1 Introduction Engine valves are mechanical components used in internal combustion engines to allow or restrict the flow of fluid or gas to and from the combustion chambers or cylinders during engine operation. Functionally, they perform similarly to many other types of valves in that they block or pass flow, however, they are a purely mechanical device that interfaces with other engine components such as rocker arms in order to open and close in the correct sequence and with the correct timing. Auto Engine Valve,Car Engine Valve,Engine Valve For Land Rover,Intake And Exhaust Valves Shijiazhuang Longshu Mechanical & Electrical Equipment Trading Co., Ltd. , https://www.longsbearings.com
Tool coating treatment is one of the important ways to improve tool performance, and the choice of coating material is the key to the performance of the tool coating. Depending on the nature of the coating material, the coating tool can be divided into two categories: "hard" coated tools and "soft" coated tools. Hard coatings such as TiC, TiN, TiCN, and TiAlN improve tool cutting performance by reducing or reducing tool wear through high hardness and good wear resistance. However, the friction coefficient of the tool using these coatings is generally high, lubrication is required during processing, and when the cutting speed is increased, the action of the lubricating fluid drops sharply. The “soft†coating of the tool prepared with solid lubricants such as MoS2, WS2, etc., because of its low friction coefficient, can reduce friction, reduce cutting force and cutting temperature, thereby reducing the bond wear of the tool, prolonging the tool life and improving The quality of the machined parts.
MoS2 solid lubricant has the advantages of low friction coefficient, large bearing capacity, good wear resistance and strong bonding with the substrate. It is widely used in aerospace, electronics, machinery manufacturing and other fields. By controlling the impurity content and grain size during sputtering, Martin and other factors reduced the friction coefficient of MoS2 under vacuum to 0.001, which fully demonstrated the excellent performance of MoS2 in reducing friction and lubrication. On the other hand, the defects of MoS2 are also very obvious: when the temperature exceeds 400 °C, MoS2 begins to oxidize, and as the temperature increases, the degree of oxidation gradually deepens, and the lubricating performance drops sharply. The reason is that the material has a tribochemical reaction. Hard particles MoO3 are formed, which increases the wear of the coating. MoS2 is very sensitive to environmental humidity, is easy to absorb moisture and directly leads to an increase in friction coefficient. When the relative humidity of the environment is increased from 10% to 90%, the friction coefficient is nearly doubled. In addition, MoS2's performance is subject to fluctuations as the test environment and contact conditions change. These shortcomings of MoS2 make certain applications limited. At present, domestic and foreign scholars have carried out research and exploration in various aspects around the hot issues of improving the performance of MoS2 and its coatings and improving the application of MoS2 “soft†coated tools in cutting.
2 Progress in MoS2 "soft" coating research at home and abroad
Factors affecting the performance of the coating include not only the physical and chemical properties of the coating material itself, but also the physical and chemical properties of the substrate, the coating process, and the matching between the substrate and the coating and between the coating and the coating. These influencing factors can be divided into the following two aspects.
2.1 Selection of substrate
As the support of the coating, the influence of the substrate on the coating properties is self-evident, and sometimes even directly determines the success or failure of the coating process. The substrate and coating should be matched in physical properties and chemical properties. It is necessary to consider whether the matrix has high hardness, whether the parameters such as elastic modulus and thermal expansion coefficient are reasonable and whether there is chemical reaction with the coating.
Jing Yang et al. compared the microhardness values ​​of MoS2 composite coatings prepared on ZL108 aluminum alloy (90-110HV) and relatively hard 1Cr18Ni9Ti (370HV) materials, and found that the latter hardness is nearly 1/5 higher than the former. Times. The authors believe that the high hardness matrix is ​​not susceptible to plastic deformation, which can delay the premature tearing and spalling of the coating due to plastic deformation of the matrix, which is similar to the hard intermediate layer in the multi-coating and composite coatings. The surface layer is supported; at the same time, the microhardness of the composite coating is also significantly improved.
