tion of exhaust gas recirculation (EGR) for a desired level of for best fuel economy within given emission constraints. For combustion stability with coefficient of variation (COV) of in- normal operation, engine spark timing is often optimized to dicated mean effective pressure (IMEP) less than 3%, where the provide minimal advance for the best torque (MBT). On the thick dash line represents the engine MBT spark timing, thick other hand, engine combustion stability and knock avoidance dash-dotted line represents the engine advanced (knock) spark requirements also constrain engine spark timing within a limit, and the thick solid line represents the retard spark limit. certain region, called the feasible spark timing region. For It can be observed that knock, MBT, and retard limits vary as a certain operational conditions, it is desirable to operate the function of EGR rate, which makes it difficult to control the op- engine at the borderline of the feasible region continuously. timal spark timing in an open-loop fashion. Further, this feasible For instance, under certain operational conditions engine MBT region varies in shape with different engine operational and en- timing is located outside of the feasible spark-timing region vironmental conditions. In current production applications, MBT timing is an open- loop feedforward control whose values are experimentally de- Manuscript received October 30, 2006; revised January 18, 2007. Manuscript received in final form January 31, 2007. Recommended by Associate Editor I. termined by conducting spark sweeps at different speed and load Kolmanovsky. points, and at different environmental operating conditions. Al- G. G. Zhu and J. Winkelman are with the Advanced Powertrain Systems, most every calibration point needs a spark sweep to see if the Visteon Corporation, Van Buren TWP, MI 48111 USA (e-mail: gzhu1@visteon. com; [email protected]). engine can be operated at the MBT timing condition. If not, a I. Haskara is with General Motors, Warren, MI 48090 USA (e-mail: ibrahim. certain degree of safety margin is needed to avoid preignition or [email protected]). knock during engine operation. Open-loop spark mapping usu- Color versions of Figs. 1, 2, 10, 14, and 15 are available online at http://iee- explore.ieee.org. ally requires a tremendous amount of effort and time to achieve Digital Object Identifier 10.1109/TCST.2007.894634 a satisfactory calibration.

Fig. 2. Closed-loop ignition timing control system.

Existing knock spark limit control utilizes an accelerom- to the knock level (see [4], [13], and [14]). It can also be pro- eter-based knock sensor for feedback control. Due to the low cessed to derive a metric for combustion quality similar to COV signal-to-noise ratio (SNR), conventional approaches are based of IMEP and closeness of combustion to partial burn and mis- on the use of a knock flag signal obtained by comparing the fire limit (see [16]), which can be used as a feedback signal for knock intensity signal of a knock sensor to a given threshold. retard limit control. Engine MBT timing can also be derived The knock intensity signal is defined as the integrated value, from in-cylinder ionization signals similar to the pressure sig- over a given knock window, of the absolute value signal ob- nals (see [3] and [9]). Since MBT criteria derived from pres- tained by filtering the raw knock sensor signal using a band-pass sure and ionization signals are solely based upon observations filter. This knock flag signal is the input to a dual-rate (slow and may change at different operating conditions, the associ- and fast correction) count-up/count-down engine knock limit ated control algorithms still require some dynamometer-based controller. The disadvantage of this control scheme is that it calibration effort. It is clear that the combustion process has to continually takes the engine in and out of knock, rather than be matched with the engine cylinder volume change to attain operating continually at the desired borderline knock limit. In the best torque. The major advantage for the ionization-based addition, at certain operating points knock observability can closed-loop MBT timing control is no additional sensing ele- be severely compromised by engine mechanical noises such ment or assembly steps are required since it uses the spark plug as valve closures and piston slap which may be picked up by as an ignition actuator and a combustion sensor. the accelerometer. Such issues result in conservative ignition This paper proposes a closed-loop ignition control architec- timing that leads to reduced engine performance. ture (see Fig. 2) that combines MBT timing control, knock, and As discussed before, during a cold start, it is desirable to op- retard timing limit control strategies into an integrated one. The erate the engine at its retard spark-timing limit for minimal HC integrated ignition control architecture allows the engine to op- emissions. The retard spark-timing limit is often constrained by erate at its true MBT timing when it is not limited by borderline engine combustion stability metrics such as COV of IMEP. Due knock limit and operate at its borderline knock limit when it is to unavailability of production ready in-cylinder pressure sen- limited by knock. Alternatively, it allows the engine to be oper- sors, the retard spark-timing limit is obtained through an offline ated at its borderline retard limit when it is limited by combus- engine mapping process, leading to conservative calibrations. In tion stability. addition, to accommodate the range of fuels used throughout a This paper is organized as follows. Section II describes market, this calibration is made even more conservative. closed-loop MBT timing control strategy. Section III proposes In recent years, various closed-loop spark timing control the stochastic limit controller architecture, which is used for schemes have been proposed based upon in-cylinder pressure both knock and retard spark timing control. The application of measurements ([1]–[8]) or spark ionization current sensing the proposed MBT timing, borderline knock limit and retard ([9]–[11], and [16]). Based upon test data, it has been found spark limit closed-loop ignition control on a 3.0-L V6 engine is that the peak cylinder pressure (PCP) usually occurs around described in Section IV. Section V adds some conclusions. 15 after top dead center (TDC) at MBT timing (see [9]). The 50% mass fraction burned (MFB) point generally occurs II. MBT TIMING CONTROL STRATEGY between 8 and 10 after TDC when MBT timing is achieved. Closed-loop MBT timing control using in-cylinder pressure The algorithm published in [3], controls PR(10) (normalized feedback was described in [5] and [8], and closed-loop igni- pressure ratio of in-cylinder and motoring pressures at 10 after tion timing control using ionization feedback was presented in TDC) around 0.55 to obtain the MBT timing. [10]. This section describes closed-loop MBT timing control, Due to the recent advance of electronics technology, ioniza- using a composite MBT criterion derived from an ionization tion current can be detected at 15 A with very low background signal as a feedback signal. For the closed-loop MBT timing noise. The high quality of an in-cylinder ionization signal makes control strategy, an individual cylinder ionization current was it possible to derive a linear knock intensity that is proportional sampled at every crank degree and processed every combustion 418 IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 15, NO. 3, MAY 2007

