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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 lim- ited. During cold start conditions it is desired to operate IC engines at its maximum retard limit subject to combustion stability con- straints 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 envi- ronmental conditions. The borderline knock limit is controlled by a dual-rate count-up/count-down closed-loop control utilizing in- formation from engine knock sensor signals. A closed-loop con- trol 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 con- trols. MBT timing is also controlled closed-loop using an MBT cri- terion 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. 1063-6536/$25.00 © 2007 IEEE ZHU et al.: CLOSED-LOOP IGNITION TIMING CONTROL FOR SI ENGINES USING IONIZATION CURRENT FEEDBACK 417 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.

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