Engine fuel control with mixed time and event based A/F ratio error estimator and controller

An improved closed-loop feedback fuel control with a model-based A/F ratio estimator, wherein the estimator, controller and portions of the model are updated on a fixed time interval basis, thereby minimizing the impact of the control on event-based throughput. Engine transport delays and oxygen sensor dynamics are modeled to estimate the sensed A/F ratio, and the estimate is compared with the sensed A/F ratio to adaptively adjust the model and to develop a closed-loop adjustment of the commanded fuel amount. The engine transport delay model is carried out on an engine event basis, but the sensor dynamics model is carried out on a time basis to accurately reflect the analog nature of the sensor. The estimator and the controller are also carried out on a time basis to reduce throughput requirements at higher engine speeds, and the control gain is scheduled to account for differences between the engine event and time update rates. The control enables numerous control enhancements, including flexibility to topology variations (such as sensor placement, sensor type and sensor characteristics), ease of calibration, and the ability to easily calibrate and schedule A/F ratio perturbations for catalytic conversion efficiency optimization.

TECHNICAL FIELD
 This invention relates to closed-loop fuel control for an internal
 combustion engine, and more particularly to a control based on a system
 model and air/fuel (A/F) ratio error estimator.
 BACKGROUND OF THE INVENTION
 The need for precise control of A/F ratio in motor vehicles has let to the
 development of controllers in which all or a portion of the engine air
 flow and exhaust system dynamics are mathematically modeled to estimate
 the sensed A/F ratio, and to adaptively adjust both the model and the base
 fuel control based on deviations of the estimated A/F ratio from the
 sensed A/F ratio. See, for example, SAE Paper N. 950846, by Fekete, Guden
 and Powell, entitled Model-Based Air-Fuel Ratio Control of a Lean
 Multi-Cylinder Engine. However, such controls tend to be complex, and when
 updated in time with the engine firing events, present excessive
 computational throughput requirements at higher engine speeds.
 Accordingly, such control strategies tend to be cost-prohibitive for most
 applications.
 SUMMARY OF THE INVENTION
 The present invention is directed to an improved closed-loop feedback fuel
 control with a model-based A/F ratio estimator, wherein the estimator,
 controller and portions of the model are updated on a fixed time interval
 basis, thereby minimizing the impact of the control on event-based
 throughput. Engine transport delays and oxygen sensor dynamics are modeled
 to estimate the sensed A/F ratio, and the estimate is compared with the
 sensed A/F ratio to adaptively adjust the model and to develop a
 closed-loop adjustment of the commanded fuel amount. The engine transport
 delay model is carried out on an engine event basis, but the sensor
 dynamics model is carried out on a time basis to accurately reflect the
 analog nature of the sensor. The estimator and the controller are also
 carried out on a time basis to reduce throughput requirements. at higher
 engine speeds, and the control gain is scheduled to account for
 differences between the exhaust gas measurements, which occur at the
 engine event frequency, and the controller time update frequency.
 The subject control strategy enables numerous control enhancements,
 including flexibility to topology variations (such as sensor placement,
 sensor type and sensor characteristics), ease of calibration, and the
 ability to easily calibrate and schedule A/F ratio perturbations for
 catalytic conversion efficiency optimization.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 Referring to FIG. 1, the fuel control of this invention is principally
 described in the context of an automotive internal combustion engine 10
 having an electronically controlled fuel delivery system 12, and an
 exhaust system 14 including a three-way catalytic converter 16, an
 upstream universal exhaust gas oxygen (UEGO) sensor 18 (also known as an
 analog or wide-range air/fuel sensor), and a downstream switching oxygen
 sensor 20. In certain other embodiments, discussed below, the locations of
 the UEGO and switching sensors 18, 20 may be swapped, or switching sensors
 may be used both upstream and downstream of catalytic converter 16. Other
 sensors depicted in FIG. 1 include a mass air flow (MAF) sensor 22 coupled
 to the engine intake manifold 24, and an engine speed (RPM) sensor 26
 coupled to the engine output shaft 28. Also in the illustrated embodiment,
 the engine 10 has a throttle 30 positioned within the manifold 24 by a
 motor-driven (or alternatively, cable-driven) throttle actuator 32, under
 the control of a driver torque command (DTC) circuit 34 via line 36. The
 DTC circuit 34 may be responsive to various un-depicted elements such as a
 driver-manipulated accelerator pedal, a cruise control circuit, a traction
 control circuit, and so on.
