What is a cylinder head in a car?
The abbreviation for cylinder-head cylinder head is one of the most important components of any internal combustion engine. Every car owner should know what is the cylinder head in the car, the principle of its operation and design features. This will help in time to notice a possible malfunction, as well as to ensure the stable operation of the power unit in various modes.
Description of the cylinder head and existing modifications
The cylinder head is the upper part of the cylinder block of the internal combustion engine. It is attached to it with bolts or special studs. The main purpose of the cylinder head is to control the flow of fuel into the working cylinders, ensure its combustion, control and distribution of gas flows. It is on the accuracy of adjustment of individual components of the cylinder head that the power and stability of the engine as a whole depends.
What the cylinder head looks like
For various power units, cylinder heads are made of cast iron or aluminum-based alloys. It is aluminum cylinder heads that are installed on most modern vehicles, which allows the overall weight of the power unit to be somewhat reduced.
For engines with an in-line arrangement of cylinders, a single cylinder head is used, and for V-shaped power plants, separate heads for each row are used. There are no other design differences.
Video about the cylinder head
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How the cylinder head works
The cylinder head casing (crankcase) is made by casting and subsequent machining (milling, drilling). The body of the product contains channels for the circulation of coolant, oil pipelines for lubrication of major components, separate combustion chambers for each of the cylinders. In addition, the crankcase has holes for installing spark plugs or injectors (for diesel engines). By design, the combustion head is considered a complex unit, which includes several different mechanisms.
- The timing mechanism, which ensures the removal of exhaust gases. The valves of the timing system open in a precise sequence depending on the operating stages of each individual cylinder.
- Timing gear actuator to open the valves when they are needed.
- The inlet and exhaust manifold mounting pads, which provide fuel supply and exhaust venting.
- To the fixed elements of the cylinder head include guide sleeves and valve seats. These elements provide sealing of the gas distribution mechanism. Mounting these parts is performed by hot-pressing, perform it yourself, especially without special equipment, it is almost impossible, especially in a private garage.
Each of these units is responsible for the performance of the engine as a whole, and the failure of any of them will cause a more serious failure. In the video clip below, you can observe the work of all the elements of the cylinder head in motion.
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How to Install a Typical Cylinder Head
Cylinder Head, Head Gasket and Engine Block.
Considering that the cylinder head has many channels for the movement of lubricant, coolant, exhaust gases, the most important condition for proper installation is reliable sealing at the connection to the cylinder block. This is done by installing a special gasket made of reinforced asbestos. This material can withstand high temperatures and considerable pressure of working fluids and exhaust gases. Note that the cylinder-head gasket is disposable, reuse will not guarantee reliable sealing of the connection with the cylinder block.
A tight fit of the cylinder head and compression of the asbestos gasket is achieved by tightening the mounting bolts or nuts on the studs. Consider the fact that any misalignment during these operations will result in insufficient sealing of the connection. That is why the tightening must be carried out with a certain torque, which should be checked with a torque wrench. At the same time each stud should be tightened strictly in a certain order, the violation of which will also cause problems with inadequate sealing.
During continuous operation it is necessary to pay attention to the tightness of the cylinder head to the surface of the cylinder block. The appearance of oil or coolant leaks indicates unreliable sealing of the connection. In this case it is necessary to tighten the cylinder head again.
During maintenance be sure to check the condition of the most stressed elements of the cylinder head. Be sure to evaluate the condition of the valves, camshaft, and don’t overlook the integrity of the oil seals.
All work related to the repair of the cylinder head or replacement of its individual mechanisms can be done by yourself only if you have the appropriate experience. Remember, any carelessness and non-observance of assembly technology will cause more serious engine damage. And the cost of such repair will be essentially greater. Therefore trust the cylinder head repair only to the professional automobile mechanic having experience and the appropriate equipment.
