Automatic transmission with neutral coast down feature

The present invention generally relates to an automatic transmission system and in particular to an automatic transmission system capable of providing improved fuel economy in connection with a negative torque condition.

A factor that effects a vehicle's fuel efficiency is the load experienced by the engine. If the engine experiences a high load, or runs at high revolutions per minute, fuel efficiency typically is reduced. On the other hand, if the engine is unloaded, or runs at a relatively lower number of revolutions per minute, the fuel efficiency commonly improves.

The load experienced by the engine may be directly related to the associated gear ratio in which the transmission is operating. Too high a numerical gear ratio may cause the engine to run too fast and fail to operate at a desired level of fuel efficiency. Conversely, an extremely low numerical gear ratio may adversely affect vehicle performance characteristics, including acceleration and ease of operation. Accordingly, communication between the engine and the transmission in determining the load experienced by the engine and determining a desired or proper gear ratio can improve fuel efficiency.

A system for controlling the engagement and disengagement of one or more clutches that are adapted to operatively connect an engine and an automatic transmission. The system including a control system for determining if a negative torque condition exists and to provide an output or signal to decrease the rotational speed of said engine in response to the determination of the existence of a negative torque condition.

FIG. 1 is a functional block diagram of a vehicle powertrain system, including an automatic transmission, according to an embodiment of the invention.

FIG. 2 is a graph of an engine speed curve that generally illustrates a speed curve when the engine is in a loaded condition and a neutral condition.

FIG. 3 is a gear selection graph according to an embodiment of the invention in which gear selection ranges are associated with throttle position and vehicle speed.

FIG. 4 is a functional flow chart of a powertrain response according to an embodiment of the invention.

FIG. 5 is a graph generally illustrating a disengagement and pre-engagement of a clutch during a negative torque condition.

Referring to FIG. 1 , a schematic illustration of an exemplary powertrain system 5 is shown. The powertrain system 5 includes an engine 10 having an output shaft 12 and an automatic transmission 14 having an input shaft 16 and an output shaft 18 . Positioned between engine output shaft 12 and transmission input shaft 16 is a torque converter 20 . Torque converter 20 may further include an impellor, a turbine and a stator. The output shaft 18 is commonly in communication with a differential assembly 22 , which in turn provides torque to a plurality of wheels 24 .

The automatic transmission 14 may be a continuously variable transmission, such as a belt driven transmission, or may be a fixed ratio transmission, such as a planetary transmission, or may be a step-change transmission, with an automated clutch and automated gear changes. Thus, it can be appreciated that the invention is not limited by the design, or type, of automatic transmission. The automatic transmission 14 may include a plurality of clutches that are capable of being engaged and disengaged, or vented, to connect and/or disconnect the output shaft 12 of the engine 10 with the input shaft 16 of the automatic transmission 14 , respectively.

The engine 10 may be in communication with an engine controller 28 and the transmission 14 may be in communication with a transmission controller 30 , either or both which may be integrated into or be provided in communication with a control module or control device 32 , such as a computer module, a powertrain control module (PCM), or the like. While the invention is described below as having an engine controller 28 and a transmission controller 30 that together form or function as part of a control device, it can be appreciated that the invention may be practiced with engine 10 and the transmission 14 in direct communication with a control device 32 that functions as or includes (or integrates) an engine controller and/or a transmission controller, thereby eliminating the need for a separate engine controller 28 and/or a separate transmission controller 30 .

In the illustrated embodiment, the control device 32 receives powertrain operating conditions from various sensors and can communicate output control signals to the engine 10 and the transmission 14 . The sensors providing input to the control device (shown in the illustrated embodiment in the form of a PCM) may include, without limitation, an engine speed sensor (not shown), a turbine output shaft sensor (not shown), an accelerator pedal input sensor 34 , and/or a vehicle speed sensor (not shown). An engine speed sensor may detect, for example, revolutions of an engine output shaft 12 and may provide or generate a signal indicative of the detected engine revolutions per minute (Ne). A turbine output shaft sensor may detect, for example, revolutions of a turbine output shaft and provide or generate a signal indicative of the detected revolutions per minute (Nt). An accelerator pedal input sensor (e.g., sensor 34 ) may detect, for example, the degree of depression or position of an accelerator pedal and provide or generate a signal indicative of the detected accelerator pedal position. Further, a vehicle speed sensor may detect for example, the speed at which the vehicle is currently traveling and provide or generate a signal indicative of a detected speed.

The control device 32 or the engine controller 28 may further include an electronic throttle control 36 . The throttle control 36 is capable of detecting, for example, the position or degree of opening of an engine throttle or throttle pedal position and providing or generating a signal indicative of detected throttle opening and/or throttle position.

The control device 32 or the transmission controller 30 may regulate, or control, the engagement and disengagement of one or more clutches 26 (which are only generically identified in connection with FIG. 1 ) based upon input or signals received from an electronic throttle control 36 and a vehicle speed sensor. It is to be understood that the invention is not limited to a specific clutch configuration, including that generally set forth in connection with FIG. 1 .

Referring to FIG. 3 , the selection, or determination, of a desired or appropriate gear for the transmission 14 may be determined based upon signals or information provided concerning throttle opening/position and vehicle speed. In turn, the gear selection will, at least in part, determine which combination of clutches 26 should be engaged or disengaged with respect to the transmission 14 . Four gears are represented in FIG. 3 . However, it can be appreciated that the system is not limited to three gears and may be practiced by including more or less gears.

The engine controller 28 and the transmission controller 30 may be configured for direct communication between each other. For instance, if the transmission 14 is in neutral, i.e., the clutches 26 are disengaged or vented, the transmission controller 30 may provide a signal to the engine controller 28 signaling that the transmission 14 is not loaded, or that a lower torque requirement may be present or occurring. In turn, the engine controller 28 may provide a signal to the engine 10 to reduce engine speed, or run at a lower number of revolutions per minute (rpm), which can result in fuel savings.

