Method and apparatus for controlling the air/fuel ratio in a diesel engine

Abstract: The air/fuel ratio of the combustion mixture in the cylinders (12) of an engine (10) is sensed and the sensed air/fuel ratio is controlled to a predetermined scheduled air/fuel ratio by controlling the amount of exhaust gases recirculated. When the exhaust gas recirculated is substantially zero, the air/fuel ratio is thereafter closed loop adjusted to a predetermined scheduled minimum air/fuel ratio when the sensed air/fuel ratio is less than the minimum and when the sensed air/fuel ratio is greater than the minimum while the operator command (26) is at a maximum thereby to provide precise fuel limiting at a predetermined fuel limit schedule.

Description: This invention relates to a method and apparatus for controlling the air/fuel ratio in a diesel engine. It is generally known that emissions of nitrogen oxides from diesel engines can be reduced by recirculating a portion of exhaust gases to the intake of the engine. Current diesel engine exhaust gas recirculation controllers provide for control of the amount of exhaust gas recirculation (EGR) based on an open loop schedule. Since the control is open loop, these systems do not provide for active correction for variability due to, for example, production tolerances, wear, dirty air filters, fuel, ambient conditions, so that the actual amounts of exhaust gases recirculated may vary from the intended amount. It is generally known that the smoke and particulate emissions from a diesel engine may be lessened by preventing the air/fuel ratio from becoming too rich. Typically, this rich limit is imposed by mapping the fuel delivery which is preset prior to delivery of the engine. Thereafter, no active corrections are made for, for example, pump wear, fuel viscosity, dirty air filters, ambient changes, so that the actual air/fuel ratio limit may vary from the intended limit. JP-A-63176647 discloses a method and apparatus in accordance with the preamble of claims 1 and 2 respectively. The present invention seeks to provide an improved method and apparatus for controlling the air/fuel ratio in a diesel engine. A method of controlling the air/fuel ratio in a diesel engine in accordance with the present invention is characterised by the features specified in the characterising portion of claim 1. Apparatus for controlling the air/fuel ratio in a diesel engine in accordance with the present invention is characterised by the features specified in the characterising portion of claim 2. This invention can provide an improved diesel engine EGR/fuel control system. More specifically, this invention can provide for closed loop control of the air/fuel ratio to a desired schedule of engine operation dependent air/fuel ratios by control of the amount of exhaust gases recirculated to the engine. Further, the air/fuel ratio may be closed loop limited to a predetermined schedule of engine operation dependent minimum air/fuel ratios. In a preferred embodiment, the air/fuel ratio of the combustion mixture in the engine cylinders is sensed (such as by an exhaust oxygen sensor) and the sensed air/fuel ratio is controlled to a predetermined scheduled air/fuel ratio by controlling the amount of exhaust gases recirculated. In this embodiment, when the amount of exhaust gases recirculated has been decreased to substantially zero, at which point closed loop control of air/fuel ratio cannot be achieved with control of EGR, the air/fuel ratio is thereafter closed loop adjusted to a predetermined scheduled minimum air/fuel ratio when the sensed air/fuel ratio is less than the minimum and when the sensed air/fuel ratio is greater than the minimum, while the operator command is at a maximum, thereby to provide precise fuel limiting at a predetermined fuel limit schedule. An embodiment of the present invention is described below, by way of illustration only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a diesel engine control system; and Figures 2a and 2b are flow diagrams illustrating the operation of the system of Figure 1 based on an embodiment of the invention. Referring to Figure 1, a diesel engine 10 is illustrated. A single cylinder of the engine is shown for illustration purposes, it being understood that the engine may have any desired number of cylinders. The cylinder illustrated includes a combustion space 12 and piston 14. Air is drawn into the combustion space 12 during engine operation from an intake manifold 16 open to the atmosphere via an air intake conduit 18. The air intake 18 includes an air meter 20 for providing a measure of the mass air flow into the engine 10. The air intake 18 is unthrottled so that the pressure in the intake manifold 16 is substantially at atmospheric pressure. Fuel is fed into the cylinders of the engine by means of a solenoid controlled fuel pump that, on the basis of engine rotation, delivers fuel pulses sequentially to fuel injectors, such as the injector 23, which deliver fuel to the combustion space 12. The injection pump 22 is controlled to control the amount of fuel fed at each injection event by an engine control computer 24. In general, the fuel quantity is primarily controlled by the computer 24 in response