A turbocharger uses the high temperature and high pressure energy of the exhaust gas to turn an exhaust turbine at high speed. The exhaust turbine is coaxial with and helps drive the compressor. Along with increasing the rpm of the compressor, it is also capable of raising the pressure in the air intake manifold so that it is greater than atmospheric pressure. From this supercharging pressure, a large volume of intake air can be supplied to the engine.
For a vehicle engine with a wide range of rpm speeds, in the medium-to-high speed operating range a sufficient supply of supercharging pressure can be maintained. However in the low speed operating range it is difficult to obtain a sufficient amount of exhaust pressure. Therefore, supercharging pressure cannot be obtained. This leads to an insufficient low speed torque.
In this case, a determining factor of the supercharging pressure in the low speed operating range is given by the ratio of the scroll cross-sectional area A, and the radius measured from the center R, that is A/R. If the area A can be made small, even in the low speed operating range where the amount of exhaust gas is small, the rpm can be increased and the rise in supercharging pressure can be quickened.
In this way, it has been required to develop the turbocharger so that even in the low speed operating range, a sufficient amount of supercharging pressure can be obtained. However, in this improvement the following problems are uncovered.
A controlling solenoid valve is necessary to improve the control of supercharging pressure. In the solenoid valve, if there are differences in tolerance from part to part or if changes in tolerance occur with use, the shifts from the target control values caused by this can be corrected by feedback control.
However, since the engine transition time, which is characteristic to the feedback control, is sometimes not in time with the feedback control, the supercharging pressure will shift from the target control value. For example, when the air intake quantity is suddenly increased by stepping on the accelerator pedal, the control value corresponding to the air intake quantity changes and it is necessary to largely make corrections for the shift. Because of this, for a short period of time, it is difficult to follow and respond to the feedback control. During this time the supercharging pressure will differ greatly from the target control value. Therefore, it is required to find a learning amount based on the feedback, correction for use in the succeeding control.
Also, when the engine transition time is not in time with the feedback control, it is desirable to use a learning control system that will remove the shift from the target control value and improve the response of the system. The learning control is a system that learns the amount of feedback correction found through the feedback control. Whether or not the proper learning amount is obtained is controlled by the period of the feedback control during which the learning is performed. Improvement in control precision due to learning control cannot be expected if the calculation timing control for the learning is not optimally performed.
Further, in the case where a variable geometry means is installed in a turbocharger to control the supercharging pressure, if only the variable geometry means is controlled with the feedback control, the control precision of the controlling range covered by the variable geometry means could be increased. However, if the arrangement of the exhaust bypass valve varies, the control precision of the controlling range covered by the exhaust bypass valve means would decrease.
In trying to increase the control precision both of the ranges covered by the exhaust bypass valve means and the variable geometry means simply by performing feedback control with multiple control means, the multiple control methods interfere with each other and proper control cannot be performed. For example, when the variable geometry means shifts from the optimum position to the closing side, the exhaust bypass valve means shifts from the optimum position to the opening side, and as a whole, the supercharging pressure is maintained at the target control value. However, in order to obtain the maximum engine capabilities, the variable geometry means and the exhaust bypass valve means must have optimum positions. If they shift from that position as in the case mentioned above, the turbine capacity becomes small and the engine output drops.
In addition, it is effective if overboost control is performed when accelerating suddenly to improve the acceleration capabilities. The overboost control, temporarily raises the target supercharging pressure so that a high supercharging pressure can be obtained improving the engine output. When applying the overboost control to a supercharging pressure control that avoids the interference of multiple control, the control mechanism will switch from the variable geometry means to the exhaust bypass valve means during overboost control. Even if the exhaust bypass valve means is completely closed in the case where the turbine capacity is large, the target supercharging pressure sometimes cannot be obtained during overboost control. That is to say, when suddenly accelerating and overboost control is performed, the target supercharging pressure is raised to the higher target supercharging pressure. In obtaining this higher target supercharging pressure, as previously mentioned, it is necessary that the control duty value in the control through the variable geometry means is large and that the opening of the variable geometry means is made smaller (moves toward the closing side). However, when suddenly accelerating in operating with a large load especially, the operating range to switch the control is attained during overboost control to be switched over to the control through the exhaust bypass valve means. Consequently, the control duty value in the control through the variable geometry means after switching becomes small. This small control duty value is unable to maintain the small opening of the variable geometry means, and adversely the opening of the variable geometry means gets large making it difficult to obtain the higher target supercharging pressure during overboost control.