Cylinder heads (cylinder heads)
The cylinder heads (cylinder caps) together with the cylinders form the cylinder head cavity, in which all thermal processes of the engine run. The cylinder head (cylinder head) not only closes the cylinder, but also serves as a cavity for complete or partial placement of the compression volume c, i.e. the combustion chamber as well as the spark plug (with external mixture formation) or the injector (in engines with internal mixture formation). The top-valve engine head contains the valve mechanism, the channels of the working body intake into the cylinder and the hot exhaust gas exhaust. To the heads are also attached intake and exhaust lines with their systems and engine auxiliary equipment.’
The walls of the head, which form the combustion chamber, are exposed to open flame and gas pressure to a greater extent than the walls of the cylinder. Therefore, make them 1.5-2.0 times thicker than the walls of the cylinder liners and intensively cool
With liquid cooling, the cylinder heads (cylinder heads), as well as the cylinders, are dressed with a cooling jacket, and in air-cooled engines – finned. Head and cylinder cooling jackets are combined by means of ducts into a common system, liquid circulation in which is organized so that its “cold” flow at the entrance to the engine cooling system has a temperature of about 80 ° C and first of all washed the hottest walls of the head (exhaust pipes). In air-cooled engines head ribbing is made especially developed, and the ribs are located along the cooling air flow so that a more efficient heat transfer is provided.
In connection with the above, the cylinder head acquires a very complex design, especially in engines with the upper arrangement of valves. Automobile engine heads do not only removable, but also cast in common for all cylinders forming a series (from two to eight inclusive), or a group of cylinders (practiced mainly with air-cooled). Only in very small two-stroke single-cylinder engines is the joint casting of the head with the cylinder still used. Combining these important structural elements of the engine skeleton makes it extremely difficult to process the cylinder mirror and perform mounting and dismounting operations during repair of multicylinder engines, so the joint casting method is currently not used in the automotive and tractor industry.
The cylinder heads (cylinder heads) of automobile and tractor engines are made of gray or alloyed cast iron, but most often of aluminum alloys (for brevity they are usually called aluminum). In air-cooled and liquid-cooled carburetor engines, aluminum heads are preferred. Aluminum alloys have good thermal conductivity, as a result of which the thermal stress of the head walls is relatively lower than that of the cast iron head. Therefore, aluminum heads help to reduce the degree of heating of the fresh charge and allow to work with higher compression ratios on the same fuel without the occurrence of detonation combustion. As a result, the use of aluminum heads can improve the power and economic performance of engines.
In order to simultaneously exploit the high thermal conductivity of aluminum and the heat resistance of cast iron in air-cooled diesels, the head base and head nozzles, especially the exhaust ports, are sometimes made of cast iron and cast with an aluminum alloy. Compared to cast iron heads, this improves heat dissipation and reduces possible head deformation during heating and cooling. However, for the heads of forced diesel engines (working with a high thermal stress) the use of aluminum alloys is recommended.
In the manufacture of heads of aluminum alloys are necessarily used insert valve seats. They are made of high-strength heat-resistant cast iron with a high linear expansion coefficient, alloyed or medium-carbon steel and aluminum bronze
For a tight and secure fit of the saddles in the head, the head is heated to about 170-200°C, and the saddles are sometimes cooled to minus 80°C (in dry ice). After such assembly, the seats are rolled back by sealing the head material around them. This is due to the fact that the hottest place of the head is the bridge between the valve seats, heated to 230 – 260 ° C, and since the mechanical strength of aluminum alloys significantly decreases when heated, the poor fit of the inserted seat leads to leakage and failure of the entire head. Valve guide sleeves made of cast iron, ceramic metal or bronze are also pressed into the heated head. Such bushings are also used in cast iron heads.
Inserts in the aluminum head of air-cooled engines also make for mounting spark plugs or nozzles, if the latter has a threaded mounting, ie screwed directly into the body of the head. These inserts are usually made in the form of simple threaded adapter sleeves (liners) and screwed into the preheated head.
