Automatic gearboxes of the automatic stage-geared gearbox type have become ever more common in heavier vehicles with the increasing development of microprocessor systems. This makes it possible, with a control computer and a number of control devices, such as servomotors, for example, to precisely regulate the engine speed, engagement and disengagement of an automatic clutch coupling between engine and gearbox, and gearbox clutch members in relation to one another, so that smooth gear changes are always achieved at the correct engine speed. The advantage with this type of automatic gearbox compared to a conventional automatic gearbox made up of planetary gear trains and having a hydrodynamic torque converter on the input side lies partly in the fact that it is more simple and robust, and can be manufactured at substantially lower cost than the conventional automatic gearbox, especially where used in heavy vehicles, and partly in that it affords greater efficiency, which means the possibility of reduced fuel consumption.
A stage-geared gearbox usually comprises (includes, but is not necessarily limited to) an input shaft, an intermediate shaft, that has at least one toothed gear meshing with a toothed gear on the input shaft, and a main shaft with toothed gears, which mesh with toothed gears on the intermediate shaft. The main shaft is then further connected to an output shaft coupled to the driving wheels via a prop shaft, for example. Each pair of toothed gears has a different gear ratio from another pair of gears in the gearbox. Different transmission ratios are obtained in that different pairs of gears transmit the torque from the engine to the driving wheels.
The development of computer technology has also had an impact on electronic control and feedback systems for a vehicle engine, and these systems have become more precise, faster and more adaptable to prevailing engine and environmental conditions. The entire combustion process can be precisely controlled according to any operating situation. The vehicle's throttle lever (an accelerator pedal, for example), which primarily controls the fuel supply to the engine, controls the vehicle's engine via electrical wiring and electronic signals. The throttle lever is therefore equipped with sensors for detecting the throttle lever position, that is to say what throttle opening is required.
The brake system in heavier vehicles is often a combined electronic and pneumatic system. A brake control unit controls modulators (relay valves) by means of electrical signals. The modulators control the air flow to the various brake cylinders on the vehicle. The system is primarily controlled by a brake control, usually a brake pedal. The brake pedal is equipped with sensors for detecting the brake control position, that is to say what brake force is required.
An automatic clutch coupling of the aforementioned type is usually controlled by means of information on the throttle lever position, the engine speed, the engine output torque and the position of the clutch coupling. The control parameter for the clutch position and hence for the degree of engagement between the engine and the gearbox primarily depends on how the driver positions the throttle lever.
When driving heavy vehicles, situations sometimes arise in which the vehicle needs to be driven at low speeds in a low gear with high transmission ratio engaged and the throttle lever fully eased off, that is to say the driver of the vehicle does not require fuel to the engine but wishes the vehicle to be driven at idling speed by means of the vehicle's low-idle speed control. Since the throttle lever is fully eased off at such low speeds, the driver of the vehicle lacks the speed-control function of the vehicle afforded by the throttle lever when he/she wishes to reduce speed further. When the lowest gear is engaged, the only possibility open to the driver is to use the brake control and hence the service brakes in order to further reduce the speed of the vehicle. In braking the vehicle to such an extent that the engine speed is reduced to less than the idling speed, however, the vehicle transmission control unit will normally ensure that the engine is isolated from the gearbox, so that the engine is not throttled to the point that it stalls. There is therefore no scope for the driver to continuously regulate the speed of the vehicle at these low speeds.
There is therefore a need, in the case of vehicles equipped with automatic stage-geared gearboxes and in precisely controlled driving at speeds lower than can be achieved in the lowest vehicle gear and at the engine idling speed, to find a way of controlling the speed of the vehicle. This is a primary object of the invention described below.