can be correlated to the maximum acceleration point of the net pressure and this point is usually between 10% to 15% MFB. The inflection point right before the second peak of the ionization signal (called the second inflection point, see Fig. 3) corre- lates well with the maximum heat release point and is located right around the 50% MFB location. Finally, the second peak location is related to the peak pressure location of the pressure signal (see Fig. 3). As described in [8] and [9], at MBT timing the maximum acceleration point of MFB (MAMFB) is located at around TDC; the 50% MFB location (50%MFB) is between 8 and 10 after TDC; and the peak cylinder pressure location (PCPL) is around 15 after TDC. Using the MBT timing criteria relationship between in-cylinder pressure and in-cylinder ionization signal, these three MBT timing criteria (MAMFB, 50%MFB, and PCPL) can be obtained using an in-cylinder ionization signal. It is well known that the second peak of the ionization signal is Fig. 3. Typical ionization signal as function of crank angle. mainly due to the high in-cylinder temperature during the combustion process. In the case that in-cylinder temperature cannot reach the reionization temperature threshold, the second peak event to generate both composite MBT timing feedback crite- of the ionization signal may disappear. For example, when the rion and closed-loop MBT timing control output. This section engine is operated either at the idle condition, with very high describes the “MBT timing signal calculation” and “closed-loop EGR rate or with lean air to fuel (A/F) mixture or a combi- MBT timing control” blocks in Fig. 2. nation of the previous, the flame temperature is relatively low and the temperature could be below the reionization tempera- A. Full Range MBT Timing Detection ture threshold. Therefore, the second peak may not appear in The mass fraction burned (MFB) is determined by the the ionization signal. well-known Rassweiler–Withrow [2] method, established in Previously mentioned MBT timing correlations over the en- 1938, through pressure measurement. Through MFB, one can tire operating range were presented in [8], [11], and [12], and find when the combustion has its peak burning velocity, ac- are omitted here for brevity. During this study, it was observed celeration, and percentage burn location as a function of crank that the following three cases cover all the possible ionization angle. Maintaining these critical events at a specific crank angle signals over the speed and load map: produces the most efficient combustion process. In other words, Case 1) normal ionization waveform, both peaks are present the MBT timing can be found through these critical events. In in this waveform; [8], instead of directly using the MFB, the connection between Case 2) first peak ionization signal, low combustion temper- MFB and net pressure is utilized to simplify analysis. The net ature resulting in no second peak; pressure and its first and second derivatives are used to rep- Case 3) second peak ionization signal, high engine speed resent the distance, velocity, and acceleration of the combus- such that the first peak merges with the ignition tion process. References [8] and [9] show that PCP location, signal due to the relatively longer ignition duration 50% MFB location, and maximum acceleration location of as a result of a relatively constant spark duration at the net pressure can each be used as MBT timing criteria for high engine speed. closed-loop control. Fig. 4 shows a representative example from each of the Fig. 3 shows a typical ionization signal versus crank angle three cases. For the Case 1) example, the engine was operated and the corresponding in-cylinder pressure signal. Different at 1500 r/min with 2.62 bar brake mean effective pressure from an in-cylinder pressure signal, an ionization signal actually (BMEP) load and without EGR, for the Case 2) example the shows more detailed information about the combustion process engine was running at the same condition as Case 1) except through its waveform. It shows when a flame kernel is formed with 15% EGR, and for the Case 3) example the engine was and propagates away from the gap, when the combustion is running at 3500 r/min with wide open throttle (WOT). accelerating rapidly, when the combustion reaches its peak It is clear from Fig. 4 that three MBT timing criteria burning rate, and when the combustion ends. A typical ioniza- (MAMFB, 50%MFB, and PCPL) are available only in Case 1), tion signal usually consists of two peaks. The first peak of the and for Cases 2) and 3), only one or two criteria are available. ion signal represents the flame kernel growth and development, This indicates that at some operating conditions, only one or and the second peak is the reionization due to the in-cylinder two MBT timing criteria can be obtained for MBT timing temperature increase resulting from both pressure increase and feedback. The proposed MBT timing estimation method com- flame development in the cylinder. bines all MBT timing criteria available at current operational The use of an ionization signal for MBT timing detection was condition into one single composite criterion for improved studied in [12]. As described in [12], the inflection point right reliability and robustness. The detailed algorithm is described after the first peak (called the first inflection point, see Fig. 3) in Section II-B. ZHU et al.: CLOSED-LOOP IGNITION TIMING CONTROL FOR SI ENGINES USING IONIZATION CURRENT FEEDBACK 419

