a system and method for engine braking

Abstract: A system and method for braking a vehicle 12 having a variable compression ratio engine 14 and a compression braking system 16 is disclosed. The system 10 is adapted to utilise the variable compression ratio engine 14 and the compression braking system 16 in a synergistic manner to selectively control or vary the generated compression braking torque by utilising high and low compression modes of operation of the engine 14.

Description: This invention relates to a system and a method for engine braking within a vehicle and more particularly to a compression braking system and method using a variable compression ratio engine to selectively augment the braking of the vehicle. Compression braking systems are typically employed within relatively large or commercial type vehicles and are effective to augment the torque provided by conventional friction braking assemblies and/or to regulate the speed of such vehicles. Compression braking is typically activated after the fuel supply has been "cut off" from the engine, such as when the vehicle is descending a steep grade. Compression braking is performed by providing resistance to airflow within the engine by changing valve timing and/or by other techniques. For example and without limitation, compression braking can be performed by opening a cylinder valve (e.g., an exhaust valve or a supplemental valve) at the end of a stroke, thereby releasing the potential energy stored in the compressed air into the atmosphere. In this manner, energy is removed from the driveshaft, thereby slowing the vehicle. Prior compression braking systems typically include several hydraulically actuated valves which communicate with various cylinders within the vehicle's engine and which are selectively opened to release compressed air from the cylinders, as can be seen for example in US 5,816,665. Efforts have been made to improve the efficiency and fuel economy of a vehicle engine by selectively varying the compression ratio within the various cylinders of the engine. For example and without limitation, a variable compression engine may selectively alter the compression ratio within its various cylinders by opening and closing auxiliary chambers within the cylinder heads, or by varying the length of the pistons and/or piston rods. These types of engines provide greater flexibility, improved fuel economy and reduced knock. However, vehicles having these types of engines are typically braked using conventional friction brakes and/or conventional compression braking systems and strategies. These conventional compression braking systems and methods do not utilize the flexibility provided by variable compression ratio engines, and therefore, are not well suited for variable compression ratio engines. It is therefore an object of this invention to provide new and improved system and method for compression braking which is adapted for use with a variable compression ratio engine. According to a first aspect of the invention there is provided a system for braking a vehicle having a variable compression ratio engine which is selectively operable in a high compression ratio mode and in a low compression ratio mode wherein the system comprises a compression braking system which selectively provides a compression braking torque and a controller which is communicatively coupled to the variable compression ratio engine and to the compression braking system, the controller being effective to activate the compression braking system and to selectively cause the variable compression ratio engine to switch between the high compression ratio mode and the low compression ratio mode while the compression braking system is activated, thereby selectively varying the compression braking torque. The system may further comprise a sensor which is adapted to measure a speed of the vehicle and wherein the controller is further communicatively coupled to the sensor and is effective to activate the compression braking system when the vehicle is being braked and the speed of the vehicle exceeds a predetermined value. The variable compression ratio engine may be of the type having a plurality of auxiliary chambers which are selectively and communicatively coupled to cylinders of the engine and which are selectively opened and closed to alter the compression ratio of the engine. Alternatively, the variable compression ratio engine may be of the type having a plurality of selectively extendable pistons. The vehicle may further include a transmission and the controller may be further communicatively coupled to the transmission and be arranged to selectively shift the transmission when the compression braking system is activated based upon the speed of the vehicle. According to a second aspect of the invention there is provided a method for braking a vehicle of the type having a compression braking system and a variable compression ratio engine characterised in that the method comprises the steps of activating the compression braking system to produce a braking torque and selectively altering the compression ratio within the variable compression ratio engine thereby altering the produced braking torque. The method may further comprise the steps of measuring a speed of the vehicle and selectively activating the compression braking system only if the measured speed exceeds a predetermined value. The method may further comprise the steps of selectively increasing the compression ratio of the engine if the speed of the vehicle is increasing and exceeds a first threshold value and the compression braking system is activated thereby increasing the produced braking torque. The vehicle may further comprise a transmission and the method may further comprise the step of selectively shifting the transmission to a lower gear if the speed of the vehicle