What is a car exhaust system? Exhaust system principle, composition and function analysis?
What is a car exhaust system?
Exhaust system principle, composition and function analysis?
With the rapid development of the automobile industry, automobiles have become an indispensable means of transportation in people's daily lives. But at the same time, vehicle exhaust emissions have also become an important source of urban air pollution. Restrictions on vehicle emissions have become a focus of politics, economics, and decision-making. The automobile exhaust system plays a vital role in purifying exhaust gas and reducing pollution emissions.
The exhaust system is a system that controls the emissions and noise of a car. It is arranged under the entire car chassis, connected to the engine exhaust manifold, and suspended under the body floor. Therefore, its shape is generally complex, and it is also a key component that affects the vibration and noise of the entire vehicle.
The function of automobile exhaust system
The automobile exhaust system mainly has the following two functions. The first is to reduce the environmental pollution caused by automobile exhaust gas, and to purify harmful gases such as CO, NOX, and HC in the exhaust gas after engine combustion.
The second is to reduce the environmental pollution caused by car noise, so that the noise of the entire vehicle is reduced to below the national noise decibel value.
The structure of automobile exhaust system
The exhaust system, as its name suggests, has the main function of exhausting exhaust gas and also has the function of reducing exhaust noise. First, let’s take a look at a typical exhaust system structure. A complete automobile exhaust system includes a manifold, three-way catalytic converter, muffler, connecting pipe and tail pipe.
The original factory divides the exhaust system into hot end and cold end according to the exhaust temperature. During daily urban driving, when the engine speed is 1500~2500 rpm, the temperature at the hot end manifold is 500~600 degrees, and the temperature at the cold end tail pipe is 200~300 degrees. In the modification market, exhaust systems are usually divided into head section, middle section and tail section according to location. The various components of the exhaust system are connected by welding, flanges or clamps to ensure air tightness.
The manifold is connected to the engine cylinder port. The geometric structure of the manifold will affect the flow pattern of the airflow in the manifold and the manifold back pressure, thereby affecting the exhaust gas discharge efficiency of the engine, and ultimately affecting the performance of the engine. The main body of the three-way catalytic converter is honeycomb-shaped ceramic. The surface of the honeycomb structure is sprayed with a coating containing precious metals (platinum, rhodium, palladium, etc.). When high-temperature automobile exhaust gas passes through the three-way catalytic converter, the toxic and harmful gases in the exhaust gas are eliminated. CO, HC and NOx undergo oxidation-reduction reactions under the action of precious metals (as catalysts) and are converted into harmless CO2, water and nitrogen. The main function of the muffler is to reduce noise and achieve comfortable driving.
How the exhaust system works?
After fresh air and gasoline are mixed into the engine for combustion, high-temperature and high-pressure gas is generated to push the piston. When the gas energy is released, it is no longer valuable to the engine, and these gases become exhaust gas and are discharged out of the engine. After the exhaust gas is discharged from the cylinder, it immediately enters the exhaust manifold. After the exhaust manifolds of each cylinder are collected, the exhaust gas is discharged through the exhaust pipe. Just like the engine manifold, the gas leaves the engine in pulses in the exhaust manifold, so the length and curvature of the exhaust manifold for each cylinder should be designed to be as consistent as possible so that the exhaust of each cylinder can be the same. of smoothness. After the exhaust gas leaves the exhaust manifold, it is connected to a catalytic converter to convert incompletely burned pollutants into harmless substances to protect the environment. It is connected to the muffler after it comes out of the catalytic converter. The cross-section of the muffler is a round or oval object, which is mostly welded with thin steel plates. It is installed in the middle or rear position of the exhaust system. It has a series of partitions inside. The chamber, orifice and pipe use the phenomenon of sound wave reflection to interfere with each other and cancel each other out, so that the sound energy gradually weakens to isolate and attenuate the pulsating pressure generated every time the exhaust valve is opened.
Understand the car catalytic converter
Before talking about the catalytic converter, let's briefly understand the composition of engine exhaust gas. Gasoline is a hydrocarbon, and gasoline molecules are almost all carbon and hydrogen atoms. When these carbon and hydrogen are burned, they should produce carbon dioxide (CO2) and water (H2O). However, because a small amount of the mixture is not completely burned, and A small amount of engine oil (either unburned or burned) will be discharged, so HC (hydrocarbons) and CO (carbon monoxide) will be produced. Furthermore, the air entering the engine contains 80% nitrogen (N2). However, after passing through the high temperature of the combustion chamber, the originally stable nitrogen will combine with oxygen (O2) in the air to produce NO and NO2, collectively referred to as NOx, HC, CO and NOx, will cause environmental pollution and be harmful to the human body. Therefore, countries around the world will formulate environmental protection laws and regulations to restrict vehicle emissions.
Since environmental protection regulations have very strict standards for vehicle emissions, they must comply with emissions standards regardless of idling, accelerating, driving at low speed, traveling at high speed or decelerating. Facing such stringent restrictions, vehicles must not only achieve a balance between performance and emissions. Otherwise, the only solution is a catalytic converter. Catalysts usually use precious metals as raw materials, and include oxidation catalytic converters, reduction catalytic converters and three-way catalytic converters currently used in most vehicles.
Let's learn more about how it purifies exhaust gas. HC and CO in the exhaust are both produced by incomplete combustion. To eliminate them, they must be burned again, that is, oxidized, so this is the task of the oxidation catalytic converter. The generation of NOx is caused by the oxidation of nitrogen, so a reduction catalytic converter is required to reduce NOx to nitrogen. The three-way catalytic converter allows the oxidation of HC and CO and the reduction of NOx to occur in the same catalyst. The [catalyst] itself does not participate in the chemical reaction of oxidation or reduction, it only plays a catalytic role in the chemical reaction.
