A method for controlling an air/fuel ratio of an internal combustion engine

Abstract: An exhaust gas oxygen sensor (43,44) is used to control the air/fuel ratio of an internal combustion engine in combination with an electronic engine control. The exhaust gas oxygen sensor (43,44) is positioned in the exhaust stream flow from the engine. The electronic engine control utilizes different air/fuel ratio feedback strategies depending upon whether the signal output from the exhaust gas oxygen sensor (43,44) is saturated indicating a rich air/fuel ratio, saturated indicating a lean air/fuel ratio or operating in a linear region.

Description: This invention relates to electronic engine control of internal combustion engines. It is known to control the air/fuel ratio (A/F) of internal combustion engines using exhaust gas oxygen sensors positioned in the exhaust stream from the engine and an electronic control module coupled to the exhaust gas sensor. Because of the response time of this system and such components as catalysts in the exhaust gas stream, there are occasions when erratic low frequency oscillations occur with feedback from EGO sensors placed after the catalyst. It would be desirable to eliminate such erratic low frequency oscillations. It is known to have A/F feedback systems for engines with exhaust gas oxygen (EGO) sensors placed behind the catalyst in an effort to achieve more precise A/F control with respect to the catalyst window. The rationale for this action is illustrated in Fig. 1A and 1B which show catalyst conversion efficiency and EGO sensor output voltage versus A/F irrespectively for sensors located both in front of and behind a typical catalyst. As this figure indicates, the switch point of the pre-catalyst EGO sensor does not coincide exactly with the catalyst window, whereas the switch point of the post-catalyst sensor generally does. Unfortunately, closed-loop A/F control systems using feedback from a post-catalyst EGO sensor frequently display erratic low-frequency oscillations under certain operating conditions. Two examples of this are illustrated in Figs. 2A and 2B which show plots of post-catalyst EGO sensor output voltage versus time obtained when the engine was operated under closed-loop A/F control using conventional low-gain integral feedback from the post-catalyst EGO sensor. In Fig. 2A, the EGO sensor output voltage shows an erratic low-frequency oscillation of approximately 0.024 Hertz, while in Fig. 2B, the sensor output voltage shows a well-defined oscillation of approximately 0.015 Hertz. Such low-frequency oscillations are somewhat unpredictable, and occur with certain combinations of catalysts and EGO sensors, but not with all. These low-frequency oscillations are undesirable both from an emissions standpoint (because they produce a loss of catalyst conversion efficiency) and from a catalyst monitoring standpoint (because they can cause erroneous indications from the catalyst monitoring system). These are some of the problems this intention overcomes. GB-A-2023885 discloses a closed loop system for the control of the air/fuel ratio of an internal combustion engine includes an open loop air/fuel ratio controller that has a closed loop correction applied to its basic pulse width control signal, the closed loop correction being based upon the bi-level switching of an oxygen sensor detecting a substantially stoichiometric condition in the exhaust gas of the internal combustion engine, and being performed by an integral controller which responds to the switching of the exhaust gas sensor to increase the air/fuel ratio for one level of the sensor and to decrease the air/fuel ratio for the other level, the limit-cycle oscillation developed by the integral controller being modified by increasing the authority and gain rate of the controller as a function of the distance the system is away from a reference point so that it responds to transient conditions rapidly and smoothly. U.S.-A-4625698 discloses a closed loop air/fuel ratio controller for an internal combustion engine is disclosed in which a proportional term is based on a fast filtered air/fuel ratio signal and an integral term is based on a slow filtered air/fuel ratio signal as well as the magnitude of the air/fuel ratio error outside of a deadband and the rich-lean conditions of the fast and slow air/fuel ratio signals. U.S.-A-4251990 discloses an air/fuel ratio control system comprises a first oxygen concentration detector arranged upstream of a three-way catalyst for purifying the exhaust gas in the exhaust system of an internal combustion engine, a second oxygen concentration detector arranged downstream of the three-way catalyst, a discrimination circuit for detecting and discriminating a specific operating condition of the internal combustion engine in response to a signal representing operating factors of the internal combustion engine such as the amount of intake air and vehicle speed, and a changeover circuit impressed with the signals from the first and second oxygen concentration detectors. The changeover circuit produces selectively the detection signal of the second oxygen concentration detector in response to the output signal of the discrimination circuit only when the discrimination circuit detects a predetermined operating condition of the internal combustion engine, and the detection signal of the first oxygen concentration detector when the internal combustion engine is under the other operating conditions. U.S.