Comprehensive Analysis of Carrier Types for Internal Combustion Engine Exhaust Purifiers: Applications from Gasoline Vehicles to General Machinery

1 Introduction

Exhaust purification technology for internal combustion engines is a critical means to address emission pollution from motor vehicles and general machinery. The purifier carrier, as the support structure for the catalyst, determines the efficiency and reliability of the entire purification system. Depending on the engine type, emission regulations, and application scenarios, exhaust purification carriers have developed various material types and structural designs. From gasoline vehicle three-way catalytic converters to diesel vehicle complex aftertreatment systems, from small metal carriers for motorcycles to simple purification devices for general machinery, carrier technology has been continuously innovating and evolving. This article aims to provide a comprehensive introduction to various types of carriers used in internal combustion engine exhaust purifiers, classified according to application scenarios and technical characteristics, including gasoline vehicle three-way catalytic carriers, diesel vehicle DOC/DPF/SCR/ASC systems, modified high-flow metal honeycomb carriers for gasoline vehicles, metal honeycomb carriers for motorcycles, and exhaust purification carriers for general machinery, providing systematic reference for professionals and technical enthusiasts in related fields.

2 Three-Way Catalytic Purifier Carriers for Gasoline Vehicles

2.1 Cordierite Honeycomb Carriers

Cordierite honeycomb carriers are currently the most widely used carrier type in gasoline vehicle exhaust purification applications. The main material is porous cordierite (2MgO·2Al₂O₃·5SiO₂). This material features low thermal expansion coefficient, excellent thermal shock resistance, strong acid and alkali resistance, and high mechanical strength. Cordierite carriers can withstand maximum continuous operating temperatures up to 1200°C, providing reliable performance under extreme exhaust conditions.

To provide a large catalytic surface in a small volume, the carrier surface is made into a honeycomb structure. This design provides an extremely high specific surface area, with ceramic carrier open porosity reaching 400-600 cells per square inch, and minimum wall thickness of about 0.16mm. The mesh number of honeycomb ceramic carriers can be 200, 300, 400, or 600, with cross-sectional shapes including circular, racetrack, oval, and some special shapes according to customer requirements, aiming to adapt to different vehicle needs.

The core function of the cordierite carrier is to provide an inert physical structure for supporting the catalyst coating. It can enhance the mechanical strength of the catalyst, improve its resistance to wear, impact, gravity, compression, high temperature, and phase transformation, improve the conductivity of the catalyst, and reduce the content of active components. Especially when using precious metal catalysts such as platinum, palladium, and rhodium, the active components can be highly dispersed, reducing usage, thereby lowering catalyst costs.

2.2 Metal Honeycomb Carriers

Metal honeycomb carriers are another important type of carrier for gasoline vehicle exhaust purification, with unique advantages compared to cordierite ceramic carriers. Metal carriers are usually made of special stainless steel materials, formed by overlapping and rolling corrugated plates with longitudinal small corrugations and special stainless steel flat plates with small corrugations.

The main advantages of metal carriers include: fast light-off speed, high mechanical strength, good heat resistance, and low heat capacity. These characteristics enable metal carriers to reach the working temperature required for catalytic reaction faster during cold start phase, thereby reducing high pollution emissions during cold start period. Additionally, metal carriers have better anti-vibration performance, which is particularly important for motorcycles and general machinery often in severe vibration environments.

The manufacturing process of metal carriers is relatively complex, especially for large-diameter carriers. Honeycomb metal carriers with diameter less than 50mm can adopt an involute form of rolling method, while those with diameter greater than 50mm use a double coaxial method for rolling. This structural design enhances the mechanical stability of the carrier and improves conversion efficiency.

2.3 China VI GPF (Gasoline Particulate Filter)

With the implementation of China VI emission standards, gasoline vehicle particulate matter emission control has become mandatory, and gasoline particulate filters (GPF) have emerged accordingly. GPF is a wall-flow filter, similar in structure to diesel vehicle DPF, but different in specific design parameters.

