Abstract:
An engine control system comprises an engine speed control module and an idle limiting module. The engine speed control module selectively controls an engine based on an idle speed request. The idle limiting module selectively reduces the idle speed request by an amount that is based on a wheel slip value.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/033,164, filed on Mar. 3, 2008. The disclosure of the above application is incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to limiting wheel slip of a vehicle, and more particularly to controlling the engine to limit wheel slip. 
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     Referring now to  FIG. 1 , a functional block diagram of an engine  102  and exhaust system  106  for a vehicle is presented. The engine  102  combusts a mixture of air and diesel fuel to produce torque. The resulting exhaust gas is expelled from the engine  102  into the exhaust system  106 . The exhaust system  106  includes an exhaust manifold  108 , a diesel oxidation catalyst (DOC)  110 , a reductant injector  112 , a mixer  114 , and a diesel particulate filter (DPF) assembly  116 . 
     The exhaust gas flows from the engine  102  through the exhaust manifold  108  to the DOC  110 . The DOC  110  oxidizes particulate unburned hydrocarbon in the exhaust gas as the exhaust gas flows through the DOC  110 . The reductant injector  112  may inject a reductant, such as ammonia or urea, into the exhaust system  106 . The mixer  114 , which may be implemented as a baffle, agitates the exhaust gas and the injected reductant. 
     The DPF assembly  116  filters particulate from the exhaust gas passing through it. This particulate may accumulate within the DPF assembly  116  and may restrict the flow of exhaust gas through the DPF assembly  116 . The particulate may be removed from the DPF assembly  116  by a process called regeneration. A heater assembly  118  may be used to initiate the regeneration process. 
     SUMMARY 
     An engine control system comprises an engine speed control module and an idle limiting module. The engine speed control module selectively controls an engine based on an idle speed request. The idle limiting module selectively reduces the idle speed request by an amount that is based on a wheel slip value. 
     A method comprises selectively controlling an engine based on an idle speed request and selectively reducing the idle speed request by an amount that is based on a wheel slip value. 
     Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a functional block diagram of an engine and exhaust system for a vehicle according to the prior art; 
         FIG. 2  is a functional block diagram of an engine, an exhaust system, and a control system according to the principles of the present disclosure; 
         FIG. 3  is a functional block diagram of an exemplary implementation of the engine control module according to the principles of the present disclosure; and 
         FIG. 4  is a flowchart depicting exemplary operation of the engine control module according to the principles of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is in no way intended to limit the disclosure, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical or. It should be understood that steps within a method may be executed in different order without altering the principles of the present disclosure. 
     As used herein, the term module refers to an Application Specific Integrated Circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
     On surfaces having a low coefficient of friction, drive wheels of a vehicle may slip, even when the engine is idling and the accelerator pedal is not being pressed. Wheel slip is more likely to occur when idle speeds are high. For example only, diesel engines may naturally have a higher idle speed. Further, various operating modes, such as diesel particulate filter (DPF) regeneration, may increase idle speed. 
     In addition, torque converters that more suddenly transmit torque to the drivetrain may increase the likelihood of wheel slip. For example, diesel engines may use tight torque converters that allow less slip. Therefore, when the brake pedal is released, engine torque may be suddenly transmitted to the drivetrain, possibly causing wheel slip. According to the principles of the present disclosure, a control system may reduce the commanded idle speed in order to reduce the amount of wheel slip. 
     Referring now to  FIG. 2 , a functional block diagram of an engine  102 , an exhaust system  106 , and a control system  202  is presented. While the engine  102  will be described herein as a diesel engine, the present disclosure also applies to other engine systems, such as spark ignition engine systems. The control system  202  includes a DPF control module  204 , an engine control module  206 , a transmission control module  208 , and a stability control module  210 . The DPF control module  204  may control the regeneration process in the DPF assembly  116 . The DPF control module  204  may receive a reading of an outlet temperature of the DPF assembly  116 . The DPF control module  204  communicates with the engine control module  206 . 
     The engine control module  206  outputs actuator values to the engine  102  to achieve a desired engine torque or engine speed. For example, the engine control module  206  may control the amount of turbocharger boost, the positions of intake and exhaust cam phasers, the amount of exhaust gas recirculation (EGR), the amount of airflow, and/or the amount of fuel injected into cylinders of the engine  102 . The engine control module  206  receives information about the currently-selected gear from the transmission control module  208 . 
     In addition, the engine control module  206  receives information about wheel slip from the stability control module  210 . For example only, the wheel slip information may be communicated over a vehicle network, such as a controller area network (CAN). The engine control module  206  may also receive an engine coolant temperature (ECT) signal from an ECT sensor  212 . 
     Referring now to  FIG. 3 , a functional block diagram of an exemplary implementation of the engine control module  206  according to the principles of the present disclosure is presented. The engine control module  206  includes an idle control module  302  that generates a desired idle RPM. 
     A multiplexer  304  receives the desired idle RPM from the idle control module  302  and a limited idle RPM from a subtraction module  306 . The multiplexer  304  outputs either the idle RPM or the limited idle RPM to an actuator control module  310 . The actuator control module  310  generates actuator values for the engine  102  to achieve the selected idle RPM. 
     The selected output of the multiplexer  304  is determined by an enable signal from an enable module  320 . For example only, when the enable module  320  outputs an enable signal, the multiplexer  304  may select the limited idle RPM from the subtraction module  306 . Otherwise, the multiplexer  304  may select the idle RPM from the idle control module  302 . 
