In a vehicle, a transmission system is disposed between the internal combustion engine and driving wheels. This transmission system changes the driving force of the driving wheels and traveling speed in accordance with traveling conditions of the vehicle which change in a wide range, thereby to derive the full performance of the internal combustion engine. In one example of a known transmission system, there is provided a continuous speed variable transmission control apparatus for transmitting power by increasing or decreasing the turning radius of a belt looped around pulleys, which pulleys each include a pulley element fixed to a rotational shaft and a movable pulley element which is separably supported relative to the stationary pulley element. By increasing and decreasing the width of a groove formed between the pulley elements, the turning radius of the belt (disposed in the groove) is changed thereby to change the belt ratio. Examples of this continuous speed variable transmission control apparatus are disclosed in Japanese Patent Early Laid-open Publication No. Sho 57-186656, Japanese Patent Early Laid-open Publication No. Sho 59-43249, Japanese Patent Early Laid-open Publication No. Sho 59-77159, and Japanese Patent Early Laid-open Publication No. Sho 61-233256.
Also, the present applicant has already developed the subject matter of Japanese Patent Early Laid-open Publication No. Sho 64-44346, namely a method for controlling a continuous speed variable transmission. This method controls speed change by providing a control unit for receiving a detection signal representing throttle opening degree, a signal representing vehicle speed, and a command for restricting the engine speed in a shift position, and for deciding a final target engine speed.
In the conventional continuous speed variable transmission control apparatus, when the line pressure is controlled, in other words, when closed loop control is performed, an engine torque value is calculated from a torque map based on the opening degree of the throttle and the engine speed. Such calculated engine torque value is initially memorized in a control unit, a target line pressure is calculated from the engine torque value, and then the actual hydraulic line pressure is fed back.
Also, when clutch pressure is controlled, the engine torque is calculated from the opening degree of the throttle when the vehicle is started, and a clutch target pressure is obtained as a feed forward quantity.
In general, the engine torque at high elevations (i.e., in low pressure area) is further lowered as the low pressure state progresses, and is apparent from the following equation:
Te.sub.1 =Te.sub.0 .times.K.times.atm/760, where PA1 Te.sub.1 =engine torque at a high elevation, PA1 Te.sub.0 =engine torque at sea level, and PA1 K=correction factor.
However, in the conventional device, because the initially memorized engine torque value is used to calculate clutch pressure, there is an inconvenience in that no correction is made for the aforementioned effect at high elevations, and the clutch pressure becomes high at vehicle starting time and engine speed is not increased. This eventually causes the driver to think that the vehicle is lacking power.
The same is true of the line pressure. As the line pressure target value becomes higher than necessary, the output load on the engine is increased with respect to the line pressure control during traveling of the vehicle, resulting in deterioration of acceleration, ascending ability on uphill roads, and traveling ability. This again causes the driver to think that the vehicle lacks power.
It is therefore an object of the present invention to provide a continuous speed variable transmission control method and apparatus which includes an absolute pressure detection type pressure sensor for detecting atmospheric pressure, and a control unit for responding to atmospheric pressure detected through said pressure sensor in accordance with the transmission shift position and other predetermined conditions and correcting operational conditions such as clutch and line pressures to proper levels based on such detected atmospheric pressure, thus permitting satisfactory acceleration, ascending ability and traveling ability to be maintained in accordance with the atmospheric pressure.
The transmission control method and apparatus of the present invention changes the belt ratio by increasing and decreasing the turning radius of the belt by respectively decreasing and increasing the width of the groove between the stationary and movable pulley elements. The method and apparatus further includes an absolute pressure detection type pressure sensor for detecting atmospheric pressure, and a control unit for responding to atmospheric pressure detected through said pressure sensor in accordance with the transmission shift position and other predetermined conditions and correcting the clutch and line pressures to proper levels based on such detected atmospheric pressure.