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Exhaust and muffler noise is a challenging problem in the transport industry. While the main purpose of the system is to reduce the intensity of the acoustic pulses originating from the engine exhaust valves, the back pressure induced by these systems must be kept to a minimum to guarantee maximum performance of the engine. Emitted noise levels have to ensure comfort of the passengers and must respect community noise regulations. In addition, the exhaust noise plays an important role in the brand image of vehicles, especially with sports car where it must be tuned to be “musical”. However, to achieve such performances, muffler and exhaust designs have become quite complex, often leading to the rise of undesired self-induced noise. Traditional purely acoustic solvers, like Boundary Element Methods (BEM), have been applied quite successfully to achieve the required acoustic tuning.

For most car manufacturers, aerodynamic noise is becoming the dominant high frequency noise source (> 500 Hz) at highway speeds. Design optimization and early detection of issues related to aeroacoustics remain mainly an experimental art implying high cost prototypes, expensive wind tunnel sessions, and potentially late design changes. To reduce the associated costs as well as development times, there is strong motivation for the development of a reliable numerical prediction capability. The goal of this paper is to present a computational approach developed to predict the greenhouse windnoise contribution to the interior noise heard by the vehicle passengers. This method is based on coupling an unsteady Computational Fluid Dynamics (CFD) solver for the windnoise excitation to a Statistical Energy Analysis (SEA) solver for the structural acoustic behavior.

Acoustics comfort is a key point for the ground transportation market and in particular in the automotive area. A significant contributor to the noise levels in the cabin in the range 200Hz to 3000Hz is the HVAC (Heating, Ventilating, and Air Conditioning) system, consisting of sub-systems such as the air intake duct, thermal mixing unit, blower, ducts, and outlet vents. The noise produced by an HVAC system is mainly due to aeroacoustics mechanisms related to the flow fluctuations induced by the blower rotation. The structure borne noise related to the surface induced vibrations and to the noise transmission through the dash or plastic panels may also contribute but is not considered in this study. This study presents a digital approach for HVAC aeroacoustics noise predictions related to the ducts and outlet vents. In order to validate the numerical method flow and acoustics measurements are performed on production HVAC systems placed in an anechoic room.

In the past years, Eberspaecher has developed active exhaust silencers for several passenger vehicles with different engines on a prototype level. In general, a substantial reduction of the exhaust noise is regularly achieved in a frequency range of 40 - 400 Hz covering the most relevant engine orders. In exhaust system development the main design conflicts are noise reduction, silencer volume/weight and backpressure. Recent progress was made in the development of the durability and industrialization of the actuator. This component could be reduced in size and weight thus allowing the integration in different design spaces of many vehicles. In some cases, the conventional dual exhaust system can even be replaced by an active single exhaust line without compromising on the acoustic or backpressure targets but saving > 50 % of space and > 30% weight. The potential impact of this technology on future vehicle designs will be discussed.

Over the past few years, the measurement procedure for the pass-by noise emission of vehicles was changed and new limit values have been set by the European Parliament which will come into force within the next few years. Moreover, also the limits for chemical emissions such as NOx, particulates and CO2 have been lowered dramatically and will continue to be lowered according to a roadmap decided not only in Europe but also in other markets throughout the world. This will have an enormous impact on the design of future passenger cars and in particular on their powertrains. Downsizing, downspeeding, forced induction, and hybridization are among the most common general technology trends to keep up with these challenges. However, most of these fuel saving and cleaner technologies also have negative acoustic side effects.

Over the past decades, interior noise from wind noise or engine noise have been significantly reduced by leveraging improvements of both the overall vehicle design and of sound package. Consequently, noise sources originating from HVAC systems (Heat Ventilation and Air Conditioning), fans or exhaust systems are becoming more relevant for perceived quality and passenger comfort. This study focuses on HVAC systems and discusses a Flow-Induced Noise Detection Contributions (FIND Contributions) numerical method enabling the identification of the flow-induced noise sources inside and around HVAC systems. This methodology is based on the post-processing of unsteady flow results obtained using Lattice Boltzmann based Method (LBM) Computational Fluid Dynamics (CFD) simulations combined with LBM-simulated Acoustic Transfer Functions (ATF) between the position of the sources inside the system and the passenger’s ears.

Nowadays vehicle quality is rated for noise and vibration and the interior sound levels have become a major target of automotive companies. Strides have been made in reducing power train, tire and external wind noise over the years. However, HVAC and blower fan flow-induced noise reaches the interior cabin without any sound isolation and can strongly impact customer comfort. In the early stage of vehicle design, it is experimentally difficult to get an estimate of the flow pattern and sound levels. The goal of this study is to develop and validate a numerical noise prediction tool for complete HVAC systems noise, defined as the arrangement of sub-systems such as air intake duct, thermal mixing unit, blower, ducts and outlet vents. This tool can then be used during the development of vehicles to evaluate and optimize the aeroacoustics performances of the system without additional or belated experiments.

In the past years Eberspaecher has installed Active Exhaust Silencers on several passenger vehicles with different diesel and gasoline engines on a prototype level. Meanwhile, a substantial reduction of the exhaust noise is regularly achieved in a broad frequency range covering all relevant engine orders. Due to the higher acoustic excitation and higher exhaust temperatures in gasoline engines it is more difficult to implement the ANC-technology on those engines. However, results from roller test benches focus on the acoustic performance as well as weight and volume reductions and demonstrate a marked improvement which was achieved with gasoline engines too. Further progress was made in the development of the durability and industrialization of all relevant components of the system. Finally, current design trends and possible fields of application will be discussed.

In all major markets, the legislation on emissions (CO2, NOx, and PM) and on pass-by noise limits have been drastically lowered over the last years and will be even lower in the near future. This will have an enormous impact on the design of future passenger cars, their powertrains, and finally their exhaust systems with an inherent cost penalty. Moreover, the individual market requirements in Europe, Asia and the Americas differ in some respect often calling for individual variants. The increasing number of vehicle models and platform derivatives e.g. sedan, hatchback, coupe, convertible, sports utility vehicle, cross-over etc. leads to a huge variety of exhaust systems even within a single OEM platform let alone a whole OEM product portfolio. This causes significant effort in development, tooling, manufacturing, part handling, and logistics. The Active Noise Cancellation technology (ANC) has been investigated in the automotive industry for many years.

The pass-by noise limits of passenger vehicles according to ISO 362 / R51.03 [1, 2] will be further reduced by 2 dB in 2024 in Europe. Since the pass-by noise is substantially influenced by exhaust noise, the effort for the exhaust system needs to be increased substantially. This results in systems with larger mufflers or higher backpressure. However, the more stringent CO2-emission targets require ever more efficient powertrains, which calls for rather lower backpressure to optimize the engine design. This paper describes how compact active exhaust lines can support a design for low backpressure and high acoustic attenuation at the same time. For two passenger vehicles with gasoline engines active exhaust lines are investigated in detail and the results are compared to the series production exhaust lines. Thus, in one exemplary case, the pass-by noise of a limousine could be reduced from 70 dB(A) to 68 dB(A) without any change in the vehicle design except the improved exhaust system.