CVT Step-Down Shift Torque Synchronization
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Solution Overview
Problem
Existing continuously variable transmissions (CVTs) face challenges in smoothly managing step-down shifts to decrease output power without affecting drivability, particularly in managing abrupt changes in rotational speeds and torque transmission during gear ratio adjustments.
Innovation Solution
A method and control routine for a CVT that determines a desired variator speed ratio and executes a step-down shift by synchronizing a preferred CVT input acceleration profile with engine torque, managing the change rate of the variator speed ratio to minimize vehicle deceleration glitches and ensure smooth powertrain operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a step-down shift is executed to decrease output power from the powertrain system, then fuel efficiency is improved, but vehicle deceleration glitches and drivability are adversely affected
Solution Approach 1:
The controller determines a desired variator speed ratio in advance before executing the step-down shift. By pre-calculating the target ratio and planning the acceleration profile, the system prepares for the transition smoothly, preventing abrupt changes that would cause drivability issues while achieving the fuel efficiency benefit of the lower output power state.
Solution Approach 2:
The system dynamically adjusts the CVT input acceleration profile during the shift transition. Rather than using a fixed shift pattern, the controller continuously monitors the present variator speed ratio and modifies the acceleration profile to synchronize with engine torque characteristics, enabling smooth transitions that maintain drivability while achieving efficient operating points.
2Loss of time
If the variator speed ratio is changed rapidly to execute a step-down shift, then shift time is reduced, but abrupt changes in rotational speed and torque transmission occur
Solution Approach 1:
The controller changes the variator speed ratio at a controlled change rate rather than instantaneously. By regulating the rate of parameter change and synchronizing it with the CVT input acceleration profile, the system achieves shifts quickly enough to be responsive while preventing abrupt rotational speed changes that would destabilize torque transmission and affect drivability.
3Stability of the object's composition
If engine torque is synchronized with CVT input acceleration profile, then smooth powertrain operation is achieved, but control complexity increases
Solution Approach 1:
The controller continuously monitors the present variator speed ratio and compares it against the desired ratio, using this feedback to adjust the CVT input acceleration profile in real-time. This closed-loop control synchronizes engine torque with the variator speed changes, ensuring smooth powertrain operation while managing the complexity through systematic feedback mechanisms rather than overly complex open-loop control strategies.
Data Source
AI summary
A powertrain system including an internal combustion engine rotatably coupled to a variator of a continuously variable transmission (CVT) is described. A method for controlling the CVT includes determining a desired variator speed ratio in response to a command to increase output power from the powertrain system and executing a step-down shift in the CVT based upon the desired variator speed ratio, the step-down shift. The step-down shift includes determining a preferred CVT input acceleration profile based upon the desired variator speed ratio, a present variator speed ratio, and a preferred shift time and synchronizing the preferred CVT input acceleration profile with engine torque. A change rate for the variator speed ratio is based upon the preferred CVT input acceleration profile, and the CVT is controlled in response to the change rate for the variator speed ratio and the desired variator speed ratio.