The working principle of GPF is to force airflow through the capillary pores of the pore wall through alternately closed channels, thereby achieving particulate matter (PM) capture with efficiency高达95%以上. Most GPFs need to be coated with catalysts to improve the combustion efficiency of soot particles, slow down the accumulation rate of soot particles, and extend the carrier cleaning regeneration cycle. A few GPFs rely solely on the wall-flow structure to intercept and capture soot particles.

To meet China VI emission standards, GPF needs to have high filtration efficiency and low backpressure characteristics, while also taking into account the catalytic conversion function. This requires GPF carriers to have precisely controlled porosity, median pore size, and pore distribution, as well as good catalyst coating compatibility. Currently, GPF has become an indispensable aftertreatment device for gasoline vehicles under China VI and subsequent stricter emission standards.

3 Exhaust Purification Carriers for Diesel Vehicles and Generator Sets

3.1 DOC (Diesel Oxidation Catalyst)

DOC (Diesel Oxidation Catalyst) is the first barrier in the diesel vehicle exhaust treatment system, usually installed in the first section of the exhaust pipe. DOC carriers usually adopt a through-flow honeycomb structure, based on cordierite or metal materials, coated with catalyst coatings containing precious metals such as platinum and palladium.

The main function of DOC is to oxidize carbon monoxide (CO) and hydrocarbons (HC) in the exhaust gas, as well as some soluble organic components, converting them into carbon dioxide and water. At the same time, DOC can also oxidize nitric oxide (NO) into nitrogen dioxide (NO₂), providing necessary conditions for the regeneration and reaction of subsequent DPF and SCR systems.

To meet strict emission standards such as China VI, the pore density of DOC carriers is generally 400-600 cells per square inch, with wall thickness of 3-4mil. The specifications and dimensions of DOC carriers are usually large, with diameter range of 190-330mm, which is related to their application environment in diesel vehicle systems.

3.2 DPF (Diesel Particulate Filter)

DPF (Diesel Particulate Filter) is the key device for diesel vehicle particulate matter capture, installed downstream of DOC. DPF adopts a wall-flow structure, with alternately closed or open ends at adjacent parallel channel ends, forcing the airflow to pass through the capillary channels of the pore wall, thereby achieving efficient capture of particulate matter (PM) with efficiency Up to 95% or more.

DPF can be divided into symmetric pore DPF and asymmetric pore DPF. The large pores of asymmetric pore DPF can accommodate more ash, extending the cleaning cycle. Most DPF need to be coated with catalysts to improve the combustion efficiency of soot particles and slow down the accumulation rate of soot particles. When the DPF device captures enough particulate matter, it will start the regeneration process, oxidizing the captured particulate matter by increasing the exhaust temperature to prevent blockage.

The production process of DPF carrier is relatively complex, requiring additional punching, plugging, and reburning processes. To balance the collection efficiency and back pressure, DPF has high requirements on the formation and consistency of porosity, median pore size, and pore distribution. Material formulas usually add more organic pore-forming agents and use special mixing processes to achieve the required microporous structure.

3.3 SCR (Selective Catalytic Reduction) Carrier

SCR (Selective Catalytic Reduction) carrier is the core component for diesel vehicle nitrogen oxide treatment. Its function is to provide attachment sites for selective catalytic reduction reactions. After the SCR carrier is coated with selective catalytic reduction agent, vehicle urea (AdBlue) selectively reacts with NOx in the exhaust gas under the action of catalyst to generate non-polluting N₂ and H₂O, with efficiency高达95%.

According to the catalyst type, SCR systems can be divided into different types such as copper-based molecular sieve, vanadium-based catalyst and iron-based molecular sieve:

• Copper-based molecular sieve (Cu-SCR): Has high low-temperature activity and broad temperature window, especially suitable for light-duty diesel vehicles and low-temperature operating environments, with good hydrothermal stability and N₂ selectivity.
• Vanadium-based catalyst (V-SCR): Uses vanadium pentoxide as the main active component, with relatively narrow operating temperature window, but low cost, widely used in heavy-duty diesel vehicles.
• Iron-based molecular sieve (Fe-SCR): Shows excellent activity under high temperature conditions, suitable for application scenarios with high exhaust temperature, but relatively poor low-temperature activity.

