Abstract:
A control system for enabling an engine to operate in a deceleration fuel cutoff (DFCO) mode is provided. The system includes: an enable module that selectively enables a DFCO mode based on an accelerator pedal position; and an engine speed module that regulates engine speed based on turbine speed during a predetermined time period after the DFCO mode is enabled.

Description:
FIELD 
     The present disclosure relates to methods and systems for controlling an engine and more particularly to methods and systems for enabling a deceleration fuel cutoff operating mode of the engine. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     Automatic transmissions use a fluid clutch known as a torque converter to transfer engine torque from the engine to the transmission. The torque converter operates through hydraulic force provided by pressurized fluid from the automatic transmission. The torque converter multiplies engine torque and directs it through the transmission. 
     A conventional torque converter includes a sealed chamber filled with hydraulic fluid. The chamber includes a pump (or impeller) driven by the engine, a turbine connected to an output shaft, and a stator that provides torque multiplication. A torque converter is said to “slip” when the impeller speed and the turbine speed are not equivalent. Some converters incorporate a lockup mechanism such as a mechanical clutch that engages at cruising speeds to physically link the impeller with the turbine. The physical link causes the impeller and the turbine to rotate at the same or near the same speed, thereby reducing or eliminating slip. The clutch is applied and released via fluid supplied through a hollow shaft at the center axis of the rotating converter assembly. 
     In some applications, the engine may employ a deceleration fuel cutoff device that is capable of operating the engine in a deceleration fuel cutoff (DFCO) mode. Operating in a DFCO mode is desirable during overrun conditions (i.e., going down a hill) or in city traffic, as well as for engine speed limitation purposes. Operation in the DFCO mode contributes to improved fuel economy. 
     In order to enter the DFCO mode, it is desirable for the torque converter clutch to be applied. This reverses the transfer of torque. More specifically, the applied clutch allows torque to be transferred from rotating drive wheels back to the engine crankshaft when the vehicle coasts. If the slip across the torque converter is too high or too low prior to entering the DFCO mode, the application of the torque converter clutch may be delayed or may not occur at all. Hence, delaying or preventing the engine from operating in the DFCO mode and thus, impacting fuel economy. 
     SUMMARY 
     Accordingly, a control system for enabling an engine to operate in a deceleration fuel cutoff (DFCO) mode is provided. The system includes: an enable module that selectively enables a DFCO mode based on an accelerator pedal position; and an engine speed module that regulates engine speed based on turbine speed during a predetermined time period after the DFCO mode is enabled. 
     In other features, a method for enabling a deceleration fuel cutoff mode of an internal combustion engine is provided. The method includes: receiving a deceleration fuel cutoff (DFCO) request wherein the DFCO request is initiated based on an accelerator pedal position; controlling engine speed based on turbine speed after receiving the DFCO request; applying a torque converter clutch when the engine speed is within a predetermined range of the turbine speed; and enabling the DFCO mode after the clutch is applied. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a functional block diagram of a vehicle including a conventional torque converter system. 
         FIG. 2  is a dataflow diagram illustrating a turbine offset matching control system. 
         FIG. 3  is a flowchart illustrating a turbine offset matching control method. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 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 executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
     Referring to  FIG. 1 , a vehicle  10  includes an engine  12  that drives a transmission  14 . Air flows into the engine  12  through a throttle  16 . Fuel is combined with the air to be combusted within cylinders  18 . The combustion process reciprocally drives pistons (not shown) within the cylinders  18 . The pistons rotatably drive a crankshaft  22  to produce drive torque. The engine  12  includes N cylinders  18 . Although  FIG. 1  depicts four cylinders (N=4), it is appreciated that the engine  12  may include additional or fewer cylinders  18 . For example, engines having 4, 5, 6, 8, 10, 12 and 16 cylinders are contemplated. 
     Torque from the engine  12  is supplied to the transmission  14  through a torque converter (TC)  24 . The torque converter  24  may be any known lockup converter including a turbine, a stator, and a torque converter clutch (TCC). The transmission  14  multiplies the drive torque generated by the engine  12  through one of a plurality of gear ratios to drive a vehicle driveline  26 . The transmission  14  includes a hydraulic pump  28  that regulates pressurized fluid within the transmission  14  and controls fluid flow to and from the TC  24  via at least one solenoid-operated valve  30 . 