TABLE I COEFFICIENT SELECTION MATRIX

Step 4: Composite MBT Timing Criterion Generation: The composite MBT timing criterion is calculated based upon the case number identiﬁed from Step 2. For all three cases, the composite MBT timing criterion can be calculated using the following equation:

Fig. 4. Three cases of ionization waveforms.

where B. MBT Timing Detection Algorithm In order to implement the MBT timing estimation strategy (2.2) using an in-cylinder ionization signal, a detection algorithm was developed. The MBT timing detection algorithm can be divided By definition, the composite MBT timing criterion is equal into the following four steps. to zero when the engine is running at its MBT timing condition Step 1: Ionization Signal Conditioning: For each given since the MBT timing criteria MAMFB is zero and the MBT cylinder, the ionization signal is sampled at every crank degree criteria 50%MFB and PCPL are shifted from their nominal after the ignition coil dwell event for 120 , as the ionization locations defined by and , signal disappears after 120 crank angle degrees. The sampled respectively. At MBT timing, both and ionization signal is conditioned by a low-pass filtering vary slightly (a few degrees) as a function of to improve the accuracy of detecting the first and second peaks engine operational conditions and they are a function of engine and inflection points. In order to minimize the phase shift speed and load. They can be obtained through an existing effects due to low-pass filtering for improved MBT timing calibration process (no extra calibration needed). Coefficients estimation, a two-way low-pass filtering technique is used, see , , and are selected based upon [8] for details. Note that the complete ionization signal array is Table I for a given case number. available for computing the spark timing control for the next C. Closed-Loop Control Strategy combustion event, so it is possible to perform this noncausal calculation. The ionization vector is filtered by the first-order The purpose of closed-loop MBT timing control is two-fold: forward filter defined as follows: keeping the engine operating at its MBT spark timing if it is not knock limited and reducing the cycle-to-cycle combustion vari- (2.1) ation through closed-loop spark timing control [8]. The control strategy associated with the “MBT timing signal calculation” where is the digital filter parameter associated with the low- and “closed-loop MBT timing control” blocks, shown in Fig. 2, pass filter bandwidth; and then the index of the ionization vector is discussed as follows. is reversed and filtered by again. The combined filtering Inputs to the “MBT timing signal calculation” block (see transfer function has zero phase delay, see [8]. Fig. 2) are the individual in-cylinder ionization signals synchro- Step 2: Operational Condition Identification: In this step, nized with engine crank angle and the current engine operational the engine operational condition is identified, and the resulting information such as the engine speed, load, etc. Speed and load output of this step is the determination of which case the sam- information are lookup table inputs for calculating MBT timing pled ionization signal belongs to, that is Cases 1), 2), or 3). A offsets ( and ) as well as the pattern recognition algorithm is used for the case identification MBT feedforward ignition timing. The output of the “MBT by using the calculated number of peaks, inflection points, and timing signal calculation” block are the composite MBT timing their distances from end of ignition. criterion for the current cylinder using the proposed al- Step 3: MBT Timing Criteria Calculation: After the ioniza- gorithm from Section II-B. tion signal case is identified, MBT timing criteria can be calcu- The closed-loop spark MBT timing control is realized using lated using a peak location detection algorithm. The inflection a proportional–integral (PI) controller whose output is used to location detection logic is implemented by applying a peak lo- correct the feedforward MBT ignition timing. The error between cation detection algorithm to the derivative of the filtered ion- MBT criterion reference and the composite MBT timing crite- ization signal. rion is used as an input to the PI controller. Under the study 420 IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 15, NO. 3, MAY 2007

Fig. 6. Ion integration location PDF. Fig. 5. Ionization signal and integration location. conditions used in this work, the MBT criteria reference signal was set to zero. However, this is not necessary. For example, under certain conditions, to meet emission requirements it may be desired to retard spark timing from its MBT timing by a few degrees. Under these conditions, the reference signal can be set to a negative value.

