Modeling and optimization for pneumatically pitch-interconnected suspensions of a vehicle

A novel analysis method of the mode and transmissibility properties is presented for the vehicle equipped with a pneumatically interconnected suspension (PIS) system. A pitch-plane 4-degree-of-freedom (4-DOF) half-car model with the front and rear air springs connected through a pipe is derived by integrating the pneumatic strut forces into the vehicle mechanical system. The air flow in the pipe is modeled through a linear differential equation based on Newton's second law in which the air mass inertial effects, the frictional and local pressure drops are considered. The vibration equation of the mechanical-pneumatic coupled system is obtained in frequency domain to describe the relationships between the vehicle motions with the air springs' internal pressures and the rough-road excitations. Based on the system vibration equation, both the vehicle free vibration modes and frequency response functions (FRFs) are compared between the half-car with a PIS and that with a standalone air suspension. The results show that the PIS can suppress the vehicle pitch vibration without affecting its bounce properties, and an additional pneumato-dominated vibration mode is observed in a low frequency range. The effects of pipe length, diameter and the local loss ratio factor on the vehicle pitch transmissibility properties are investigated. The design of experiments (DOE) approach is further applied to obtain an optimal design of the pipe to achieve the desired pitch vibration responses, i.e. the resonance frequency and amplitude and the minimal vibration level, under road random inputs. It shows that the vehicle pitch performance can be conveniently enhanced by designing a pipe with suitable length and diameter of the PIS.