Normal internal combustion vehicle engines with automatic transmissions produce torque fluctuations caused by inertia due to the reciprocating movement of pistons, pressure fluctuations produced in the internal combustion chambers of cylinders, etc. which are transmitted to driving axles and the car body of the vehicle, resulting in increased vibrations of the car body and a deterioration of fuel economy.
Generally, automatic transmissions are cooperated with torque converters. The normal torque converter allows some slippage, even at cruising speeds. This is due to the fact that the only connection between pump and turbine is the transmission fluid. To prevent this slipping action, and thus improve fuel economy, a number of torque converters are equipped with a lockup feature. For better fuel economy and far more comfortable driving, some automatic transmissions are operative in different drive modes, namely an economy drive mode in which fuel economy is regarded as more important and a power drive mode in which driving power is regarded as more important, which are selected according to driver's preference or driving conditions.
In an attempt at eliminating the transmission of torque fluctuations and, thereby, performing a sufficient transmission of generating power of the vehicle engine, an automatic transmission with a torque converter disclosed in, for example, Japanese Pat. Unexamined Publication No. 57-33253 entitled Torque Transmitter filed Aug. 4, 1980 and laid open Feb. 2, 1982, is adapted to detect slippage produced between input and output shafts of the torque converter and vary the fluid coupling efficiency of a lockup clutch so as to control the torque converter to make the detected slippage substantially equal to an intended slippage.
In more detail, in order to control this slippage action, the lockup clutch is controlled by a slip control device such as one comprising means for detecting the release of an accelerator pedal, timer means for counting the duration of time for which the accelerator pedal is continuously released, feed-forward control means for controlling the lockup clutch with a signal corresponding to a preselected or predetermined slippage to be allowed by the torque converter for a preselected or predetermined period of time after the release of the accelerator pedal, and feed-back control means for controlling the lockup clutch to cause the torque converter to allow an intended target slippage until the release of the accelerator pedal is suspended after the preselected period of time. The torque converter is controlled to allow slippage by controlling the lockup clutch in feed-forward control until the preselected or predetermined duration of time after the release of the accelerator pedal is counted up. Until the release of the accelerator pedal is suspended after the preselected duration of time, the torque converter is controlled to vary slippage by controlling the lockup clutch in feed-back control so as to converge it to the intended target slippage.
According to the above slip control device, during a wide range of fluctuations of engine torque which will be produced immediately after the accelerator pedal is released, the torque converter can be controlled to regulate slippage quickly in feed-forward control for the preselected period of time so as to cancel or absorb such fluctuations of engine torque. After the duration of the preselected period of time at which fluctuations of engine torque become small, the torque converter is controlled in feed-back control so as to allow a minimum slippage which is enough to cancel or absorb the small fluctuations of engine torque.
This slip control device can prevent more effectively the torque converter from being subjected to such an unsteady control as accompanied by hunting which is apt to occur immediately after the release of the accelerator pedal followed by a wide range of fluctuations of engine torque, in comparison with torque converters of the type being controlled only in feed-back slip control. In addition, the slip control device can make a delay of control as small as possible, so as to thereby prevent the vehicle engine from rapidly lowering the speed of its own rotation due to the delay of control. This leads to a prolonged time period of fuel cut-off.
A problem in association with the automatic transmission of the type varying the fluid coupling efficiency of the lockup clutch and being shiftable between the different drive modes, economy and power drive modes, is that the automatic transmission exerts a load on the vehicle engine due to the input and output shafts of the torque converter locked together. The vehicle engine therefore can not exhibit its best accelerating performance due to the load exerted on the engine by the automatic transmission with its input and output shafts locked together. The loads also leads to a loss of the generating power of the vehicle engine, notwithstanding that the vehicle engine should operate with a high accelerating performance during the power drive mode.
Another problem is a difficulty of properly changing the torque converter control between feed-forward and feed-back slip controls responding to various decreasing speeds of rotation of the vehicle engine. This difficulty is caused due to the fact that the slip control device controls the torque converter to operate in feed-forward control during a predetermined period of time immediately after the release of accelerator pedal and in feed-back control after the predetermined period of time and that no engine speed is reflected on changing the torque converter control between feed-forward and feed-back slip controls.
Because, when an engine is decelerated by releasing an accelerator pedal, engine speed or vehicle speed will be lowered at different rates generally according to changing road and load conditions, if engine speed or vehicle speed is lowered at a high rate, the engine speed sometimes becomes excessively lower than a fuel injection recovery speed before the predetermined period of time passes. In this case, it is impossible to increasingly change the time period of fuel injection cut-off by feed-back control around the fuel injection recovery speed at which fluctuations of engine torque is in an narrow range.
It may be permissible to shorten the predetermined period of time during which the slip control device controls the torque convertor to operate in feed forward control, in order to deal with the fact that it is impossible to increase the fuel injection cut-off time period. However, if the predetermined period of time is shortened too much, the vehicle engine sometimes remains in a speed range wherein relatively large fluctuations of engine torque after the shortened predetermined period of time are caused. In such a case, the torque converter by feed-back control, can not cancel the large fluctuations of engine torque, resulting in unsteady slip control such as hunting.