SNW tells you about the exhaust system
SNW tells you about the exhaust system parts mainly includes exhaust manifolds, headers, downpipes, cat-back systems.
Why is the supercar exhaust so loud?
From the perspective of product positioning, first of all, the exaggerated shape of the supercar highlights its sports genes. Manufacturers not only want you to see it, but also let you hear it. The roaring sound is the most intuitive expression of the sense of power. Secondly, for consumers, sound is an important factor in meeting their psychological needs. . When you blast the street with a supercar, you want the whole world to know you exist. Coolness and style are what consumers want, and the existence of sound waves exactly meets this demand of consumers; in addition, there are many sports cars from Lamborghini to Ferrari, with different models and engine types, the effect of sound waves is different. the same. Today, when labels are prevalent, sound also needs labels, and different sound effects mark different car brands.
On the other hand, it is also determined by the special mechanical structure of the supercar. The structure of the exhaust pipe of a supercar with a mid-mounted or rear-mounted engine is different from that of a traditional front-mounted, rear-wheel drive car. The exhaust length becomes shorter and the exhaust resistance becomes smaller. If the muffler is removed, it will help reduce the engine The loss of power is exactly what a high-power, high-torque supercar needs. The advantage of this is that the sound of the engine becomes louder, and after the manufacturer's special sound tuning, the engine will make an exciting sound (the specific principle will be explained in detail later). Therefore, its own structural characteristics also determine that supercars must be loud.
V12 engine and exhaust system used in Ferrari LaFerrari
Maybe some friends will say that changing the exhaust of my hundreds of thousands of cars is like changing the feeling of a supercar! Is it really that simple? In order to clarify this problem, we must first start with the structure of the exhaust system.
How much do you know about the exhaust system?
Schematic diagram of a single exhaust system
Taking the single exhaust system as an example, when the engine crankshaft is running, the exhaust gas enters the exhaust system through the exhaust manifold. Usually the exhaust pipe is segmented, this is to install different components in the middle of the exhaust system. Although the exhaust system can be divided into single row and single outlet, single row and double outlet, double row and single outlet, and double row and double outlet, their main components are inseparable from several major parts: exhaust manifold, exhaust pipe, Sensors, three-way catalytic converters, mufflers, exhaust tailpipes.
Among them, the function of the exhaust manifold is to introduce the exhaust gas during the engine exhaust process into the exhaust system; the function of the three-way catalytic converter is to convert the three harmful gases of CO, HC and NOx in the exhaust into carbon dioxide, nitrogen, Harmless gases such as hydrogen and water, which is why it is called a three-way catalytic converter.
Structural diagram of three-way catalytic converter
The main function of the muffler is to control the exhaust sound. It is because of its existence that the car exhaust sound meets the requirements of regulations. Therefore, in addition to reducing noise, the main function of the exhaust system must also meet the requirements of exhaust gas treatment.
Exhaust gas treatment can be achieved through chemical reactions. So, how is the silencer function implemented? What determines the length and thickness of the pipe? In order to clarify these problems, we must also understand how the exhaust sound is formed.
How is exhaust noise produced?
Exhaust sound mainly consists of four parts: air noise, impact noise, airflow friction noise and radiation noise.
Air noise refers to the pressure wave generated when the engine is working, which propagates in the exhaust pipe to form air noise. In the duct, this airflow is steady. The acoustic design in the exhaust system is aimed at this type of noise; impact noise refers to the impact noise caused by the impact of unstable airflow in the exhaust pipe on the pipe. If the curvature of the exhaust manifold is too small, the engine airflow will strongly impact it, making a "bang, bang, bang" impact sound; radiation noise mainly refers to the noise radiated by structural vibration caused by the impact of the engine and airflow, so Although the suspension damping structure of the exhaust system and the car body looks inconspicuous, it also needs careful design to achieve the effect of damping vibration and noise; air flow friction noise: when the gas flow rate in the pipe is very high, it is between the pipe wall and the pipe wall. There is friction and a loud noise. Both friction noise (unstable airflow) and airborne noise (steady airflow) make up tailpipe noise. And the higher the engine speed, the greater the friction noise. This is one of the reasons why performance cars with high-powered engines can blow up the streets.
For cars that need to comply with national noise regulations, we hope that the noise is as low as possible. This requires that during the design stage of the exhaust pipe, the cross-sectional area of the pipe cannot be too large. If it is too large, the gas flow resistance will become smaller and the noise will get bigger. In order to make it easier to understand this problem, here is a concept introduced to you: exhaust system back pressure.
Exhaust system back pressure?
The so-called exhaust system back pressure refers to the difference between the pressure at the exhaust manifold outlet and atmospheric pressure. The power loss of the engine (here refers to the loss at the maximum engine speed) has a direct relationship with the back pressure value. The relationship between the two can be obtained through a large amount of test data processing. The following picture shows the average back pressure of the exhaust system and relationship with engine power.
