The cylinder head bolts onto the top of the cylinder block where it forms the top of the combustion chamber.
In-line engines of light vehicles have just one cylinder head for all the cylinders. Larger in-line engines can have 2 or more.
V-type and horizontally-opposed engines have a separate cylinder head for each bank of cylinders.
Just as with engine blocks, cylinder heads can be made of cast iron, or aluminium alloy.
A head made of aluminium alloy is lighter than if it were made of cast iron. Aluminium also conducts heat away more quickly than iron. So with an aluminium-alloy head, the heat of combustion can be conducted away into the coolant more quickly.
Manufacturing the head is similar to manufacturing the block. A casting mold is made. Sand cores are put in to form any hollow areas. Depending on the engine, these can be for coolant and lubricant passages, and inlet and exhaust ports.
Air-cooled engines have cooling fins cast into the cylinder head. The underside of the head is shaped to form the combustion chamber.
Molten metal is poured in, and allowed to cool. The cores are broken out and removed, and the cylinder head cleaned of any sand. After casting comes machining. Surfaces that must seal are machined flat. Holes are drilled and tapped for attaching bolts and studs.
In sand-cast heads, the large holes that had contained sand are machined, then fitted with soft metal plugs, called core plugs.
Cylinder heads are designed to help improve the swirl or turbulence of the air-fuel mixture, and prevent fuel droplets settling on the surfaces of the combustion chamber or cylinder walls.
When air-fuel mixture is compressed between the piston and the flat part of the cylinder head, it produces what’s called “squish”. That means squeezing of the gases to increase their velocity and turbulence.
In gasoline engines, the three most popular combustion chamber designs are called hemispherical pent roof, bath-tub and wedge.
A hemispherical, or pent-roof combustion chamber has the intake valve on one side of the chamber and the exhaust valve on the other. This provides crossflow. Air-fuel mixture enters on one side, and exhaust gases exit on the other. Positioning the valves in this way leaves room for relatively large valves and ports, and that helps the engine breathe. Breathing refers to the engine taking in the air or air-fuel mixture. Fuel starts to burn at the plug, then burning travels outward in all directions. This is called flame propagation. With the plug in the middle of the hemisphere, the flame front has less distance to travel than in some other designs, which gives rapid and effective combustion. This design is common in a lot of passenger vehicles.
The bath-tub combustion chamber is oval-shaped, like an inverted bathtub. Valves are mounted vertically and side by side, making them simple to operate. The plug is to one side, and that creates a short flame path. It all helps increase turbulence.
The wedge-shaped combustion chamber tapers away from the plug which is at the thick end of the wedge. The valves are in line and inclined from the vertical. This design usually has a smaller surface area than the others, with less area where fuel droplets can condense. Less fuel is left unburned after combustion, which reduces hydrocarbon exhaust emissions. And since the flame is directed toward the small end of the wedge, damage caused by detonation is reduced.
Automotive services Diesel combustion chambers
Diesel combustion chambers come in 2 main types. Direct and indirect injection. Both are designed to promote turbulence, to help the compressed air and injected fuel mix well.
Engines using direct injection have cylinder heads with a flat face. The combustion chamber is formed in the top of the piston.
Sometimes, the rim of the piston provides “squish”, forcing the air to the centre of the combustion chamber. This causes turbulence as fuel is injected into the cylinder.
In indirect injection, the piston is fairly flat, or has a shallow cavity. The main combustion chamber is between the cylinder head and the top of the piston, but a smaller, separate chamber is in the head. Fuel is injected into this smaller chamber. It can have various designs. A swirl chamber is spherical, and connected to the main chamber by an angled passage. Both the injector and glow plug are screwed into the head. The glow plug preheats the air inside to help start the engine.
During compression, the spherical shape makes the air swirl in the chamber. This helps make a better mixture of the air and fuel, which improves combustion. This combustion chamber is divided into a main combustion chamber and an air cell, joined by a throat. The injector is in the throat.
