As with all cylinder blocks, the aluminium alloy castings are made with ribs, webs and fillets to provide rigidity.
The cylinder block for the four-cylinder in-line engine shown in Figure 4.5( as you have seen in last post "Cylinder blocks, assembly and construction) is made in two parts — the cylinder block and the base plate. Both parts are made from aluminium. The cylinders have dry liners that are pressed into the block and these are not replaceable.
The main bearing caps are combined with the base plate which has webs that carry the lower halves of the crankshaft main bearings. The cylinder block and the base plate are matched during manufacture and cannot be replaced separately.
In addition to the two parts of the cylinder block, there is a cast aluminium oil pan bolted to the underside of the base plate. This further increases the rigidity of the assembly.
Cylinder blocks can be designed with cylinder sleeves (or liners). These are fitted to cast iron blocks and are different to the liners that are cast into aluminium alloy cylinder blocks.
With cast iron blocks, the sleeves are cast separately and then fitted into place in the block when the engine is being assembled. Separate sleeves allow more control over the manufacturing process and a grade of iron that is different from the block can be used. The cast iron for the block can be of a grade which is easy to cast, while the sleeve is made more resistant to wear.
There are three types of cylinder sleeves: dry sleeves, flanged dry sleeves, and wet sleeves (Figure 4.6).
A thin dry sleeve is of uniform diameter throughout its length and is a press fit in its bore in the cylinder block. The wall of the sleeve is about 2 mm thick, and its outer surface is in contact with the block for its full length. The top of the sleeve is finished flush with the top of the cylinder block and is hardly visible.
This type of sleeve is sometimes fitted to new engines, but is used to recondition badly worn or damaged cylinders that cannot be re-bored without removing excess metal.
The bore in the block must be true because the accuracy of the sleeve depends on its contact with the block. This also has a bearing on heat transfer from the cylinder.
This type of sleeve is not generally used for passenger or light commercial vehicles.
The outer surface of a wet sleeve forms part of the water-jacket around the cylinder. It is called a wet sleeve because it has coolant against its outer surface. This eliminates problems of heat transfer between the sleeve and the coolant in the water-jackets. However, the sleeve must be sealed to prevent coolant leaks. Seals are used at the top to prevent coolant from entering the combustion chamber and at the bottom to prevent coolant from entering the crankcase.
Figure 4.7 shows a cylinder block and wet sleeve. The block does not have full-length bores, but has mounting holes at the top and bottom. Wet sleeves have thicker walls than dry sleeves. They do not have the same support in the block and they depend on their wall thickness to prevent distortion.
Engines with wet sleeves can be reconditioned by fitting new sleeves and pistons which are supplied ready to install.