Air/fuel control for engines

Abstract: Fuel vapours are periodically purged from a fuel system into an engine's air/fuel intake (44). A measurement of the massive inductive fuel vapours is provided by a purge compensation signal is derived from an exhaust gas oxygen sensor (76) output. Lean air/fuel operation is enabled when the purge compensation signal is below a predetermined value.

Description: The invention relates to air/fuel control for engines having a lean burn air/fuel mode and a fuel vapour recovery system coupled between the fuel supply and the engine's air/fuel intake. Engine air/fuel control systems are known in which fuel delivered to the engine is adjusted in response to the output of an exhaust gas oxygen sensor to maintain average air/fuel ratios at a stoichiometric value. Such systems may also include a fuel vapour recovery system wherein fuel vapours are purged from the fuel system into the engine's air/fuel intake. An example of such a system is disclosed in U.S. Patent No. 5,048,493. The inventors have discovered numerous problems when prior air/fuel control systems are employed with engines having a lean burn operating mode. Such engines will operate at air/fuel ratios substantially leaner than stoichiometry to achieve improved fuel economy. However, when fuel vapours are purged into the engine air/fuel intake during lean burn operating modes, the engine will not run as lean as it is capable of running and fuel economy will therefore not be maximised. An object of the invention herein is to purge fuel vapours from an engine fuel system into the engine air/fuel intake while maintaining engine operation at a desired lean air/fuel ratio during lean burn operating modes. According to the present invention, there is provided a control method controlling air/fuel ratio of an internal combustion engine coupled to a fuel system, comprising the steps of: purging air through the fuel supply system to purge a mixture of air and any fuel vapours from the fuel supply system into an air/fuel intake of the engine; providing an indication of fuel vapour presence during said purging step; and enabling operation of the engine at an air/fuel ratio lean of a stoichiometric air/fuel ratio when said fuel vapour presence indication is below a predetermined value. An advantage of the present invention is that lean air/fuel operation can be provided at a desired lean value while accommodating the purging of fuel vapours from the fuel system. The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of an embodiment in which the invention is used to advantage; and Figures 2-6 are high level flowcharts illustrating various steps performed by a portion of the embodiment shown in Figure 1. Internal combustion engine 10 comprising a plurality of cylinders, one cylinder'of which is shown in Figure 1, is controlled by electronic engine controller 12. Engine 10 includes combustion chamber 30 and cylinder walls 32 with piston 36 positioned therein and connected to crankshaft 40. Combustion chamber 30 is shown communicating with intake manifold 44 and exhaust manifold 48 via respective intake valve 52 and exhaust valve 54. Intake manifold 44 is shown communicating with throttle body 58 via throttle plate 62. Intake manifold 44 is also shown having fuel injector 66 coupled thereto for delivering liquid fuel in proportion to the pulse width of signal fpw from controller 12. Fuel is delivered to fuel injector 66 by a conventional fuel system including fuel tank 67, fuel pump 68, and fuel rail 69. Catalytic converter 70 is shown coupled to exhaust manifold 48 upstream of nitrogen oxide trap 72. Exhaust gas oxygen sensor 76 is shown coupled to exhaust manifold 48 upstream of catalytic converter 70. In this particular example, sensor 76 provides signal EGO to controller 12 which converts signal EGO into two-state signal EGOS. A high voltage state of signal EGOS indicates exhaust gases are rich of a desired air/fuel ratio and a low voltage state of signal EGOS indicates exhaust gases are lean of the desired air/fuel ratio. Typically, the desired air/fuel ratio is selected at stoichiometry (14.3 lb. of air per pound of fuel, for example) which falls within the peak efficiency window of catalytic converter 70. During lean burn air/fuel operating modes, the desired air/fuel ratio is selected at a desired lean value considerably leaner than stoichiometry (18-22 lb. of air per pound of fuel, for example) to achieve improved fuel economy. Fuel vapour recovery system 94 is shown coupled between fuel tank 67 and intake manifold 44 via purge line 95 and purge control valve 96. In this particular example, fuel vapour recovery system 94 includes vapour canister 97 which is connected in parallel to fuel tank 67 for absorbing fuel vapours therefrom by activated charcoal contained within the canister. Further, in this particular example, valve 96 is a pulse width actuated solenoid valve responsive to pulse width signal ppw from controller 12. A valve having a variable orifice may also be used to advantage such as a control valve supplied by SIEMENS as part no. F3DE-9C915-AA. During fuel vapour purge, air is drawn through canister 97 via inlet vent 98 absorbing hydrocarbons from the activated charcoal. The mixture of