The difference in thermal expansion coefficient and elastic modulus between the substrate and the coating or coating and the coating results in a residual stress field of unequal size and uneven distribution between the coating interfaces. After Jing Yang et al. deposited the TiN-MoS2/TiN composite coating on the surface of YG8 and YT14, it was found that the residual stress state between the layers was the residual tensile stress of the coating and the residual compressive stress of the matrix. The reason was thermal expansion. The coefficient δTiN>δYG8 or δYT14. Finally, the stress state inside the coating is: the residual compressive stress of the YT14 matrix is ​​reduced, and the YG8 matrix has a larger difference in thermal expansion coefficient compared with the TiN coating, so that not only the compressive stress disappears, but also a certain tensile stress is generated. The state of stress before and after has changed. The existence of the residual stress field affects the bonding force between the coating and the substrate, and the greater the difference between the layer-base thermal expansion coefficient, the greater the residual stress, and the lower the bonding force between the layer and the base, which adapts to the wide temperature difference environment. The ability is worse. Therefore, when selecting the substrate, materials with different parameters such as thermal expansion coefficient and elastic modulus and the matrix should be selected as much as possible to reduce the residual stress and improve the bonding force between the coating interfaces.
The literature also tested MoS2/TiN on the surface of Cu and carbon steel, and the coating failed. The authors found that the coating reacted with the matrix during the deposition process. The CuO contained in the impure Cu reacted with the H+ which was decomposed into the H2S gas flowing into the deposition chamber to form water vapor, which produced a so-called hydrogen phenomenon. When the water vapor expands, the formed crystal grains are broken, resulting in pits of about 0.5 mm on the surface of the Cu substrate, so that the coating layer cannot be deposited at all. Similar conclusions have been made on the experimental analysis of carbon steel substrates. Therefore, the chemical properties of the layer-base are also considered when selecting the substrate. It should be noted that the use of materials with similar chemical properties gradually forms a transition layer (gradient coating), which has been widely used in multi-coating and composite coatings: the closer the performance of the material, the more reasonable the material matching performance, between the coating interfaces. The stronger the bonding force, the easier it is to form a transfer film, and the better the wear resistance, the longer the life of the soft coating.
2.2 MoS2 coating process
(1) Coating method
The MoS2 coating method is divided into a chemical vapor deposition (CVD) method and a physical vapor deposition (PVD) method. Compared with the CVD method, the PVD coating method has a low processing temperature, and the internal state of the coating is compressive stress, which is more suitable for the hard and complex tool coating of the cemented carbide, and has no adverse effect on the environment, and conforms to the development direction of modern green manufacturing. At present, the sputtering technology of PVD method, ion plating technology (or a combination of both) is used to prepare a "soft" coating of MoS2. However, as the current mainstream deposition method for magnetron sputtering MoS2 coating, the coating quality and deposition rate obtained have been unsatisfactory. Teer et al. developed a deposition method called closed magnetic field unbalanced magnetron sputtering ion plating (CFUBMSIP), which is gradually being applied to the preparation of "soft" coatings.
Increasing the ion current density during magnetron sputtering is the key to improving the performance and efficiency of sputter coatings. The generation of ions is initially based on a balanced magnetron source. The CFUBMSIP system is characterized by the use of an unbalanced magnetron source in the vacuum chamber. The magnetic fields of adjacent magnetrons in the system have opposite polarities, resulting in a toroidal magnetic field throughout the vacuum chamber. After the secondary electron escapes from the trap of the parallel magnetic field on the surface of the cathode target, it cannot directly fly to the anode, but then falls into the trap of the closed magnetic field with an approximate cycloid motion, thereby increasing the collision probability of electrons and gas molecules. The amplitude increases the ionization rate of the gas and the ion current density that can be obtained by the cathode target, allowing the system to have a higher sputtering rate. The performance of the composite coating was significantly improved by comparing the performance of the MoST (MoS2+ metal or compound) composite coating prepared on the surface of M42 steel with the pure MoS2 coating.
Han Chengming et al. proposed a non-equilibrium nanocomposite plasma coating method (NCUPP) based on the concept of “multiphase materials†in the development of materials. The principle is to use gas discharge to vaporize or evaporate under specific process parameters. The material is ionized, and ion bombardment is generated while depositing vapor or other reactants on the substrate. In this way, several nanometers of different materials can be finely nanocomposited, so that several layers or even dozens of multiphase nanocomposite layers are contained in the coating of 2 to 3 μm thick.