III. STOCHASTIC IGNITION LIMIT CONTROL This section describes a stochastic ignition limit control algorithm that is used for both advanced and retard ignition timing limit controls.

A. Combustion Stability Criterion This subsection discusses how to generate a feedback measure for the retard limit control, which is the functionality of the “Combustion stability calculation” block shown in Fig. 2. A typ- Fig. 7. Mean and standard deviation of integration locations. ical ionization signal with its integration window is displayed in Fig. 5. Before defining the combustion stability criterion, let us define an integral ratio function as follows: firing events at the same spark timing) of data are used to create the probability density function (PDF) or histogram of the integration location, where the solid line is a Gaussian distribution fit of PDF data. (3.1) Based on the PDF shown in Fig. 6, the statistics of the ionization integration locations seem to be close to a Gaussian random where is a 120 element ionization vector sampled at the process. As the spark timing becomes more retarded, the PDF of start of ignition with one crank degree resolution (same vector integration location starts skewing towards the retard direction for the MBT timing estimation), is the crank angle index at (see [16] for details). But more importantly, at the spark timing the start of integration window (see Fig. 5), and is the crank with a desired combustion stability level, the PDFs are close to degrees representing the integration window width. The inte- a Gaussian random process. gration location (IL) of a given percentage is an integer The mean and standard deviation of the ionization integra- that satisfies the following equation: tion locations (90%) during a spark sweep at 1500 r/min with 2.62 bar BMEP are shown in Fig. 7, where stars represent the (3.2) test data and the solid lines are fitted curves using polynomials. It can be observed that both mean and standard deviation of in- Fig. 5 shows a 90% integration location . Note that, tegration location increases as the spark timing retards. 100% integration location is ideally reached at the end of the integration window. B. Knock Intensity Calculation Fig. 6 shows the stochastic properties of with spark This subsection is associated with the “Knock intensity timing at 21 before TDC. 300 cycles (number of consecutive calculation” block in Fig. 2. Under engine operational condi- ZHU et al.: CLOSED-LOOP IGNITION TIMING CONTROL FOR SI ENGINES USING IONIZATION CURRENT FEEDBACK 421

Fig. 9. Mean and standard deviation of knock intensity. Fig. 8. Knock intensity PDF.

C. Stochastic Limit Control tions that result in knock, knock intensity can be calculated The objective of the stochastic limit controller is to provide using in-cylinder ionization signals in a similar way to using dynamic ignition-timing limits for the overall spark controller an in-cylinder pressure sensor signal (see [4], [13], and [15]). to avoid engine knock in advanced direction or to assure com- Knock intensity calculation utilizes the high frequency combustion stability in retard direction. This subsection describes ponent (between 3 and 15 kHz corresponding to the knock the strategies of both the “Stochastic knock limit control” and frequency range) of ionization signal over a given knock the “Stochastic retard limit control” blocks in Fig. 2. Since the window defined in Fig. 5. An analog circuit was used to stochastic knock limit controller structure is the same as the re- calculate knock intensity. Define as the analog tard limit control, it will not be discussed in detail. ionization signal and the band-pass filtered Fig. 10 shows the architecture of the closed-loop stochastic ionization knock signal that is obtained by filtering limit controller for the retard limit application. Inputs to this using a fourth-order Butterworth band-pass filter with corner controller block are user specified confidence level targets, made frequencies of 3 and 15 kHz, respectively. The knock intensity up of two parts (reference confidence number and level can be calculated using the following formula: ), and the stochastic limit feedback signal . The control objective is to maintain a reference confidence number (percentage) of the controlled feedback signal (3.3) below the reference confidence level . There are three main feedback actions of the proposed control scheme shown in Fig. 10. Their functionalities are discussed as where is the time corresponding to the beginning of the knock follows. window defined in Fig. 5 and is the time associated with the Adaptive Seeking Feedback (Thin Black Lines): The purpose end of the knock window. Fig. 8 shows a PDF of knock inten- of this loop is two-fold: reducing the calibration conservative- sity signal obtained from the ionization signal, where an analog ness by providing the regulation set point with its “TRUE” mean circuit was used for calculating knock intensity . The en- target value and improving robustness of stochastic limit con- gine was operated at 1000 r/min with WOT. The spark timing is troller when the engine operates under different conditions. This at 18 before TDC. Comparing the PDF drawings of Figs. 6 and control loop is associated with two blocks in Fig. 10. They 8, the knock intensity PDF histogram is not symmetric and it is are the “stochastic analyzer” and the “adaptive seeking algo- obvious that a Gaussian random process cannot approximate it. rithm” blocks. The “nominal mean target” block consists of In fact, [20] shows that the knock intensity is a log-normal a multi-dimensional lookup table using reference confidence random process. number , engine speed, and load as input, and the output The mean and standard deviation of knock intensity is the estimated mean target from a calibration table. during a spark sweep at 1500 r/min with 2.62 bar BMEP are is the desired value for the mean of the feedback signal. The shown in Fig. 9, where stars represent the test data and the solid “stochastic analyzer” block forms a buffer of lines are fitted curves using polynomials. It can be observed with a calibratable length (number of consecutive combus- that both mean and standard deviation of integration location tion events). At each event, the oldest date is replaced by the decreases as the spark timing retards. For stochastic limit con- new one in the buffer. The mean of is calculated by trol, we use the mean of knock intensity and the information contained in PDF such as percentage of knock intensity below (3.4) a given threshold as the feedback signals. 422 IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 15, NO. 3, MAY 2007

