After the spring, the national wheat will enter the stage of greening, getting up and jointing. It is the main period of wheat spring management, an important period to promote the growth of weak seedlings and control the stable growth of seedlings, and also a key stage for regulating the rational development of wheat population.
According to news from the Ministry of Agriculture, last year's winter wheat planting and sowing period was concentrated, standardized seeding technology was implemented better, seeding quality was higher, soil moisture was suitable, wheat emergence was neat, seedlings were uniform, and the population was sufficient. Year. The main features are: First, the seedlings grow well. Overall, the proportion of first- and second-class seedlings of winter wheat in the main producing areas increased, the proportion of the three types of seedlings decreased, and the population was sufficient. Second, the soil is full of love. Since September last year, the main producing areas have had more rainfall and better soil moisture, which is more favorable for wheat emergence and tillering, especially in the early December, which is conducive to the safe wintering of wheat. Third, the proportion of Wang Miao is large. Due to the high accumulated temperature before winter and the lack of illumination, some plots with early sowing or large amount of sowing are prosperous. Fourth, the pests and diseases are heavy. Before wintering, the soil moisture is better, and the temperature is suitable for high temperature, which is conducive to weed germination and pests. In some areas, there are seedling pests such as golden worms, and strip rust and powdery mildew occur. In some areas, especially the southern paddy wheat, weeds grow and have a high density. Experts put forward the main objectives of spring management for this year's wheat seedling characteristics: scientific classification guidance, strengthening suppression and planning, controlling Wang Miao seedlings to maintain longevity, promoting weak seedling root growth, strengthening body jointing fertilizer, and promoting foot The grain is multi-grain weight, preventing and controlling pests and diseases, and capturing the harvest of wheat. This edition will publish specific field management recommendations for each major wheat producing area.
North China, Huang-Huai wheat area to include Hebei, Shandong, Henan and northern Jiangsu, northern Anhui, southern Shanxi, Shaanxi Guanzhong Plain. The current characteristics of the seedlings in this region are: the foundation of autumn is good, the soil moisture is suitable, and the current seedling situation is the best year in recent years; but there are also large areas with large length, large weeds in wheat fields, and large number of wintering pests and diseases. Unfavorable factors such as complex seedlings. Spring wheat management earlier Zhuazao tube, guidance, control repression Wang, reasonable logistics fertilizer, promote multi-grain weight spike enough grain to lay a good foundation for the high-yielding wheat.
It is an important agronomic measure to improve the control of the market and increase the efficiency of the grain in the early spring. The repression of the overgrown wheat field during the rejuvenation to the beginning of the body can inhibit the growth of the aboveground parts and play a role in controlling the growth.
Although the precipitation in the autumn last year was large, the current wheat field was generally better. However, after the spring, with the increase of temperature, the soil evaporation increased, and “spring rain is as expensive as oilâ€, there are uncertain factors in precipitation. Therefore, in order to prevent the drought in the spring, it is very important to do everything possible to keep the underground. Early in the early spring, when the surface soil is frozen by 2 cm (topping period), all kinds of wheat fields are slashed to maintain soil moisture, increase surface temperature, eliminate winter weeds, and strive for active management of wheat fields in the middle and late stages. Especially for the wheat fields with small populations and weak individuals, it is necessary to take the shovel as the primary measure for the management of wheat fields in early spring. In addition, watering in the spring or raining should also be timely. When you are squatting, you must make sure that you are fine, even, flat, and smooth, without leaving any waste, not pressing the wheat seedlings, and not weeding, to improve the effect of the stroke.
Promote the combination of control, fertilizer and water management, and manage the water-smelting wheat field in North China and Huanghuai wheat area. The water quality of the wheat field, the seedling condition and the soil fertility capacity are different. The spring fertilizer management must be based on the conditions of seedlings and classification. Spring management should first manage three types of wheat fields, and then two types of wheat fields, followed by a type of wheat field; for Wang Miao, first of all, it must be controlled during the rejuvenation period, and then fertilize and water according to the size of the group during the up or jointing period.
1. Focus on control, and do a good job in Wang Changmiao management. Generally, the number of stems per mu of wheat seedlings is more than 800,000, the plants are taller, the leaves are longer, the differentiation process of spikes of main stems and low tillers is advanced, and freezing damage occurs in early spring. After the jointing, it is easy to cause the field to be covered, the light is poor and lodging, and the spring fertilizer management should be dominated. The first is the repression of the rejuvenation period. This is the most effective measure to control the growth of the seedlings and transform them into strong seedlings. This year, the area of ​​the wheat seedlings is large, and we must do a good job of repression. The second is to spray the Zhuangfengan and other regulators during the period to shorten the base joints and prevent the late lodging. The third is to manage the fertilizer and water of Wang Miao according to the size of the group. The seedling wheat field without defermentation should be used to suppress seedlings in early spring, avoid excessive spring tillering, fertilize water in jointing stage, apply urea 10 to 15 kg per mu; for wheat fields with de-fertilization, if the group is not large, early spring The total number of stems per mu is below 800,000. In the initial stage of the body, the topdressing and watering; if the group is too large, the topdressing and watering in the middle of the body, the general application of urea 10 to 15 kg, to prevent the seedlings to weaken the seedlings.