     An engine speed sensor  38  generates an engine speed signal based on a rotational speed of the crankshaft  22 . A turbine speed sensor  40  generates a turbine speed signal based on a rotational speed of the turbine within the torque converter  24 . The control module receives the signals and commands a current and/or pulse width modulated signal to the solenoid  30  in order to vary the supply of pressurized fluid to the torque converter  24 . The control module  32  controls a slip rate of the TC  24  by varying the pressurized fluid. 
     A vehicle operator manipulates an accelerator pedal  34  to regulate the throttle  16 . A pedal position sensor  36  senses the position of the accelerator pedal  34  and generates a pedal position signal that is communicated to the control module  32 . The control module  32  generates a throttle control signal based on the pedal position signal. A throttle actuator (not shown) adjusts the throttle  16  based on the throttle control signal to regulate air flow into the engine  12 . Such method of controlling the throttle  16  is referred to as electronic throttle control (ETC). The control module  32  adjusts fuel quantity and generates a fuel signal to the engine  12  based on the air flow. 
     When the pedal position signal indicates that the pedal  34  has been released and the vehicle is operating in a coast mode, the control module  32  communicates with the engine  12  and various sensors and actuators to control the activation of a deceleration fuel cutoff (DFCO) mode. In order to smooth the transition into the DFCO mode, the control module  32  controls the throttle  16  based on engine speed, turbine speed, and a turbine offset matching method and system according to the present disclosure. 
     More particularly, the control module  32  determines when the DFCO mode is desired and controls the throttle  16  and fuel such that the engine speed is equal to the turbine speed plus a predetermined offset. Controlling the engine speed to be within a predetermined range of the turbine speed allows for the torque converter clutch to be applied. Once the torque converter clutch is applied, the DFCO mode is enabled thereby disabling fuel to the engine  12 . Thereafter, the transmission  14  backdrives the unfueled engine  12  through the torque converter  24  to maintain a default engine speed. 
     Referring to  FIG. 2 , a dataflow diagram illustrates various embodiments of a turbine offset matching system that may be embedded within the control module  32 . Various embodiments of turbine offset matching systems according to the present disclosure may include any number of sub-modules embedded within the control module  32 . The sub-modules shown may be combined and/or further partitioned to similarly control the engine during activation of the DFCO mode. Inputs to the system may be sensed from the vehicle  10 , received from other control modules (not shown) within the vehicle  10 , and/or determined from other sub-modules within the control module  32 . In various embodiments, the control module  32  of  FIG. 2  includes a DFCO enable module  50  and an engine speed control module  52 . 
     The DFCO enable module  50  receives as input the accelerator pedal position  54 . Based on the accelerator pedal position  54 , the DFCO enable module  50  selectively sets a DFCO enable flag  56 . The DFCO enable flag  56  is set to TRUE when the accelerator pedal position  54  indicates that an accelerator pedal tip-out has occurred (e.g., the operator has released the pedal  34  ( FIG. 1 )). Otherwise the DFCO enable FLAG  56  remains set to FALSE. The engine speed control module  52  receives as input the DFCO enable flag  56 , engine speed  58 , turbine speed  60 , and gear  62 . When the DFCO enable flag  56  is TRUE, the engine speed control module  52  controls a desired engine speed to be nearly the same as the turbine speed. In various embodiments the desired engine speed is controlled to be near the turbine speed plus a predetermined offset. More particularly, the engine control module  52  controls the throttle  16  ( FIG. 1 ) via a throttle control signal  64  as a function of gear  62  and engine speed  58  such that the desired engine speed is achieved. The throttle  16  is controlled for a predetermine time period. Once the time period expires, the desired engine speed is gradually adjusted back to a default value and the throttle is controlled via the throttle control signal  64  accordingly. 
     Referring now to  FIG. 3 , a method of turbine offset matching for deceleration fuel cutoff is shown. The method can be run continually during engine operation. DFCO enable conditions are evaluated at  100 . If the DFCO mode is desired at  100 , control evaluates a timer at  102 . If a time since accelerator pedal tip-out has not expired at  102 , engine speed is controlled via the ETC at  104 . More particularly, the throttle is controlled based on gear and current engine speed such that the ultimate engine speed is near the turbine speed plus a predetermined offset (e.g., 100 RPM). Control continues to control the engine speed via the throttle at  104  until the timer expires at  102 . When the timer expires at  102 , the throttle is controlled back to a default value at  106 . 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present disclosure can be implemented in a variety of forms. Therefore, while this disclosure has been described in connection with particular examples thereof, 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, specification, and the following claims.