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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.

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.

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.

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.

In recent years, the ANC technology was adapted for intake and exhaust systems on several passenger vehicles with a variety of diesel and gasoline engines. In these predevelopment activities, the active components (actuators, electronic hardware and software) were rigorously improved and its durability professionally validated leading to an increased system performance even under very harsh conditions. Depending on the customer targets, this yielded several benefits when compared to conventional exhausts. For example, a passive conventional dual exhaust system of a highly efficient 4-cylinder gasoline engine could be replaced by an active single exhaust line without compromising on the acoustic targets but saving a considerably amount of design space and weight. In particular, the space advantage can be utilized to create innovative exhaust layouts. To support future downsizing of engines, for example, an ultra-compact exhaust layout is proposed.

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 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.