FR2729343A1 - Management of continuously variable automobile transmissions - Google Patents

Abstract

An automobile fitted with continuously variable transmission has an electrical generator connected to the internal combustion engine. The generator supplies an electrical traction motor and the system allows two modes of transmission (A), (B). The first mode (A) gives continuously variable transmission. The second mode (B) normally causes an immediate changing down of gear suitable for subsequent acceleration or deceleration at a constant gear ratio or a constant rate of acceleration or deceleration. Deceleration at a constant rate or at a constant gear ratio is selected according to whether the vehicle is on a flat or sloping road.

Description

 The present invention relates to motor vehicles; more particularly this invention relates to a management method intended for continuously variable transmissions.

 Continuously variable automatic transmissions for cars have more and more improvements to improve their approval: the electronic management of the transmission ratio has led to improvements such as auto-adaptivity, availability of several modes of dynamic behavior ("sport" and "economy" for example).

Known systems have flaws
The level of satisfaction is not maximum, in terms of choice of transmission ratio. I1 indeed happens that certain desirable changes in the transmission ratio must be decorrelated with any other action: for example downshifting at constant speed before overtaking, or when passing over a risky road segment (to make the vehicle more reactive), or to obtain an engine brake before a singularity (roundabout, etc.); these changes are planned in at least one known system thanks to a dedicated command; but the feel of the continuously variable transmission confuses the driver accustomed to conventional gearboxes
- with constant support on the accelerator, the increase in velocity with a conventional gearbox is a succession of segments with constant acceleration (this comes from the characteristics of petrol engines, whose maximum torque is substantially constant over the range of use) ; with a continuously variable transmission, the increase in velocity takes place with decreasing acceleration (the power and the engine speed are constant).

 - the initial response of a conventional transmission is direct; that of a continuously variable transmission is more progressive and gives a false impression of softness.

 To overcome this drawback, at least partially, self-adaptive management systems have been produced which adapt the dynamic behavior to the driver's behavior. In addition, known systems make available to the driver, via direct control, several modes of dynamic behavior such as short and "long".

 In the case of continuous variation, the acceleration is immediate when the accelerator is pressed (downshifting takes place in engagement, therefore without interruption of traction); for acceleration, the selection of a "short" mode has the advantage only of starting the engine in advance, the management of the acceleration being modified only quantitatively, but not qualitatively, that is to say that the engine will work at higher speed in "short", but will always be regulated in the same way (its speed curve as a function of speed will have the same speed).

 It is possible to manage a continuously variable transmission in emulation of a conventional transmission; the transmission then works at constant ratios, and switches these ratios. The advantages of the continuously variable transmission are then lost.

 An object of the present invention is to allow the driver to rediscover the sensations of a conventional gearbox under the conditions where he feels the need, while retaining the advantages of continuous variation the rest of the time.

 Another object of the present invention is to give the driver the choice, at any time, between a "default" mode substantially at constant power for efficiency, and a so-called "alternative" mode which allows a feeling of near acceleration. that of conventional gearboxes or a pleasant engine brake adapted to the singularities of the road, which the driver signals to the management body by said choice.

 Another object of the present invention is to have an "alternative" mode whose initial ratio is perfectly suited to the changes expected by the driver, the default mode being assumed to be perfectly suited to the conditions prevailing just before the selection command is activated.

 Another object of the invention is to adapt the "alternative" mode to the conditions where it is most useful, namely in recovery or in engine brake: in these conditions there is also a need for downshifting; the method according to the invention makes it possible to associate during the same action of the driver, the switching of the mode and the demotion.

 A final object of the method according to the invention is not only to have an efficient and pleasant engine brake, but also to allow the choice between two engine braking levels.

 Figure 1 shows a block diagram of the device, of known type, which is the support of the method according to the invention.

 The method according to the invention uses a continuously variable transmission (11) of known type, for example belt or electric, a management system (12) of self-adapting type of known type using a computer or a fuzzy logic device, and a manual or foot control (13) of a known type also.

 We will now specify the terms used.

In the following, for simplification, the description of the different types of management, or modes, is limited to the conditions of constant engine load or support on the constant accelerator.
The transients of the acceleration command are considered to be processed in a manner known for so-called self-adaptive management devices.

 The terms constant ratio "and constant speed have a meaning on the central speed range, it being understood that when the mechanical limitations intervene (range of variation of the ratio, engine speed), these latter limitations take precedence over the type of management.

 Similarly, these definitions are not absolute but are a good approach to characterize these modes, shown schematically in Figures 2 and 3.

 FIG. 2 shows a transfer function diagram of a constant speed type mode ".

 FIG. 3 shows a transfer function diagram of a constant ratio type mode ".

 On a graph with the speed on the abscissa and the speed on the ordinate, at engine load (or press on the accelerator) given, a "constant speed" mode (fig.2) will have its central range (21) represented by a segment substantially vertical right; a "constant ratio (fig.3) mode will have its central section (31) containing at least one segment of a straight line passing through the origin; a constant deceleration management mode" will have a central section representing the inverse of the value of the engine brake torque as a function of the speed, so that the engine brake on the wheels is constant whatever the speed in this range.

 The constant power mode is identical to the "constant speed" mode: indeed, by pressing the constant accelerator, the torque at a given speed is constant: therefore the power is constant.