The pore density of SCR carrier is generally 400-600 cells per square inch, with wall thickness of 3-4mil, and diameter range of 190-330mm. The production process of SCR carrier is similar to that of DOC carrier, but due to the different requirements of catalyst types on ceramic carrier water absorption, its material formula is different.

3.4 ASC (Ammonia Slip Catalyst)

ASC (Ammonia Slip Catalyst) is an auxiliary device for SCR system, used to treat unreacted ammonia gas. During the operation of the SCR system, there may be situations where urea injection is excessive or the reaction is incomplete, resulting in ammonia gas (NH₃) leakage. ASC oxidizes the leaked ammonia gas into nitrogen gas, preventing secondary pollution caused by ammonia gas emission.

There is no essential difference in structure between ASC carrier and SCR carrier, only the coated catalyst is different. ASC is usually installed downstream of SCR, and the two are closely located and chemically connected. The production process and requirements of ASC carrier are basically the same as those of SCR carrier.

3.5 Characteristics of Exhaust Purification for Diesel Generator Sets

The exhaust purification system of diesel generator sets is similar in principle to that of vehicle diesel engines, but has its unique characteristics due to installation space and operating conditions. Generator sets are usually installed in fixed or semi-fixed positions with relatively small space constraints, but need to adapt to harsh conditions of long-term continuous operation.

The exhaust purification system of diesel generator sets usually adopts a combined technical route of DOC+DPF+SCR+ASC to meet increasingly stringent emission regulations. Especially for large generator sets above 500KW, the exhaust purification system is more complex, requiring appropriate intake and exhaust channels to meet normal ventilation and reduce noise propagation.

4 Modified High-Flow Metal Honeycomb Carriers for Gasoline Vehicles

In the field of vehicle modification, high-flow metal honeycomb carriers have become an important means to improve engine performance. Compared with original cordierite carriers, modified metal honeycomb carriers have lower exhaust backpressure and better thermal conductivity, which can improve engine power and torque output to a certain extent.

The core advantage of high-flow metal honeycomb carriers lies in their special structural design. Metal carriers are usually made of ultra-thin foils (up to 0.03mm), and through different combinations of corrugated and flat foils, higher cell density (up to 1200 cpsi) and larger geometric surface area can be achieved, while maintaining low exhaust resistance. This design makes engine exhaust smoother, reduces pumping loss, and thus improves engine efficiency.

In addition, the rapid light-off characteristics of metal carriers are particularly important for modified vehicles. Modified vehicles often need to operate under drastically changing conditions in a short time. Due to low heat capacity and fast heat conduction, metal carriers can quickly reach working temperature and provide catalytic efficiency. This means that under conditions of rapid acceleration and high load, pollutant emissions can be effectively controlled.

However, modified high-flow metal honeycomb carriers for gasoline vehicles also face some challenges. First is the issue of regulatory compliance. Many countries and regions have strict restrictions on vehicle emission system modifications. Uncertified modifications may cause vehicles to fail annual inspections or be driven on the road. Second is the issue of catalyst coating compatibility. Metal carriers need to match specific catalyst formulas to achieve optimal performance. Therefore, it is crucial to choose qualified professional modification products and experienced modification manufacturers.

5 Metal Honeycomb Carriers for Motorcycles

Motorcycle exhaust purification faces special challenges, including strict space limitations, harsh vibration environment, large temperature fluctuations, and high cost pressure. In response to these characteristics, metal honeycomb carriers have become an ideal choice for motorcycle exhaust purification.

Metal honeycomb carriers for motorcycles feature fast light-off, small size, high strength, and good heat resistance. An innovative design is to braze two cores along the axis direction in the shell, with a buffer gap between the two cores. The width of the buffer gap is 5% to 90% of the entire shell length. This design increases the disorder of the airflow and improves the conversion rate of the catalyst.

