Engine cylinders, when cast in a single housing, are known as the engine block. Usually, the engine block Figure 2.48 V6 crank
Figure 2.49 V8 crank
Figure 2.51 Engines vary, here is a four cylinder inline design. (Source: Ford Media)
Figure 2.50 Typical fi ring orders – V8 engine
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is manufactured from cast iron (Fig. 2.52) or aluminium alloy. In the latter case, cast iron or steel liners form the cylinder bore. The engine block forms the major component of a ‘short’ motor.
The cylinder bores are formed via a machining process with a boring tool to give the correct form to the cylinder within closely specifi ed tolerances. Cast iron is a mixture of iron with a small amount of carbon (2.5–4.5% of the total). The carbon added to the iron gives a crystalline structure that is very strong in compression. In addition, it is slightly porous and this helps to retain a fi lm of lubricating oil on working
surfaces. This property makes cast iron particularly suitable for cylinder bores that can be machined directly into the casting. On many engines cylinder liners are used.
Cylinder liners fall into two main categories, wet and dry (Fig. 2.53). Wet liners are installed such that they are in direct contact with the coolant fl uid. They are fi tted into the block with seals at the top and bottom and are clamped into position by the cylinder head. Spacers are fi tted at the bottom to adjust the protrusion of the liner to achieve the correct clamping force.
Defi nition
Cast iron: A mixture of iron with a small amount of carbon (2.5–4.5% of the total).
Key fact
Cylinder liners fall into two categories, wet or dry.
Dry liners are not in direct contact with the coolant.
In general, they are fi tted into the casting mould and retained by shrinkage of the casting via an interference fi t. Alternatively, they can be pressed into place in a precast cylinder block. When repairing or reconditioning the engine, the former type can be rebored whereas the latter type is replaceable.
Figure 2.52 Cast iron engine block
Figure 2.53 Wet and dry liners
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Most modern engines (Fig. 2.54) have specifi c treatments applied to the cylinder bores and, as such, cannot be rebored or honed. Replaceable liners mean that the liner and piston assembly can be replaced without the need for specialist reboring equipment.
Commercial vehicle engines often use replaceable liners to reduce repair times.
Cast iron has been used for cylinder block construction in the past as the cylinders can be bored directly into the material; in addition, these bores can be remanufactured or repaired by reboring oversize. Cast iron is porous and hence the cylinder bore is capable of retaining lubricating oil for lubrication of the contact surfaces. The disadvantage of cast iron is weight. Modern engines use aluminium and can achieve the same strength and stiffness as cast iron via advanced design techniques.
Aluminium alloy cannot provide a suitably durable surface for piston ring contact. Therefore, cylinder liners or sleeves, made from cast iron or steel, are normally fi tted into an aluminium cylinder block (Fig. 2.55).
Key fact
Aluminium alloy cannot provide a suitably durable surface for piston ring contact, so liners made from cast iron or steel are used.
Recent developments in material technology have produced a coating of nickel phosphate and silicon carbonate which provides a suitably durable surface for the cylinder bores (Fig. 2.56). Note that these bores cannot be rebored, so if excessive wear occurs the block must be replaced.
A crankcase is usually integrated into the cylinder block and is machined in-line to form the crankshaft main bearings. This process is known as line boring.
The main bearings are split in two halves; one half locates in the block, the other in the bearing cap.
The bearing caps are secured before the machining process and thus each cap is matched in position with its opposite half. It is important to note this when disassembling and reassembling the bearings.
The caps are located via dowels and fastened via high-tensile steel bolts (Fig. 2.57). It is important to follow manufacturer guidance if the bolts are removed and refi tted; replacement of the bolts and tightening procedures must be followed if specifi ed.
Between the cylinder walls and the outside surface of the cylinder block, voids and channels are formed during the casting process; this is known as the water jacket (Fig. 2.58) and is used for engine cooling purposes. A sand former creates this space during casting and when the cast block has cooled, the sand is evacuated via holes in the side of the block. These holes are then sealed using core plugs (Fig. 2.59).
Figure 2.54 Modern engine block
Figure 2.55 Liners in an aluminium block
Figure 2.56 Cylinder coating: 1, engine block; 2, silicon carbonate; 3, nickel phosphate
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Figure 2.57 Main bearing bolts
Figure 2.58 Water jacket
Figure 2.59 Core plugs
crankshaft and cylinder head. Additional drillings connect the oil pump and pressure control valve to complete the oil supply system. The block is prepared, drilled and threaded to attach additional components such as the oil sump pan and oil pump assembly.
Key fact
Pistons are generally manufactured from an aluminium alloy, which reduces weight and increases heat dissipation.