 The remaining elements (also referred to herein as circuits) depicted in
 FIG. 1 relate to the fuel control of engine 10, a represent functional
 blocks of hardware and/or software residing within a microprocessor-based
 engine control module 40. In the illustrated embodiment, the engine
 control module 40 is responsible for suitably activating the engine fuel
 delivery system 12 via control lines 42. The fuel control acts primarily
 in response to the DTC signal, which is applied as one of several inputs
 to an Open-Loop Fuel Control (OLFC) circuit 44 via line 46. The OLFC
 circuit also receives the MAF signal via line 48, and operates in an
 open-loop and generally conventional manner to produce a commanded A/F
 ratio (CAFR) signal on line 50, and a corresponding fuel injection base
 pulse width (BPW) signal on line 52. Alternate control strategies based on
 engine speed and intake manifold pressure (instead of MAF) are also
 conventional and well known. Various methods of scheduling the commanded
 A/F ratio CAFR are also well known in the art, and not described here. The
 BPW signal on line 52 is modified by a vector of multipliers 54 to produce
 an adjusted pulse width (PAPW) signal on line 56, as described below. In
 turn, the APW signal is corrected for transient fueling conditions by the
 Transient Fuel Controller (TFC) 58 (which alternatively, may be
 implemented within the OLFC 44), and further adjusted for
 cylinder-to-cylinder variations of engine 10 by the Individual Cylinder
 Fuel Controller (ICFC) 60, which generates the individual fuel control
 signals on lines 42. The controllers 58 and 60 may implement any of a
 number of known and conventional control strategies, and are not
 particularly relevant or critical to the control of the present invention.
 The control of this invention is concerned primarily with the development
 of a suitable closed-loop feedback term which is applied to the multiplier
 54 for the purpose of adjusting the delivered fuel quantity so that the
 A/F ratio at the catalytic converter 16 will actually correspond to the
 commanded A/F ratio CAFR, or in the preferred embodiment, to a perturbated
 version (CAFR') of CAFR. The control involves utilizing a model-based
 estimator 62 to estimate the A/F ratio that should be sensed by the UEGO
 sensor 18, and to develop a leading control error (CE) signal on line 63
 based on the deviation between the estimated and sensed A/F ratios; and a
 controller 64 for developing a closed-loop multiplier (CLM) on line 65
 based on the control error CE signal on line 63 and the engine speed
 signal RPM on line 27. The estimator 62 and controller 64 are described in
 detail below in reference to FIGS. 3 and 4, respectively. In the
 illustrated embodiment, the closed-loop multiplier CLM is applied as an
 input to a block learn module (BLM) 66, which includes a number of fuel
 correction tables that are adaptively adjusted based on the CLM, resulting
 in the generation of a closed-loop feedback signal on line 67 for
 application to multiplier 54.
 Perturbation of the CAFR is customarily practiced in automotive engine
 controls as a means of enhancing the conversion efficiency of a catalytic
 converter. The perturbation frequency and amplitude characteristics are
 typically determined experimentally and indirectly via control gains for a
 given powertrain configuration, but the techniques employed to identify
 the optimal characteristics vary widely, and typically entail considerable
 calibration effort. Accordingly, a significant aspect of this invention
 resides in the implementation of perturbation circuits 68 in the overall
 control strategy described above. As fully described below in reference to
 FIG. 2, the perturbation circuit (PERT) 68 enables direct control of the
 perturbation frequency, amplitude, and bias offset. The PERT circuit 68
 generates a perturbated commanded A/F ratio (CAFC') signal on line 69 for
 application to the estimator 62, and a corresponding perturbation
 multiplier (PM) on line 70 for application to the multiplier 54. As a
 result, the estimator 62 and controller 64 cooperate to produce a
 closed-loop multiplier (CLM) that causes the scheduled A/F perturbations
 to occur at the sensing location of UEGO 18. An additional, but related,
 function of perturbation circuit 68 concerns an on-board development tool
 for sweeping various combinations of perturbation frequency and amplitude
 in order to streamline the calibration process.
