(1) Field of the Invention
The present invention generally relates to an apparatus for controlling an internal combustion engine, and more particularly to an engine control apparatus for adjusting a control parameter of an internal combustion engine equipped with an automatic transmission using a torque converter with a lock-up clutch, the control parameter being so adjusted that a cycle-by-cycle variation of torque output by the engine substantially agrees with a target torque variation when the engine is in a prescribed operating condition.
(2) Description of the Related Art
Conventionally, there is a known engine control device in which a cycle-by-cycle variation of torque generated in each of a plurality of cylinders of an internal combustion engine is detected and it is corrected so as to substantially agree with a target torque variation by adjusting an air-fuel ratio of the engine to make the air-fuel mixture as lean as possible, or by increasing or decreasing the quantity of exhaust gas recirculation (EGR quantity) therein. The primary purpose of the conventional device is to improve the fuel consumption of an internal combustion engine and reduce the amount of nitride oxides (NOx) in the exhaust gas thereof.
Japanese Laid-Open Patent Application No. 2-67446, for example, discloses a conventional engine control device of this type. In this engine control device, only a torque decrease is detected in each operation cycle and a cycle-by-cycle torque variation is calculated by totaling such torque changes in a number of operation cycles. The calculated torque variation is compared with a target torque variation, and an engine control parameter such as the air-fuel ratio or the EGR quantity is corrected on the basis of the result of the comparison, so that an air-fuel mixture is substantially at its lean limit. This method of controlling the engine control parameter is called herein a lean limit control.
In a case in which such a conventional device is applied to an internal combustion engine which is equipped with an automatic transmission using a torque converter with a lock-up clutch, there is a problem in that the target torque variation is determined in response to the engine operating conditions such as engine speed and load thereon, irrespective of whether or not the lock-up clutch is in ON state.
An automatic transmission for automotive vehicles in general performs automatically the starting clutch operation and the shift operation for producing a desired traction force of the vehicle. From the aspect of functional operation, the system of the automatic transmission may be divided into three parts, which are a torque converter, a sub-transmission and a control part. The torque converter in the automatic transmission system serves to amplify power produced by an internal combustion engine and transmit the same from an input shaft of the torque converter to an output shaft thereof. A fluid-type torque converter makes use of fluid for the power transmission, but the torque converter of this type often causes a loss of the trasmitted power due to the slip in the fluid used therein. For preventing the loss of the transmitted power, a lock-up clutch is used in the above mentioned torque converter, and this lock-up clutch mechanically connects the input shaft of the torque converter to the output shaft thereof for the power transmission.
FIG. 1 shows the construction of a torque converter with a lock-up clutch. In FIG. 1, a torque converter 1 generally has a pump impeller 2, a turbine liner 3, a stator 4 and a lock-up clutch 7. The pump impeller 2 on the front side thereof is connected to a crankshaft (not shown) of an engine via a front cover 5 on the outer periphery of the torque converter 1. The turbine liner 3 is fitted to an OD input shaft 6 by a spline gear. The stator 4 which is located at an intermediate portion between the pump impeller 2 and the turbine liner 3 is so arranged that the stator 4 is rotatable only in one direction around the shaft 6. The lock-up clutch 7 which is fixed at its one end portion to the OD input shaft 6 by a turbine liner hub 8 is so arranged that the lock-up clutch 7 is connected in pressure contact with the front cover 5 and disconnected from the same in response to a difference in fluid pressure between the input side and the output side.
Next, a description will be given of the operation which is performed by the torque converter 1, with reference to FIG. 1. Power produced on a crankshaft by an internal combustion engine is transmitted to the pump impeller 2 in the torque converter 1. The pump impeller 2 is rotated around the shaft 6, and fluid between impeller blades flows from the central portion of the pump impeller 2 to the outer peripheral wall and the turbine liner 3, and it moves from the outer portion of the turbine liner 3 to the central portion thereof. Such movement of the fluid causes the driving and rotation of the turbine liner 3. The rotating force of the turbine liner 3 is transmitted to the OD input shaft 6, and the OD input shaft 6 is rotated integrally with the turbine liner 3. The flow of the fluid from the turbine liner 3 is changed in direction by the stator 4, and the fluid flowing from the stator 4 serves to increase the rotation of the pump impeller 2.
Next, the operation which is performed by the lock-up clutch 7 will be explained, with reference to FIGS. 2A and 2B. In FIGS. 2A and 2B, those parts which are essentially the same as those corresponding parts shown in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted. FIG. 2A shows schematically the torque converter 1 in which the lock-up clutch 7 is in ON state and connected in pressure contact with the front cover 5. In this case, the operation of a control valve 9 is so controlled by a control signal that the fluid flows in a direction indicated by an arrow X in FIG. 2A. The flow of the fluid allows the lock-up clutch 7 to be connected in pressure contact with the front cover 5 by a difference in fluid pressure between the input side and the output side, and it is rotated integrally with the front cover 5. Therefore, the power generated in the internal combustion engine is transmitted to the OD input shaft 6 from the lock-up clutch 7.
FIG. 2B shows the torque converter in which the lock-up clutch 7 is in OFF state and disconnected from the front cover 5. In this case, the operation of the control valve 9 is so controlled by a control signal that the fluid flows in a direction indicated by an arrow Y in FIG. 2B, which is opposite to the direction indicated by the arrow X in FIG. 2A. The flow of the fluid allows the lock-up clutch 7 to be disconnected from the front cover 5 by a difference in fluid pressure between the input side and the output side. Thus, the power generated in the internal combustion engine is transmitted to the OD input shaft 6 through the pump impeller 2 and the turbine liner 3.
Thus, when the lock-up clutch 7 is in operation (ON state), the torque generated in the internal combustion engine is transmitted directly to a driven shaft in the torque converter 1. But, when the lock-up clutch 7 is not in operation (OFF state), the torque produced by the engine is attenuated in the torque converter 1 and hardly transmitted to the driven shaft. As described above, in the case of the conventional engine control device, the target torque variation is predetermined, irrespective of whether or not the lock-up clutch 7 is in ON state. Accordingly, if the actual torque variation is substantially the same, the surge of an automotive vehicle when the lock-up clutch is in ON state is greater than that when the lock-up clutch is in OFF state, and therefore the driveability becomes worse in the case where the conventional engine control device is applied to an internal combustion engine which is equipped with an automatic transmission using a torque converter with a lock-up clutch.