In this paper, the excessively transient Vibration phenomenon of a vehicle equipped with a 3-cylindered 1.2T engine was studied during start-up process. Firstly, the transient vibration of the vehicle was measured at the driver seat position in the car. The peak value of the acceleration along x direction was prominent, and the subjective feel during the start-up process was unacceptable and more aggressive then the same class competitor with 1.5T I4 engine. Secondly, the main excitons including combustion force and inertia force during start-up of 3-cylindered engine were analyzed, and the approximation relationship between powertrain motion and car body vibration was derived. And then a dynamic model of powertrain system with mounting system was built to simulate the start-up vibration of the vehicle under toque and inertia force, the simulation results coincided well with the experiment.

Based on this model, the relationships between force input and vibration output of the car body was discussed, the results showed that the combustion induced torque fluctuation transfer though the mounting system to the car body was the main contribution of the transient vibration during engine start-up process, and the couple between fore-aft mode (along the x direction) and the pitch mode (revolution around crankshaft) of the powertrain mounting system led to a deterioration vibration. Finally, a transfer function H (ω) was defined to describe transmission of the torque fluctuation to the powertrain acceleration at the center of gravity.

By optimization of the mounting system to minimize the amplitude of H (ω), the peak value of transient vibration was reduced by about 15% in physical test. And start-up procedure calibration was adjusted, by increasing the cranking speed from 210 rpm to 300rpm, using a more powerful starter (1.6kW), and abating the combustion torque fluctuation by limiting the combustion pressure, the peak value of the seat vibration was effectively reduced by 40% totally, and the subjective feeling achieved a significant improvement.

Dr. Yan Jin, China Automotive Engineering Research Institute Co. Ltd, CHINA