 Referring specifically to FIG. 2, the perturbation circuit 68 includes a
 square-wave generator 72 and an amplitude sweep tool 74. The output of the
 square-wave generator on line 76 is added to the commanded A/F ratio CAFR
 in summer 78 to form the perturbated version CAFR' on line 69. The
 perturbation multiplier PM on line 70 is obtained by dividing CAFR by
 CAFR' in the arithmetic block 80. The various inputs designated PF, BO and
 PA are calibration constants utilized by the square-wave generator 72, and
 correspond to the desired perturbation frequency (PF), perturbation
 amplitude (PA) and bias offset (BO). The perturbation frequency PF is
 applied to a frequency generator 82, which generates a corresponding clock
 signal (C) to trigger a state change of the components within the boses 74
 and 84. The coupled memory 86 and flip-flop 88 provide a first input of
 alternating polarity to the multiplier 90, and the perturbation amplitude
 PA forms a second input. The alternating polarity output of multiplier 90
 is added to the bias offset BO by summer 92 to form the square-wave
 generator output on line 76. With this simple arrangement, the calibration
 engineer can independently adjust the PF, BO and PA to achieve the best
 catalytic conversion efficiency, thereby significantly reducing the
 calibration effort, compared to prior control arrangements. In practice,
 PF, BO and PA are scheduled by table look up as a function of engine
 operating conditions, such as exhaust gas flow rate and temperature.
 The amplitude sweep tool 74 is calibration tool that provides a
 perturbation amplitude signal on line 94 for selectively overriding the
 calibrated perturbation amplitude PA. The switch 96 is used to select the
 desired perturbation amplitude input for square-wave generator 72; in the
 indicated position, the calibrated PA value is selected, while in the
 opposite position, the amplitude signal on line 94 is selected. The
 amplitude sweep tool 74 is designed to sweep the amplitude signal on line
 94 between base and maximum amplitude values BA, MA, upon activation of
 the amplitude reset (AR) input. The calibration engineer can use the
 amplitude sweep tool 74 to quickly determine the optimum combination of PF
 and PA by sweeping the amplitude signal for each of a number of PF
 settings, while monitoring the conversion efficiencies for specified
 exhaust gas constituents. When the optimum settings are determined, the
 optimum PA and PF settings are stored, and the switch 96 is positioned as
 shown in FIG. 2.
 Within the amplitude sweep tool 74, a multiplier 98 is reset to zero by
 either of the AR input and the compare circuit 100. The multiplier output
 is supplied to a memory 102, and the output of memory 102 is supplied
 along with the step rate (SR) 104 to the summer 106. The output of summer
 106 is supplied as an input to multiplier 98, and is added to the base
 amplitude (BA) by summer 110 to form the perturbation amplitude signal on
 line 94. When the perturbation amplitude signal on line 94 reaches the
 maximum amplitude (MA) 112, the compare circuit 100 resets multiplier 98,
 to begin a new amplitude sweep.
 Referring now to FIG. 3, the estimator 62 comprises an engine delay mode
 120, a UEGO sensor dynamics model 122 and a bias estimator 124. The engine
 delay model imparts a variable engine event-based delay to the perturbated
 command A/F ratio CAFR' on line 50, producing a delayed version CAFR'(d)
 on line 126 corresponding to the expected A/F ratio upstream of catalytic
 converter 16 at the location of the upstream sensor 18. The signal
 CAFR'(d) is adjusted by summer 128 in accordance with a bias estimator
 feedback signal on line 130, forming a corrected estimate of the A/F ratio
 in the exhaust upstream of the catalytic converter 16, designated as
 EAFRexh. The output of summer 128 is then applied as an input to sensor
 model 122, which models the sensor dynamics and produces a signal EAFRsen
 on line 132 corresponding to the estimated A/F ratio at the location of
 UEGO sensor 18. The actual output voltage of sensor 18 (designated as V18)
 is sampled on an engine firing event basis, and is combined in summer 134
 with a rear trim bias voltage (RBV) developed by the rear trim circuit 136
 in response to the output voltage of downstream switching sensor 20
 (designated as V20). The rear trim circuit 136 may be conventional in
 nature, and serves to calibrate the UEGO sensor 18 relative to the voltage
 target of rear switching sensor 20. The table 138 converts the trimmed
 oxygen sensor voltage on line 140 to a measured A/F ratio, designated as
 AFRm. In bias estimator 124, the summer 142 determines and A/F ratio error
 (AFRerr) according to the difference between AFRm and EAFRsen. Integral
 and proportional feedback terms based on AFRerr are combined in summer 144
 to form the above-mentioned bias estimator feedback signal on line 130.