Using this method, several materials (Ti, N, Mo, S, etc.) were finely nanocomposited. TiN-MoS2/Ti multiphase nanocomposite coatings were prepared on 1Cr18Ni9Ti stainless steel sheets and φ8 twist drills. X-ray photoelectron spectroscopy (XPS) tests show that part of Ti exists in the form of oxides, forming a dense oxide film on the surface of the coating, preventing further oxidation of the coating, thereby improving the moisture resistance of the nanocomposite coating. . The wear comparison test shows that the friction coefficient of nanocomposite coatings hardly changes with the wear life, indicating that the wear life of TiN-MoS2/Ti multiphase nanocomposite "soft" coating deposited by NCUPP method is much higher than that of ordinary TiN-MoS2/Ti coating.
(2) Coating process
The parameters of the coating process also affect the bonding force of the "soft" coating interface, which in turn affects the overall performance of the coating.
These parameters include: Ar gas pressure, cathode current density, substrate negative bias and magnetron sputtering conditions (target distance, metal or compound addition amount, etc.). The literature examined the effects of Ar gas pressure, magnetron power mode, sputtering target type, liquid nitrogen cold trap, etc. on coating properties. The results show that the coating obtained at a lower Ar gas pressure (0.40 Pa in the test) is superior to the coating obtained at a higher pressure (0.88 Pa); the wear volume of the coating prepared by using a single DC power source is greater than that of the double Coating under pulsed DC power supply; wear volume of coating prepared under liquid nitrogen cold trap conditions is smaller than coating under liquid nitrogen-free cold trap; cold target preparation under conditions of relative humidity and low Ar gas pressure (0.44 Pa) The coating has a slightly higher wear volume than the hot target, but the opposite is true for coatings at higher Ar pressures.
Jing Yang et al. conducted drilling tests on the AZ5032 drilling machine under atmospheric conditions, and investigated the performance of TiN-MoS2/Ti composite coating prepared on the surface of φ8mm 6542 steel twist drill by NCUPP method, and the performance and magnetic control of the coating. The relationship between sputtering conditions (target distance, deposition pressure, and Ti additive content). The authors found that the drilling life of the coated tool is directly related to the Ti content, but it does not increase linearly with the increase of Ti content, but reaches the highest when the Ti content is about 12.5%. As the target distance decreases and the deposition pressure increases, the Ti content increases. When the target distance is too small (less than 50 cm) and the deposition pressure is too large (more than 3.0 Pa), the Ti content increases significantly, resulting in coating. The internal lattice is severely distorted, and the distortion can be rapidly increased, resulting in a rapid decrease in the wear life of the coating and loss of the desired lubricating effect. After many experiments, the authors concluded that the composite coating obtained with the target distance of 50cm and the deposition pressure of 3.0Pa (Ti content of about 12.5%) is the best.
In addition, the pre-sputter cleaning of the substrate before deposition can remove impurities that are not conducive to the bonding of the coating to the substrate. For the MoST coating, a metal sputtering target (such as a Ti target) can be opened at the same time as the cleaning process, and the vacuum can be reduced. The concentration of indoor water vapor; ion bombardment of the coating by applying a certain negative bias to the substrate during the deposition process can improve the interdiffusion ability of the layer-base component and the atomic reactivity of the coating surface, thereby reducing The generation of defects in the coating. Considering the above factors, the MoS2 with the lowest friction coefficient and the best wear resistance in the test was obtained under the conditions of ion pressure of 0.40Pa, cathode current density of 10A/cm2 and simultaneous application of -100V negative bias. coating.
3 Application effect of MoS2 “soft†coated tool
The average milling force comparison and the surface quality of the workpiece after machining (cutting amount: V=150m/min, f=0.04mm/r, ap=4mm) can be seen by using two different coating tools for end milling AISI 304 stainless steel. Carbide milling cutters with MoST coating on TiCN have a significantly reduced average milling force under dry friction conditions and a significant improvement in machined surface quality. The impact of both sides has led to a significant increase in the quantity and quality of the final product.