In contrast, if the transmission 14 is in gear, i.e. the clutches 26 are engaged, the transmission controller 30 may provide a signal to the engine controller 28 signaling that the transmission 14 is loaded, or that an increased torque requirement may be present or occurring. In turn, the engine controller 28 may provide a signal to the engine 10 to increase or ramp up the engine speed or rpm. FIG. 2 illustrates a general example of an engine speed curve in accordance with an embodiment of the invention. The illustrated graph depicts an engine speed curve through a period of time in which the engine 10 is loaded, i.e., running at X rpm, and when the engine 10 is unloaded, or in a neutral condition.

In certain situations, the powertrain system 5 may experience a negative torque condition. A negative torque condition may occur, for example, when the engine 10 is not producing an overall positive torque within the powertrain system 5 , when the turbine is revolving at a higher rate than the impeller within the torque converter 20 , or when the turbine is revolving at a higher rate than the engine 10 , i.e. when the transmission 14 is attempting to drive the engine 10 . By way of example, and without limitation, a negative torque condition may be present when the following conditions exist:

(1) the engine's revolutions per minute is less than the turbine's revolutions per minute plus a constant (Ne<Nt+a constant); and (2) the electronic throttle control sensor senses a throttle opening less than a given number of degrees (e.g., 2 degrees (thr<2°)); and (3) the above conditions are present for a select or predetermined time, for example, 1.3 seconds.

Such a situation may occur, for instance, when the operator of a vehicle requests a deceleration of the vehicle by engaging the brakes, or by initiating a neutral coast down situation by removing or reducing pressure on the accelerator pedal. When such a situation occurs, the engine 10 may be running at a higher rpm than necessary based upon the torque output of the transmission. Accordingly, the engine's rpm may be reduced to help improve associated fuel efficiency.

Referring now to FIGS. 4 and 5 , a flowchart illustrates a response by the powertrain system 5 to a negative torque condition which, among other things, may improve the fuel efficiency of the powertrain system 5 . In the illustrated embodiment of the response, a control device 32 (or its functional equivalent) determines if a negative torque condition is or may be occurring based upon signals provided by or received from an engine controller 28 and a transmission controller 30 (step S 1 ). If it is determined that a negative torque condition is not present or occurring, the powertrain system 5 may continue as it is currently operating, the clutches 26 engaged to the transmission 14 remain engaged (step S 2 ) and the system returns or loops back to the initial inquiry (step S 1 ).

Conversely, if the control device 32 (or its functional equivalent) determines that a negative torque condition is present, then the control device 32 provides a signal to the transmission controller 30 to initiate the release, or venting, of the engaged clutches 26 (step S 3 ). Once the transmission controller 30 receives the signal from the control device 32 , all clutches 26 engaged to the transmission 14 can be vented, or disengaged (step S 4 ). When all clutches 26 are vented, the transmission 14 is in neutral gear condition.

The control device 32 then may continue to analyze current vehicle operating conditions, including vehicle speed and throttle position, to determine which clutches 26 should or would be desirably engaged once the negative torque condition is canceled or no longer exists. Once the control device 32 has determined which clutch combination is appropriate, a select or predetermined amount of pressure may be provided to at least one of the clutches 26 in preparation for the engagement of the appropriate clutch configuration once the negative torque condition has passed or no longer exists (step S 5 ). For instance, the select or predetermined amount of pressure may be slightly less than the pressure necessary to keep the clutch(es) from slipping, or may be a percentage of the total pressure applied to the clutch(es) during engagement.

Referring to FIG. 5 , a graph is presented that generally illustrates the pressure applied or removed from the clutches during steps S 4 and S 5 . As illustrated, during step S 4 , pressure is removed from the engaged clutches such that the clutches may slip, or become disengaged. Once that occurs, during step S 5 , pressure is applied to at least one clutch so that once the negative torque condition is abated or removed and only a minimal amount of pressure may be necessary to engage the clutch. Applying the select or predetermined amount of pressure to one of the clutches 26 typically reduces the amount of time necessary for clutches 26 to engage the transmission 14 once the negative torque condition is no longer present. As a result, the transmission 14 may transition more smoothly from neutral to a loaded or in-gear condition.

Once the negative torque condition is canceled or no longer exists, the appropriate clutches 26 may be engaged to the transmission 14 and a signal may be provided by the transmission controller 30 to the engine controller 28 to increase engine speed accordingly. For instance, the negative torque condition may be canceled by a driver when an accelerator pedal is depressed to increase the vehicle's acceleration. In another example, the negative torque condition may be canceled automatically if the control device 32 senses that vehicle speed is increasing despite the prior application of brakes. In a further example, if the control device 32 senses that vehicle acceleration is changing beyond a select or predetermined rate, the clutches may be automatically engaged to the transmission for safety purposes. Either of the foregoing situations could be present if the vehicle is heading down a hill or slope.

The present invention has been particularly shown and described with reference to the foregoing embodiments, which are merely illustrative of the best modes for carrying out the invention. It should be understood by those skilled in the art that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention without departing from the spirit and scope of the invention as defined in the following claims. It is intended that the following claims define the scope of the invention and that the method and apparatus within the scope of these claims and their equivalents be covered thereby. This description of the invention should be understood to include all novel and non-obvious combinations of elements described herein, and claims may be presented in this or a later application to any novel and non-obvious combination of these elements. Moreover, the foregoing embodiments are illustrative, and no single feature or element is essential to all possible combinations that may be claimed in this or a later application.