Vehicle NVH characteristics not only contribute to the luxury level of the car, it is an issue that affects customer satisfaction, warranty cost and brand image.
Vehicle NVH (Noise, Vibration and Harshness) characteristics not only contribute to the luxury level of the car, they also have an impact on customer satisfaction, warranty cost, brand image and company’s efforts to gain a competitive edge in the automotive market. The purpose of this article is to increase the awareness level and explore ideas that can lead to better understanding of how we can manage the vehicle NVH effectively without compromising the cost component.
The industry is still facing challenges in the area of vehicle NVH. The complexity of these challenges varies depending on the source and the nature of the NVH. Challenges exist due to known factors such as:
• Complex components interaction in a wide range of driving behaviour, time in service and environmental conditions
• Vehicle NVH annoyance is a subjective term in general; a clear and specific metric that represents all vehicle NVH concerns is not well defined
• Lack of correlation between subjective rating and objective measurements
• Vehicle or sub-system modelling is a challenging multidisciplinary problem in mechanics and contact mechanics
• Lack of full understanding of unintended aspect of changes on vehicle NVH attributes
It is a common practice in the automotive industry to set up a subjective or objective vehicle NVH target at vehicle level followed by down-cascading these vehicle level targets into subsystem and component level targets. This process has not been fully successful due to the non-deterministic nature and lack of robustness of the process. A higher level of understanding is greatly needed to relate component and sub-system requirements to the NVH phenomenon at the vehicle level. This leads to the need to understand each component parameters that have influence on the system or sub-system NVH characteristics. Although great steps towards understanding the vehicle NVH attributes were achieved, the development and assessment technology to understand the component design parameters and components interaction is not yet mature to integrate the NVH into the design process at a very early and preventative stage. Many characteristics of NVH are typically not found until a reasonably representative hardware is built.
There are many sources and causes of noise and vibration in a vehicle— engine, driveline, tires, brakes, wind and electrical drives. While some NVH characteristics are actually admired in certain cases (such as the characteristic noise of a certain brand’s engine or even the sound of the door when slammed), most noise and vibration in the vehicle must be minimised in order to ensure a totally comfortable driving experience. Brake system is another important source of noise and vibration.
The brake system NVH will be used to demonstrate the complexity and the challenges that the automotive industry is facing in one of the NVH areas. General NVH characteristics of brake system include high frequency squeal, low frequency squeal, moan/groan noise and vehicle judder or roughness. The industry achieved great success in detecting and to some degree good understanding of the mechanisms of these NVH attributes. However, many challenges still exist regarding how to control and prevent them at the component design level.
Although, the brake system is viewed as a well known technology from a functional point of view, it is a very complex system from NVH point of view. It consists of many mechanical, electrical, hydraulic and composite components that interact in a complex manner (Figure 1).
The complexity of the brake system is due to its components interfaces and its boundary conditions. Also, it is due to the expectation that the brake has to function consistently in different weather conditions, different driving behaviours and over a long period of time. In order to achieve customer satisfaction, the automotive industry needs to focus on six customer NVH attributes for brake system: front and rear noise, front and rear roughness, actuation and modulation noise.
Huge efforts and millions of dollars are being invested by vehicle manufactures and their suppliers to build NVH laboratories, perform research in developing simulation tools and understanding the physics behind this topic. Improvements have been achieved but total control and cost of brake NVH remain a challenge.
The current best practice that is followed during the brake NVH development at Bosch is shown in figure 2. It is clear that simulation and experiments are used in parallel during the brake NVH development. Until the NVH assessment technologies like CAE advance enough to predict the brake NVH characteristics before building hardware, it is highly recommended that the current approach be followed.
The automotive industry can go forward and achieve customer satisfaction in brake and vehicle NVH without adding huge costs by adopting the following practices:
• Treat sub-systems’ or components’ NVH characteristics as part of the whole vehicle system
• Vehicle NVH is a system issue that requires both OEMs and suppliers to work together as partners
• NVH should be viewed as an important function of the system and considered in all engineering and manufacturing changes and planning
• Increase the level of understanding of components functionality including NVH, and of components’ and sub-systems’ interactions on overall vehicle NVH
• Discipline in project development with respect to timing, supplier selection, late design changes and material or manufacturing processes that require NVH rework or compromising NVH validation
• NVH is a complex phenomenon, and this complexity is generally not well recognised in engineering and manufacturing
• View the cost of NVH in the development phase as an investment not as a burden
• Secure an adequate testing facility and simulation tool capacity to accommodate most of the operating conditions and increase tests samples to reduce NVH risk
• Assure product quality to reduce part variations and ensure consistent NVH performance. More attention needs to be given to the manufacturing process, quality and control. This will minimise unintended NVH concerns
• Align the project’s milestones to include design, manufacturing, quality, system performance and NVH development
• Continue improvements on simulation tools and laboratory procedures to provide reliable and cost-effective NVH diagnosis and solutions.
It was noticed that the OEMs and suppliers who produce products with better NVH characteristics at a reasonable cost adhere more to the above practices. Though all NVH causes are not well known, NVH issues can be managed if the NVH is well integrated into engineering and manufacturing processes, and is made part of the company culture.
It is important that the vehicle NVH is considered and NVH engineers are involved in the early engineering decisions and that an NVH process is an integral part of product development. This will ensure that good NVH characteristics are designed into the product in the same sense as product performance. The result will be a more optimal design to address trade-offs at an early stage and minimising cost of the NVH that is due to late design changes that required NVH reworks or due to lack of NVH validation.
