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Vehicles with electrified powertrains are being introduced at an increasing pace. On the level of interior sound, one is often inclined to assume that NVH problems in EV have disappeared together with the combustion engine. Three observations demonstrate that this is not the case. First of all, only the dominant engine sound disappears, not the noise from tire, wind or auxiliaries, which consequently become increasingly audible due to the removal of the broadband engine masking sound. Secondly, new noise sources like tonal sounds from the electro-mechanical drive systems emerge and often have, despite their low overall noise levels, a high annoyance rating. Thirdly, the fact that engine/exhaust sounds are often used to contribute to the “character” of the vehicle leads to an open question how to realize an appealing brand sound with EV. Hybrid vehicles are furthermore characterized by mode-switching effects, with impact on both continuity feeling and sound consistency problems.

This paper presents the results of an experimental test campaign carried out on a city bus powered by serial hybrid power train. The driveline system combines an Internal Combustion Engine with a battery pack and two electric motors. Tests were aimed at identifying the salient signal characteristics of the noise spectra recorded during operating conditions and to assess the acoustic comfort in the passenger compartment. Transfer Path Analysis technique was applied to identify airborne and structure borne vibro-acoustic loads, to measure transfer functions linking source locations to target locations and to estimate the internal vibro-acoustic comfort in operating conditions.

The Transfer Path Analysis method is at the core of the Source-Transfer-Receiver approach to address noise and vibration problems. While originally developed for analyzing structure-borne noise transmission, its application range has been extended to airborne noise. Various frequency and time domain approaches have been developed with a focus of supporting specific design engineering problems. One such application is the source contribution analysis in the context of vehicle pass-by-noise. The upcoming changes in the pass-by noise regulation will not only require more complex tests in different conditions but most importantly, the new directive will force car manufacturers to further reduce the emitted noise levels of their vehicles.

The increasing contribution of electronic and mechatronic content to the vehicle value requires rethinking the vehicle design and engineering processes. The present paper describes an approach hereto, based on virtual and physical prototype testing of heterogeneous systems. A key element is the integration between 3D geometry-Based Models (FE, MBS) and 1D multi-physics system-theoretic models for simulating complex devices (hydraulics, actuators, specific sensors) and processes (combustion, thermal, flow). Embedding control laws paves the way to Model-ln-the-Loop (MIL) and Software-In-the-Loop (SIL) concepts. By linking the virtual models to hardware systems on a physical test-bench, the static and dynamic performance of the rest of the vehicle system (suspension, body…) can be represented, enabling “Hardware in the Loop testing” (HIL). The “Vehicle-in-the-Loop” (VIL) validation finally allows the evaluation of all system dependencies and system interconnections.