23-07-2012, 04:41 PM
Car Muffler
MUFFLERS.doc (Size: 200.5 KB / Downloads: 70)
Introduction
A car muffler is a component of the exhaust system of a car. The exhaust system has mainly 3 functions:
1. Getting the hot and noxious gas from the engine away from the vehicle
2. Reduce exhaust emission
3. Attenuating the noise output from the engine
The last specified function is the function of the car muffler. It is necessary because the gas coming from the combustion in the pistons of the engine would generate an extremely loud noise if it were sent directly in the ambient air surrounding engine through the exhaust valves. There are 2 techniques used to dampen the noise: absorption and reflection. Each technique has its advantages and disadvantages.
The absorber muffler
The muffler is composed of a tube covered by sound absorbing material. The tube is perforated so that some part of the sound wave goes through the perforation to the absorbing material. The absorbing material is usually made of fiberglass or steel wool. The dampening material is protected from the surrounding by a supplementary coat made of a bend metal sheet.
The advantage of this method is low back pressure with a relatively simple design. The inconvenience of this method is low sound damping ability compared to the other techniques, especially at low frequency.
The mufflers using the absorption technique are usually sports vehicle because they increase the performances of the engine because of their low back pressure. A trick to improve their muffling ability consists of lining up several "straight" mufflers.
Definition of destructive interferences
Let's consider the noise a person would hear when a car drives past. This sound would physically correspond to the pressure variation of the air which would make his ear-drum vibrate. The curve A1 of the graph 1 could represent this sound. The pressure amplitude is a function of the time at a certain fixed place. If another sound wave A2 is produced at the same time, the pressure of the two waves will add. If the amplitude of A1 is exactly the opposite of the amplitude A2, then the sum will be zero, which corresponds physically to the atmospheric pressure. The listener would thus hear nothing although there are two radiating sound sources. A2 is called the destructive interference.
Back pressure
Car engines are 4 stroke cycle engines. Out of these 4 strokes, only one produces the power, this is when the explosion occurs and pushes the pistons back. The other 3 strokes are necessary evil that don't produce energy. They on the contrary consume energy. During the exhaust stroke, the remaining gas from the explosion is expelled from the cylinder. The higher the pressure behind the exhaust valves (i.e. back pressure), and the higher effort necessary to expel the gas out of the cylinder. So, a low back pressure is preferable in order to have a higher engine horsepower.