Relationship between exhaust system average back pressure and engine power
Among them, the abscissa represents the exhaust back pressure, and the ordinate represents the proportion of engine output power. From the picture above you can see:
1) The power of the engine is linearly related to the exhaust back pressure.
2) The greater the back pressure, the greater the engine power loss, that is, the greater the engine power loss. Therefore, in order to pursue engine power output, supercars adjust the resistance of the exhaust system during the exhaust process, such as increasing the cross-sectional area of the exhaust pipe and making the exhaust pipe very thick, so as to reduce the exhaust resistance and increase the power. output. Of course, this can also bring another benefit. The engine exhaust sound is very loud, and with the help of sound tuning, it can output perfect sound waves.
Therefore, in daily refitting, if you simply pursue the sound effect, you can directly refit the exhaust tailpipe. At this time, the engine power will not be greatly affected, because it is equivalent to only changing the sound effect at this time. Of course, improvements can also be made to the exhaust middle section, manifold, etc., which requires considering both cost and power output effects. One thing to note here is that for the low-speed area, if the exhaust back pressure is too small, it is not conducive to the discharge of exhaust gas. This is why when the exhaust pipe is modified, if the pursuit of high-speed performance is blindly pursued, the low-speed will be lost. dynamic reasons. Therefore, when modifying the exhaust, the power requirements of low speed must be properly considered.
Different from performance cars, everyday luxury brands have reached the ultimate in pursuit of quietness. So, how is the exhaust noise analyzed above dealt with?
Silencer, how to silence it?
This is naturally inseparable from the key component of the exhaust system - the muffler. Maybe some readers think this thing is very simple, a loud voice goes in and a small voice comes out. In fact, after careful study, there are really many doorways inside.
Generally speaking, traditional passive mufflers can be divided into resistive mufflers, resistive mufflers and composite mufflers.
The so-called resistive muffler, simply understood, is to use sound-absorbing materials (such as fiber materials) to absorb noise energy and convert it into heat energy, thereby achieving the purpose of silencing noise. This muffler mainly absorbs high-frequency broadband noise. Sound-absorbing materials are generally placed inside the muffler. Its sound absorption effect depends on the structure of the material, the diameter of the hole and the density of the material.
Schematic diagram of resistive muffler
The so-called resistance muffler is simply understood as using the reflection of sound to cancel each other out of energy, and then realize the function of noise reduction. Resistant mufflers are widely used in automobile exhaust muffler. The resistant muffler can be divided into expansion muffler and side branch muffler.
The principle of the expansion muffler is to use the sudden change of the cross-section to make the acoustic energy enter the large chamber from the small chamber, and part of the sound wave is reflected back to achieve the attenuation of the energy, thereby achieving silencing.
Expansion muffler principle
The blue dotted line in the figure represents the incident sound wave, and the red represents the reflected sound wave. The area changes and part of the sound waves are reflected back, reducing the noise energy. There are actually a lot of formulas that can be explained here, but considering the convenience of everyone's reading, the formulas are not listed for everyone. Interested friends can refer to relevant information by themselves.
Common side branch pipe mufflers are mainly Helmholtz mufflers.
Helmholtz muffler schematic diagram
The principle of a Helmholtz muffler is similar to a dynamic vibration absorber in the field of vibration. It is formed by connecting the cavity A with the connecting pipe B on the main road. When the gas passes through the Helmholtz resonance structure, part of the gas passes through B and enters A. At this time, due to the compressibility of air, the gas in A is equivalent to the spring k in the picture on the right, and the air column in B is equivalent to the mass block m in the picture on the right. With the spring and mass, a spring-mass system is formed , and the natural frequency of this system can be calculated. This frequency is related to the structural parameters of the Helmholtz muffler and will not be expanded here. When the fluctuation frequency of the engine exhaust is close to the natural frequency, it causes gas resonance in B and consumes part of the energy. Therefore, this type of muffler can attenuate exhaust noise within a specific frequency range. Once you understand this principle, you can also understand that there are various "weird" "bypass structures" in the car exhaust pipes we see in daily life.
Exhaust system with Holmhertz muffler
Of course, in addition to resistive and resistant mufflers, there is also a composite structure muffler. This type of muffler combines the characteristics of the above two mufflers to achieve exhaust noise reduction in a wider frequency range.
Schematic diagram of composite muffler
Composite muffler structure diagram
Among composite mufflers, the Holmhertz muffler is generally used for low-frequency muffler, generally in the range of 40-200Hz, and the porous tube muffler is used for mid-frequency muffler, with the range of 100-500Hz, plus the resistance formed by the outer sound-absorbing material. Sexual mufflers are generally used for high frequency silencers above 500Hz. Based on the above frequency bands, this type of muffler can basically meet the requirements of all frequency bands. This is also a very widely used muffler structure.
Structural diagram of composite muffler adopted by Toyota
Schematic diagram of exhaust system equipped with composite muffler
Physical picture of exhaust system equipped with composite muffler
The previous article analyzed the generation, principle and noise reduction principle of engine exhaust noise. Next, let me tell you how the performance car "Sound" of the current mainstream brand is tuned.