-A-4878472 discloses a method of controlling the air/fuel ratio of a mixture being supplied to an internal combustion engine, in a feedback manner responsive to an output signal from an O 2 sensor arranged in the engine exhaust system for sensing the concentration of a component in exhaust gases from the engine to produce as the output signal a normally fluctuating output signal indicative of the concentration. The air/fuel ratio is controlled to a desired value by means of at least one of proportional control applying a first correction value to correct the air/fuel ratio when the output signal changes from a rich side to a leap side or vice versa with respect to a predetermined reference value, and integral control applying a second correction value to correct the air/fuel ratio whenevet a predetermined period of time elapses so long as the output signal remains on the lean side or on the rich side with respect to the predetermined reference value. A correcting amount is determined in response to the rate of change in the output signal of the O 2 sensor. The second correction value is to be increased or decreased by means of the correcting amount, depending upon whether the output signal remaining on the lean side or on the rich side. JP-A-6079132 discloses an air/fuel ratio controller which will obtain an error between target air/fuel ratio and actual air/fuel ratio in an error amplifier then an output proportional to said error and an output produced through time integration of said error are added in an adder/amplifier. Thereafter its output is converted by a pulse width operator into pulse width and applied onto a solenoid valve to feed fuel through said solenoid valve to an engine. Here, an operational amplifier having such characteristic as to produce a proportional output for an error input higher than predetermined level but not produce an output for lower level error input is provided in the prestage of a proportional amplifier for producing an output proportional to said error. JP-A-58445 discloses a fuel controller of an engine invention an excess air rate is controlled in such a manner that of exhaust gas is detected by a sensor and fed to a controller further its output signal is collated to an air flow signal to perform proportional plus integral control of injection pulse width. Here for constant proportional and integrating values and I/To, if a speed signal and the signal are changed, a period T of injection pulse changes. In this change, the value P, I is extremely increased at low speed and low intake air quantity time such as idle time, and limit-cycle amplitude of I exceeds a range of the purification window in a three-dimensional catalyser, then the value of P and I/To is controlled to be decreased at the idle time. According to the present invention there is provided a method for controlling air/fuel ratio of an internal combustion engine controlled by an electronic engine control and having and exhaust gas oxygen (EGO) sensor positioned in an exhaust stream flow from the engine, said method including the step of utilising different air/fuel ratio feedback control strategies depending upon whether the exhaust gas oxygen sensor is saturated, rich or lean, or operating in a linear region, the method further including (a) utilising a linearly ramping lean feedback signal, when exhaust gas oxygen output sensor voltage indicates a rich condition, so as to lean out the engine air/fuel ratio as a function of time and determining a lean ramp rate by increasing the ramp rate until a limit-cycle oscillation results and then reducing the ramp rate and (b) utilising a linearly ramping rich feedback signal, when the exhaust gas oxygen sensor output indicates a lean condition, so as to enrich the engine air/fuel ratio as a function of time and determining a rich ramp rate by increasing said ramp rate until a limit-cycle oscillation results and then reducing the ramp rate. A structure in accordance with an embodiment of this invention prevents low-frequency oscillations, such as described above, from occurring with post-catalyst A/F feedback systems. When the EGO sensor output voltage indicates a rich condition (V out > 0.7 volts, for example), the feedback signal would be a linear ramp which slowly leans out the engine A/F as a function of time. When the EGO sensor output voltage indicates a lean condition (V out < 0.15 volts, for example), the feedback signal would be a linear ramp which slowly enriches the engine A/F as a function of time. When the EGO sensor voltage is between the rich and lean limits, the feedback signal would be proportional to the difference between the output of the EGO sensor and an appropriate reference voltage such as 0.45 volts. In addition, in an effort to reduce steady-state offset errors, it may be advantageous to include a small amount of integral feedback along with the proportional feedback when the EGO sensor is between the rich and lean limits. In some applications, it may be desirable to freeze the feedback signal when the EGO sensor is between the rich and lean limits, thereby producing a dead band. