Types, structure and principle of operation of a car muffler
The automobile silencer is designed to reduce exhaust noise in the exhaust system to comply with international standards. It is a metal body, inside which are made partitions and chambers, forming channels with complex routes. When the exhaust gases pass through the latter, sound vibrations of different frequencies are absorbed and converted into thermal energy.
Muffler function in the exhaust system
In the engine exhaust system, the muffler is installed after the catalytic converter (for gasoline-powered vehicles) or the particulate filter (for diesel engines). In most cases there are two:
- Pre-muffler (muffler resonator) – designed to roughly suppress noise and stabilize fluctuations in the flow of exhaust gases coming out of the engine. It is installed first, so it is often called “front”. One of its main functions is to distribute the exhaust gases in the system.
- The main silencer is designed for final noise suppression.
In practice, the vehicle silencer arrangement provides the following noise-reducing transformations of the exhaust:
- Changing the cross-section of the exhaust flow. It is realized due to the presence in the design of chambers of different cross-section, which allows absorbing noises of high frequency. The principle of the technology is simple: first, the moving flow of exhaust gases narrows, which creates some acoustic resistance, and then expands sharply, as a result of which the sound waves are dispersed.
- The redirection of the exhaust gases. This is accomplished by baffles and shifting the axis of the tubes. By reversing the exhaust gas flow at an angle of 90 degrees or more, damping of high-frequency noise is achieved.
- Change of gas oscillations (interference of sound waves). It is achieved due to the presence of perforations in the pipes through which the exhaust flows. This technology allows damping of noises of different frequencies.
- “Self-absorption” of sound waves in the Helmholtz resonator.
- Absorption of sound waves. In addition to the chambers and perforations in the body of the muffler there is a sound-absorbing material that isolates the noise.
Features and types of mufflers
Two types of mufflers are used in modern cars: resonant and straight-through mufflers. Both can be installed in conjunction with a resonator (pre-muffler). In some cases, the straight-through design may replace the front muffler.
Structurally, the muffler’s resonator, also called a flame arrestor, is a perforated tube located in a sealed housing, divided into several chambers. It consists of the following elements:
- body (has a cylindrical shape);
- Insulating layer (exhaust gases have a very high temperature);
- Blank baffle (for turning the gas flow);
- perforated pipe;
- throttle (enables adjustment of the exhaust gas flow cross-section).
Construction of the resonant muffler
In contrast to the preliminary one, the main resonance muffler is more complicated. It consists of several perforated tubes, installed in the common body, which are separated by partitions and are on different axes (see fig. Muffler in the section):
- front tube with perforations;
- rear pipe with perforation;
- inlet pipe;
- front baffle;
- middle baffle;
- rear baffle;
- outlet tube;
- body (oval cross-section).
Thus, the resonant muffler uses all kinds of conversion of sound waves of different frequencies.
Features of the straight-through muffler
The main disadvantage of the resonant muffler is the effect of creating backpressure, which results from the redirection of the flow of exhaust gases (when it collides with the baffles). In this regard, many motorists perform tuning of the exhaust system by installing a straight-through muffler.
Structurally, a straight-through muffler consists of the following elements:
- sealed body;
- exhaust and intake pipe;
- perforated pipe;
- Noise isolation material – most often glass fiber is used, which is characterized by resistance to high temperatures and good sound-absorbing properties.
In practice, the straight-through silencer has the following principle of operation: one perforated tube passes through all chambers. Thus, there is no noise damping by changing the direction and cross-section of the gas flow, and the noise suppression is realized exclusively through interference and absorption.
Due to unobstructed passage of an exhaust through a straight-through silencer, the arising back pressure is very small. But in practice it does not provide a big increase in power (from 3% to 7%). On the other hand, the car has a sound characteristic of sports cars, because the noise-absorbing technologies present in it eliminate only high frequencies.
