CVT Virtual Gear Ratio Control to Prevent Low-Speed Stalling

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Solution Overview

Problem

Existing continuously variable transmissions (CVT) with virtual fixed gears risk stalling due to insufficient rotation speed, causing driver discomfort from unexpected drivetrain behavior when the vehicle's rotation speed approaches a minimum threshold.

Innovation Solution

A method and computer program to control the CVT gear ratio, ensuring it matches a virtual fixed gear ratio during acceleration, maintaining a monotonic increase in motor rotation speed to simulate a fixed-gear transmission experience.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of operation

If the transmission operates with a virtual fixed gear ratio, then the acceleration behavior matches driver expectations, but the motor rotation speed decreases proportionally and risks falling below the minimum threshold

Engineering Contradiction:

Improveacceleration behaviorVSAvoidmotor rotation speed

Core Design Contradiction:

Ease of operationVSReliability

Solution Approach 1:

The transmission control system dynamically adjusts the gear ratio based on motor rotation speed. When the motor rotation speed approaches the minimum threshold during acceleration, the control unit increases the gear ratio above the virtual fixed gear ratio, thereby maintaining motor rotation speed while preserving acceptable acceleration behavior.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The control unit continuously monitors the motor rotation speed and compares it against the minimum threshold. Based on this feedback, the control unit automatically adjusts the transmission gear ratio to prevent the motor rotation speed from falling below the threshold, ensuring reliable operation while maintaining driver-expected acceleration characteristics.

Inventive Principle:

Principle #23Feedback

2Reliability

If the gear ratio deviates from the virtual fixed gear ratio to prevent stalling, then motor rotation speed is maintained, but driver discomfort occurs due to unexpected drivetrain behavior

Engineering Contradiction:

Improvemotor rotation speedVSAvoiddrivetrain behavior

Core Design Contradiction:

ReliabilityVSEase of operation

Solution Approach 1:

The control unit changes the gear ratio parameter only when necessary, specifically when motor rotation speed approaches the minimum threshold. This targeted parameter adjustment prevents stalling while minimizing deviations from the virtual fixed gear ratio, thereby maintaining driver-expected drivetrain behavior during normal acceleration.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The virtual fixed gear ratio is applied locally during normal acceleration conditions, while a corrected gear ratio is applied only in the specific local condition where motor rotation speed approaches the minimum threshold. This localized application of different gear ratio strategies maintains driver comfort while preventing stalling.

Inventive Principle:

Principle #3Local quality

Data Source

PatentUS12359720B2Acceleration with virtual gear ratios from low speeds

Publication Date: 2025.07.15 ZF FRIEDRICHSHAFEN AG

AI summary

The invention relates to a method for accelerating a vehicle with a continuously variable transmission (1050) where a gear ratio of the transmission is at first larger than a gear ratio of a virtual fixed gear. While the vehicle is accelerating, the gear ratio of the transmission is increased to the gear ratio of the virtual fixed gear. The transmission is then operated with the gear ratio of the virtual fixed gear.