Cylinder-Head and Valves

Cylinder-Head and valves

Two different arrangements of cylinders heads and valves are used in engines of passenger cars and light-commercial vehicles. Both these arrangements have the valves in the cylinder head, but they have different designs of valve trains.

Overhead valve (OHV) engines have their camshafts mounted in the cylinder block and use push rods to transfer movement from the crankshaft to the valve mechanism on top of cylinder-head. Over the head camshaft (OHC) engines have the camshaft mounted directly on top of the cylinder-head. There are many variations of these two basic arrangements which will be considered.

While some information in this chapter applies particularly to petrol type engines, much also applies to diesel engines. However there are separate chapters later on diesel engines.

Cylinder heads

Fig 2.1

In-line engines of passenger cars and light-commercial vehicles have a single Cylinder-Head that covers all the cylinders. Larger in-line engines have a separate and have two or more Cylinder-Heads, the type and engines and horizontally opposed engines have a separate Cylinder-Head for each bank of cylinders.

Cylinder head casting

Figure 2.1 shows the top and bottom views of a Cylinder-Head for a V-8 overhead valve engine. A Cylinder-Head such as this is produced as a casting of aluminum alloy or cast iron.

During the casting process, molten metal is poured into shaped moulds. Spaces are left within the casting for the water-jackets and cooling system passages, and holes are left for the intake and exhaust ports. The underside of the casting is shaped to form the combustion chambers.

The casting is finished by a number of machining operations that produce a Cylinder-Head with the following: a flat surface on the underside where it fits onto the top of the cylinder block; machined surfaces on the top for camshaft and valve mechanism; holes for the valve guides; threaded holes for the spark plugs and for securing other parts; mounting surfaces For the manifolds and a machined surface on top for valve cover.

Cylinder head designs

Fig.2.2

While all Cylinder-Head from the same function, there are number of different designs. To a large extent, this will depend on whether it is for an overhead valve engine, and overhead camshaft engine, A V-type engine or a horizontally opposed engine.

 The parts of a Cylinder-Head for a V-type overhead valve engine were identified in figure 2.1, and parts of the cylinder head for a four cylinder petrol engine are shown in figure 2.2. This is the basic arrangement of for an overhead camshaft Cylinder-Head assembly, which includes the camshaft, rocker assembly and the valves and associated parts.

The Cylinder-Head and valve assembly in figure 2.3 are quite different. They are for a horizontally opposed engine with four cylinders. The Cylinder-Head shown is for two cylinders on one side of the engine. There is another Cylinder-Head Of the same design for the two cylinders on the opposite side of the engine. The camshaft is supported and in the Cylinder-Head casting. The rocker arms are of cast aluminum alloy and are fitted with hydraulic lash adjusters (see later heading). In various parts can be identified on the illustration.

Cylinder head cooling

Fig. 2.3

Cylinder-Head for Liquid-cooled engines are provided with water-jackets through which the coolant is circulated. This absorbs the heat from combustion and transfers it to the radiator. Particular attention is given to cooling the exhaust port area, which is the hottest part of the cylinder head.

The coolant is the mixture of distilled or deionized water and chemical Corrosion inhibitors. Chemicals are needed to prevent corrosion that can restrict coolant passages, enlarge holes or cause Leaks. This applies particularly to the aluminum parts, which are very susceptible to corrosion.

•       Air-cooled engines have cooling fins cast into the Cylinder-Head and these with the assistance of a fan, dissipate the heat to the atmosphere.

Continued

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