The comfort of the driver, passengers and pedestrians depends on how the silencer works. So in case of long exploitation the increased noise can cause serious inconveniences. Nowadays the installation of the straight-through muffler for the cars, driving in the city limits, is an administrative violation, which threatens with fines and order to dismantle the device. It is associated with exceeding the noise limits set by the standards.
The car silencer. What’s inside?
The muffler is an important structural element of the exhaust system, without which the operation of a modern car is simply impossible. A car silencer performs the following main functions:
– reduction of exhaust gas noise level;
– energy conversion of exhaust gases, reduction of their speed, temperature and pulsation.
Exhaust gases leaving the engine cylinders have high pressure. As the exhaust gases move through the exhaust system, sound waves are created that propagate faster than the gases. The silencer converts the energy of the sound waves into heat energy, thus reducing the noise level to a certain (specified) value. At the same time the use of a silencer creates back pressure in the exhaust system, resulting in some reduction in engine power.
The silencer uses several noise reduction technologies:
☑ 1: Muffler design :
1: housing; 2: thermal insulation; 3: blind baffle; 4: perforated tube; 5: choke
– sound waves interference; – sound waves absorption. – Flow expansion is realized by means of several chambers of different volumes, separated by partitions. – Allows effective damping of low-frequency sound vibrations. – Along with the expansion in the silencer, the flow is narrowed with the help of a diaphragm orifice (throttle). It is used for damping of high-frequency noise.
Baffle pipe 6. front perforated tube 7. intake pipe 8. middle baffle 9. exhaust pipe 10. front baffle 11. rear perforated tube 12. rear baffle 13. casing
In the silencer, except for straight-through silencers, the direction of exhaust gas flow reversal is provided. The flow rotation angle is within 90-360°, thus damping medium- and high-frequency sound vibrations.
Sound wave interference, depending on the nature of their superposition, leads to an increase (constructive interference) or decrease (destructive interference) of vibration amplitude. The silencer uses both types of interference. The technology is implemented by means of perforation holes in the muffler tubes. By changing the size of the holes and the volume of the chamber surrounding the tube it is possible to achieve damping of sound vibrations in a wide range of frequencies.
1: sound-absorbing material; 2: body; 3: direct flow exhaust pipe; 4: steel mesh; 5: perforated pipe; 6: all-welded joints
When sound waves pass through a special sound-absorbing material, they are absorbed. This method is effective in damping high-frequency sound vibrations.
To achieve the greatest effect, these technologies in mufflers are usually used in combination.
One to five mufflers are installed in modern vehicles, mainly two. The silencer closest to the engine is called the preliminary (front) silencer or resonator. This is followed by the main (rear) silencer. A different set of mufflers is used for each specific car model and engine brand.
☑ Resonator design
The resonator serves to preliminarily reduce the noise level and balance the pulsation of the exhaust gas flow. Structurally, the resonator is a perforated tube placed in a metal casing. To increase the efficiency of damping of vibrations in the tube is made throttling hole.
☑ Design of the main silencer
The main silencer provides the ultimate in noise reduction. It has a more complex design. The metal casing houses several perforated tubes. The casing is divided into 2 to 4 chambers. Some chambers can be filled with sound-absorbing material. In the main silencer, the flow of exhaust gases changes its direction many times – the labyrinthine silencer.
Of all the structural elements of the exhaust system, the silencer is the most subject to modernization (tuning). When tuning the exhaust system, the so called direct flow muffler is installed (one direct flow pipe for all chambers without changing the flow direction). Such a muffler has less back pressure, but it does not give a significant increase in engine power. The main advantage of the straight-through muffler is “noble” or “sporty” sound of the car (depending on what you like more).
☑ The construction of the straight-through muffler
The construction of a straight-through muffler combines a stainless steel housing in which a perforated pipe wrapped in a steel mesh and sound-absorbing material is placed. The steel mesh mainly protects the sound-absorbing material from being blown out. Fiberglass is used as the sound-absorbing material. In a straight-through muffler, sound waves pass unhindered through the pipe holes, the metal mesh and are absorbed by the fiberglass (converted to thermal energy).