Plug-in valve seats are used in cast iron heads, but they usually put under the exhaust valves, as they work in more severe conditions (the average heating temperature reaches 800 ° C). The inserts in these cases are made of heat-resistant materials.
The joint surfaces of the cylinder head and cylinder block are sealed with steel-asbestos or all-metal gaskets, which are put at once under the entire head. When tightening the cylinder head studs, such as the right cylinder block, the gasket is clamped between the upper bearing plane of the right block and the fitting plane of its head.
Asbestos gaskets are now used in the vast majority of automotive-type engines. They are based on a fire-resistant fibrous mineral – asbestos, prepared in the form of a thin sheet (sheet asbestos). To give the pads the necessary strength, they are reinforced with a thin perforated steel sheet or steel wire mesh. In the latter case, an asbestos sheet is obtained, from which the head gaskets are cut out. Gaskets are made with the same inner and outer contours, which are present at the fitting plane of the cylinder block with a complex pattern of holes for the combustion chambers, studs mount the connecting channels of the cooling jacket, etc. The thickness of the gaskets in the working (compressed) position is approximately 1.5 mm.
On the contour of the combustion chambers and in places with thin bridges between the cylinders steel-asbestos and other reinforced gaskets edging thin relatively soft steel sheet. Metal trim improves the mechanical properties of the gaskets, and most importantly, helps increase the local tightness of the joint around the combustion chamber, which is essential for reliably sealing the cylinders. To protect the gasket from burning, its edged edge is diverted from the contour’of the combustion chamber inner walls by about 1-2 mm. In designs with wet liners, for this purpose, the supporting flange of the liner is sometimes provided with a special shoulder, which protects the sealing gasket from the open flame and its possible burnout
To reduce the adhesion of the gaskets to the fitting planes of the cylinder head or cylinders and their subsequent breakage during engine disassembly, the surfaces of the gaskets are abundantly coated with graphite. Earlier all the asbestos engine gaskets were completely lined with thin sheet brass (foil). This resulted in the so-called copper-asbestos gaskets, which could withstand repeated use, but due to their relative complexity and high cost, in the vast majority of cases they are now replaced by relatively simple, cheap, although less reliable, reinforced gaskets.
All-metal head gaskets are made of sheet metal – aluminum, copper, or mild steel. Aluminum gaskets are used, for example, on the B-2 diesel engine. They are cut from a whole sheet under the entire cylinder head (engine block) so as to overlap the supporting flange of the liner, which, in the cases of wet liners, is usually 0.1 – 0.2 mm higher than the fitting plane of the cylinder block. The B-2 wet liner mounting flange has a series of annular sealing grooves in contact with the gasket and a shoulder on the inner lip to protect the aluminum liner from direct attack by an open flame.
Steel head gaskets are a set of several sheets of thin, relatively soft material, spackled in a certain way. Such gaskets are used, in particular, on two-stroke diesel engines of the Yaroslavl Motor Plant.
Copper gaskets, made in the form of thin rings, are put under the cast iron cylinder heads of air-cooled engines. Aluminum heads in these engines are usually installed without gaskets.
Reliable sealing of the cylinder head in any case depends on its correct and uniform tightening when installed on the cylinders. The cylinder head should be tightened only when the engine is cold, in a strictly defined sequence, and always with a torque wrench that allows controlling the amount of force applied. Head tightening usually begins with the middle area of the head with gradual transition to the peripheral areas. The tightening torque of heads in carburetor engines with liquid cooling averages 7-12 kG-m (≈70-120 N-m), and in diesel engines reaches 20 kG-m (≈200 N-m). When selecting the sequence and the correct tightening torque in each individual case, refer to the manufacturer’s instructions. Incorrect head tightening reduces or completely nullifies the effectiveness of any gaskets. It is also necessary to ensure that under the nuts studs (bolt heads) mounting aluminum heads necessarily put the usual steel thick washers, otherwise the nuts will cut into the soft body of the head and destroy the surface of its walls. Cast iron heads are fastened without the use of washers.