In terms of catalyst coating, motorcycle metal carriers usually adopt the method of adding low-ceria rare earth composite oxide oxygen storage materials to the coating to reduce the catalyst light-off temperature, broaden the catalyst working window, and improve the durability of the catalyst. This can reduce the use of precious metals and save catalyst costs while achieving the same emission target.

The density of ventilation holes on the core of motorcycle metal carriers is usually 50-400 mesh, varying according to engine displacement and emission requirements. Compared with ceramic carriers, metal carriers can better withstand the severe vibration environment of motorcycles and have a longer service life, up to 80,000 kilometers above.

The manufacturing process of motorcycle exhaust purification metal carriers includes multiple key steps: first, surface pretreatment of the metal honeycomb carrier to generate a transition layer; then coating the transition layer with active coating containing rare earth elements such as Ce, La, Pr; finally loading active components containing precious metals such as Pt, Rh. This multi-layer structure ensures good adhesion of the coating and excellent high temperature resistance.

6 Exhaust Purification Technology for General Machinery

6.1 Exhaust Pollution Problems of General Machinery

General machinery such as lawn mowers, gasoline generators, and small engineering machineryAlthough the displacement of a single machine is relatively small, its contribution to environmental pollution cannot be ignored due to the large number and often lack of effective exhaust treatment devices. These types of machinery typically use two-stroke or four stroke gasoline engines, with emission characteristics including high concentrations of CO, HC, and particulate matter

The special challenges faced by exhaust purification of general machinery include: extremely strict space limitations, high cost sensitivity, harsh working environment (high temperature, high vibration), and poor maintenance awareness. These factors make the development of exhaust purification technology for general machinery relatively lag behind the automotive field.

6.2 Current Technical Solutions and Challenges

At present, exhaust purification of general machinery mainly uses simple catalytic converters, with carriers mostly Choose a small metal honeycomb or cordierite honeycomb structure. Due to cost considerations, these catalytic converters usually use catalysts with low precious metal loading, or even non-precious metal catalysts, such as some rare earth-transition metal composite oxide catalysts.

The technical challenges of exhaust purification for general machinery mainly include: high requirements for high temperature resistance (especially for generators and other machinery that operate for long times under high load), high requirements for anti-vibration performance, limited catalytic converter volume due to space constraints, and limited technology application due to cost pressure.

6.3 Future Development Trends

With increasingly stringent environmental regulations, exhaust purification technology for general machinery is also constantly developing and innovating. Future trends include: developing efficient non-precious metal catalysts to reduce costs, designing integrated purification systems to save space, optimizing carrier structure to improve purification efficiency and reduce back pressure, and introducing passive regeneration technology to extend service life.

In addition, with the development of electric technology, some small general machinery is gradually being electrified, fundamentally solving the problem of exhaust pollution. However, for high-power application scenarios, internal combustion engines still have advantages, so the innovation of exhaust purification technology is still of great significance.

7 Conclusion

Internal combustion engine exhaust purifier carrier technology has become an indispensable key technology for controlling mobile source emissions. From cordierite and metal honeycomb carriers for gasoline vehicles to DOC+DPF+SCR+ASC systems for diesel vehicles, from high-flow metal carriers for modified vehicles to small metal carriers for motorcycles, and then to simple catalytic converters for general machinery, different types of carriers have developed specific technical routes for the characteristics of their respective application scenarios.

In the future, with increasingly stringent global emission regulations and competition from new energy technologies, internal combustion engine exhaust purification carrier technology will continue to develop towards high efficiency, low back pressure, long life and low cost. The application of new materials such as silicon carbide and silicon nitride, technological innovations such as 3D printed honeycomb structures, and the deep integration of intelligent control technology and exhaust purification systems will all bring new development opportunities for internal combustion engine exhaust purification carrier technology.

Even in the context of the rapid development of new energy vehicles, internal combustion engines will continue to play an important role for a considerable period of time in the future. Therefore, the innovation and development of exhaust purification carrier technology is still of great significance and will make important contributions to global air quality improvement and climate change response.