 And finally, the integral feedback term is divided by CAFR'(d) in
 arithmetic circuit 146 to form the control error CE signal on line 63.
 The engine delay model is engine event based, and constructs a variable
 delayed version of CAFR' by storing successive samples of CAFR' in
 successive registers of delay unit 150, and selecting an appropriate
 sample for application to line 126 via selector 152. The selector 152, in
 turn, is controlled by a calibrated front sensor delay FSD value (0, 1 . .
 . N) on line 154. The FSD values are selected by the calibration engineer
 based on measured on estimated independent parameters of engine 10, and in
 practice, are scheduled by table look up as a function of engine operating
 conditions representative of exhaust mass flow rate. This calibration can
 be facilitate by setting the perturbation frequency PF of perturbation
 circuit 68 to a very low frequency, and counting the number of engine
 events required for the perturbations to reach the UEGO or switching
 sensors 18, 20.
 The sensor model 122 mimics the dynamics of UEGO sensor 18 by filtering
 EAFRexh with a time-based first-order filter defined by the calibration
 values .DELTA.T and TC. The term .DELTA.T is the filter update time
 increment (corresponding to clock frequency C), and TC is the filter time
 constant. The block 155 (U) represents a unity offset. The arithmetic unit
 156 divides the update time increment .DELTA.T by TC, and supplies the
 result to multiplier 158 and summer 160. The summer forms a difference
 between .DELTA.T/TC and the offset U, and supplies the result to
 multiplier 162, which also receives a previous value of the model output
 from memory 164. The model input EAFRexh is multiplied by .DELTA.T/TC with
 multiplier 158, and the result is summed with the output of multiplier 162
 in summer 166 to form the output signal EAFRsen on line 132.
 As mentioned above, the estimator bias feedback signal on line 130 is
 formed in bias estimator 124 by combining proportional and integral
 feedback terms in summer 144. The proportional term is simply obtained by
 applying the proportional gain (Gp) 170 to the error AFRerr. The integral
 term is obtained by applying the integral (Gi) gain 172 to the error
 AFRerr, and summing the result in summer 174 with a previous value of the
 integral term, supplied by memory (M) 176. The arithmetic unit 146
 normalizes the integral term relative to CAFR(d) to form the control error
 signal CE. As a result, the control error signal CE represents a
 percentage A/F ratio error, allowing the controller 64 to use the same
 gains for A/F ratio operating range.
 FIGS. 4A and 4B, taken together, depict an alternate embodiment of
 estimator 62, designated as 62', which is adaptable to mechanizations in
 which the UEGO sensor 18 is located either upstream or downstream of the
 catalytic converter 16. In certain instances, locating the sensor 18
 downstream of the converter 16 facilitates combining the senor 18 with a
 NOX sensor, for example. FIG. 4A mirrors FIG. 3, except for the
 simplification of elements 120, 122, 124 and the addition of a catalytic
 converter model 180 and a switch 182. In common respects, the reference
 numerals used in FIG. 3 have been repeated. FIG. 4B depicts the catalytic
 converter model 180 in detail. With the switch 182 in the position
 indicated in FIG. 4A, the bias estimator 62' is equivalent to the bias
 estimator 62 depicted in FIG. 3. However, when the switch 182 is
 positioned to contact the terminal 184, the catalytic converter model 180
 is interposed between summer 128 and the sensor model 122. In such event,
 the input into sensor model 122 on line 185, represents the estimated A/F
 ratio at the outlet of catalytic converter 16, on EAFRcatout.