continues to increase after the compression ratio of the engine has been increased. The method may further comprise the steps of selectively decreasing the compression ratio of the engine if the speed of the vehicle is decreasing and is below a second threshold value and the compression braking system is activated. The vehicle may further comprise a transmission and the method may further comprise the step of selectively shifting the transmission to a higher gear if the speed of the vehicle continues to decrease after the compression ratio of the engine has been decreased. The method may further comprise the step of deactivating the compression braking system if the speed of the vehicle continues to decrease after the vehicle has been shifted to a higher gear. The invention will now be described by way of example with reference to the accompanying drawing of which:- Figure 1 is a block diagram of a compression braking system for use with a vehicle having a variable compression ratio engine in accordance with a first aspect of the invention; and Figure 2 is a flow diagram illustrating a method in accordance with a second aspect of the present invention to perform compression braking. Referring now to Figure 1, there is shown a braking system 10 which is adapted to provide compression braking within a vehicle 12 having a variable compression ratio engine 14. The system 10 is adapted to use the variable compression ratio engine 14 and the compression braking system 16 in a synergistic manner to selectively control or vary the generated compression braking torque. The vehicle 12 as shown is a commercial type truck or industrial vehicle having a multi-gear transmission 18 but could be any vehicle requiring additional braking. The system 10 includes a conventional controller 20 operating under stored program control. Controller 20 is communicatively coupled to compression braking system 16, to variable compression ratio engine 14, and to transmission 18. Controller 20 includes one or more microprocessors which cooperatively perform the below-described compression braking strategy or method. As will be apparent to one of ordinary skill in the art, the controller 20 may comprise a plurality of commercially available, conventional, and disparate chips or devices, which are operatively and communicatively linked in a cooperative manner or preferably the controller 20 comprises a portion of a conventional engine control unit ("ECU"). The controller 20 is effective to control the compression braking function provided by system 16, the variable compression ratio function of engine 14 and the gear-switching functions of transmission 18. It should be appreciated that controller 20 may also control other vehicle and/or engine functions such as valve and/or spark timing and calibration. Vehicle attribute sensors 22 comprise a plurality of conventional and commercially available sensors which measure information pertaining to the speed of vehicle 12 and other vehicle attributes. The sensors 22 include one or more conventional wheel speed sensors and pedal sensors which provide data, such as vehicle speed and driver pedal position to the controller 20. The controller 20 uses these values to control the compression braking system 16, the variable compression ratio engine 14 and the transmission 18 in a synergistic manner to achieve a continuously varying and controllable compression braking torque so as to regulate the speed of vehicle 12. The variable compression ratio engine 14 is a conventional variable compression ratio engine having several cylinders (not shown) and which is able to selectively vary the compression ratio within the cylinders. One example of such an engine is a variable compression ratio engine of the type having auxiliary chambers which are selectively and communicatively coupled to the various cylinders of the engine 14 and which are selectively opened and closed to alter the compression ratio of the engine 14. Another example of such an engine is a variable compression ratio engine of the type having selectively extendable pistons which are selectively extended or retracted to alter the compression ratio within the engine 14. In either case the engine 14 has at least two modes of variable compression operation, a high compression ratio mode in which the engine 14 operates at a relatively high compression ratio, and a low compression ratio mode in which the engine 14 operates at a relatively low compression ratio. In auxiliary chamber type variable compression ratio engines the auxiliary chambers are closed in high compression ratio mode, and the auxiliary chambers are open in low compression ratio mode. In extendable piston type variable compression ratio engines the pistons are extended in high compression mode and the pistons are retracted in the low compression ratio mode. It should be appreciated that the present invention is not limited by the specific type of variable compression ratio engine utilised and the present invention can operate with any type of variable compression ratio engine. Vehicle 12 further includes a conventional compression braking system 16 which is communicatively coupled to and/or which forms a portion of engine 14. In the preferred embodiment, compression braking system 16 includes several valves (not shown), each of which is disposed within a unique cylinder of engine 14 and each of which is selectively and hydraulically activated in a conventional manner, effective to selectively release compressed air from the cylinders at certain times when vehicle 12 is in a compression braking mode. In operation, controller 20 coordinates the functions of the compression braking system 16, the variable compression ratio engine 14 and the transmission 18 to achieve a continuously varying and controllable compression braking torque. Referring now to