to a fuel request input from the driver of the vehicle. This input is provided by a pedal position sensor 26 that monitors the position of the accelerator pedal 28. The exhaust gases resulting from the combustion events in the cylinders of the engine 10 are discharged into an exhaust manifold 30 and thereafter into the atmosphere via an exhaust conduit 32. The engine 10 further provides for controlled recirculation of exhaust gases from the exhaust manifold 30 to the intake manifold 16 in order to reduce the amount of nitrogen oxides (NO x ) in the exhaust gases discharged into the atmosphere. Accordingly, a conduit 34 is provided for directing exhaust gases from the exhaust manifold 30 to the intake manifold 16. The conduit 34 includes a normally closed electromagnetic valve 36 for regulating the amount of exhaust gases recirculated. This valve is controlled by the computer 24, such as by means of a pulse-width or duty cycle modulated signal, to control the position of the valve to establish the desired amount of EGR. In general, the valve 36 is controlled by the computer 24 so as to achieve a predetermined scheduled air/fuel ratio, as will be described. An exhaust gas sensor 38 is positioned in the exhaust manifold to monitor the air/fuel ratio of the mixture of air and fuel from the combustion space of the cylinders. The sensor 38 is of the type which provides an output which is a measure of air/fuel ratio over a wide range of ratios including the ratios of interest in operation of the diesel engine 10. This air/fuel ratio signal is fed to the computer 24 to enable closed loop control of air/fuel ratio. An engine speed signal RPM is also fed to the computer 24. This signal may be provided by a conventional toothed wheel driven by the engine output shaft and by an electromagnetic pickup. The resulting signal from the pickup has a frequency which is a measure of engine speed. The computer also receives inputs from other sensors as required, such as engine position, coolant temperature, as is common in diesel engine controllers. Similarly, a number of other actuators, not related to this embodiment, such as the engine glow plug heating system, can be controlled by the computer 24. The engine control computer 24 takes the form of a standard digital computer, with the embodiment described herein being implemented in the form of an operating program stored in the computer's memory. This program is illustrated in Figures 2a and 2b and takes the form of an air/fuel feedback control interrupt routine executed by the computer 24 at interrupt intervals established by a once per cylinder timing signal. Referring to Figures 2a and 2b, the air/fuel feedback control routine is entered at step 40 and proceeds to step 42 where the various input parameters used for fuel and EGR control are read and stored in memory. These signals include the signal from the air meter 20 providing a measure of the mass air flow, the vehicle operator load command output of the pedal position sensor 26, and the output of the exhaust oxygen sensor providing a measure of the engine air/fuel ratio. The operator load command is converted to a percentage of maximum pedal position and is referred to as the fuel request (FR) percent. Further, this step determines and stores in memory engine speed from the engine speed signal RPM. The routine next determines the fuel quantity to be injected into each cylinder of the engine. In general, the fuel quantity is a predetermined function of FR percent and engine speed, and is closed loop limited at a minimum air/fuel ratio value which is a predetermined function of engine speed. First, an air/fuel ratio feedback fuel modifier (AFFM) is retrieved at step 44 from a two-dimensional AFFM look-up table storing such modifiers as a function of engine speed. In this regard, the engine speed range is divided into a predetermined number of individual operating speed regions hereafter called operating speed points. The AFFM look-up table includes a same number of memory locations, each being associated with a respective one of the operating speed points and having stored therein an AFFM associated with the corresponding operating speed point. The AFFM look-up table is addressed by the measured value of engine speed to retrieve the AFFM corresponding to the engine operating speed point defined by the measured speed value. At step 46, a predetermined air/fuel ratio minimum limit is retrieved from a read only memory look-up table storing minimum allowable air/fuel ratio values as a predetermined function of engine speed. These engine calibration values are determined through engine development to provide desired engine performance objectives including, for example, exhaust particulate emissions. The air/fuel ratio minimum limit retrieved from memory at the location corresponding to the engine speed stored at step 42 is next adjusted by summing with it the air/fuel ratio feedback modifier retrieved from memory at step 44. Step 50 then determines the fuel quantity associated with the adjusted air/fuel minimum limit. This is simply the computation of the amount of fuel quantity at the mass air flow stored at step 42 to establish the adjusted air/fuel ratio minimum limit. This