     The enable module  320  may generate the enable signal based on wheel slip and other conditions. For example, the enable module  320  may generate the enable signal when the amount of wheel slip is greater than a threshold. In various implementations, hysteresis may be used. For example, the enable module  320  may begin generating the enable signal when the wheel slip increases above a first threshold, and may stop generating the enable signal once the wheel slip falls below a second threshold that is less than the first threshold. 
     The enable module  320  may communicate with a timer  322 , which may limit the amount of time the enable signal will be generated. For example, once the wheel slip increases past the first threshold, the timer  322  may be reset, and the enable module  320  may stop producing the enable signal once the timer  322  reaches a predetermined value. The predetermined value may be based upon operating conditions and/or may be calibrated. For example, the predetermined period may be 15 seconds. Once the timer is exceeded, the enable module  320  may wait to generate the enable signal until the wheel slip falls below the second threshold. 
     The enable module  320  may limit generation of the enable signal to times when the selected gear is either first gear or reverse. In addition, the enable module  320  may limit generation of the enable signal to when the engine coolant temperature is above a threshold. The engine coolant temperature threshold may be established to avoid engine smoking at low engine temperatures. In various implementations, hysteresis may be used and two engine coolant temperature thresholds defined. 
     The enable module  320  may also limit generation of the enable signal to times when the DPF is not undergoing regeneration. However, if the DPF is undergoing regeneration and a temperature, such as the outlet temperature, of the DPF is high enough, the enable module  320  may still generate the enable signal. Hysteresis may also be used with the DPF outlet temperature. The enable module  320  may also limit generation of the enable signal to when the driver is applying little or no pressure to the accelerator pedal. 
     The enable signal may be sent to the PI module  308 . When the enable signal is first received, the PI module  308  may be initialized. For example, the PI module  308  may be initialized to the values in use when the enable signal was last generated. The PI module  308  may generate an offset that is subtracted from the idle RPM from the idle control module  302  by the subtraction module  306 . 
     The offset is based on a term that is proportional to an error value and a term that is based on an integration of the error value. The error value may be determined by subtracting acceptable wheel slip from the measured wheel slip. The acceptable wheel slip may be a calibratable value, such as two percent or three percent. The proportional term may be equal to a proportional constant times the error value, while the integral value may be equal to an integral over time of the error value multiplied by an integral constant. Upon initialization, the integral may be set to zero. 
     A maximum reduction and/or a minimum idle RPM may be defined. For example, the PI module  308  may be prevented from reducing the idle RPM by more than a predetermined value, such as 200 RPM. Alternatively, the subtraction module  306  may be prevented from producing a limited idle RPM less than a predetermined value, such as 600 RPM. 
     Referring now to  FIG. 4 , a flowchart depicts exemplary operation of the engine control module  206 . Control begins in step  402 , where thresholds are initialized and a timer is reset. For example, a wheel slip threshold, an engine coolant temperature threshold, and a DPF outlet temperature threshold may be defined. 
     For each of these thresholds, a first and second value may be defined. Having two values allows for hysteresis. For example, idle RPM limiting may be enabled when the wheel slip increases past a first threshold and may be disabled when the wheel slip decreases below a second threshold, where the second threshold is less than the first threshold. 
     In step  402 , slip, ECT, and outlet threshold variables are set to first (upper) values. Control continues in step  404 , where limiting of the idle RPM is disabled. Control continues in step  406 , where control determines whether the measured wheel slip is greater than the slip threshold. If so, control transfers to step  408 ; otherwise, control transfers to step  410 . In step  410 , the slip threshold variable is set to the upper value. Control continues in step  412 , where the timer is reset and control returns to step  404 . 
     In step  408 , the slip threshold variable is set to a second (lower) value. Control continues in step  414 , where control determines whether the timer is greater than a predetermined threshold. If so, control transfers to step  415 ; otherwise, control transfers to step  416 . In step  415 , control disables limiting of the idle RPM and continues in step  417 . In step  417 , control determines whether the wheel slip is less than the slip threshold. If so, control transfers to step  410 ; otherwise, control remains in step  417 . 
     In step  416 , control determines whether pressure on the accelerator pedal is less than a predetermined threshold. If so, control transfers to step  418 ; otherwise, control returns to step  404 . In various implementations, control may transfer to step  418  when the driver is applying no pressure to the accelerator pedal. 
     In step  418 , control determines whether the engine coolant temperature is greater than the ECT threshold variable. If so, control transfers to step  419 ; otherwise, control transfers to step  420 . In step  420 , the ECT threshold variable is set to the upper value. Control then returns to step  404 . 
     In step  419 , the ECT threshold variable is set to a second (lower) value. Control continues in step  422 , where control determines whether the transmission is in either first gear or reverse. If so, control transfers to step  424 ; otherwise, control returns to step  404 . In step  424 , control determines whether DPF regeneration is off. If so, control transfers to step  426 ; otherwise, control transfers to step  428 . In step  428 , control determines whether the DPF outlet temperature is greater than the outlet threshold variable. If so, control transfers to step  430 ; otherwise, control transfers to step  432 . 
     In step  432 , the outlet threshold variable is set equal to the upper value and control returns to step  404 . In step  430 , the outlet threshold variable is set equal to a second (lower) value and control continues is step  426 . In step  426 , idle RPM limiting is enabled, and control returns to step  406 . The idle RPM may be limited based on the amount of measured wheel slip. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification, and the following claims.