Comparison of the number of drilled holes in three high-speed steel coated drill bits (cutting amount: V=30m/min, f=0.12mm/r; workpiece is JIS S50C steel), it is visible that a layer of MoST is deposited on the hard-coated TiN. "The coated drill has a life expectancy of 2.1 times higher than that of the TiN coated drill alone and 2.8 times better than the TiAlN coated drill.
The literature discusses the limitations of MoST "soft" coating applications. After the turning test, the presence of oxygen was found in the heat affected zone of the insert, indicating that the coating was oxidized and worn due to the relatively high cutting temperatures during turning. The authors therefore believe that MoST “soft†coatings are not suitable for continuous high-speed turning processes; in low-speed milling, MoST “soft†coated tools typically have a life of 1.15 to 2 times higher than uncoated tools. In summary, MoST “soft†coated tools are suitable for low speed interrupted cutting.
It can be seen from the comparison of the cutting rate when the aluminum alloy parts are milled and forged by different bases and coatings. The cutting speed of the high-speed steel tool with MoS2 deposited on the surface is 2 times higher than that of the cemented carbide tool, which is higher than that of the uncoated high-speed steel tool. 6 times.
Comparison of wear life when cutting 1045 steel and 302 steel with ceramic tools with/without MoS2 coating (cutting amount: V1045=180m/min, V302=103m/min, f=0.1mm/r, ap=0.25mm) When cutting 1045 carbon steel, the wear life of Si3N4 and Ti(CN) ceramic tools deposited with MoS2 is 50% longer than that of uncoated tools. When cutting 302 stainless steel, the wear life of WC-based ceramic tools coated with MoS2 is longer than that of uncoated tools. 140%.
4 Conclusion
The research and development of MoS2 “soft†coated tools provides new ideas for improving the cutting performance of tools. The MoS2 soft coating can significantly reduce the coefficient of friction during tool cutting, reduce tool wear and extend tool life. Development of coating methods such as closed magnetic field unbalanced magnetron sputtering ion plating and unbalanced nanocomposite plasma plating, and techniques and measures prepared by rational selection of substrates, optimization of deposition processes, and preparation of appropriate matrix pre and post treatments The MoS2 “soft†coating structure is more compact, the bond between the coating and the substrate is enhanced, and the tool life is extended. Combining nanotechnology and composite coating technology, expanding the new MoS2 coating technology, further optimizing the coating process parameters, expanding the cutting test range and application range of “soft†coated tools, and further exploring the “soft†coating friction and wear properties. The mechanism for obtaining improvement will be the development direction of MoS2 “soft†coated tool research in the future.
The term engine valve may also refer to a type of check valve that is used for air injection as part of the emission control and exhaust gas recirculation systems in vehicles. This type of engine valve will not be addressed in this article.
Engine valves are common to many types of combustion engines, whether they run off a fuel such as gasoline, diesel, kerosene, natural gas (LNG), or propane (LP). Engine types vary by the number of cylinders which are the combustion chambers that generate power from the ignition of fuel. They also vary by the type of operation (2-cycle or 4-cycle), and by the design placement of the valves within the engine [overhead valve (OHV), overhead cam (OHC), or valve in block (VIB)].
This article will briefly describe the operation of engine valves in typical combustion engines, as well as present information on the types of valves and their design and materials. More information concerning other about other valve types may be found in our related guide Understanding Valves.
Engine Valve Nomenclature
Most engine valves are designed as poppet style valves because of their up and down popping motion and feature a conical profile valve head that fits against a machined valve seat to seal off the passage of fluids or gases. They are also called mushroom valves because of the distinctive shape of the valve head. Figure 1 shows the nomenclature for the different elements in a typical engine valve.
Diagram showing the nomenclature of a poppet valve.
Figure 1 - Nomenclature for a standard poppet style engine valve.
Image credit: https://dieselnet.com
The two primary elements are the valve stem and the valve head. The head contains a fillet that leads into a seat face that is machined at a specified angle to match the machining of the valve seat to which it will match. The seating of the valve face to the valve seat is what provides the seal for the valve against combustion pressure.
The valve stem connects the valve to the mechanical elements in the engine that operate the valve by creating a force to move the stem against the seating pressure provided by a valve spring. The keeper groove is used to hold the spring in position, and the tip of the valve stem is repeatedly contacted by a rocker arm, tappet, or lifter that actuates the valve.