Fig. 10. Stochastic closed-loop retard limit controller. and actual confidence number can be calculated by generated by a lookup table using the error signal as input. When the error is greater than zero, the output is zero, and when the error is less than zero, the output is positive and increases as the input becomes more negative. (3.5) Feeding the instant correction to integral term is equivalent to where if , otherwise, . the role of counter up/down logic. Instant correction refers to a The actual confidence level of a given confidence spark retard action at the next combustion event; the gain de- number is another parameter of interest. Define termines how much spark retard is generated for the excessive as a reordered vector of with its elements arranged in an knock intensity value over the threshold. increased order. Then the actual confidence level can be defined For knock limit control, the same controller structure was as follows: used. The stochastic limit feedback signal is replaced by calculated , and the details can be found in [15]. (3.6) The interaction of the stochastic limit controllers with the MBT controller is managed by the “spark timing limit man- where is the closest integer of . The “adaptive agement” block in Fig. 2. This interaction may be illustrated seeking algorithm” utilizes adaptation error using the retard limit control as an example. If the baseline spark as input, and the output is mean target correction (MTC) ob- timing is more advanced than the current retard limit, then the tained by integrating the adaptation error with a calibratable baseline spark is used as it is. In that case, the retard limit con- gain. This control loop is used to reduce the conservativeness troller pushes the limit in the maximum retard direction by itself. of the mean target for the regulation controller discussed This is due to the fact that the limit controller integrator has a next. negative input and keeps integrating until the maximum retard Regulation Stochastic Feedback (Thick Black Line): The reg- allowed is reached (an anti-windup scheme is used). If the base- ulation loop is used to regulate the mean value of the stochastic line spark controller pushes the spark timing to a level at which limit feedback signal to a mean target value. The regulation con- the feedback signals generate corrections, then the retard spark troller is structured as a PI controller and a feed-forward term limit moves from its maximum retard spark limit to a less retard based on engine operating conditions. The error input to the PI level as a variable saturation limit on the baseline spark. On the controller is other hand, if the baseline spark continues pushing the spark in (3.7) the retard direction even when baseline spark is saturated, the seeking and instant correction actions of the retard controller Despite the variability of the stochastic retard limit feedback will adjust the retard limit online. signal its mean value is a well-behaved signal for regulation purposes. The regulation controller is tuned to pro- IV. EXPERIMENTAL STUDY vide the desired settling time and steady-state accuracy for the The experimental study of the proposed spark timing control response. strategy consists of three subsections. These subsections present Instant Correction Feedback (Thick Grey Line): This block the experimental results for closed-loop MBT timing, advanced, calculates an instant correction signal to be fed into the inte- and retard limit controllers. At the end of this section, the results gration portion of the PI controller. The instant correction is of an integrated ignition control system that combines the MBT ZHU et al.: CLOSED-LOOP IGNITION TIMING CONTROL FOR SI ENGINES USING IONIZATION CURRENT FEEDBACK 423