Where is the catalytic converter located? In the early days, the catalytic converter was mostly installed in the middle of the exhaust pipe, but recently it is often installed immediately after the exhaust manifold, so that the catalyst can reach the operating temperature quickly. The catalyst must operate at a high temperature close to 500 degrees in order to obtain good conversion efficiency. At low temperatures, it has almost no conversion ability, so the amount of pollutant emissions from cold cars is quite large.
Many studies have shown that although precious metal catalysts have high catalytic activity for automobile exhaust, they also have many shortcomings, such as easy poisoning, catalytic activity greatly affected by air-fuel ratio, unsatisfactory high-temperature performance, and high cost. Therefore, in order to give full play to the efficiency of the three-way catalytic purifier, the prerequisites for installing a three-way catalytic purifier in a car must be:
1. Good fuel quality means low content of lead, sulfur, phosphorus, arsenic, silicon, magnesium and iron. Excessive leaded gasoline can cause catalytic converter poisoning, damage the catalytic converter and oxygen sensor in the exhaust system, and can also cause harm to human health and the social environment. High sulfur content in fuel will reduce the efficiency of the catalyst, delay the engine ignition time, increase the exhaust temperature, and reduce engine efficiency.
2. The engine is equipped with an electronically controlled gasoline injection system (closed-loop control)
The electronically controlled gasoline injection system can ensure that the engine works within a small air-fuel ratio range. After a cold start, the catalytic converter temperature reaches above 400 degrees before closed-loop control can be started. The engine's throttle is controlled based on the feedback signal of the oxygen sensor (high and low voltage analog signals). The opening of the fuel rail nozzle and the injection time of the fuel rail nozzle are used to control the air-fuel ratio around 14.7:1, so that the conversion efficiency of the catalytic converter is high, the amount of harmful gases produced by the engine is small, and the exhaust gas treatment rate reaches more than 90%.
The oxygen sensor plays a very critical role in signal monitoring of exhaust emissions. The highest feedback signal is 800MV and the lowest is 100MV. The ECU controls the injection time of the injector according to these two signals to adjust the excess air coefficient λ so that it always oscillates near λ=1, which can intermittently supply air to the catalytic converter. CEO2 replenishes oxygen and uses CO and HC to consume the adsorbed oxygen. When the waveforms of the front and rear oxygen sensor signals are consistent, it proves that the catalytic converter has aged and failed, and the OBD alarm light will light up to remind the driver that maintenance is required.
Get to know car mufflers
Noise has all the characteristics of sound waves. All kinds of sound waves are generated by the vibration of objects. The objects that vibrate and radiate sound waves are called sound sources. The propagation of sound waves is essentially the propagation process of medium vibration, that is, what is propagated is the movement of the material without knowing the material itself. In other words, the propagation of sound waves is an energy transfer process, and this form of motion is called wave. Vibration and fluctuation are closely related to each other. Vibration is the source of fluctuation, and fluctuation is the propagation process of vibration. So sound waves are essentially the propagation process of mechanical vibrations, also called mechanical waves.
The basic properties of noise include physical aspects and physiological aspects. The former describes the objective attributes of noise, such as sound pressure, sound intensity, frequency, etc., while the latter involves people's subjective feelings, that is, the impact of noise on people.
Noise evaluation indicators: The description of sound is divided into objective evaluation and subjective evaluation. Objective evaluation uses the basic physical quantities of sound such as sound pressure, sound intensity, and sound power to describe the sound; subjective evaluation uses the human ear's auditory sensation to describe the sound.
When the engine of a car is working, it will make a lot of noise, which will have a great impact on the social environment and cause great damage to the human auditory system. Therefore, the main purpose of installing a muffler on a car is to reduce the noise of the car and reduce the pollution of the noise to the environment.
Mufflers are devices with sound-absorbing linings or special airflow ducts that effectively reduce airflow noise. The muffler generally includes the entire muffler component starting from the muffler air inlet, excluding the engine exhaust manifold and exhaust pipe.
Muffler and exhaust pipe design
The main function of the muffler is to reduce the exhaust noise of the engine and enable high-temperature exhaust gas to be discharged safely and effectively. As a part of the exhaust pipe, the muffler should ensure smooth exhaust, low resistance and sufficient strength. The muffler must withstand high-temperature exhaust gases of 500 to 800 degrees to ensure that it is not damaged or loses its muffler effect within the specified driving mileage of the car. Car mufflers can be divided into three categories: resistance mufflers, resistive mufflers and impedance composite mufflers according to their muffling principles and structures.
1) Resistance muffler
When the resistant muffler is composed of expansion chambers, resonance chambers and other muffler units internally through pipes, partitions and other components, sound waves are reflected and interfered when propagating, reducing the sound energy to achieve the purpose of silencing. Resistant mufflers have a limited silencing frequency band. They usually have good silencing effects in low and mid-frequency bands, but have poor silencing effects in high frequencies. Trucks mostly use resistant mufflers.
2) Resistive muffler
Resistive mufflers are mufflers that are filled with sound-absorbing materials around the pipes through which the internal exhaust gas passes to absorb sound energy to achieve the purpose of silencing. It has good silencing effect on medium and high frequencies. It is rarely used simply as a car exhaust muffler. It is usually used in combination with a resistant muffler.
3) Impedance composite muffler
The impedance composite muffler is a muffler composed of a combination of resistive muffling units and sound-absorbing materials. It has the common characteristics of resistive and resistive mufflers. It has a good silencing effect on low, medium and high frequency noise.
About SNW
SNW is mainly engaged in the Manufacturing and Wholesale of automobile exhaust system parts,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.
