Generation of Component Noise from Electric Vehicles

Electric Vehicle refers to the vehicle which is powered by battery power supply, converting the power, driven by motor, and conforms to the requirements of traffic safety regulations. Electric vehicles are now being seen the alternative to the conventional vehicles. Electric cars are commonly known not only to be environmentally friendly, but also to be very quiet. While a quiet car is generally a good thing, safety concerns have been raised that Electric Vehicles (EVs) may cause more frequent accidents involving pedestrians than Internal Combustion Engine (ICE) cars, as pedestrians may not notice an approaching EV. The specific standards are established by regulatory authorities for noise emission of electric vehicles. This paper describes different aspects of noise related to electric vehicles. Also, different methods and experiments conducted by researches for better understanding of noise from electric vehicles.

In the 21st century, EVs saw a resurgence due to technological developments, and an increased focus on renewable energy. Government incentives to increase adoptions were introduced, including in the United States and the European Union. Nowadays, the tendency towards the more electric cars is obvious. But Electric (EV) or Hybrid Electric Vehicles (HEV) bring new noise, vibration, and harshness (NVH) challenges such as: high frequency electric motor generator noise, power control unit high frequency switching noise, power-split system gear whine and engine start/stop noise and vibration. The intensity of noise from some of these components depend on the speed of the car, electrical components used and body structure of vehicles. In addition to this when the speed of the electric car is low the noise from tyre/road, wind and other noises from EV are not sufficient to signify the presence of car to road users including pedestrians, cyclist, or visually impaired peoples. The regulations for sound of EV have been set by most of the countries for safety of road user. Some consideration is then given to the methods by which Interior Noise Targets may be set for hybrids and EVs, and how they are cascaded to components around the vehicle, before focussing on some specific issues arising on quiet vehicles with high power traction motors.

Classification of Noise Generation from Electric Vehicles:

Existing noises that become audible on hybrids: gear whines, electric machine noise and driveline booms resulting from a new degree-of-freedom in the driveline. The noises generated in the electric vehicles can be classified into two types: (1) Interior noise, (2) Exterior noise.

Interior noise consists of high frequency noise of electrical motors, Power control unit high frequency switching noise, power- split system gear while and noise because of Engine starting and Stopping. These pose new NVH challenges such as road noise and powertrain noise as they become more noticeable.

In the Internal Combustion engine, the start event can be described in two clearly discrete phases: (A) the initial application of torque by the starting machine, and (B) the firing of the engine as it accelerates.

Electric motors will generally provide maximum torque at low speed, which is precisely the opposite of what the NVH engineer would like. Ideally, torque should be applied progressively, rather than with a harsh initial step. This characteristic is infeasible with a standard, low-cost starter motor with minimal control electronics. A significant challenge for the NVH engineer is to provide a sufficient approximation to the ideal characteristic by either modifying the existing start system at minimum cost or utilizing the traction motor present on all hybrid vehicles without compromising other attributes, or by employing some alternative start device.

The technology used in electric and hybrid vehicle concepts is significantly different from conventional vehicle technology with consequences also for the noise and vibration behaviour, which is dramatically different from conventional vehicles. NVH refinement is an important aspect of powertrain development and the vehicle integration process, being often critical to satisfy customer expectations. Particular attention should be paid to the NVH performance of the vehicles, especially in relation to the subjective perception by driver and passengers.

Second, the extensive use of electric drives and actuators brings different sound than in the former cars. Especially in EV or HEV, where the conventional combustion engines are replaced or doubled by electrical motors, new types of components are introduced in the cars. These components often generate noise of higher frequency. At the same time, the masking effect from the engine noise is lost for EV and random for HEV, meaning that noise from these new components will be more dominant.

At low and medium vehicle speeds, the electric vehicle is about 10 dBA quieter, i.e. about 25% of the noise. At higher speeds, the shares of wind and tire noise (which are independent of the powertrain concept) are increasing, causing the difference to be reduced.

Mechanical noise can be produced by unbalanced rotors, overall rotor condition, rubbing and rolling bearing motions, and mechanical resonances of the stator core and end shields. Stator cores, end shields and fan covers may respond in two ways to the internal transmitted noise. First, they act as an acoustical enclosure, reflecting the noise energy back into the system. Secondly, they act as an acoustic

transmitter and radiator, converting the noise energy to airborne noise radiation or to structure-borne vibrations. EM forces generated by electric motors are generally lower than the combustion and reciprocating mass forces of an IC engine, and significantly, they are at a much higher frequency. As a result, the rubber isolation systems used to mount the electric motor to the body can be tuned more efficiently and achieve a much higher level of isolation than with an IC engine. Also, the noise radiated directly from the motor is generally quite high in frequency (>1000 Hz), which is easier to block and absorb with conventional acoustical materials than to lower frequencies.