Fig. 11. MBT criterion g from ionization signal. timing controller, along with both advanced and retard limit controllers are also shown. Fig. 12. MBT timing criteria versus timing sweep. The proposed control system was validated in an engine dynamometer. The engine was controlled by the engine dynamometer except for engine spark timing. The engine dynamometer controlled the engine throttle position, EGR rate, and fuel injection. It also controlled the engine speed and load. A rapid prototype controller was used for prototyping both open-loop and closed-loop spark timing control. Laboratory grade pressure sensors were installed in every cylinder for comparing with in-cylinder ionization signals. The digital waveform capture card inside the rapid prototype controller generates the interrupt based upon the crankshaft en- coder pulses that triggers the data sampling process. A calculated crank angle also generates a software interrupt to initiate the combustion event-based closed-loop ignition control calculation. The spark timing is calculated during exhaust stroke of the corresponding cylinder to make the spark control commands available before intake stroke. The closed-loop control algorithms, shown in Fig. 2, run every combustion event. Fig. 13. CL MBT timing control with ionization feedback. A. Closed-Loop MBT Timing Control Before the results of closed-loop MBT timing control are dis- a result of spark-timing sweep when the engine was operated at cussed, results demonstrating the validity of the composite MBT 1500 r/min with 7.0 bar BMEP load. The spark timing varies timing criterion are presented on an example operating point. from 13 to 25 before TDC. The top graph of Fig. 12 shows the Fig. 11 shows the estimated MBT timing criterion when MBT timing criteria for cylinder number three, and the bottom the engine was running at 1500 r/min with 7.0 bar BMEP load, one shows the average MBT timing criteria over all the cylin- and the ignition timing is at its MBT timing (21 before TDC). ders. It is clear that the ionization MBT timing criterion The data shown in Fig. 11 is 100 cycles of data for cylinder is very close to the MAMFB. three only. It can be observed that the MBT criterion confirms From the average MBT timing criteria plot, it can be con- that the engine is operated close to its MBT timing since the av- cluded that the engine MBT timing is around 21 before TDC erage of the calculated is close to zero. This ionization since both and MAMFB are close to zero at that timing. MBT timing detection algorithm was validated over the entire An important observation of both top and bottom graphs is that engine operational map using offline test data. all four MBT timing criteria are almost linear against the spark Fig. 12 shows the relationship between estimated ionization timing sweep, that provides for good closed-loop control char- MBT criterion and in-cylinder pressure MBT criteria acteristics. Test data for the other cylinders is also similar to MAMFB, 50%MFB, and PCPL, where MAMFB is the max- cylinder three. imum acceleration point of MFB calculated from in-cylinder Fig. 13 shows the response of PCP location, 50%MFB pressure signal (see [8]), which is defined as the peak location location, and under closed-loop MBT timing control of the second derivative of MFB. The data shown in Fig. 12 is at 1500 r/min with 7.0 bar BMEP load. It is clear that all 424 IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 15, NO. 3, MAY 2007

process lasted for about 10 min and resulted in almost 10 000 engine cycles. The starting engine coolant temperature is about 34 C and the ending temperature is about 93 C. The top graph of Fig. 15 shows the relationship between engine coolant temperature and engine MBT timing. In order to keep the composite ionization MBT timing criterion at or around TDC location, the ignition timing has to be advanced to compensate for relatively slow combustion when engine is cold. It is clear that during the 10-min warm-up process, the closed-loop MBT timing controller moves the spark timing in a retard direction from around 28 before TDC to 21 before TDC. During this warm-up process, the burn-rate increases and the corresponding MBT spark timing moves back in a retard direction. Fig. 14. Open-/closed-loop variance comparison. The second graph from the top in Fig. 15 shows the average PCP location of all cylinders and the bottom graph shows the average 50%MFB locations for all cylinders. It can be observed that the mean PCP location is between 15 and 16 after TDC during the engine warm-up process, and that the average 50%MFB location for all four cylinders is between 8 and 10 after TDC. This validates that engine operates at its MBT timing during the engine warm-up process, and it also shows that closed-loop MBT timing control using ionization feedback is able to operate the engine at its MBT timing during the temperature transition.

B. Closed-Loop Retard Limit Control During an engine cold start process, the stochastic retard limit manager seeks the maximum retard possible while assuring that misfire and partial-burn are avoided with the objective of increasing the catalyst temperature rapidly to minimize tailpipe Fig. 15. CL control for transient temperature. emissions. Delaying the combustion through high values of ignition retard can shorten the time that it takes the catalyst to reach its light-off temperature. Therefore, the conventional three MBT timing criteria on average remain at their MBT three-way catalyst becomes effective much sooner in reducing locations, respectively. That is, the PCP location is around 14 tailpipe emissions ([17]–[19]). However, if the ignition retard to 16 after TDC, 50%MFB location is around 8 to 10 after is too much, engine-out HC emissions become excessive due TDC, and is close to TDC. The closed-loop controller to incomplete combustion (partial-burn) as well as misfire. An demonstrated in this paper, utilizes ionization signals from open-loop retard calibration needs to provide enough margins all the cylinders continuously to generate a global ignition to avoid misfire under all conditions and with all fuels. It, timing control signal. Constant timing offsets are then used to therefore, is inherently conservative. compensate for individual cylinder unbalance. The PI gains are The calibration of the stochastic retard limit controller (see tuned to have sufficient stability margin. Fig. 10) for cold start retard limit control can be explained as Another aspect of analyzing closed-loop control of en- follows. Suppose that we want to make sure that a given per- gine MBT timing is from a stochastic perspective. It is well centage of the integration locations will not go beyond known that for a linear dynamic system with a stationary a certain crank angle (say, 110 after TDC). Since the integra- stochastic process input, closed-loop controllers, such as a tion location practically represents the end of the combustion, linear quadratic Gaussian (LQG) controller, are able to reduce this is equivalent to saying that for a percentage of the closed-loop system output variances. Fig. 14 shows output consecutive combustion events the combustion will be over be- variances of MBT timing criteria (PCP and 50%MFB loca- fore the desired crank angle (110 after TDC in this example). tions) for both open-loop and closed-loop control. The engine This location is then the desired confidence level target operating condition is the same as the open-loop control case for the feedback control. In this sense, represents the shown in Fig. 11. It is clear that closed-loop control using acceptable combustion retard in crank degrees, which will be ionization-based MBT timing feedback reduces cycle-to-cycle continuously monitored from the ion current processing. Using variances shown in Fig. 14 by about 5%–10% resulting in a the standard deviation of the measured data, a nominal target smoother running engine. mean for the regulation controller is calculated by subtracting a Finally, Fig. 15 shows the results of closed-loop ionization certain multiple of the standard deviation of the measured data in MBT timing control during engine warm-up process. The en- the buffer. That initial mean target is then increased by the adap- gine was operating at 1500 r/min with 64 Nm load. The whole tive seeking loop slowly if the actual confidence level (say with ZHU et al.: CLOSED-LOOP IGNITION TIMING CONTROL FOR SI ENGINES USING IONIZATION CURRENT FEEDBACK 425