 Referring to FIG. 4B, the catalytic converter model 180 mimics the dynamics
 of catalytic converter 16 by filtering EAFRexh with a time-based
 first-order filter defined by the calibration values .DELTA.T and TC. The
 term .DELTA.T is the filter update time increment, TC is the filter time
 constant (which of course is different than the time constant used in
 sensor model 122). The arithmetic unit 186 divides the update time
 increment .DELTA.T by TC, and supplied the result to multiplier 188 and
 summer 190. The summer forms a difference between .DELTA.T/TC and the
 unity offset (U) 191, and supplies the result to multiplier 192, which
 also receives a previous value of the model output from memory 194. The
 model input EAFRexh is multiplied by .DELTA.T/TC with multiplier 188, and
 the result is summed with the output of multiplier 192 in summer 196 to
 form the output signal EAFRcatout on line 185.
 Referring to FIG. 5, the controller 64 develops a time-based closed-loop
 feedback multiplier CLM on line 65 for modifying the base fuel pulse width
 BPW, forcing the A/F ratio at the senor 18 to conform with the CAFR'. The
 feedback multiplier comprises proportional and integral terms, which are
 applied to the summer 200. The proportional term is obtained simply by
 applying the proportional gain Gp 202 to the control error CE signal on
 line 63. The integral gain term is formed by applying an integral gain to
 the control error CE in multiplier 204, and summing the result in summer
 206 with a previous value of the integral term, supplied by memory 208.
 The previous integral term value supplied by memory 208 is limited to
 predefined minimum and maximum values, as indicated by limiter 210. The
 output of summer 200 (and hence, the output of controller 64) is also
 limited to predefined minimum and maximum values by the limiter 212, and
 the output of limiter 212 is applied to an arithmetic unit 214, which
 converts the limited feedback signal into the closed-loop multiplier CLM.
 The integral gain comprises first and second components determined
 respectively as a function of the control error CE and the engine speed
 RPM. The first component, Gi(err), generated by table 216, is high for
 large values of control error CE, but rapidly decreases when the magnitude
 of the control error CE falls below a threshold, as indicated by the table
 graph. Thus, the feedback is high for aggressive closed-loop fuel
 correction when fast dynamic response is needed, and low for stability
 enhancement when the A/F ratio is at or near the commanded value. The
 second component, Gi(rpm), generated by table 218, progressively increases
 with engine speed RPM so that the closed-loop multiplier CLM generated by
 the time-based controller 64 matches the dynamics of the fuel delivery
 system 14 (and the corresponding rate of new information at UEGO 18),
 which is inherently engine event-based. Thus, the integral gain component
 Gi(rpm) is low at low engine speeds when the time increment rate of the
 controller 64 exceeds the event rate of engine 10, and high at high engine
 speeds when the event rate of engine 10 exceeds the time increment rate of
 the controller 64.
 Calibration of the controller gains for disturbance rejection can be
 facilitated by replacing the perturbated input CAFR' to estimator 62 on
 line 69 with the un-perturbated signal CAFR on line 50, while retaining
 the perturbation multiplier input PM to multiplier 54 on line 70. This
 produces a known and controllable (via perturbation circuit 68) A/F ratio
 disturbance for judging the suitability of the controller gains.
 In summary, the control of this invention provides a practical and
 cost-efficient implementation of a model-based A/F ratio estimator by
 carrying out the slow and calculation-intensive portions of the control on
 a time basis, adjusting the response of the control to match the
 event-based engine fuel delivery system. In addition to accurate A/F ratio
 control, the control topology allows an easily calibrated method of
 perturbation the controlled A/F ratio, and permits the flexibility to
 adapt the control to different powertrain mechanizations. Calibration of
 the perturbation schedule is also facilitated by the on-board catalyst
 sweep tool, which eliminates the need for special external equipment
 during development. Further, direct control of the perturbation
 characteristics facilitates calibration of the engine delay model and the
 controller gains. While the present invention has been described in
 reference to the illustrated embodiments, it is expected that various
 modification in addition to those mentioned above will occur to those
 skilled in the art. For example, the various sensor models may be enhanced
 to represent more complex dynamic behavior, the calibration sweep tool
 could be designed to sweep frequency instead of amplitude, and so on.
 Thus, it will be understood that methods incorporating these and other
 modifications may fall within the scope of this invention, which is
 defined by the appended claims.