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figs. 1A and 1B are graphic representations of the three way catalyst conversion efficiency and the exhaust gas oxygen sensor output voltage versus air/fuel ratio, respectively; Figs. 2A and 2B are plots of post catalyst exhaust gas oxygen sensor voltage versus air/fuel ratio, on a time line; Figs. 3A and 3B are graphic representations of post catalyst exhaust gas oxygen sensor voltage versus air/fuel ratio for pure integral controller and a tri-state feedback controller, respectively; Fig. 4 is a block diagram of a feedback system in accordance with an embodiment of this invention; Figs. 5A, 5B, and 5C are graphical representations of engine air/fuel ratio, EGO sensor output, and feedback control signal with respect to time, in accordance with an embodiment of this invention; and Figs. 6A, 6B, and 6C are graphical representations of engine air/fuel ratio, EGO sensor output, and feedback control signal with respect to time, in accordance with an embodiment of this invention. When an internal combustion engine is operating on the rich side of a catalyst window (i.e., rich of stoichiometry as indicated by a post-catalyst EGO sensor), the output of the EGO sensor is essentially saturated at a "high" output voltage and does not give any meaningful information as to how much the engine A/F is rich of stoichiometry (See Figs. 1A and 1B). The feedback strategy in this case is to simply ramp the engine A/F back toward stoichiometry until the sensor output voltage starts to switch toward its lean state. Since the catalyst presents an appreciable time delay to the exhaust gasses which pass through it, the rate at which the feedback signal commands the engine A/F toward stoichiometry must be restricted to a very low value. This is necessary so that the A/F won't pass through stoichiometry faster than the EGO sensor can detect and subsequently hold it in the window of the catalyst. For example, if the non-saturated (or linear) region of the EGO sensor characteristic is 0.05 A/F wide,and the time delay through the engine and catalyst is 10 seconds, the maximum A/F ramp rate would be 0.05/10 = 0.005 A/F per second. This value will insure that once the A/F enters the sensor's non-saturating region, the sensor will be able to initiate a change in the A/F and subsequently detect the effect of the change before the A/F has caused the sensor voltage to reach its other saturated level. The A/F ramp rate can be automatically adjusted to provide the fastest possible feedback correction without causing unstable system operation. This automatic rate control could be implemented by periodically increasing the A/F ramp rate until the system begins to oscillate in a well defined limit-cycle, and then reducing the ramp rate by an appropriate amount. In pre-catalyst applications of the invention, the time delay through the engine will be a function of rpm (and torque). The optimum value for the ramp rate will therefore be a function of engine rpm (and torque), and will be contained in an appropriate table in the engine control computer. Now when the engine is operating at an A/F which is in the catalyst window (i.e., in the non-saturated region of the EGO sensor characteristic), the output voltage of the EGO sensor will be approximately linearly related to A/F as suggested by the post-catalyst EGO sensor plot shown in Fig. 1B. Since the EGO sensor output voltage in this case does provide information as to how far the engine A/F is away from stoichiometry, the strategy is to feed back a signal that is proportional to the difference between the output of the EGO sensor and a suitable reference voltage such as 0.45 volts. Since the catalyst will exhibit an appreciable amount of time delay irrespective of the feedback mode, the value of the proportional feedback gain must be kept to a low value so that the feedback system will not become unstable and oscillate. The gain should be high enough to correct possible A/F disturbances as fast as possible without causing oscillations. In some applications where the need to provide oscillations is paramount, the gain might be reduced to zero so that the linear region effectively becomes a dead band. It may be desirable to add a small amount of integral feedback to the proportional feedback signal in this "linear" operating region in order to eliminate any steady-state A/F offsets that may arise. The value of the gain used for this integral feedback would be chosen to be sufficiently high to eliminate steady-state errors, but not too high to cause unstable (i.e., oscillatory) operation. Further, it may be advantageous to "truncate" the lower end of the linear region of