2. Promote the three types of seedling management. The total number of stems of the three types of wheat fields at the beginning of the green period is less than 450,000, which is mostly late seeding and weak seedlings. The management of spring fertilizer and water should be promoted. In spring, the topdressing is carried out twice. The first time in the regreening period, when the temperature is 5 cm, the temperature is stable at 5 °C, and the topdressing and watering are started. The urea is applied with 5-7 kg of urea and the appropriate amount of diammonium phosphate to promote spring tillering and consolidate the tiller before winter. Increase the number of ears per mu; the second time in the middle of jointing, fertilize and water, increase the number of grains per ear. In the southern part of the Huanghuai wheat area, there are no irrigated plots in the north and south of Henan and the southern part of Henan, and should be applied to the green fertilizer when the spring falls.
3. Promote the combination of control and do a good job in the management of the second type of seedlings. The total number of stems of the second type of wheat field is 450,000 to 600,000. The focus of spring fertilizer management is to consolidate the tillering before winter, appropriately promote the occurrence of spring tillering, and increase the rate of tillering. The level of soil fertility is general. The second type of wheat field with a number of stems of 450,000 to 500,000 is topdressed in the early stage of wheat, and 10 to 15 kilograms of urea is combined with watering. The level of soil is relatively high, and the number of stems is 500,000 to 600,000. In the second type of wheat field, the wheat is topdressed and watered in the middle of the wheat. In the southern part of the Huanghuai wheat area, there are no irrigated plots in the north and south of Henan and the southern part of Henan, and the rain should be applied to the body during the spring precipitation.
4. Control and promote the combination, do a good job in a type of seedling management. The level of soil fertility in a type of wheat field is relatively high. The total number of stems per acre in the beginning of greening is 600,000 to 800,000. It belongs to the strong wheat field, which should be controlled to promote the combination, increase the rate of tillering and increase the number of grains. First, the body is sprayed with Zhuangfeng'an and other regulators, shortening the base internodes, controlling the plant growth, promoting the root system and preventing the late growth. The second is to topdress the water in the jointing stage of wheat, and the urea is 12 to 15 kg. In the southern part of the Huanghuai wheat area, there are no irrigated plots in the northern and southern parts of the south and south of Henan, and the rain should be applied to the rain during spring precipitation.
Repression and tamping, rain and topdressing, and management of dry land wheat for dryland wheat fields without irrigating conditions, spring management should be the key measures for spring wheat field management. After the wheat field is crushed, the capillary is formed in the soil, and the deep soil moisture rises along the capillary to the upper layer soil, which is beneficial to moisturizing the root growth and improving the drought resistance of the wheat. At the same time, after the soil is returned to the early spring or the light rain, the application of nitrogen fertilizer with chemical fertilizers has a prominent effect on increasing the number of ears per pan and the number of grains per ear, increasing grain weight and increasing yield. Generally, about 10 kg of urea is applied. The application of diammonium phosphate to the base fertilizer without the application of phosphate fertilizer.
Prevention and control of pests and diseases, chemical weeding, and comprehensive prevention and control of spring pests and diseases should vigorously promote the treatment of stages, mixed application of drugs and treatment of a variety of pests and diseases. Regreening to jointing stage is another peak of infection and other viral diseases such as sheath blight, total rot disease, root rot and other diseases such as bushy dwarf disease and yellow dwarf disease. It is also a hazard of wheat spiders, underground pests and grass pests. At the peak, it is one of the key links in comprehensive wheat control. All local plant protection departments should do a good job in forecasting and forecasting pests and diseases, and guide farmers in timely prevention and control. Due to high temperature before winter, heavy rainfall, heavy weed occurrence, and small chemical weeding area before winter, chemical weeding should be strengthened after returning to green. When using herbicides, strictly follow the concentration and technical procedures to avoid phytotoxicity. Agricultural technicians everywhere should guide farmers to choose the right medicine, spray concentration and spraying time.
Paying attention to the weather, coping with it in advance, preventing early spring freezing damage to early spring frost damage (inverted spring cold) is a frequent disaster in early spring in North China and Huanghuai wheat area. The most effective measure to prevent early spring frost damage is to pay close attention to weather changes and water before cooling. Since the heat capacity of water is larger than the heat capacity of air and soil, watering before the early spring cold current can increase the water vapor in the near-surface air, and when condensation occurs, the latent heat is released to reduce the variation of the ground temperature. Therefore, in areas with watering conditions, watering before the cold wave can adjust the microclimate near the ground layer, which has a good effect on preventing early spring frost damage.
Wheat is a crop with tillering characteristics. The wheat field that suffers from early spring freezing will not freeze all the parts of the wheat, and the wheat axillary buds can grow into long ears. As long as management is strengthened, a good harvest can still be obtained. Therefore, if frost damage occurs in early spring, it is necessary to remedy it in time. The main remedy: First, seize the time and apply fertilizer. According to the degree of damage, the wheat field subjected to freezing damage should be seized, and the quick-acting nitrogen fertilizer should be applied to promote the early emergence of seedlings and increase the rate of formation of small tillers. Generally, about 10 kg of urea is applied to the mu; the second is to cultivate and raise the ground temperature. Timely cultivating, storing water and warming, promoting root development, can effectively increase the number of tillers and make up for the loss of the main stem.
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.
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.
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