Process for operating a stepless drive unit consisting of an int

Details Process for operating a stepless drive unit consisting of an int Process for operating a stepless drive unit consisting of an int

1998-01-05

1999-06-15

Wright, Dirk

Interrelated power delivery controls, including engine control

Transmission control

Including fluid drive

B60K 4112

Patent

active

059116455

DESCRIPTION:

BRIEF SUMMARY
The invention is concerned with a method for operating a drive unit which is composed of an internal combustion engine and of a continuously variable power-split gear and the gear of which is composed of a continuously controllable hydrostatic engine/pump unit and of a summator gear with range gears for successive stepup ranges, the changeover from one range to the next taking place at a predetermined shift point by the closing of the positive clutch assigned to the next range and, subsequently, the opening of the positive clutch assigned to the preceding range.
A power-split of the type mentioned and a method of this type are known from DE-A 41 04 167. In this gear, during acceleration the hydrostatic unit is first controlled in a first range from a nominal speed in one direction of rotation to the nominal speed in the opposite direction of rotation. In the summator gear, the rotation of this hydrostatic branch is superposed on the mechanical branch and is brought to the output speed in a downstream range gear. In order to continue acceleration, a changeover is made from the first to the second of various members of the range gear at a predetermined shift point, at which the output speeds of these members are identical. In this second range, the hydrostatic unit is controlled back again to the nominal speed in the initial direction of rotation. This can then be followed by further ranges. It is thus possible, at a constant engine speed, to accelerate to a selected vehicle speed by adjusting the hydrostatic unit, with the result that the engine is always operated at the most favorable operating point of its characteristic map.
However, when the shift method specified is used for a working machine, a problem arises when an operating point is selected at the boundary between two ranges, that is to say in the vicinity of the shift point. Small load fluctuations may then lead to repeated upshifts and downshifts. To solve this problem, DE-A 41 04 167 proposes simply not opening the clutch of the preceding range, so that the stepup-ratio is retained at the shift point. That is, in effect, a shift block.
For this purpose, it proposes a hysteresis device activated by a signal which results from a specific time variable and/or a difference of the load variable and/or as a function of an operating variable.
It is known from EP-A 429,503 to provide shift suppression in the region of the shift points. In this case, if the range limit is exceeded, the stepup is kept constant from this point and the rotational speed of the engine is varied. In order to keep the stepup constant, however, the hydrostatic unit would have to be retained exactly in its instantaneous position, which is not possible on account of its hydraulic control. The unavoidable fluctuations, for example due to a change in the hydrostatic slip in the event of load fluctuations, could nevertheless lead to unintended range shifting. Moreover, such fluctuations also impair the regulation of the engine speed.
When an operating point near the upper range limit is to be approached, a considerable increase in the engine speed is necessary, thus increasing the wear of the engine and reducing the efficiency. Finally, at the moment when the range limit is exceeded, it is not yet possible to establish whether the range is merely to be passed or whether an operating point in the range has been selected for a longer time. The driver may, of course, change his mind at any time and actuate the accelerator lever. If, for example, he discontinues acceleration in order to select an operating point within the range limits, shift suppression is put out of action. If he wishes to accelerate from an operating point within the range limits, the engine must do this at the increased speed which it reached at the upper range limit.
The object of the invention is therefore, to provide a generic method which makes it possible to operate a drive unit of the type described even at operating points in the vicinity of the shift points, at the same time ensuring reliable shift suppressi

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