computed fuel quantity represents the maximum allowable amount of fuel to be injected. Step 52 next determines the desired fuel quantity from a look-up table storing a schedule of fuel quantities as a predetermined function of the FR percent and engine speed. This desired fuel quantity may be adjusted at this step, if desired, as a function of conventional parameters such as engine temperature and engine transients. Step 54 then limits, if required, the determined fuel quantity at the limit established at step 50. The fuel pulse-width required to inject the desired fuel quantity (limited at step 50) is determined at step 56 from a read only memory calibration look-up table of pulse-width values as a predetermined function of fuel quantity and engine speed. Finally, step 58 provides for actuating the fuel pump on the basis of the determined fuel pulse-width. The AFFM value stored at each AFFM memory location referred to in relation to step 44 is updated in response to the air/fuel ratio sensor 38, as will be described, while the engine operates at the corresponding speed operating point as the engine goes through its service life so that the AFFMs stored in the AFFM look-up table provide closed loop air/fuel ratio adjustment of the fuel quantity injected into the engine 10 based on step 58 so as to limit precisely the air/fuel ratio at the desired minimum limit value. The routine next provides for controlling the EGR valve to control the amount of exhaust gases recirculated from the exhaust manifold 30 to the intake manifold 16. The EGR amount is established on the basis of a desired air/fuel ratio which is a predetermined function of the fuel quantity injected and engine speed and closed loop adjusted based upon the feedback signal from the exhaust oxygen sensor 38. First, the desired air/fuel ratio is determined at step 60 from a look-up table of calibration air/fuel ratios stored in read only memory as a predetermined function of engine speed and fuel quantity determined in the preceding steps. This schedule is determined through engine development to achieve desired operating characteristics such as reduced nitrogen oxides (NO x ) content in the exhaust gases. The air/fuel ratio retrieved from memory at the memory location corresponding to the engine speed and fuel quantity may be modified, if necessary, for basic engine operating conditions such as coolant temperature or transient conditions. The EGR air/fuel ratio is then modified at step 62 by summing the EGR air/fuel ratio with an air/fuel ratio feedback EGR modifier (AFEM) retrieved from a three-dimensional AFEM look-up table storing such modifiers as a function of fuel quantity and engine speed. In this regard, the engine operating range defined by fuel amount and engine speed is divided into a predetermined number of individual operating regions hereafter referred to as operating points. The AFEM look-up table includes a same number of memory locations each being associated with a respective one of the operating points and having stored therein an AFEM associated with the corresponding operating point. The AFEM look-up table is addressed by the fuel quantity established at steps 52 and 54 and the measured value of engine speed to retrieve the AFEM corresponding to the engine operating point defined by the established fuel quantity and engine speed values. The AFEM value stored at each AFEM memory location is updated in response to the air/fuel ratio sensor 38, as will be described, while the engine operates at the corresponding operating point as the engine goes through its service life, so that the AFEM's stored in the AFEM look-up table provide closed loop air/fuel ratio adjustment of the EGR amount so as to control precisely the air/fuel ratio at the desired value. The modifiers are corrected, as will be described, on a continual basis in response to the feedback of the air/fuel ratio by the sensor 38 so that the desired air/fuel ratio is precisely obtained. At step 64, the air/fuel ratio represented by the measured mass air flow and the fuel quantity injected via steps 44 to 58 is determined and compared to the modified air/fuel ratio determined by step 62 and the duty cycle of the signal to the EGR valve 36 is adjusted in a direction to restore the determined air/fuel ratio at the modified air/fuel ratio. By continually closed loop adjusting the modifiers used at step 62 in response to the actual air/fuel ratio sensed by the sensor 38, the resulting actual air/fuel ratio in the combustion chambers will be established at the predetermined desired air/fuel ratio. The routine next functions to update the AFFM and AFEM modifiers in response to the exhaust oxygen sensor 38 output to provide closed loop control of the air/fuel ratio at the scheduled air/fuel ratios. In general, the AFEM values are updated if EGR is on and the AFFM values are updated if EGR is off. The EGR on/off condition is sensed at step 66 and may be based on the state of the EGR control signal established via steps 60 to 64. If the duty cycle of the signal applied to the EGR valve is greater than zero, EGR is considered on and the EGR air/fuel ratio modifiers AFEM are adjusted. When this condition is sensed at step 66, the