purged air and vapours is then inducted into intake manifold 44 via purge control valve 96. Concurrently, fuel vapours from fuel tank 67 are drawn into intake manifold 44 via purge control valve 96. Controller 12 is shown in Figure 1 as a conventional microcomputer including: microprocessor unit 102, input/output ports 104, an electronic storage medium for executable programs and calibration values shown as read only memory chip 106 in this particular example, random access memory 108, and a conventional data bus. Controller 12 is shown receiving various signals from sensors coupled to engine 10, in addition to those. signals previously discussed, including: measurement of inducted mass air flow (MAF) from mass air flow sensor 110 which is coupled to throttle body 58; engine coolant temperature (ECT) from temperature sensor 112 coupled to cooling sleeve 114; a profile ignition pickup signal (PIP) from Hall effect sensor 118 coupled to crankshaft 40; throttle position signal TP from throttle position sensor 120; and signal HC from hydrocarbon sensor 122 coupled between purge valve 96 and intake manifold 44. The liquid fuel delivery routine executed by controller 12 for controlling engine 10 is now.described beginning with reference to the flowchart shown in Figure 2. Fuel delivery signal Fd, which is subsequently converted to fuel pulse width signal fpw for actuating fuel injector 66, is first generated as shown in step 202. More specifically, measurement of inducted mass airflow MAF is divided by the product of feedback variable FV (which is generated by the flowchart shown in Figure 3) and desired air/fuel ratio A/Fd. Purge compensation signal PCOMP (the generation of which is described later herein with particular reference to the flowchart shown in Figure 4) is then subtracted from the above quotient. Purge compensation signal PCOMP provides a measurement of the mass of fuel vapours inducted from fuel vapour recovery system 94 into air/fuel intake manifold 44 of engine 10. Closed-loop or feedback air/fuel conditions are then checked during step 206 such as engine coolant temperature ECT being above a threshold value. When feedback control conditions are present, feedback variable FV is read from the subroutine shown in Figure 3 (step 208). And desired air/fuel ratio A/Fd is set equal to a stoichiometric air/fuel ratio such as 14.3 lb. of air per pound of fuel (step 212). On the other hand, when feedback control conditions are not present (step 206), feedback variable FV is set equal to a value corresponding to a stoichiometric air/fuel ratio. Such value is unity in this particular example (step 216). When controller 12 is not causing air/fuel operation in either the closed-loop mode (step 206) or the lean burn mode (step 220), desired air/fuel ratio A/Fd is set equal to the stoichiometric air/fuel ratio (step 224). When not operating in the feedback control mode (step 206), but when operating in the lean air/fuel ratio mode (step 220), desired air/fuel ratio A/Fd is set equal to a preselected value which typically ranges between 18 and 22 1b. of fuel per pound of air. In some applications, however, such as those encountered in direct injection engines utilising stratified fuel charges, the desired air/fuel ratio A/Fd may be as lean as approximately 40 or 50 pounds of air per pound of fuel. Continuing with Figure 2, when not operating in the feedback control mode (step 206), but while operating in the lean mode (step 220), purge compensation signal PCOMP is frozen at the last value it had when operating during feedback air/fuel control (step 228). The air/fuel feedback routine executed by controller 12 to generate fuel feedback variable FV is now described with reference to the flowchart shown in Figure 3. This subroutine will proceed only when feedback control or closed-loop control conditions are present (step 310) and controller 12 is not in the fuel vapour learning mode (step 312). The fuel vapour learning mode is described in greater detail later herein with particular reference to Figure 4. When the above conditions are satisfied, two-state signal EGOS is generated from signal EGO (step 314) in the manner previously described herein with reference to Figure 1. Preselected proportional term Pj is subtracted from feedback variable FV (step 320) when signal EGOS is low (step 316), but was high during the previous background loop of' controller 12 (step 318). When signal EGOS is low (step 316), and was also low during the previous background loop (step 318), preselected integral term Aj is subtracted from feedback variable FV (step 322). Similarly, when signal EGOS is high (step 316), and was also high during the previous background loop of controller 12 (step 324), integral term Δi is added to feedback variable FV (step 326). When signal EGOS is high (step 316), but was low during the previous background loop (step 324), proportional term Pi is added to feedback variable FV (step 328). In accordance with the above described operation, feedback variable FV is generated from a proportional plus integral controller (Pl) responsive to exhaust gas oxygen sensor 76. The integration steps for integrating signal EGOS in a direction to cause a lean air/fuel correction are provided by integration