When injection commences, combustion pressure forces the air to flow from the air cell where it mixes with fuel from the injector. The rush of air from the air cell produces a rotary motion of gas in the main chamber which helps make combustion more efficient.
This pre-combustion chamber is screwed into the cylinder head. The injector is mounted in the upper end.
Injection occurs near the top of the compression stroke. Only part of the fuel is burned in the pre-combustion chamber because of the limited amount of air there. The high rise in pressure forces burning fuel into the main chamber. This happens very rapidly, which helps make more efficient combustion.
Automotive services Intake and exhaust passages
The size of passages in the head can affect engine output. Smaller intake and exhaust passages and ports allow more torque at low engine speeds. This is because smaller passages improve mixing of air and fuel at low speeds, which causes more efficient combustion.
At high speeds however, these smaller passages restrict airflow. To reduce the effect of this, this engine has two inlet valves. One opens at low speed and the other operates at higher engine speeds. Larger passages produce greater power at high engine speeds.
Each intake and exhaust passage can be formed separately in the head. Intake passages for adjacent passages may have a common, thin wall between them. This is called siamesed. Exhaust ports in the same head can also be siamesed.
When all intake and exhaust ports are on one side, it is called a counter-flow head. They can be cast separately or siamesed.
When all of the intake ports are on one side and exhaust ports are on the other, it is called a cross-flow head. This allows for straighter passageways and higher efficiency.
Automotive services Gaskets
Gaskets form a seal by being compressed between stationary parts where liquid or gas could pass. Most gaskets are made to be used only once. They can be made of soft materials such as cork, rubber, paper, asbestos. They can also be made of soft alloys and metals such as brass, copper, aluminium or soft steel sheet metal.
Choosing which material and design to use depends on the substance to be sealed, the pressures and temperatures involved. And, the materials and mating surfaces to be sealed.
Head gaskets seal and contain the pressures of combustion within the engine, between the cylinder head and block. They also seal oil passages between block and head. And control the flow of coolant between the block and the head.
Some gaskets provide or adjust clearances. Some joints between surfaces on modern engines are being sealed with special sealants which eliminate the use of gaskets in some applications.
Gaskets around a rotating part would quickly wear out and leak. To seal these parts, oil seals are needed. Many different kinds have been developed, including oil slinger rings. The most widely used is the lip type dynamic oil seal. It has a shaped dynamic rubber lip that’s held in contact with the shaft to be sealed by a circular coil spring called a garter spring.
A similar sealing principle is used to seal the valve stem to prevent oil entering the engine combustion chamber.
Rotating or sliding shafts can also be sealed by using “O” rings, but generally they are not as durable in most applications as the lip-type seal.
As a general rule, oil seals must be replaced when a component is overhauled.
Automotive services Gaskets and oil seals
Gaskets form a seal by being compressed between stationary parts where liquid or gas could pass. Most gaskets are made to be used only once. They can be made of soft materials such as cork, rubber, nitrile, paper, heat resistant materials or graphite; or they can also be made of soft alloys and metals such as brass, copper, aluminum or soft steel sheet metal. Such materials may be used individually or in some cases as blends to produce the required functional material.
Choosing which material and design to use depends on the substance to be sealed, the pressures and temperatures involved, and the materials and mating surfaces to be sealed.
Head gaskets seal and contain the pressures of combustion within the engine, between the cylinder head and block. Modern head gaskets have to be constructed to resist high temperatures and engine detonation.
Some modern high temperature head gaskets are called 'anisotropic' in nature. This means that the gasket is designed to conduct heat laterally to transfer heat from the engine to the coolant faster. They are normally constructed with a steel core. Special facing materials are added to both sides of the gasket core to provide a comprehensive seal under varying torque conditions.
