Monday, May 30, 2011

Cylinder blocks, assembly and construction

The cylinder block is the largest part of the engine and is the base to which many other parts are assembled or attached. The upper part of the block carries the cylinders and pistons, and the lower part forms the crankcase which supports the crankshaft. The cylinder block has to be rigid to withstand the stresses and vibration to which it is subjected, and must be unaffected by temperatures reached when the engine is in operation.
Balance is an important part of engine design and so this has also been included in this blog.

Cylinder blocks
Fig 4.1
Cylinder blocks are made of cast iron or of aluminium alloy. Aluminium cylinder blocks are cast from aluminium alloyed with other materials. Cast iron cylinder blocks are usually cast from grey iron alloyed with other metals such as nickel or chromium.
During manufacture, patterns that are the shape of the cylinder block are used to form a sand mould. Molten cast iron is then poured into the mould where it cools to become a rough casting of a cylinder block.
Before casting, the shapes of water-jackets, cylinders and some other parts are made up as sand cores. The cores are fitted into the moulds so that these parts of the block do not become solid cast iron during the casting process.
After casting, the core sand is removed through holes in the sides and ends of the block provided for this purpose. This leaves internal spaces for the water-
jackets and other parts. The holes are later fitted with steel plugs which are referred to as expansion plugs, core plugs, or welsh plugs. They can be seen in the cylinder block in Figure 4.1. When installed, they are held in place by sealer and expansion of the plug against the hole.

·       The plugs in the side of the block are cup-shaped and the one in the rear end of the block
    is convex.

During manufacture, the casting undergoes a number of machining operations which include boring and finishing the cylinders, machining surfaces, drilling holes and cutting threads to produce the finished cylinder block.

Cylinder-block assembly

Fig 4.2
The components of a basic cylinder-block assembly are shown in Figure 4.2. These are some of the main parts of the engine. An assembled cylinder block (with the crankshaft, connecting rods and pistons) is sometimes referred to as a short motor. A short motor does not include the cylinder head, the oil pan, the timing pulleys, timing belt or the flywheel.
The lower section of the engine block forms the crankcase, and the part of the crankcase that extends below the main bearings is called the skirt. In some engines, the skirt is extended well below the centreline of the crankshaft and is called a full skirt. This is done to give the engine rigidity. Other engines have a very short skirt or none at all and rigidity is obtained in other ways.

Cylinder-block construction
In most engines, the cylinders, cylinder block and crankcase are all cast together. This is known as mono-block construction, as distinct from engines that have their cylinders as separate parts. This is the case with air-cooled engines where the cylinders are made separately to the crankcase.
Mono-block engines with cast-iron blocks have the cylinders bored directly in the cast iron casting. Cast iron is a suitable material for cylinders because it wears well and resists the effects of heat. Cylinder walls can be plated with chromium, which is very resistant to wear, although this is not common for passenger car engines.

Aluminium cylinder-blocks

Fig 4.3
Some cylinder blocks are cast in aluminium alloy. This has the advantage of being lighter than cast iron. Aluminium alloy on its own is not suitable for cylinders. It is a soft material that wears rapidly under the action of the pistons and piston rings. For this reason, liners of a harder material are often used in the cylinders.
The liners are cast iron sleeves that are either cast or pressed into the block. Liners that are cast into the block have grooves on the outside that form a key between the liner and the block. This prevents any possible movement and the sleeve becomes a permanent part of the block. (The cylinder blocks in
Figure 4.5 arc of cast aluminium alloy with cast iron liners.)

The terms cylinder liner and cylinder sleeve are generally interchangeable.

Aluminium cylinder block without liners

Aluminium cylinder blocks can be designed without liners, but special alloys are needed. Figure 4.3 shows a V-8 cylinder block that has no cylinder liners. The aluminium alloy of the cylinders has been specially treated to expose hard silicon crystals. This provides a low-wearing surface for the pistons and piston rings.
Aluminium alloy is used in the cylinder block to reduce the weight of the engine, but there is also an advantage that it is better at transferring heat than cast iron. The aluminium of the block has a similar expansion rate to the pistons.

Fig 4.5

 Continued
See two-part cylinder block>>>>>








3 comments:

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