steps Δi, and the proportional term for such correction provided by Pj. Similarly integral term Δj and proportional term Pj cause rich air/fuel correction. Description of the fuel vapour learning mode in which purge compensation signal PCOMP is generated is now described with particular reference to Figure 4. More specifically, when both closed-loop or feedback control air/fuel conditions are present (step 402) and fuel vapour purge of fuel vapour system 94 is enabled (step 404), the fuel vapour learning mode is entered (step 408). Feedback variable error signal FVe is generated by subtracting reference feedback variable FVr from feedback variable FV (step 412). Reference feedback variable FVr is the value which is associated with stoichiometric combustion. In this particular example, reference feedback variable FVr is set equal to unity. Purge compensation signal PCOMP is then generated by integrating feedback error signal FVe and multiplying the integral by gain constant k (step 416). Controlling the rate of fuel vapour flow is now described in more detail with reference to Figure 5 and Figures 6A-6F. Referring first to Figure 5, purge is enabled as a function of engine temperature during step 560. Desired purge flow signal Pfd is generated during step 562. In this particular example, signal Pfd is the maximum purge flow obtainable through purge control valve 96 (i.e., 100% duty cycle) to prevent emissions of hydrocarbons, operate engine 10 more efficiently, and reduce fuel system pressure. Unlike prior approaches, maximum purge flow is obtainable without exceeding the operating range of authority of air/fuel feedback control. During step 564, signal Pfd is multiplied by a scaling factor shown as signal Mult. As described in greater detail below, signal Mult is incremented in predetermined steps to maximum value of unity for controlling the turn on of purge flow. The product Pfd * Mult is converted to the corresponding pulse width modulated signal ppw in step 566. For example, if signal Mult is 0.5, signal ppw is generated with a 50% duty cycle. During steps 570-574, purge is disabled under sudden deceleration conditions when there is an appreciable fuel vapour concentration to prevent temporary driveability problems. More specifically, a determination of whether fuel vapours comprise more than 70% of total fuel (fuel vapour plus liquid fuel) is made during step 570. In this particular example, signal PCOMP is divided by the sum of signal Fd plus signal PCOMP. If this ratio is greater than 70%, and the throttle position is less than 30°, (see step 572), then purge is disabled by setting signal Mult and signal PCOMP to zero (see step 574). However, if the ratio PCOMP/(Fdm + PCOMP) is less than 70%, or throttle position is greater than 30°, the process continues with step 580. During steps 580 and 582, signal Mult is decremented a predetermined amount if the fuel vapour contribution of total fuel is greater than 50%. When the fuel vapour contribution is less than 50%, but greater than 40%, the program is exited without further changes to signal Mult (see step 584) such that the rate of purge flow remains the same. When fuel vapour concentration is less than 40% of total fuel, the program advances to step 590. It is noted that the functions performed by steps 580-584 may be accomplished by other means. For example, a simple comparison of signal PCOMP to various preselected values may also be used to advantage for either decrementing purge flow during initiation of purging operations, or holding it constant, when there are high concentrations of fuel vapours. During step 590, fuel injector pulse width signal fpw is compared to a first minimum value (min1) which defines an upper level of a pulse width dead band. If signal fpw is greater than min1, processing continues with program step 600. On the other hand, when signal fpw is less than min1, but greater than a minimum pulse width associated with the lower level of such dead band (min2), the rate of purge flow is not altered and the program exited (see step 592). Under such conditions the fuel injector pulse width signal fpw is within the dead band. However, when signal fpw is less than min2, the rate of purge flow is decremented a predetermined amount by decrementing signal Mult a corresponding predetermined amount (see steps 592 and 594). When fuel injector pulse width signal fpw is above the dead band (i.e., greater than min1) the program continues with steps 600-606. Signal Mult is incremented a predetermined amount when signal EGO has switched states since the last program background loop (see steps 600 and 602). If there has not been an EGO switch during a predetermined time, such as two seconds, signal Mult is decremented by a predetermined time (see steps 602 and 606). However, if there has been an EGO switch during such predetermined time, the rate of purge flow remains the same (see step 604). Accordingly, during initiation of the purging process, the rate of purge flow is gradually increased with each change in state of exhaust gas oxygen sensor 76. In this manner, purge flow is turned on at a gradual rate to its maximum value (i.e., signal Mult incremented to united when indications (EGO switching) are provided indicating