Direct Estimation of Acoustic Source Characteristics of the Internal Combustion Engine Exhaust System and Analysis of Complex Muffler Configurations
Reduction in noise radiated from the IC engines is needed to meet the ever increasingly stringent noise regulations. The unmuffled exhaust noise of an IC engine is the biggest contributor to the overall engine noise. Prediction of the unmuffled sound pressure level (SPL) spectrum calls for prior estimation of both the source characteristics, namely, source pressure and source impedance, whereas prediction of the insertion loss (IL) spectrum of a muffler needs only source impedance. The present study aims at estimation of both these source characteristics using a novel direct evaluation method. Computation of source impedance and exhaust gas mass flow rate calls for the aero-thermodynamic computation in the time-domain making use of the values of pressure and temperature of the in-cylinder gas at the exhaust valve opening (EVO) along with the basic geometrical details of the cylinder, intake and exhaust valves or ports. Equivalent source impedance of the time-varying source is estimated as the reciprocal of the crank-angle-area-weighted average of the admittance of the source. A simple, albeit approximate, parametric expression of source impedance is derived in terms of the cylinder capacity for ready reference of the muffler designers. This novel direct evaluation method is now applied to the naturally aspirated (NA) as well as turbocharged (T/C) multi-cylinder engine exhaust system by estimating the source characteristics of each cylinder assuming that the other cylinders are not exhausting at the same time. The estimated source characteristics for each cylinder at the downstream end of the exhaust valve are then transferred downstream to the end of the runner. These are then combined and transferred across the manifold (runners) and turbine to estimate the source characteristics of the complete engine at any desired source-load junction. For turbocharged engines, transfer matrix of the turbine is derived using the mass continuity upstream and downstream of the turbine along with its static pressure ratio. Finally, the estimated source characteristics of the complete engine are used to predict the unmuffled SPL spectrum which is shown to compare reasonably well with the measured spectrum. Efficient mufflers are needed to comply with the stringent noise regulations as well as the requirement of low backpressure to ensure low brake specific fuel consumption. Design and analysis of some commercial complex mufflers are therefore studied here. For mufflers used in multi-cylinder engines, it is observed that anechoic source assumption is reasonable, and therefore, the IL spectrum is computed using characteristic impedance of exhaust pipe as source impedance. The mufflers are analysed using the 1-D integrated transfer matrix (ITM) approach and validated against the 3-D finite element analysis. Three different types of mufflers; double flow-reversal muffler with a few holes in baffles, multiply-connected co-axial (MCCA) perforated element muffler, and side-inlet side-outlet (SISO) perforated element muffler are investigated here. The parametric studies on one of the configurations out of the four different MCCA muffler configurations highlights the crucial role of mean flow on the flow-acoustic performance. Some design guidelines have been developed for the MCCA mufflers with higher specific IL (ratio of overall IL (in dBA) to the muffler-to-engine volume ratio) as well as modest backpressure. In general, large volume of the muffler for a diesel generator (DG) set engine is not a concern, but logistics of an acoustic enclosure require mounting of the exhaust muffler on the top of the enclosure with side-inlet and side-outlet. Four different, yet somewhat similar, SISO perforated element muffler configurations with approximately same backpressure have been investigated. Finally, some design guidelines have been evolved for the SISO mufflers.