Electric Bus NVH Challenges and Refinement Using Innovative Approach

<div class="section abstract"><div class="htmlview paragraph">The trend towards electrification propulsion in the automotive industry is highly in demand due to zero-emission and becoming more significant across the world. Battery electric vehicles have lower overall noise as compared to conventional I.C Engine counterparts due to the absence of engine combustion and mechanical noise. However, other narrowband and tonal noises are becoming dominant and are strongly perceived inside the cabin. With the ongoing push towards electrification, there is likely to be increased focus on the noise impact of gearing required for the transmission of power from the electric motor to the road. Direct coupling of E-motors with Axle has resulted in severe tonal noises from the driveline due to instant e-motor torque ramp up from 0 rpm and reverse torque on driving axle during regenerative braking. The tonal noises from the rear axle during vehicle running become very critical for customer perception. For automotive NVH engineers, it has become a challenge to balance expected noise performance and manufacturing limitations of hypoid gears for rear axles and transmissions in case they are carried forward from existing designs used for ICE configurations.</div><div class="htmlview paragraph">In this paper, the challenges and refinement strategy for driveline NVH issues in a battery electric bus were discussed. The study found that in-cabin noise was unacceptable in both the speed sweep and coasting down between 35-40 km/hr and beyond 50 km/hr speed. Transfer path analysis, operational deflection shape analysis, modal analysis, and order analysis methodologies were adopted to identify the source of noise. The analysis revealed the effect of gear macro and micro geometry on rear axle whine, and the occurrence of resonance frequencies during both speed sweep and coast down. The torsion resonances as well as other subsystem resonances were also getting coupled in the same problematic frequency bands.</div><div class="htmlview paragraph">To improve the NVH, both source and transfer path refinement opportunities were explored. Hypoid gear macro & micro geometry optimized to improve contact ratio & minimize transmission error. The driveline modes were decoupled using propeller shaft stiffness optimization, Inertia ring on the propeller shaft, modal decoupling using mass and torsional dampers, and improvements in attachment stiffness.</div><div class="htmlview paragraph">When all design modifications were assessed on the electric bus, the overall noise level in the problematic speed zone was reduced by around 8-10 dB (A) in speed-sweep as well as coasting down. The electric bus with all feasible solutions was presented to juries, and the subjective rating was improved to 'Fair'.</div></div>