Idle NVH (Noise Vibration Harshness) is one of the major quality parameters that customer looks into while buying the vehicle. Idle shake is undesirable vibrations generated from Engine while it is in idling condition. These low frequency vibrations affects both driver and passenger comfort. Vibrations are perceived by customer through the interfaces such as the seats, floor, and steering wheel. The frequencies of vibration felt by customer ranges between 10-30 Hz and varies based on engine configurations. There are two factors that are critical to the vehicle idle NVH quality, 1. Engine excitation force and 2. Vehicle sensitivity to excitation forces (Transfer function). Even though the engine excitation forces are governed by cylinder combustion process inside the cylinder and engine mass, it is also largely affected by how well the engine and transmission are supported on vehicle through isolators. The location and stiffness of these powertrain mounts play an important role in reducing the energy transfer from engine to vehicle. The conventional approach of mount configuration and design is mainly focused on rigid body modes of the powertrain and mount characteristics to isolate the energy. However, it is realized that this approach is not robust enough to ensure first time right idle NVH behavior.
In this paper, a new holistic approach for analyzing idle shake is discussed using multi-body simulations. In this approach, a full Functional Digital Car is modeled which consists of Powertrain and Vehicle systems modeled and integrated together. Here, pressure vs. crank angle forces combined with the dynamic disturbances of suspended powertrain are used simultaneously as an input to calculate the vibrations at engine mount locations. These vibrations in turn gets transferred through mounts and chassis to Driver Seat and Steering Wheel (at various customer touch interfaces). The complex modal behavior of the chassis and body determines the amplitude and frequencies of vibrations observed at customer touch points mentioned above.
This approach provides substantial flexibility to optimize the powertrain mount locations, stiffness and damping characteristics to directly control the vibrations observed at customer touch points. The optimized mount configuration through this approach has shown better Idle NVH characteristic and also good correlation with NVH Test results. Hence this concept of predicting Objective Idle Vibrations at Full Vehicle level using multi-body simulations has proved to be useful at an early stage of powertrain mount development.