Electric Motor Noise in a Lightweight Steel Vehicle 2011-01-1724

The present work attempts a complete noise and vibration analysis for an electric vehicle at concept stage. The candidate vehicle is the Future Steel Vehicle (FSV), a lightweight steel body with an electric motor developed by WorldAutoSteel [1,2,3]. Measurements were conducted on two small Mitsubishi vehicles that both share the same body, yet one is equipped with an internal combustion engine and the other with an electric motor. The outcome was used as a starting point to identify assets and pitfalls of electric motor noise and draw a set of Noise Vibration and Harshness (NVH) targets for FSV. Compared to a combustion engine, the electric motor shows significantly lower sound pressure levels, except for an isolated high frequency peak heard at high speeds (3500 Hz when the vehicle drives at top speed). The prominence of this peak is lowered by increased use of acoustic absorbent materials in the motor compartment. For low and mid frequencies, moderate electric motor forces imply less stringent noise and vibration design constraints and a possibility to reduce the body mass. To take full advantage of this opportunity, NVH is integrated early into the FSV design cycle and optimized in parallel with crash performances. Finite element simulations at low and mid frequencies lead to reshaping the suspension mounts, the rear roof, the front header and the cowl top connection area, each change driving large reductions of noise levels while adding little to no mass. Damping sheets prove unnecessary. Lighter damping solutions such as vibration damping steels are examined and proved to be successful in the mid frequency range. Overall, the change from combustion engine to electric motor is compatible with mass reductions and similar or better noise and vibration performances.