Computational study of the gas-dynamic approach for noise reduction in the two-stroke engine’s exhaust system

BACKGROUND: The traditional approach to designing exhaust mufflers relies mainly on energy dissipation. In the gas-dynamic approach, the flow of exhaust gases is equalized by introducing long channels into the muffler to separate impulses and to shift them in time. It is assumed that this ensures noise reduction without generation of significant counterpressure.

AIM: Evaluation of the prospects of the gas-dynamic approach to reducing the noise level of the exhaust system of two-stroke internal combustion engines.

METHODS: The study has a computational and theoretical nature. The study object is the RMZ-551i two-stroke gasoline two-cylinder engine, which exhaust system includes a resonator (ensures gas-dynamic supercharging) and a muffler. The processes in the gas-air circuit of the piston engine were calculated using the 1D model. The noise characteristic was the effective sound pressure at a specified point in the environment, calculated using the 2D model of propagation of disturbances in elastic medium. Initially, the engine parameters and sound pressure level with the stock muffler at full load and close to nominal engine speed were calculated. Then, the structure of the stock muffler was modified by adding a channel between its two chambers. The parameters of the modified muffler were optimized based on the criterion of gas pulsations reduction at the outlet. The noise reduction of the muffler implementing the gas-dynamic approach was evaluated relatively to the stock muffler and expressed in terms of sound pressure levels in dB. The parameters and sound pressure were finally calculated over a wide range of engine speeds.

RESULTS: According to the computational estimation, the optimal implementation of the gas-dynamic approach in the muffler reduces exhaust noise by 7 dB, while engine power decreases by 2.5%. Calculation of the sound pressure level based on the full-load curve showed that at an engine speed of 3000 rpm, the calculated sound pressure exceeds the minimum (99 dB), obtained for the optimally tuned muffler at an engine speed of 5000 rpm, by 8 dB. It is suggested that the gas-dynamic approach with optimization is also applicable for uniform noise reduction over a wide range of engine speeds, with a more complicated design of the exhaust muffler.

CONCLUSION: Theoretical evaluation of the muffler with a tuned channel connecting its two chambers was carried out. The RMZ-551i two-stroke engine with a stock muffler is a basis for comparison. At the optimum point on the full load curve, the exhaust noise was reduced by 7 dB, while the calculated power decrease was insignificant. The authors note the suitability of the methodology for rapid assessments and automated computational optimization of mufflers that utilize wave effects. They also point out the limitations of the models used, which require validation or calibration based on the experimental data. The necessity in the development of applied models of acoustic effects and measuring devices for domestic CAE packages is pointed out as well.