NVH Reduction and Brake Design in Auto Manufacturing
Noise, Vibration and Harshness reduction in automotive design is nothing new, but increasingly stringent EU legislation is putting more pressure on manufacturers to develop quieter, more ‘acoustically comfortable’ vehicles.
Due to the wide-ranging causes of NVH, very few areas of manufacturing do not take some account of noise reduction in design. Much focus has been on improving internal acoustics for driver and passengers through a number of innovations from aerodynamics to engine noise control, but external noise is also an issue. The aim of reducing external noise from road vehicles has been addressed by the EU’s pass-by regulations, which came into force in 2016, in order to limit noise pollution particularly in urban areas.
In terms of braking, efforts to reduce NVH are developing in parallel with the trend towards electrification and there is considerable crossover in terms of design. As manufacturers move towards automatic emergency braking and systems more reliant on electronic controls, there is potential for new braking technology to come to the fore. This, of course, must be balanced with safety, functionality, and efficiency to meet existing parameters.
The EU paper, ‘Guidelines for Road Traffic Noise Abatement’, states that road traffic is a main source of noise in urban areas, accounting for about 80% of total noise pollution. Among many initiatives to reduce noise in urban areas, the European Commission’s regulations aim to help reduce the noise produced by passenger cars, light commercial vehicles, buses, light trucks, coaches, and trucks by 25%.
The regulations, which came into force in July 2016, include the following changes to EU noise emission limits:
New Test Method - A new test method, developed by the Economic Commission for Europe of the United Nations, has been introduced which better reflects current driving behaviour and real-world noise emissions from urban driving.
Lower Limit Values - Passenger cars, buses and light trucks will be subject to a lowering of noise limits in two steps of 2 decibel A-weighting each. For heavy-duty vehicles, the lowering of limit values will be by 1 dB (A) at the first step, and by 2 dB (A) at the second step.
Additional Sound Emission Provisions (ASEP) - Type-approval procedures which will ensure that noise levels are tested in real driving conditions, outside of the type-approval driving cycle.
Minimum Noise - This annex covers the minimum noise (Approaching Vehicle Audible Systems) of electric and hybrid electric vehicles.
Noise labelling - a protocol to increase requirements of noise labelling at the dealership, obliging manufacturers to display the sound level of each vehicle during sale.
Impact of Brakes on NVH
Disc brake noise, in addition to the general contribution to NVH, is also a significant contributor to OEM warranty costs. While excessive noise rarely has an effect on the performance of braking systems, consumers perceive the noise as a problem and expect it to be ‘fixed’ under warranty. In this respect, optimising NVH performance can reduce warranty costs and brand rejection in addition to contributing to overall noise reduction.
NVH from braking systems is generally well understood, and the automotive industry has sought to minimise friction noise and vibrational energy through a number of methods. These include mechanical fixes, selection of ‘quiet’ friction materials, and the modification of components’ natural frequencies to avoid close coincidences. The electrification of vehicles and the development of new brake technology has presented new challenges, but also offers a wide range of potential solutions through new materials, computer-aided design simulation, and electronically-controlled systems. Much of the focus is on improving safety and functionality, reducing weight and bettering fuel efficiency, and advances in noise reduction must be developed alongside these critical concerns.
Trends in Braking Technology
The trends in braking technology are being driven by all of these factors, and electrification could be the biggest driver in the coming years.
As automakers increase their range of electric and hybrid electric vehicles range becomes an important factor, while reduced noise levels of the powertrain can serve to sharpen the focus on other causes of noise and vibration such as the braking system. Regenerative braking has also become a hot issue in the industry in electric vehicles due to the contribution such systems can make to fuel efficiency and range. In the US, 20 manufacturers, including BMW, Audi, VW and Volvo, have committed to ensuring all new cars feature Automatic Emergency Braking (AEB) by September 2022.
In terms of increasing range of electric vehicles and reducing unwanted noise and vibration, a hot topic is the reduction of residual drag. Any contact between brake pads and discs when not in use can adversely affect fuel efficiency and therefore range, while having the potential to cause unnecessary noise or vibration. In conventional systems this can be caused by brake disc imperfections, and corrections - such as moving pad away from disc - can have a negative effect on brake pedal feel. Newer systems based on electronic controls whereby the brake is decoupled from the pedal have the potential to mitigate against this problem.
A 100% reduction in noise and vibration across the audible range (20Hz to 20kHz) is a huge challenge in any friction brake system, but the trend towards EV’s and HEV’s make the issue more critical due to considerably quieter drivetrains. This is a key area for the industry to tackle, and much research is being undertaken in the use sophisticated computer modeling to aid design.
Another consideration with EV’s and HEV’s is the capacity for regenerative decceleration through the operation of electric motors as generators. If future brake applications are included in the regenerative decceleration system, this will impact upon brake pressure, temperature and other factors, which may significantly alter the typical operation and life cycle of brake components.
A brief look at two of the largest brake suppliers to the automotive industry gives an indication of the current state of development of brake design.
ZF TRW’s Integrated Brake Control (IBC) is a non-vacuum, integrated electro-hydraulic brake apply control unit which can replace the electronic stability control, vacuum booster, vacuum pump, and associated cables, sensors, switches and controllers. The module is 6kg lighter than conventional systems, requires around half the space, and enables up to 10% more braking regeneration. Brake pedal feel is delivered through simulation, and the system offers huge potential for application in electric vehicles.
ZF TRW is due to start volume production for a US-based automaker in 2018, and expects the market to grow to 8% application in vehicles worldwide by 2020, and 20% application by 2025.
In August this year Continental introduced its New Wheel Concept, specifically designed to optimise the braking system for electric vehicles. The design uses a large aluminium brake disc to solve the problem of poor performance due to corroded brake discs, while reducing overall weight of the wheel and brake.
The wheel rim consists of two aluminium parts - an inner aluminium carrier star with the aluminium brake disc, and the outer aluminium rim well with the tyre. The New Wheel Concept engages the brake disc from the inside, allowing it to have a particularly large diameter thus improving brake performance. To increase electric vehicle range decceleration generates as much electricity as possible through recuperation, so the wheel brake is used less frequently. The corrosion-free aluminium disc prevents the buildup of rust which can affect performance.
The New Wheel Concept is based on a new division between wheel and axle. The wheel, in Continental’s concept, consists of two parts - the aluminium carrier star which remains permanently bolted to the wheel hub, and the rim well which is bolted to the star. The wheel brake is fastened to the wheel carrier of the axle and engages from the inside with an annular aluminium brake disc, which in turn is bolted to the carrier star. The internal brake permits a wide brake disc friction radius as the space available in the wheel is optimally utilised.
Continental’s new brake design comes hot on the heels of its MK C1 brake system, which made its debut in 2016 in the Alfa Romeo Giulia.
NVH reduction will continue to be a big issue for the automotive industry in the coming years. The challenge of reducing noise from various components and parts of a vehicle is made more difficult by a moving landscape of automotive design that must also take into account a wide range of other criteria.
Safety, functionality, automatic emergency braking, electronically controlled systems, regenerative braking and various other trends all have an effect on considerations of noise reduction, but can also provide innovative solutions to problems which would otherwise be constrained by conventional, mechanical systems. The braking concepts that succeed in the future will be those that converge best with these trends.