the EGO sensor output voltage by raising the lean switch voltage (from 0.15 volts to 0.5 volts, for example,) and also increasing the reference voltage (from 0.45 volts to 0.6 volts, for example). The reason for this is to provide a slightly rich shift in the effective linear operating range of the EGO sensor in order to enhance the ability of the A/F feedback control system to provide optimum catalyst conversion efficiency. Some engine/dynamometer studies have shown that the highest simultaneous conversion efficiency for HC, CO, and NOx occurs when the post-catalyst sensor control voltage is approximately 0.6 volts. The actual control voltage is a function of the operating temperature of the EGO sensor. When the engine is operating on the lean side of the catalyst window (i.e., lean of stoichiometry as indicated by the post-catalyst EGO sensor), the output of the EGO sensor is essentially saturated at a low output voltage and does not give any meaningful information as to how much the engine A/F is lean of stoichiometry (See Fig. 1B). The feedback strategy in this case is to simply ramp the engine A/F back toward stoichiometry until the sensor output voltage starts to switch toward its rich state. This is the same strategy that was used when the engine was operating on the rich side of the catalyst window except now the engine A/F is ramped rich rather than lean. As previously discussed, the rate at which the feedback signal ramps the engine A/F toward stoichiometry must be restricted to a very low value so that the A/F won't pass through stoichiometry faster than the EGO sensor can detect and subsequently hold it in the window of the catalyst. Also, as previously discussed, the ramp rate of the A/F feedback signal could be automatically adjusted to provide the fastest possible feedback correction without causing system oscillation. In pre-catalyst applications of the invention, the optimal ramp rate will be a function of engine rpm (and torque), and will be contained in an appropriate table in the engine control computer. A tri-state control method, in accordance with an embodiment of this invention, can be applied to a system with pre-catalyst and post-catalyst A/F feedback to eliminate erratic oscillations. An example of the invention's ability to eliminate low-frequency oscillations is presented in Figs. 3A and 3B which show the post-catalyst EGO sensor output voltages versus time for a pure integral post-catalyst A/F feedback controller (Fig. 3A) and for this tri-state controller (Fig. 3B). As the figures indicate, the low-frequency oscillation that occurs with the pure integral feedback is eliminated when tri-state feedback is used. An embodiment of this invention can also be used to enhance the operation of certain catalyst monitoring schemes. For example, the tri-state A/F post-catalyst feedback system can be used to enhance the catalyst monitoring scheme by providing a more uniform A/F versus time characteristic. Referring to Fig. 4, an engine 41 has an exhaust stream coupled to a catalyst 42. A pre-catalyst EGO sensor 43 is positioned upstream of catalyst 42 and a post-catalyst EGO sensor 44 is positioned downstream of catalyst 42. A post feedback controller 46 receives a signal from sensor 44 and provides an air/fuel ratio trim signal to a pre-catalyst feedback controller 45 which also receives a signal from sensor 43. The output of feedback controller 45 is applied to a base fuel controller 47 which provides a fuel control signal to engine 41. As shown in Fig. 4, a post-catalyst tri-state A/F controller can be combined with a pre-catalyst A/F controller in order to realize the high-frequency correction capabilities of the pre-catalyst feedback loop. Post-catalyst A/F feedback controller 46 serves as a trim for pre-catalyst A/F feedback controller 45. The A/F trim will maintain post-catalyst EGO sensor 44 at stoichiometry by appropriately changing the "dc" value of the pre-catalyst feedback loop. It should be noted that the actual A/F trim can be accomplished in one of several different ways. For example, the feedback signal from post-catalyst A/F controller 46 can be used to change the switch point of pre-catalyst EGO sensor 43. Alternately, the feedback signal from post-catalyst controller 46 can be used to change the relative values of the up-down integration rates and/or the jump back in pre-catalyst controller 45. The tri-state control method can be applied to the control of any A/F feedback loop utilizing an EGO sensor. As such, it can be directly applied to the pre-catalyst feedback loop as well as the post-catalyst feedback loop. Using tri-state control in the pre-catalyst feedback loop can eliminate the limit-cycle mode of operation normally associated with the pre-catalyst feedback loop. To explain in more detail how the invention would work, consider the rich, linear and lean regions shown in Figure 1B. Furthermore, referring to Fig. 5, assume that the engine A/F is initially rich of stoichiometry and that the A/F feedback loop is closed at t = t 1 . Since the EGO sensor would initially see a rich A/F, its output