program determines at step 68 if the engine has been operating at the same operating point for a predetermined period of time which accounts for any system time lag and ensures that the engine is in a stable operating condition. The AFEM's are updated only when this condition is met. If not met, the program exits the routine at step 69. If a stable operating condition is determined at step 68, the actual measured air/fuel ratio indicated by the output of the exhaust oxygen sensor 38 is determined at step 70 and then compared with a desired air/fuel ratio. The desired air/fuel ratio may be obtained from the same look-up table of calibration air/fuel ratios used at step 60 for EGR control or may be obtained from a separate look-up table establishing a corresponding schedule but having fewer memory locations. In this case, the engine operating range represented by fuel amount and engine speed is divided into fewer operating regions, or points, each corresponding to one of the look-up table memory locations. In either case, the desired air/fuel ratio is retrieved from memory at the location addressed by the fuel quantity and measured engine speed. If the next step 74 determines the actual air/fuel ratio is rich relative to the desired air/fuel ratio, step 76 first addresses the AFEM look-up table by the last determined values of fuel quantity and engine speed to retrieve the current value of the AFEM for the engine operating point corresponding to those values and then decreases that modifier to effect a decrease in the EGR amount via step 62. If the next step 78 determines that the adjusted modifier is within calibration limits, the adjusted modifier is stored at step 80 in the AFEM look-up table at the same memory location at which the original value was retrieved. Returning to step 74, if the comparison of step 72 indicates the actual air/fuel ratio is lean relative to the desired air/fuel ratio, step 82 addresses the AFEM look-up table by the last determined values of fuel quantity and engine speed to retrieve the current value of the AFEM for the engine operating point corresponding to those values and then increases that modifier to effect an increase in the EGR amount via step 62. If step 78 indicates that the adjusted value is within limits, the adjusted AFEM is stored at the same memory location at which the original value was retrieved. Returning to step 66, if it is determined that EGR is off, the air/fuel feedback fuel modifiers are updated on the basis of the output of the exhaust oxygen sensor 38. This routine begins at step 84 where the program determines if the engine is in a stable operating condition, in the same manner as described with reference to step 68. If not, the program exits at step 69. If a stable operating condition is found, the actual air/fuel ratio represented by the output of the exhaust oxygen sensor 38 is determined at step 86 and then compared to the desired air/fuel ratio at step 88. The desired air/fuel ratio may be obtained from the same look-up table of calibration air/fuel ratio minimum limits used at step 46 for fuel control or may be obtained from a separate look-up table establishing a corresponding schedule but having fewer memory locations. In this case, the engine operating speed range is divided into fewer operating regions, or points, each corresponding to one of the look-up table memory locations. In either case, the desired air/fuel ratio is retrieved from memory at the location addressed by measured engine speed. Step 90 then determines if the comparison of step 88 indicates a rich or lean combustion condition relative to the desired air/fuel ratio. If rich, step 92 first addresses the AFFM look-up table by the last measured value of engine speed to retrieve the current value of the AFFM for the engine operating point corresponding to that speed and then decreases that modifier to effect a decrease in the fuel amount when the limit is applied at step 54. If the next step 94 determines that the modifier is within calibration limits, the adjusted modifier is stored at step 96 in the AFFM look-up table at the same memory location from which the original AFFM was retrieved. Otherwise, the step 96 is bypassed and the routine exits at step 69. Returning to step 90, if the comparison of step 88 indicates a lean air/fuel ratio relative to the desired air/fuel ratio, the AFFM is updated to increase fuel only if the operator is commanding full load represented, for example, by the fuel request percent being at a high value such as 95%. This condition is determined at step 98. If full load is being commanded, a step 100 first addresses the AFFM look-up table by the last measured value of engine speed to retrieve the current value of the AFFM for the engine operating point defined by that speed and then increases that modifier to effect an increase in the fuel quantity when the limit is applied at step 54. If step 94 indicates that the adjusted modifier is within limits, it is stored at the same look-up table address from which the original value was obtained, otherwise step 96 is bypassed. In the foregoing manner, the AFFM's are continuously updated in a direction to restore the air/fuel ratio at the desired minimum limit.