Fig. 16. Control for cold start run-up.

Fig. 18. Knock intensity statistics.

Fig. 17. Temperature versus controlled ignition retard.

Fig. 19. Knock intensity actual confidence levels. 90% confidence number) computed from the measured data is less than desired confidence level of 110 after TDC. C. Application to Knock Limit Control Figs. 16 and 17 show responses from a cold-start run using Before applying stochastic limit control to knock limit man- integration location as feedback signal. The 90% confidence agement, knock controllability was studied using ionization level is also included in Fig. 16 as a performance knock intensity feedback. Due to the high-resolution knock measure. Note that it was kept around 110 after TDC at intensity signal obtained from in-cylinder ionization signals, the steady state and did not exceed 124 after TDC during both mean and standard deviation of the knock intensity signal the transient operation, which was the exhaust valve opening show high correlation to engine spark timing (see Fig. 18). timing for the particular engine tested. Therefore, combustion Both mean and standard deviation of knock intensity increase was completed before the exhaust valves were opened, which when the engine spark timing varies from 10 before TDC to is critical for engine HC emissions. Fig. 17 demonstrates the 26 before TDC. This demonstrates good controllability using corresponding exhaust temperature rise during the run. An the knock intensity obtained from ionization signals. The mean open-loop temperature profile was also included in Fig. 17 and standard deviation data is processed using 300 cycle ioniza- to show the improved temperature rise-time with the proposed tion data. Similar results are obtained over the whole speed and control. For the open-loop case, the ignition timing was held at load range of the engine. TDC, which was the initial ignition timing for the closed-loop The knock intensity actual confidence levels , controller. Based on Fig. 17, the time it takes the exhaust , and are shown in Fig. 19. The temperature to reach 500 C was reduced from 18 to 12 s using actual confidence levels of 90%, 95%, and 100% increase as the the closed-loop controller. spark timing advances. The criterion used for adaptive seeking 426 IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 15, NO. 3, MAY 2007

Fig. 21. CL advanced and retard limit control.