Engine Operation
Four stoke or four-cycle internal combustion engines make use of two primary types of valves – the intake valve and the exhaust valve. Intake valves are opened to allow the flow of an air/fuel mixture into the engine`s cylinders prior to compression and ignition, while exhaust valves open to permit the expulsion of exhaust gases from the combustion process after ignition has occurred.
In normal operation, a crankshaft in the engine to which the pistons are attached is tied to a camshaft as part of a valve train arrangement for the engine. The movement of the crankshaft transfers motion to the camshaft through a timing chain, timing belt, or other geared mechanism. The timing and alignment between the position of the crankshaft (which establishes the position of the Piston in the cylinder) and the position of the camshaft (which determines the position of the valves for the cylinder) is critical not only for peak engine performance but also to preclude interference between pistons and valves in high compression engines.
In the intake cycle, the intake cylinder piston cycles downwards as the intake valve opens. The piston movement creates negative pressure that helps draw the air/fuel mixture into the cylinder. Just after the piston reaches the lowest position in the cylinder (known as bottom dead center), the intake valve closes. In the compression cycle, the intake valve is closed to seal off the cylinder as the piston rises in the cylinder to the highest position (known as top dead center), which compresses the air/fuel mixture to a small volume. This compression action serves to provide a higher pressure against the piston when the fuel is ignited as well as pre-heating the mixture to assist with an efficient burning of the fuel. In the power cycle, the air/fuel mixture is ignited which creates an explosion that forces the piston back down to the lowest position and transfers the chemical energy released by burning the air/fuel mixture into the rotational motion of the crankshaft. The exhaust cycle has the piston again rising upward in the cylinder while the intake valve remains closed and the exhaust valve is now open. The pressure created by the piston helps force the exhaust gases out of the cylinder through the exhaust valve and into the exhaust manifold. Connected to the exhaust manifold are the exhaust system, a set of pipes that includes a muffler to reduce acoustical noise, and a catalytic converter system to manage emissions from the engine combustion. Once the piston reaches the top of the cylinder in the exhaust cycle, the exhaust valve begins to close and the intake valve starts to open, beginning the process over again. Note that the cylinder pressure on intake helps to keep the intake valve opened and the high pressure in the compression cycle helps to keep both valves closed.
In engines that have multiple cylinders, the same four cycles repeat in each one of the cylinders but sequenced so that the engine proves smooth power and minimizes noise and vibration. The sequencing of piston movement, valve movement, and ignition is accomplished through the precise mechanical design and electrical timing of ignition signals to the spark plugs that ignite the air/fuel mixture.
Engine Valve Motion
The motion of the engine valves is driven by the camshaft of the engine, which contains a series of lobes or cams that serve to create linear motion of the valve from the rotation of the camshaft. The number of cam lobes on the camshaft is equal to the number of valves in the engine. When the camshaft is in the cylinder head, the engine is called an overhead cam (OHC) design; when the camshaft is in the engine block, the engine is called an overhead valve (OHV) design. Regardless of the engine design, the basic movement of the engine valves occurs by the cam riding against a lifter or a tappet that provides a force that presses against the valve stem and compresses the valve spring, thereby removing the spring tension that keeps the valve in the closed position. This movement of the valve stem lifts the valve off the seat in the cylinder head and opens the valve. Once the camshaft rotates further and the cam lobe moves so that the eccentric portion is no longer directly in contact with the lifter or tappet, the spring pressure closes the valve as the valve stem rides on the centric portion of the cam lobe.
Maintaining the proper valve clearance between the valve stem and the rocker arm or cam is extremely important for the proper operation of the valves. Some minimal clearance is needed to allow for the expansion of metal parts as the engine temperature rises during operation. Specific clearance values vary from engine to engine, and failure to maintain proper clearance can have serious consequences to engine operation and performance. If the valve clearance is too large, then the valves will open later than optimally and will close sooner, which can reduce engine performance and increase engine noise. If the valve clearance is too small, valves will not close fully, which can result in a loss of compression. Hydraulic valve lifters are self-compensating and can eliminate the need for valve clearance adjustments.
