This paper presents a detailed investigation of the dry clutch engagement process, and vehicle start-up judder phenomenon that could result in the deterioration of vehicle ride comfort. Elasto-plastic friction model is elaborated through the slider-pulley system, which shows some friction characteristics such as presliding, stick-slip motion, Stribeck effects, etc., and applied to dry clutch. The axial compression characteristics of three elastic parts, which include diaphragm spring, cushion spring, and link strip have been taken into consideration, and nonlinear relationship between the release bearing travel and the clutch clamp force is also established. The powertrain system model of front-engine and front-wheel-drive vehicle equipped with manual transmission is set up to recreate the start-up judder phenomenon in the numerical simulation and analyze its mechanism. The sudden transfer of the engine torque during the clutch engagement process results in the initial judder, which can be supposed as the step response of system and is initially weakened due to the damping of the powertrain system. Then the judder gradually strengthens and gets in the most severe vibrance when the clutch is about to get in to the fully engaged state, which is related to the frictional characteristics that forms a closed-loop positive feedback system, as well as the frequent state transitions between sliding state and engaged state. The positive slope of Stribeck effect as well as the reduction of absolute value of negative slope can both effectively suppress the start-up judder, and the apparent judder occurs only if the negative slope is outside of a certain range, instead of in all of the range. In addition, the fluctuation of clutch clamp force can aggravate the start-up judder, in which a more chaotic oscillation is emerged.

This paper presents a profound mechanism investigation for vehicle start-up judder phenomenon using a combination of experiment and simulation. First, from the experimental analysis, the characteristic frequency of start-up judder is mainly concentrated at about 9 Hz. A 13-degree-of-freedom powertrain branched model is established to numerically reproduce experimental phenomenon. The validity and accuracy of simulation model in reflecting the characteristics of start-up judder are verified by the experimental results in time–frequency domain. Second, through analyzing clutch friction torque, it can be concluded that the closed-loop positive feedback mechanism caused by the negative gradient characteristic of Stribeck effect is the determining factor for the start-up judder. It promotes aggravated fluctuation in rotational speed of clutch driven plate. The introduction process of negative damping that makes powertrain system divergent is explained in detail. Finally, two theoretical measures are proposed to suppress the vehicle start-up judder. One of the measures is to diminish the absolute value of the negative gradient. It weakens the aggravation effect of the closed-loop positive feedback and hence attenuates the start-up judder. Another measure is to change to positive gradient. It forms a closed-loop negative feedback process that causes the almost disappearance of start-up judder. The effectiveness of the two suppression measures verifies the correctness of the start-up judder mechanism proposed in this paper.

In this paper, a mechanism-oriented control strategy is proposed to cut off the introduction process of negative damping caused by the Stribeck effect for suppressing the vehicle start-up judder. From the experimental analysis, the characteristic frequency of start-up judder is mainly concentrated at about 8.25 Hz, coinciding with the first-order natural frequency of the driveline system. A 13-degree-of-freedom powertrain branched model is established, whose validity and accuracy in reflecting the characteristics of start-up judder are verified by comparing with the experimental results in time–frequency domain. The start-up judder mechanism is profoundly revealed through explaining the introduction of negative damping and the origin of characteristic frequency. The positive-feedback closed loop caused by the negative gradient characteristic of the Stribeck effect is the determining factor that promotes the aggravation of the fluctuation in the rotational speed of clutch-driven plate, which is the manifestation of the negative damping. The core idea of the mechanism-oriented control strategy is to cut off the positive-feedback closed loop, which is achieved by fine-tuning the position of release bearing. The judder-suppression performance behaves well both in simulation and in experiment.

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