Figure 2, there is shown a block diagram 30 which illustrates a method used by the present system 10 to control the compression braking function. The method 30 begins in functional block or step 32, where the controller 20 receives vehicle speed data from sensors 22. Controller 20 then compares the present vehicle speed to a maximum speed threshold value, V d,max . If the measured vehicle speed does not exceed V d,max then the controller 20 proceeds to step 50. Whereas, if the vehicle speed exceeds V d,max , controller 20 proceeds to step 36 and determines whether the compression braking system 16 is activated. If the compression braking system 16 is not operating, the controller 20 proceeds to step 38 and activates the compression braking system 16, thereby slowing the vehicle. Normally the controller 20 ensures that other conditions are met prior to activating compression braking system 16. For example the controller 20 will ensure that the vehicle's driver pedal or accelerator is not being depressed or that fuel is not currently being supplied to the engine. Once the compression braking system 16 has been activated, the compression braking function provides a torque which controls the speed of vehicle 12 without the use of the vehicle's friction braking system or which supplements the braking torque produced by the vehicle's friction braking system. The controller 20 then proceeds to step 40 and determines whether the vehicle's speed is increasing. If the vehicle's speed is no longer increasing, then the controller 20 proceeds to step 50. Otherwise, if the vehicle's speed continues to increase then the controller 20 proceeds to step 42 and determines whether the engine 14 is operating in a low compression ratio mode. If the engine 14 is in high compression ratio mode then the controller 20 proceeds to step 44 and downshifts the vehicle until the vehicle speed stops increasing. To achieve this the controller 20 communicates a signal to the transmission 18, effective to cause transmission 18 to shift into the next lowest gear. If the vehicle's speed continues to increase, controller 20 communicates another signal to transmission 18 to shift into the next lowest gear. This process will continue until the transmission is in its lowest gear, the vehicle's speed ceases to increase or the speed of the engine will exceed a pre-determined maximum engine speed if a lower ratio is selected. Once the vehicle's speed "levels off" or begins to decrease, controller 20 proceeds to step 50. If in step 42, the controller 20 determines that the vehicle's engine 14 is operating in a low compression ratio mode, controller 20 proceeds to step 46 and switches to high compression ratio mode. To achieve this the controller 20 communicates a signal to engine 14 effective to cause the engine 14 to switch from low compression ratio mode to high compression ratio mode. By switching to high compression ratio mode, the braking torque provided by the compression braking function is considerably increased. The controller 20 then proceeds to step 48 and determines whether the vehicle's speed continues to increase. If the vehicle's speed is still increasing after switching to high compression ratio mode then the controller 20 will proceed to step 44. Otherwise the controller 20 proceeds to step 50. Controller 20 continues to monitor the vehicle's speed once the speed begins to decrease, and in step 50, controller 20 determines whether the vehicle speed has fallen below a predetermined minimum threshold value, V d,min . If the vehicle's speed is not less than V d,min then the controller 20 takes no further action and the strategy is repeated. However, if the vehicle's speed falls below V d,min controller 20 proceeds to step 51 and determines whether compression braking is activated. If compression braking is not activated then the controller 20 takes no further action and repeats the strategy but if compression braking is activated then the controller 20 proceeds to step 52 and determines whether the vehicle is operating in a high compression ratio mode. If the vehicle is not operating in high compression ratio mode, controller 20 proceeds to step 56. Otherwise, controller 20 proceeds to step 54. In step 54, controller 20 switches to low compression ratio mode by communicating a signal to engine 14, effective to cause engine 14 to switch from high compression ratio mode to low compression ratio mode. By switching to low compression ratio mode, the braking torque provided by the compression braking function is desirably decreased. Controller 20 then proceeds to step 56 and determines whether the vehicle's speed continues to decrease. If the vehicle's speed continues to decrease in step 56, controller 20 proceeds to step 58. If the vehicle's speed does not continue to decrease, controller 20 continues to use compression braking to slow vehicle 12, and once the vehicle's speed begins to decrease again, controller 20 proceeds to step 58. In step 58, controller 20 communicates a signal to transmission 18 to shift to the next highest gear. Controller 20 will continue to "up-shift" the transmission 18 until the highest gear is reached. Controller 20 then ends compression braking and communicates a signal to engine 14, effective to begin delivering fuel to the various cylinders of the engine 14. In this manner, system 10 and method 30 controllably vary the compression braking torque. System 10 and method 30 thereby provide a smoother and more flexible compression braking function with additional braking torque capabilities. In other alternate embodiments, system 10 can be adapted to provide further flexibility and control by selectively activating compression braking in less than all of the cylinders of the engine, or by selectively altering the compression ratio in less than all of the cylinders of the engine.