Modern combustion engines can use a different number of valves per cylinder depending on the design and the application. Smaller engines such as those used in lawnmowers may have only a single intake valve and one exhaust valve. Larger vehicle engines such as 4-, 6- or 8-cylinder engines may use four valves per cylinder or sometimes five.
Engine Valve Materials
Engine valves are one of the components in internal combustion engines that are highly stressed. The need for reliable engine operation dictates that engine valves be capable of exhibiting resistance to repeated and continuous exposure to high temperature, high pressure from the combustion chamber, and mechanical loads and stresses from the engine dynamics.
The intake valves on internal combustion engines are subjected to less thermal stress because of the cooling effects of the incoming air/fuel mixture that passes by the valve during the intake cycle. Exhaust valves, by contrast, are exposed to higher levels of thermal stress by being in the pathway of the exhaust gases during the exhaust cycle of the engine. In addition, the fact that the exhaust valve is open during the exhaust cycle and not in contact with the cylinder head means the smaller thermal mass of the combustion face and valve head has a greater potential for a rapid temperature change.
Intake valves, because of their lower operating temperatures, are typically made of materials such as chrome, nickel, or tungsten steel. The higher temperature exhaust valves may use more heat resistant metals such as nichrome, silicon‑chromium, or cobalt-chromium alloys.
Valve faces that are exposed to higher temperatures are sometimes made more durable by the welding of Stellite, which is an alloy of cobalt and chromium, to the valve face.
Other types of material used for the fabrication of engine valves include stainless steel, titanium, and tribaloy alloys.
In addition, coatings and surface finishes can be applied to improve the mechanical properties and wear characteristics of the engine valves. Examples of this include chromium plating, phosphate plating, nitride coating, and swirl finishing.
Types of Engine Valves
Besides the characterization of engine valves by function (intake versus exhaust), there are several specific types of engine valves that exist based on design and materials. The primary types of engine valves include:
Monometallic engine valves
Bimetallic engine valves
Hollow engine valves
Monometallic engine valves, as their name implies, are fabricated from a single material that forms both the valve stem and valve head. These types of engine valves provide both high heat resistance and exhibit good anti-friction capabilities.
Bimetallic engine valves, also known as bimetal engine valves, are made by joining two different materials together using a friction welding process to create a valve that has austenitic steel on the valve head and martensitic steel for the valve stem. The properties of each of these steels serve an optimal purpose, wherein the austenitic steel on the valve head provides high-temperature resistance and corrosion resistance, and the martensitic steel for the valve stem offers high tensile strength and abrasive wear resistance.
Hollow engine valves are a special bimetallic valve that contains a hollow cavity that is filled with sodium. The sodium liquifies as the valve temperature rises and is circulated by the motion of the valve, which helps dissipate heat from the hotter valve head. The hollow design facilitates greater heat transfer through the stem than with solid valves because the martensitic stem material is a better conductor of heat than the austenitic head material. Hollow valves are especially suited for use in modern engines that are delivering more power out of smaller, denser engine designs that have higher exhaust gas temperatures which solid valves are not capable of handling. These higher exhaust temperatures are the result of several conditions, including:
A desire for a lean-burn combustion process that reduces greenhouse gas emissions
Engine designs with higher compression ratios and higher combustion pressures which offer greater efficiency
Integrated manifold designs that support turbochargers for more engine performance from smaller engines
There are several other types of engine valve designs. So-called sleeve valves consist of a tube or sleeve that sits between the cylinder wall and the piston, and which slide or rotate driven off a camshaft as with other engine valves. The movement of the sleeve valve causes ports that are cut into the sleeve to align with corresponding ports in the cylinder wall at different points in the engine cycle, thus functioning as a simple engine intake and exhaust valve without the complexities of rocker arms and lifters.
Engine Valve Specifications
Typical engine valves are specified by the parameters outlined below. Note that this data is intended for information purposes and be aware that variations in the parameters used for specifying engine valves may exist from manufacturer to manufacturer. By understanding the specifications, buyers are better equipped to engage in discussions of their specific needs with suppliers of engine valves.
Abstract 1 Introduction Tool coating treatment is one of the important ways to improve tool performance, and the choice of coating materials is the key to the performance of tool coating. Depending on the nature of the coating material, the coated tool can be divided into "hard" coated tools and...