The september 18, 2017 Update

Electric and hybrid vehicles take centre stage among environmentally-friendly vehicles¹. In addition to generating less atmospheric pollution, they are also recognized as being very quiet in urban traffic conditions. This gives them the potential to reduce urban noise pollution.

The sources of light vehicle noise

The noise generated by vehicle passage has two main components: noise from the power train and rolling noise caused by contact when the tyre rolls over the pavement. There is little tyre-pavement contact noise at low speeds, but it becomes dominant when vehicles move faster.

In the case of a vehicle with a conventional engine, either petrol or diesel, the noise from the power train dominates at low speed and therefore adds to noise pollution in urban areas. In the case of an electrically-powered vehicle (an electric vehicle, or a hybrid vehicle in fully electric mode), the engine noise is low, and the rolling noise is therefore the main source of noise over a large range of speeds. These vehicles have the advantage of being almost silent when stationary. It should nevertheless be remembered that vehicle manufacturers have developed some vehicles with conventional engines which also have a very low noise emissions. The noise emitted by electric and hybrid vehicles is therefore the subject of many studies, at the LAE in particular, in the framework of the FOREVER project^2.

Heavy vehicles are also involved

Heavy vehicle manufacturers are also developing electric or hybrid vehicles, for example delivery lorries or household refuse collection vehicles. In this connection, the LAE taken part in the acoustic evaluation of a hybrid lorry developed in the framework of the GEODE project³. As far as public transport is concerned, the authorities in many cities are becoming increasingly interested in hybrid or fully electric buses, and manufacturers are developing new technologies to increase the electrical range of vehicles. This explains LAE’s participation in the ElLiSup project⁴ which aimed to develop quick charge hybrid or electric buses. These projects highlight the benefits provided by the fully electric mode, while acoustic performance in the hybrid mode depends very much on the hybridization technologies employed. 

Impact on traffic as a whole

While a very significant noise reduction can be achieved for a single electric vehicle, there will only be a marked impact for traffic as a whole if it contains a high proportion of low-noise vehicles. This is due to the characteristic arithmetic that applies to noise levels, which are expressed in decibels. If we imagine a situation in which half of the urban traffic mix consists of electric vehicles whose noise emissions are 10 dB(A) lower and the other half consists of conventional vehicles, the overall noise reduction compared with conventional traffic will only be few decibels and barely noticeable.

Reducing or increasing noise?

The issue of the risk silent vehicles pose for other road users has been raised because they are so quiet at low speeds that pedestrians and cyclists may fail to notice them. In some countries it is advisory or compulsory for vehicles to emit warning signs when travelling at low speeds to make the vehicles more audible. Achieving both the safety of road users and noise reductions for residents therefore opens up a new area for urban noise research.

¹ A hybrid vehicle has both an internal combustion engine and an electric engine, one or other being used depending on the type of hybridization and the situation (speed, acceleration, battery charge level, etc.).
² http://forever.fehrl.org/
³ The GEODE project (GEstion Optimisée De l’Energie), financed by the FUI and Région Rhône-Alpes and managed by Renault Trucks.
⁴ The ElLiSup project (Bus Electrique à recharge rapide batteries Lithium et supercapacités) financed by ADEME and managed by Iveco Bus.