Modeling and Analysis of Clutch Engagement Judder in Commercial Vehicle Powertrain Systems

Transient events in a vehicle driveline such as tip in tip out, clutch engagement-disengagement, engine start, etc. are very important considering NVH characteristics and hence comfort of a vehicle system overall. Clutch engagement (especially during vehicle launch from a rest condition) is one of the most important transient events in commercial vehicles utilizing automated manual transmissions (AMTs). This paper presents common NVH issues observed during vehicle launch in a commercial heavy duty truck. One of the most important NVH issues is clutch engagement judder. Judder is seen when torque fluctuations are generated in a slipping clutch which induce undesired driveline vibrations. This paper concentrates on friction judder as well as geometric judder. Different friction materials are compared for their performance against friction judder. Performance of friction materials with respect to slip speed, temperature, and pressure is also studied. Geometric judder and how cushion discs can be effectively used as a counter measure against geometric judder are also discussed in later part of the paper. Cushion disc stiffness comes out to be a very important parameter considering the performance of the clutch against geometric judder. Both simulation and vehicle test results show the importance of friction material selection and cushion disc design for better design of clutches against judder. Apart from clutch engagement judder, few other issues during vehicle launch like inconsistent vehicle launch due to driveline backlash and impact of clutch controls are also presented with their mitigation actions.