What is Engine Cylinder Head?

 

Cylinder head- 

In an Internal combustion engine, The cylinder head is the roof of the engine. It is the most important part where the airflow and the fuel injection are controlled. It is a largely unnoticed and valuable engine part. The top of the cylinder is covered by a separate cast piece known as the cylinder head. The cylinder head is bolted to the top of the cylinder block. It contains a combustion chamber. A spark plug and sometimes valves (in I-and F-head) are mounted in it. It incorporates passages for the flow of cooling water. Depending upon the valve layout, the cylinder head may carry the camshaft, rockers, and valves. Water passages are provided adjacent to the valve and plug seatings.

Making materials- 

The cylinder head is usually made of gray iron or aluminum alloy. Aluminium has the advantages of lightness in weight and high heat conductivity. It is cast separately from the cylinder block so that it may be removed for cleaning carbon and grinding valves. To retain compression in the cylinder, a flat piece of gasket is placed between the cylinder head and the cylinder block. In certain cases, such as a racing car engine. a separate head is not used. But a single piece of the cylinder block and the head is difficult and costlier to make; the internal parts of the engine are not as accessible.

Advantages of Aluminium Alloy- 

1. Higher heat conductivity which is about three times that of cast iron.
2. Light in weight.
3. Increased compression ratio without detonation due to the heat conductivity.
4. Better cooling while the engine is running.
5. Greater power output and low fuel consumption due to increased compression ratio and better cooling effect. 

6. Engine warms up more quickly and requires a smaller radiator.

Disadvantages of Aluminium Alloy-

1.  The engine costs more due to the costly aluminum alloy.
2. It is liable to be crusted out of shape by holding down studs due to low modulus of elasticity.
3. Due to higher thermal expansion, greater clearance is required between the piston and cylinder.
4. The Greater possibility of corrosion by cooling water.
5. Sometimes, it is necessary to use cast iron valve seatings and spark.plug inserts.
6. Due to inter-metallic corrosion between steel studs and aluminum alloys, the head may stick to the cylinder block.

Design- 

From the mechanical point of view, the design details of a detachable cylinder head are perhaps the most difficult. Care should be taken to the following points when detachable heads are made :

1. The bore of the cylinder or liner should not be distorted by the pull of the holding-down studs.
2. The circulation of coolant in the cylinder head should be carried as nearly as possible to the top
end.
3. To ensure a sound gas-tight joint, the holding-down studs must be distributed as uniformly as possible around the circumference of each cylinder.

Fig. (a) is the popular side valve construction with all valves in line, the detachable head being of turbulence type. The piston closely approaches the flat portion of the cylinder head.

Fig. (b) is shown the reverse valve construction of the cylinder head which gives good volumetric efficiency, as a large diameter inlet valve may be used; and the valve port gives direct access to the bore.

Fig. (c) is shown a cylinder head with vertical valves in a single line permitting the simple push rod and rocker gear.

Fig. (d) is an ideal hemispherical head that is used in many high-performance designs. In this design, the flame travel is short and a high compression ratio can be readily provided by a domed piston crown.

Cylinder Head Gasket- 

A gasket is placed between the cylinder head and cylinder block to retain compression in the cylinder, prevent leakage, and ensure a metallic tight-fit joint. The gasket should be able to withstand not only high pressure but also extreme temperature. 

A metal sandwich-type gasket is made of either copper and asbestos or steel and asbestos. It is mainly used on passenger cars and trucks. 

Embossed steel or shim-type gasket is generally used on an I-head engine.

Shimbestos is a combination of a thin steel heat shield on one side and a highly resilient specially treated metal-reinforced asbestos on the top side.

Felbestos is made of perforated steel sandwiched between two treated asbestos sheets.