that air/fuel feedback control and fuel vapour control are properly compensating for purging of fuel vapours. Referring now to Figure 6, switching between the lean burn mode and the fuel vapour learning mode is now described. The lean burn air/fuel operating mode is entered (step 620) when purged fuel vapours are negligible or below a preselected value (step 612) and lean burn operating conditions are present (step 616). Lean burn operating conditions include operations such as when engine 12 is not accelerating. An indication of the presence of purged fuel vapours (step 612) is provided in this particular example when purge compensation signal PCOMP is at a low value or zero. Other parameters indicative of the presence of fuel vapours in the purged mixture from fuel vapour recovery system 94 may be used to advantage such as feedback variable FV being at a value different from unity or a range about unity. Still another indication may be provided from a hydrocarbon sensor positioned in the purge vapour lines such as hydrocarbon sensor 122. When engine 10 has been operating in the lean burn mode for time period T1 (step 624), the lean burn mode is disabled (step 628) and the fuel vapour learning mode enabled (step 632) to determine whether fuel vapours are present in the fuel system. When an indication is provided that fuel vapours are present, such as when purge compensation signal PCOMP is greater than zero, the fuel vapour learning mode continues (steps 632 and 636). Stated another way, engine 10 continues to operate in the closed-loop or feedback air/fuel control mode wherein fuel vapours are purged from fuel vapour recovery system 94 into engine air/fuel intake 44. When fuel vapours are no longer present or are at a preselected threshold (step 636), the routine continues and will enter the lean burn mode when lean operating conditions are satisfied (step 612, 616, and 620).

Claims: A control method controlling air/fuel ratio of an internal combustion engine coupled to a fuel system, comprising the steps of: purging air through the fuel supply system to purge a mixture of air and any fuel vapours from the fuel supply system into an air/fuel intake of the engine; providing an indication of fuel vapour presence during said purging step; and enabling operation of the engine at an air/fuel ratio lean of a stoichiometric air/fuel ratio when said fuel vapour presence indication is below a predetermined value. A method as claimed in claim 1, wherein said step of providing a fuel vapour indication is responsive to a feedback variable generated from an output of an exhaust gas oxygen sensor. A method as claimed in claim 2, wherein said step of providing a fuel vapour indication is responsive to an error signal related to a difference between said feedback variable and a reference value associated with a stoichiometric air/fuel ratio. A method as claimed in claim 1, wherein said step of providing a fuel vapour indication is responsive to a hydrocarbon sensor communicating with said purged mixture. A control method as claimed in claim 1 including delivering fuel to the engine air/fuel intake from the fuel supply system in response to a fuel delivery signal, and comprising the further steps of: adjusting the fuel delivery signal in response to a feedback signal indicative of engine air/fuel ratio to maintain air/fuel ratio at a stoichiometric air/fuel ratio during a feedback control mode; providing an indication of fuel vapour presence in said purged mixture entering said air/fuel intake during said feedback control mode; disabling said purging step and enabling an engine air/fuel ratio lean of said stoichiometric air/fuel ratio during a lean burn mode when said fuel vapour indication is below a predetermined value; and disabling said lean burn mode and enabling said feedback control mode at preselected times to determine whether fuel vapours are present via said fuel vapour indication step. A method as claimed in claim 5, wherein said step of providing an indication of fuel vapour presence is responsive to a fuel vapour measuring signal derived from a feedback variable generated during said feedback control mode. A method as claimed in claim 6, wherein said adjusting step is further responsive to said fuel vapour measuring signal so that said fuel vapour measuring signal reduces the fuel delivery signal which causes said feedback variable to be driven to a value corresponding to a stoichiometric air/fuel ratio. A method as claimed in claim 6, wherein said fuel vapour measuring signal is derived from an error signal related to a difference between said feedback variable and a reference value associated with said stoichiometric air/fuel ratio. A method as claimed in claim 5, wherein said step of providing an indication of fuel vapour presence is responsive to a feedback variable generated during said feedback control mode. A method as claimed in claim 5, wherein said purging step is disabled during said lean burn mode.

Inventors: FARMER DAVID GEORGE, ORZEL DANIEL V, SURNILLA GOPICHANDRA, FARMER, DAVID GEORGE, ORZEL, DANIEL V., SURNILLA, GOPICHANDRA
Applicants: FORD GLOBAL TECH INC, FORD GLOBAL TECHNOLOGIES, INC.
Application year: 1998
Main classification: F02D41
IPC codes: F02M25, F02D45, F02D41