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Demands for engines to operate at low-frequency firing order are increasing in the automotive market. This requirement is driven by consumer and regulatory demand for vehicles which are more efficient in the use of fuel. As a result, engine and transmission technologies have been developed which permit operation of engines with fewer cylinders at increasingly low RPM’s. The resulting low frequency exhaust noise is more difficult to attenuate than in vehicles in years past. At the same time, vehicles often have less packaging space for mufflers, when larger volume would otherwise be needed to attenuate at lower frequencies. A further challenge is the demand for increasingly refined performance sounds from the exhaust systems of premium cars despite the technical obstacles involved in even maintaining sound quality. Finally, legally permissible sound levels are decreasing in some markets. These market and regulatory demands require new solutions.

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.

Cylinder deactivation is an efficient way to significantly improve fuel economy without compromising the horsepower and torque. For an exhaust system to manage both V4 and V8 modes, a unique development approach for exhaust system design needs to be incorporated. Simulation tools and correlation testing can be used to achieve great sound quality without the use of an active valve design classically (believed to be required) implemented. A quasi-Computer Aided Engineering (CAE) 1-Dimensional code drove the design of the passive (without a valve) system. The 1-Dimensional code analyzes pressure waves, mass flow and energy losses in ducts and junctions. To achieve the objective for cylinder deactivation, this tool also allows significant changes with relatively fast results for trends to be evaluated. The goal for this cylinder deactivation system was to have a smooth transition between the two modes and achieve the commercial performance with low backpressure.

The exhaust system is one of the major P/T systems for sound quality tuning. The many varieties in exhaust pipe routing and the flexibility in muffler design make it possible to design an exhaust system to deliver tailpipe sound for specific sound quality requirements. It is essential that the tailpipe sound be balanced with other P/T sound to yield the overall sound targets. The primary contribution of an exhaust system is the firing and sub-firing orders. The typical tailpipe sound target contains banded targets for “good” orders as well as “do-not-exceed” targets for the rest. Every order target needs to be met in order to yield the right tailpipe sound. In most cases, the pipe routing and the muffler volumes of mufflers are dictated by package constraints, however, the internal design of muffler with a given volume can create quite different tailpipe sounds.