How Does An Exhaust Silencer Work?

The pulsating flow from each cylinder's exhaust process of an automobile petrol or diesel engine sets up pressure waves in the exhaust system-the exhaust port and the manifold having average pressure levels higher than the atmospheric. This varies with the engine speed and load.

At higher speeds and loads the exhaust manifold is at pressures substantially above atmospheric pressure. These pressure waves propagate at speed of the sound relative to the moving exhaust gas, which escapes with a high velocity producing an objectionable exhaust boom or noise.

A suitably designed exhaust silencer or muffler accomplishes the muffling of this exhaust noise. The basics of silencing can be understood by recalling a few principles of physics. The velocity of sound in the gas at a given temperature is directly proportional to the square root of the product of the pressure and the ratio of the specific heats (at constant pressure to that at constant volume), and inversely to the square root of the density of the gas.

As the temperature varies, the velocity also varies directly as the temperature by another square root law involving the product of the coefficient of thermal expansion of the gas and the temperature. The exhaust noise can be reduced appreciably by providing resonance chambers to offset the noise wave effects.

This is accomplished by the principle of the Helmholtz resonator. In principle, it comprises the exhaust pipe, which goes through the large volume of a chamber.

The axial holes in the exhaust pipe enclosed by the chamber allow the gases to vibrate with the large mass of the gases in the chamber (forming a spring-mass vibrating system) and generate the sound of the same frequency but in opposite phase to that which has to be nullified (called anti-sound).

To achieve this the muffler volume should be proportioned to the engine piston displacement, and inversely proportioned to the engine speed and the square root of the number of engine cylinders. The usual length to diameter (l/d) ratio of the resonator is about 4:1 to 8:1.

A small l/d ratio muffler attenuates the sound well for a narrow frequency band, where as the large l/d muffler attenuates the sound to a lesser degree but over a wider frequency band. The effectiveness of the exhaust system in silencing the exhaust depends also on the relative lengths of the exhaust pipe (from the exhaust manifold to the muffler) and the tail pipe. A ratio of 1:2 is better than 4:1, and 1:1 is the poorest ratio.

Since the narrow frequency range limits the resonant chamber application, other features are incorporated in the resonant chamber to produce friction effects and filter off noise effects of other offending frequencies. Provision of baffles, resonator mufflers with end baffles, resonator with centre baffle chamber and four-chamber muffler are illustrative examples. In early stationary engines, muffling of the sound was accomplished by allowing the gases to expand by changing the direction of flow or by cooling them with injected water.

Published in The Hindu on April 11, 2002.