Claims: A method of controlling the air/fuel ratio in a diesel engine which includes an intake space (16) coupled to atmosphere and from which in use air is drawn into one or more cylinders (12) of the engine, an exhaust passage (30) into which exhaust gases are discharged from the cylinders, one or more fuel injectors (23) for injecting fuel into the or a respective cylinder (12) to provide an air and fuel mixture for combustion, and recirculating means (34,36) for recirculating a portion of the exhaust gases from the exhaust passage (30) into the intake space (16), the recirculating means including a valve (36) operable between closed and open positions for regulating the amount of exhaust gases recirculated; the method comprising the steps of sensing the air/fuel ratio of the air and fuel mixture in a cylinder (12); storing (A) a first schedule of air/fuel ratio values as a predetermined function of engine speed and (B) a second schedule of air/fuel ratios as a predetermined function of engine speed and injected fuel amount; controlling a fuel injector (23) to inject an amount of fuel to the cylinder (12) on the basis of a driver fuel requirement; determining if the valve (36) is closed; when the valve is not closed, (A) sensing engine speed and injected fuel amount and (B) adjusting the valve (36) in a direction to vary the amount of exhaust gases recirculated so as to restore the sensed air/fuel ratio to the air/fuel ratio in the second schedule corresponding to the sensed engine speed and injected fuel amount; characterised by, when the valve (36) is closed, (A) sensing engine speed, (B) determining if the sensed air/fuel ratio is rich or lean relative to the air/fuel ratio in the first schedule corresponding to the sensed engine speed, (C) when the sensed air/fuel ratio is determined to be rich, decreasing the amount of fuel injected by the injector (23) so as to restore the sensed air/fuel ratio to the air/fuel ratio in the first schedule corresponding to the sensed engine speed, and (D) when the sensed air/fuel ratio is determined to be lean, increasing the amount of fuel injected by the injector (23) so as to restore the sensed air/fuel ratio to the air/fuel ratio in the first schedule corresponding to the sensed engine speed only when the driver fuel requirement is at full load. Apparatus for controlling the air/fuel ratio in a diesel engine which engine includes an intake space (16) coupled to atmosphere and from which in use air is drawn into one or more cylinders (12) of the engine, an exhaust passage (30) into which exhaust gases are discharged from the cylinders, one or more fuel injectors (23) for injecting fuel into the or a respective cylinder to provide an air and fuel mixture for combustion, and recirculating means (34,36) for recirculating a portion of the exhaust gases from the exhaust passage (30) into the intake space (16), the recirculating means including a valve (36) operable between closed and open positions for regulating the amount of exhaust gases recirculated; the apparatus comprising sensing means (38) for sensing the air/fuel ratio of the air and fuel mixture in a cylinder; a memory (24) for storing (A) a first schedule of air/fuel ratio values as a predetermined function of engine speed and injected fuel amount and (B) a second schedule of air/fuel ratios as a predetermined function of engine speed; processing means (24) for controlling a fuel injector (23) to inject an amount of fuel to the cylinder (12) on the basis of a driver fuel requirement and for determining if the valve (36) is closed; the processing means (24) being adapted, when the valve (36) is not closed, (A) to sense engine speed and injected fuel amount and (B) to adjust the valve (36) in a direction to vary the amount of exhaust gases recirculated so as to restore the sensed air/fuel ratio to the air/fuel ratio in the second schedule corresponding to the sensed engine speed and injected fuel amount, characterised by, when the valve (36) is closed, (A) to sense engine speed, (B) to determine if the sensed air/fuel ratio is rich or lean relative to the air/fuel ratio in the first schedule corresponding to the sensed engine speed, (C) when the sensed air/fuel ratio is determined to be rich, to decrease the amount of fuel injected by the injector (23) so as to restore the sensed air/fuel ratio to the air/fuel ratio in the first schedule corresponding to the sensed engine speed, and (D) when the sensed air/fuel ratio is determined to be lean, to increase the amount of fuel injected by the injector (23) so as to restore the sensed air/fuel ratio to the air/fuel ratio in the first schedule corresponding to the sensed engine speed only when the driver fuel requirement is at full load.

Inventors: BROWN MICHAEL JOHN, CURRIE JAMES HOWARD, MITCHELL MARK ALAN, PEDEN RICHARD ALLEN, SCZOMAK DAVID PETER, BROWN, MICHAEL JOHN, CURRIE, JAMES HOWARD, MITCHELL, MARK ALAN, PEDEN, RICHARD ALLEN, SCZOMAK, DAVID PETER
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1992
Main classification: F02D21
IPC codes: F02D41, F02D21, F02B3, F02M25