How is the sound wave tuned out?
Reduce the function of the muffler: For supercars, their engine power is generally relatively large. Therefore, the engine exhaust noise itself will be louder than that of ordinary family cars. In order to make the exhaust sound more pleasant, engineers will adjust the muffler and appropriately weaken its silencing effect. This method brings two benefits: first, it can reduce engine power loss and meet high power requirements; second, it can It makes the engine sound in a specific frequency band stand out and more in line with the hearing requirements of the human ear, that is, it makes the sound sound more pleasant.
Design pipeline structure: For the Y-shaped exhaust system in the picture below, the length of the exhaust pipes on the left and right sides connected to the exhaust manifold also determines the sound effect of the exhaust. For cars, try to make the lengths of the two equal as much as possible, so that the ignition order and harmonic frequencies in the exhaust sound (taking a four-stroke six-cylinder engine as an example, the ignition order is 3rd order, and the harmonic frequencies are 6 and 9 , 12...and other orders), half-order and other whole-order components are relatively low, which is the sound quality that most people like. Because this order can make passengers feel comfortable and harmonious. The sound tuning of supercars is different. Engineers can change the length relationship of the two exhaust pipes to highlight the half-order sound components. This sound effect is the "passionate" sound we hear.
Y-pipe exhaust system
Install an exhaust bypass valve: Since some supercars use turbocharging, part of the exhaust gas is not directly discharged through the exhaust system, but is used for intake boosting. Therefore, this will affect the sound effect to a certain extent. In order to solve this problem, engineers installed an exhaust bypass valve (Exhaust Bypass Valve) in the exhaust system. This is a bypass valve located after the exhaust manifold and before the turbocharger. The valve is controlled mechanically or electronically. opening and closing. The advantage of this is that at low speeds, the valve can be closed to ensure power. At high speed, open the valve to ensure the horsepower while obtaining a pleasant sound.
Working principle of exhaust bypass valve
Sound transmission: In order to further improve the driver's driving experience, Porsche and Ford have installed sound ducts on the intake manifold and introduced them into the cab, so that some sound waves are transmitted into the cab, fully stimulating the driver. nerves. The principle is to connect a special sound wave duct to the intake manifold, and resonate with the air inhaled by the engine through a paddle made of a mixed material to produce the effect of sound wave enhancement. When the driver deeply depresses the accelerator, the sound duct will automatically open. At this time, the sound waves in the engine compartment will be amplified and transmitted to the cockpit, producing a pleasant roar. In the same way, when driving normally, the sound duct will be closed, and the noise of the engine will naturally be isolated. Of course, this design is more inclined to "self-play", because such an obvious effect may not be felt outside the cab. In order to meet the needs of the sound wave, the engineers really took great pains~
Can sound waves be "fake"?
It is said that what the ears hear is false and what the eyes see is true. The same goes for sound waves. There are more or less certain difficulties in the improvement and adjustment of the above-mentioned structure, and there are certain cost problems. The sound simulator avoids these problems. By simulating the sound waves of supercars, relevant manufacturers have developed corresponding sound wave simulation systems. Volkswagen installed the Soundaktor system on the Golf GTI in 2011 to simulate engine sound. Its sound is emitted by speakers installed on the firewall in order to improve the driver's driving experience.
Of course, BMW has not been idle either and has developed a system called Active Sound, which has a similar principle. This system is quite cool when used on the i8. Can you imagine that a hybrid i8 equipped with a three-cylinder engine can make the sound of a supercar? Different from sound wave conduction, the i8 equipped with 5 speakers is not only very exciting inside the cab, but also the outside by arranging speaker No. 6.
BMW i8 Active Sound system, 2, 3, 7, 8 are internal speakers; 6 is external speakers
Indeed, no matter how realistic the simulated sound is, it is still fake. There is still some grudge in my heart. However, with the development of electric vehicles, in pursuit of a better driving experience, there seems to be more demand for this kind of sound simulation.
In addition, the arrival of the electrification era has also put forward new requirements for NVH engineers.
On the one hand, from a safety perspective, the driver of a traditional internal combustion engine car does not have to worry about pedestrians not being able to hear the car due to its engine. Therefore, the active voice of electric vehicles has been clearly stipulated in relevant laws and regulations. How to better remind pedestrians is a problem that needs to be solved.
On the other hand, without the masking effect of the engine, other noises in the cab become more prominent, such as air conditioning noise, gear noise, high-frequency noise from the motor, road noise, etc. How to block these annoying noises and how to reasonably adjust the sound quality in the cab is not a simple matter for NVH development.
Thank you SNW for providing the information. I hope it can help you learn more about auto parts and the structure of the exhaust system. SNW specializes in manufacturing The exhaust system parts mainly includes exhaust manifolds, headers, downpipes, cat-back systems,aluminum alloy intake pipes, T-pipe, Y pipe, X pipe, H pipe and other performance parts; the shock absorption system parts mainly includes coilovers and arms. If you want to know more about product models, please contact SNW, info@snwmetal.com, sales@snwmetal.com.