Claims: A system for braking a vehicle (12) having a variable compression ratio engine (14) which is selectively operable in a high compression ratio mode and in a low compression ratio mode characterised in that the system (10) comprises a compression braking system (16) which selectively provides a compression braking torque and a controller (20) which is communicatively coupled to the variable compression ratio engine (14) and to the compression braking system (16), the controller (20) being effective to activate the compression braking system (16) and to selectively cause the variable compression ratio engine (14) to switch between the high compression ratio mode and the low compression ratio mode while the compression braking system (16) is activated, thereby selectively varying the compression braking torque. A system as claimed in claim 1 wherein the system further comprises a sensor (22) which is adapted to measure a speed of the vehicle (12) and wherein the controller (20) is further communicatively coupled to the sensor (22) and is effective to activate the compression braking system (16) when the vehicle (12) is being braked and the speed of the vehicle (12) exceeds a predetermined value. A system as claimed in claim 1 or in claim 2 wherein the vehicle (12) further comprises a transmission (18) and the controller (20) is further communicatively coupled to the transmission (18) and is arranged to selectively shift the transmission (18) when the compression braking system (16) is activated based upon the speed of the vehicle (12). A method for braking a vehicle (12) of the type having a compression braking system (16) and a variable compression ratio engine (14) characterised in that the method comprises the steps of activating the compression braking system (16) to produce a braking torque and selectively altering the compression ratio within the variable compression ratio engine (14) thereby altering the produced braking torque. A method as claimed in claim 4 wherein the method further comprises the steps of measuring a speed of the vehicle (12) and selectively activating the compression braking system (16) only if the measured speed exceeds a predetermined value. A method as claimed in claim 4 or in claim 5 wherein the method further comprises the steps of selectively increasing the compression ratio of the engine (14) if the speed of the vehicle (12) is increasing and exceeds a first threshold value and the compression braking system (16) is activated thereby increasing the produced braking torque. A method as claimed in any of claims 4 to 6 wherein the vehicle (12) further comprises a transmission (18) and the method further comprises the step of selectively shifting the transmission to a lower gear if the speed of the vehicle (12) continues to increase after the compression ratio of the engine (14) has been increased. A method as claimed in any of claims 4 to 7 wherein the method further comprises the steps of selectively decreasing the compression ratio of the engine (14) if the speed of the vehicle (12) is decreasing and is below a second threshold value and the compression braking system (16) is activated. A method as claimed in any of claims 4 to 8 wherein the vehicle (12) further comprises a transmission and the method further comprises the steps of selectively shifting the transmission (18) to a higher gear if the speed of the vehicle (12) continues to decrease after the compression ratio of the engine (14) has been decreased. A method as claimed in claim 9 wherein the method further comprises the step of deactivating the compression braking system (16) if the speed of the vehicle (12) continues to decrease after the vehicle (12) has been shifted to a higher gear.

Inventors: KOLMANOVSKY ILYA V, KOLMANOVSKY, ILYA, V.
Applicants: FORD GLOBAL TECH INC, FORD GLOBAL TECHNOLOGIES, INC.
Application year: 2001
Main classification: F02D13
IPC codes: F02D13