 A constant acceleration mode "is close to the" constant ratio "mode for many internal combustion engines, but nevertheless at a given engine load (or torque), the transfer function of the gearbox (output speed / engine speed) is corrected by the torque / speed transfer function of the engine, so as to restore a constant torque at the outlet of the gearbox, despite the fact that the engine torque is not strictly constant as a function of the speed (especially for a turbocharged engine).

 For decelerations, a "constant deceleration" mode is notably different from a constant ratio mode ": the engine brake torque is not constant depending on the speed the transfer function of the gearbox (output speed / speed engine) is corrected by the transfer function (torque-braking / engine speed), so as to restore constant braking to the wheels.

 In flat deceleration ", it is interesting to be at" constant deceleration ".

When going downhill it is nevertheless preferable to keep a "constant ratio" for the speed stability of the vehicle, said "constant" ratio is nevertheless variable depending on the gradient encountered by the vehicle (a steep slope will be synonymous with a short ratio)
The method according to the invention allows the selection of at least two management modes (A) and (B) by a command actuable by the driver, and is characterized in that the selection of the mode (B) en route always results , except in the case of an aberration (if the engine speed is high for example), by an immediate downshift and in that the initial transmission ratio of the mode (B) just after its selection is optimized for, from the vehicle velocity during the selection of said mode (B), an acceleration with constant transmission ratio or with constant acceleration and also, always starting from the same velocity, for a deceleration with constant transmission ratio or with constant deceleration.

 Said command is preferably of the flip-flop type: a pulse selects a mode, a second pulse returns to the other mode.

 The method according to the invention is also characterized in that, after the selection of the mode (B), said mode (B) manages the accelerations in a manner substantially at constant transmission ratio or substantially at constant acceleration.

Typically, the default mode "A" manages accelerations substantially at constant power or "constant speed;
The method according to the invention is also characterized in that, at least said mode (B) manages the decelerations in a manner substantially at constant transmission ratio or substantially at constant deceleration, depending on whether the road is downhill or flat respectively. The gradient is measured in a known manner, for example by the energy balance of the car, or by accelerometers associated with a measurement of the braking power.

 Mode "B" manages decelerations in the same way as mode "A" but corrected in the direction of shortening by a monotonic function of speed, for example a linear function.

 The process allows the driver, when he judges that the conditions require it (he will double for example, or else he wants flat engine brake or more engine brake downhill), to find the sensations of mechanical transmission thanks to the selection of said mode "B"; the first object is reached.

 Thanks to the tilt-type command, the driver can make this choice at any time and in a very short time: the second object is reached.

 With the continuously variable transmission, there are an infinite number of ratios between the minimum ratio and the maximum ratio: when selecting the alternative mode ", the management device can choose the downshift ratio that best suits an acceleration or a deceleration from the current speed and this with an infinite resolution, therefore much better than with a mechanical gearbox, thus the "alternative" mode selected is very finely adapted to the developments expected by the driver taking into account the objective conditions prevailing during mode selection "B": the third and the fourth object are reached.

 If we consider an embodiment where the managements of modes "A" and "B" are similar as regards deceleration, mode "B" will simply be "shorter" than mode "A", and this fact will provide a greater engine brake: there will be a choice between two engine brake powers according to the same parameters: the last object of the method according to the invention is achieved.

 We now describe an embodiment of the physical part of the management method; a continuously variable transmission of the electrical type is chosen, that is to say comprising at least one electric generator driven by the heat engine and at least one electric traction motor.

 The realization of the types of management described above can be done simply, and in a known manner in electrical transmissions, by regulating the induction of electrical machines so as to follow the desired curve at the speed of the heat engine.

 The method according to the invention allows a more pleasant use of the vehicle, in particular for people used to conventional transmissions. It is intended for passenger vehicles as well as industrial vehicles.

Claims (5)

 -1- Method of management of continuously variable transmission allowing the selection of at least two management modes (A) and (B) by a command actuable by the driver, characterized in that the selection on the way of the mode (B) except in the case of an aberration, always results in an immediate downshift and in that the initial transmission ratio of mode (B) just after its selection is optimized for, from the vehicle velocity when selecting said mode (B) , acceleration at constant transmission ratio and also, always from the same velocity, for deceleration at constant transmission ratio or at constant deceleration.  -2- Continuously variable transmission management method allowing the selection of at least two management modes (A) and (B) by a command actuable by the driver, characterized in that the en-route selection of the mode (B) except in the case of an aberration, always results in an immediate downshift and in that the initial transmission ratio of mode (B) just after its selection is optimized for, from the vehicle velocity when selecting said mode (B) , an acceleration of the constant acceleration type and also, always starting from the same velocity, for a deceleration at constant transmission ratio or at constant deceleration.  -3- A continuously variable transmission management method according to claim 1 characterized in that, after the selection of the mode (B), said mode (B) manages the accelerations in a manner substantially at constant transmission ratio.  -4- A method of managing continuously variable transmission according to claim 1 characterized in that, after the selection of the mode (B), said mode (B) manages the accelerations in a manner substantially at constant acceleration.  -5- A method of managing continuously variable transmission according to claim 1 characterized in that, at least said mode (B) manages the decelerations in a manner substantially at constant transmission ratio or substantially at constant deceleration, depending on whether the road is respectively downhill or flat.