would be approximately equal to 0.8 volts, and the A/F feedback controller would therefore slowly ramp the A/F leaner. When the engine A/F reached the linear region of the EGO sensor, the feedback controller would switch from a simple ramping mode to a proportional (or proportional plus integral) feedback mode. When this occurs (at t = t 2 ), the controller would drive the engine A/F to a pre-programmed setpoint (for example, 14.7). Assuming there were no other changes, the engine A/F would remain at this point. Idealized waveforms of the engine A/F, the EGO sensor output, and the feedback control signal corresponding to this example are shown in Figures 5A, 5B, and 5C as a function of time. If the engine A/F were initially lean of stoichiometry rather than rich, the EGO sensor would initially see a lean A/F, and its output would be approximately equal to 0.1 volts. In this case, when the A/F feedback loop is closed, the A/F feedback controller would slowly ramp the A/F richer until the engine A/F reached the linear region of the EGO sensor. At that time, the feedback controller would switch from a simple ramping mode to a proportional (or proportional plus integral) feedback mode, and the controller would drive the engine A/F to the pre-programmed setpoint. Assuming there would no other changes, the engine A/F would remain at this point. Idealized waveforms of the engine A/F, the EGO sensor output, and the feedback control signal corresponding to this situation are shown in Figures 6A, 6B, 6C as a function of time. It should be noted that the time scales in Figures 5 and 6 are not defined. This is because the actual times depend on whether the feedback system is pre-catalyst or post-catalyst, and the invention will apply to both situations. For clarity, no signal noise is shown on the various traces in Figures 5 and 6.

Claims: A method for controlling air/fuel ratio of an internal combustion engine controlled by an electronic engine control and having an exhaust gas oxygen (EGO) sensor (43,44) positioned in an exhaust stream flow from the engine, said method including the step of utilising different air/fuel ratio feedback control strategies depending upon whether the exhaust gas oxygen sensor (43,44) is saturated, rich or lean, or operating in a linear region, the method further including (a) utilising a linearly ramping lean feedback signal, when exhaust gas oxygen output sensor voltage indicates a rich condition, so as to lean out the engine air/fuel ratio as a function of time and determining a lean ramp rate by increasing the ramp rate until a limit-cycle oscillation results and then reducing the ramp rate and (b) utilising a linearly ramping rich feedback signal, when the exhaust gas oxygen sensor output indicates a lean condition, so as to enrich the engine air/fuel ratio as a function of time and determining a rich ramp rate by increasing said ramp rate until a limit-cycle oscillation results and then reducing the ramp rate. A method as claimed in claim 1, further comprising the step of utilising a feedback signal which is a function of the difference between the output of the EGO sensor and an appropriate reference voltage, when the exhaust gas oxygen sensor voltage is between rich and lean saturation limits. A method as claimed in claim 2, wherein said function of the difference is a proportional function. A method as claimed in claim 2, wherein said function of the difference is a proportional function plus an integral function. A method as claimed in claim 1, further comprising the step of making the feedback signal to be invariant so that a feedback deadband results, when the exhaust gas oxygen sensor voltage is between the rich and lean saturation limits. A method as claimed in claim 1, further including the steps of: providing a catalyst in the engine exhaust stream; providing an upstream exhaust gas oxygen sensor positioned upstream of the catalyst; and utilising an output from the upstream exhaust gas oxygen sensor as an input to the air/fuel ratio feedback control strategy. A method as claimed in claim 1, further including the steps of: providing a catalyst in the engine exhaust stream; providing a downstream exhaust gas oxygen sensor positioned downstream of the catalyst; and utilising an output from the downstream exhaust gas oxygen sensor as an input to the air/fuel ratio feedback control strategy. A method as claimed in claim 1, further including the steps of: providing a catalyst in the engine exhaust stream; providing downstream and an upstream exhaust gas oxygen sensor positioned downstream or upstream, respectively, of the catalyst; and utilising outputs from both the upstream and the downstream exhaust gas oxygen sensor as inputs to the air/fuel ratio feedback control strategy.

Inventors: CULBERTSON THOMAS RAYMOND, CURRAN JUDITH MECHAM, HAMBURG DOUGLAS RAY, CULBERTSON, THOMAS RAYMOND, CURRAN, JUDITH MECHAM, HAMBURG, DOUGLAS RAY
Applicants: FORD FRANCE, FORD MOTOR CO, FORD WERKE AG, FORD FRANCE S. A., FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1993
Main classification: F02D41
IPC codes: F02D41