With the advent of environmental factors and a reduction in the use of asbestos, replacement materials have been developed. Some of these modern special materials that are now used for the side layers of head gaskets are designed to withstand temperatures up to 2100 degrees F or 1150 degrees C. Such materials are also designed to allow the cylinder head and block, some of which which have considerable distortion rates, to move slightly on the head gasket as they expand during engine warm-up. This feature is vital for preventing head gasket failure.
Some head gaskets also incorporate stainless steel fire rings to help to contain heat and pressure within the cylinder. In addition, many head gaskets also have an added silicone based outer coating on both sides of the side material layers to provide additional cold sealing ability during start-up and warm-up. Head gaskets also seal oil passages, and control the flow of coolant between the cylinder block and head and are fitted with beads or rings to prevent leakage and corrosion.
Some joints between surfaces on modern engines are being sealed with special sealants which eliminate the use of gaskets in some applications. Pure rubber, or conventional cork-rubber is unable to deal with the stresses and pressures in modern engines.
Modern gasket manufacturers are producing improved material combinations such as nitrile and cork blends to deal with 'high tech' engine demands. Such combinations are more able to deal with issues such as compressibility and wicking.
Some materials are designed to 'swell' in application and increase sealing ability. For instance when oil inside a valve cover penetrates the edge of the gasket material, it is designed to swell by approximately 30%. This swelling effect increases the sealing pressure between the head and valve cover sealing surfaces and helps to seal potential leaks.
Some gasket materials are designed to have high tensile strength. They are designed to resist breakage during dismantling or installation processes.
Gaskets around a rotating part would quickly wear out and leak. To seal these parts, oil seals are needed. The most widely used is the lip type dynamic oil seal. It has a shaped dynamic rubber lip that’s held in contact with the shaft to be sealed by a circular coil spring called a garter spring.
A similar sealing principle is used to seal the valve stem to prevent oil entering the engine combustion chamber.
Rotating or sliding shafts can also be sealed by using “O” rings, but generally they are not as durable in most applications as the lip-type seal.
Various materials are used in modern oil seals, some being impregnated with special coating materials that are designed to increase their sealing ability on worn shafts.
As a general rule, oil seals must be replaced when a component is overhauled.
Automotive services Head gaskets
Head gaskets seal and contain the pressures of combustion within the engine, between the cylinder head and the block.
Some high temperature head gaskets are called 'anisotropic' in nature. This means that the gasket is designed to conduct heat laterally to transfer heat from the engine to the coolant faster. They are normally constructed with a steel core. Special facing materials are added to both sides of the gasket core to provide a comprehensive seal under varying torque conditions.
Some head gaskets incorporate stainless steel fire rings to help to contain heat and pressure within the cylinder. Many head gaskets also have an added silicone based outer coating on both sides of the side material layers to provide additional cold sealing ability during start-up and warm-up. Head gaskets also seal oil passages, and control the flow of coolant between the cylinder block and head and are fitted with beads or rings to prevent leakage and corrosion.
Gasket manufacturers have produced improved material combinations such as nitrile and cork blends because pure rubber, or conventional cork-rubber, is unable to deal with the stresses and pressures of 'high tech' engines. Such combinations are more able to deal with issues such as compressibility and wicking.
Some materials are designed to 'swell' in application and increase sealing ability. For instance when oil inside a valve cover penetrates the edge of the gasket material, it is designed to swell by approximately 30%. This swelling effect increases the sealing pressure between the head and valve cover sealing surfaces and helps to seal potential leaks.
Some materials are also designed to allow the cylinder head and block, some of which have considerable distortion rates, to move slightly on the head gasket as they expand during engine warm-up. This feature can be vital for preventing head gasket failure.
Automotive servicesTurbulence
Turbulence refers to the swirling motion of a liquid or a gas.
It helps to maximise the mixing of air and fuel, which helps make sure the combustion process occurs efficiently. Without turbulence, the air-fuel mixture can form local areas of high pressure and temperature that can cause detonation during combustion. A high level of turbulence can prevent this.
