Approximating Engine Tailpipe Orifice Noise Sound Quality Using a Surge Tank and In-Duct Measurements

Because of the need to safely vent exhaust gases, most engine dynamometer facilities are not well suited to measuring engine exhaust orifice noise. Depending on the location of the dyno facility within the building, the exhaust system may need to be extended in order to properly vent the exhaust fumes. This additional ducting changes the acoustic modes of the exhaust system which will change the measured orifice noise. Duct additions downstream of the original orifice location also alter the termination impedance such that in-duct pressure measurements with and without the extended exhaust system can vary significantly. In order to minimize the effect of the building’s exhaust system on the desired engine exhaust system measurements, the present approach terminates the engine exhaust into a large enclosed volume or surge tank before venting the gases into the building’s ventilation system. The large volume of the surge tank produces acoustic reflections similar to those of an orifice open to the atmosphere. Two pressure transducers upstream of the surge tank are used to separate out the forward and backward traveling acoustic waves in the duct and approximate the particle acceleration at the exhaust orifice location using linear acoustic assumptions. The radiated orifice noise is then estimated from the particle acceleration using a simple source model. Comparisons of this approximated orifice noise with the actual external measurements reveal that this method correctly captures trends in the dominant engine orders and predicts the amplitudes of these orders within about 5 dB. INTRODUCTION Measuring engine exhaust orifice noise on a dynamometer typically requires a facility built specifically with exhaust noise measurements in mind. For indoor facilities, the tailpipe is usually routed to an isolated anechoic chamber which allows the engine radiated noise to be separated from the tailpipe orifice noise. This requires a dedicated room for measurement, however, along with a separate ventilation system to remove the exhaust gases from this room. Alternately, if the dyno room is constructed adjacent to the exterior wall of the building, the tailpipe can be routed through the wall and the noise measurements taken outside the building. This type of arrangement is not possible in many buildings though. The present work investigates a technique for measuring the exhaust orifice noise when a dedicated facility similar to those described above is not available and the engine exhaust must be connected to a ventilation system within the building. In the present approach, the tailpipe is routed into a large chamber or surge tank before entering the building ventilation system. If the surge tank is properly designed, the reflections from the tailpipe at the surge tank will be similar to the reflections from an open ambient. As long as the original tailpipe length is maintained when the surge tank is added to the system, the acoustic properties of the original system are retained. Following this introduction, the methodology section describes how two pressure transducers placed in the tailpipe can be used to separate out the forward and backward travelling waves in the tailpipe using the two-microphone technique (ASTM, 1990; Chung and Blaser, 1980). This is then used to approximate the particle acceleration at the tailpipe opening and estimate the radiated noise at a distance from the tailpipe. Next, the experimental setup used to validate this approach is discussed. Experiments were performed both with and without the surge tank in order to separate out deviations caused by the 2-microphone methodology from those introduced through the addition of the surge tank. Results for these two configurations are then presented and compared to the actual externally measured orifice noise.