intensity staying beyond 0.1 V reference confidence level. At Fig. 20. CL knock limit control. the 60th second, the adaptive seeking algorithm is enabled with a 90% reference confidence number, and the KI percentage over 0.1 V target increases to around 10% (or equivalent to 90% ac- is the actual confidence level with reference tual confidence level). The spark timing is further advanced to confidence number for stochastic limit control. This adaptive between 14 and 13 before TDC. seeking control loop adjusts the reference signal for mean Fig. 21 shows the test results of the combined advance control loop such that the given confidence number percentage (knock) and retard limit control. The thin dark line is the engine of the actual knock intensity signal stays below the baseline spark timing starting at 15 before TDC. Since the target confidence level . engine is neither knock limited nor retard limited, both advance The closed-loop control results of the knock intensity confi- (thick gray) and retard (thin gray) limit stay at their maximum dence number and level, using the proposed stochastic limit con- levels (40 before TDC for advanced limit and 5 before TDC trol of Fig. 10, are shown in Fig. 20, where the top plot shows for retard limit). When the baseline spark timing moves in the both actual mean knock intensity and actual confidence level advanced direction and causes engine knocking, the advanced of knock intensity with reference confidence number 90%. The limit reduces due to the closed-loop knock limit control, the second plot from the top shows spark advance limit and actual baseline spark timing is limited to about 23 before TDC and spark timing, the third shows the instantaneous knock intensity the retard spark limit still stays at its maximum retard limit (5 (KI) signal, and the reference confidence level knock intensity before TDC). At the 30th second, the baseline spark timing at 0.1 V. The bottom plot shows the percentage of KI over the is manually moved in the retard direction. At about the 38th 0.1-V threshold V . Note that, the con- second, the retard limit control moves the retard limit in the trol calibrations ( is 90% and for KI is 0.1 V) set advanced direction due to the reduced combustion stability and the knock limit control objective as keeping 90% of the consec- the retard spark timing stabilizes at about 12 before TDC while utive knock intensity levels below 0.1 V. During the first 18 s, the knock limit controller independently returns the advanced the closed-loop knock limit control is not active, baseline spark limit to its maximum at 40 before TDC due to its integral timing starts at around 13 before TDC, the advanced limit is at action since knock is below the target. This plot demonstrates its maximum of 20 before TDC, and the knock intensity mean both steady state and transitional control utilizing both knock is relatively low (less than 0.1 V). At the 16th second, the base- advanced limit control and combustion stability retard limit line spark timing is manually advanced to 20 before TDC and control. It also shows how each limit control interacts with mean and actual confidence level knock intensity increases to the baseline (or MBT) ignition timing control as independent over 0.40 and 1.4 V, respectively right before the closed-loop timing limits in both directions. knock limit control is activated. After the mean knock limit control is enabled at the 18th second, knock intensity is reduced V. C ONCLUSION to desired knock intensity level and the advanced limit, gen- A closed-loop ignition control architecture is proposed to erated by the closed-loop knock limit controller, moves to the combine three closed-loop ignition control strategies into a range before TDC. Note that the advanced spark is single one. They are closed-loop MBT timing control, border- digitized from the advanced limit with 1 resolution due to the line knock limit control, and retard limit control. The integrated control hardware limitations. Between the 18th and 60th second, ignition control architecture allows the engine to operate at its the bottom plot shows that there is about 5% of the actual knock true MBT timing when it is not limited by borderline knock ZHU et al.: CLOSED-LOOP IGNITION TIMING CONTROL FOR SI ENGINES USING IONIZATION CURRENT FEEDBACK 427 limit and operate at its borderline knock limit when it is knock [19] P. Tunestal, M. Wilcutts, A. T. Lee, and J. K. Hedrick, “In-cylinder limited. During a cold start, the closed-loop controller operates measurement for engine cold-start control,” in Proc. IEEE Int. Conf. Control Appl., 1999, pp. 460–464. the engine at its maximum retard limit for fast catalyst light-off [20] J. D. Naber, J. R. Blough, D. Frankowski, M. Goble, and J. E. while maintaining combustion stability at desired level. The Szpytman, “Analysis of combustion knock metrics in spark-ignited control strategy has been validated under steady state and slow engines,” SAE, Warrendale, PA, Tech. Rep. 2006-01-0400, 2006. [21] I. Haskara, G. Zhu, and J. Winkelman, “Multivariable EGR/spark transient operations and the fast transient tests (such as FTP) timing control for IC engines via extremum seeking,” in Proc. Amer. have not been completed yet. Combining this quasi-steady-state Control Conf., 2006, pp. 1173–1178. oriented ignition timing control strategy with a feedforward controller for improved transient performance remains a work in progress. Guoming G. (George) Zhu received the B.S. degree in mechanical engineering and the M.S. degree in REFERENCES electrical engineering from Beijing University of Aeronautics and Astronautics, Beijing, China, in [1] J. D. Powell, M. Hubbard, and R. R. Clappier, “Ignition timing controls 1982 and 1984, respectively, and the Ph.D. degree method and apparatus,” U.S. Patent 4 063 538, Dec. 20, 1977. in aerospace engineering from Purdue University, [2] G. M. Rassweiler and L. Withrow, “Motion picture of engine flames Lafayette, IN, in 1992. correlated with pressure cards,” SAE Trans., vol. 42, pp. 185–204, May Currently, he is a Technical Fellow at Visteon 1938. Corporation, Van Buren TWP, MI, in engine com- [3] M. C. Sellnau, F. A. Matekunas, P. A. Battiston, C.-F. Chang, and bustion controls. From 1994 to 2000, he worked as D. R. Lancaster, “Cylinder-pressure-based engine control using pres- a Technical Advisor at Cummins Engine Company, sure-ratio-management and low-cost non-intrusive cylinder pressure Columbus, IN, where he developed diesel combustion, charge handling, and sensor,” SAE, Warrendale, PA, Tech. Rep. 2000-01-0932, 2000. after treatment control systems. Over the last six years, he has led various [4] R. J. Hosey and J. D. Powell, “Closed loop, knock adaptive spark timing combustion feedback projects using in-cylinder pressure and ionization current control based on cylinder pressure,” Trans. 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Tagomari, “Cold-start hydrocarbon emissions to the area of power systems. From 1987 to 1997, he was with the Ford in port-injected gasoline engines,” Progr. Energy Combustion Sci., vol. Motor Company, Dearborn, MI, where he was involved in the development 25, pp. 563–593, 1999. of electronic vehicle controls ranging from vehicle speed control to active [18] J. Zhu and S. C. Chan, “An approach for rapid automotive catalyst suspension systems. He managed the Motorsports Electronics Department at light off by high values of ignition retard,” J. Inst. Energy, vol. 69, pp. Ford from 1993 to 1997, where they developed electronic controls for both 167–173, 1996. chassis and powertrain systems.