Engine cylinders, when cast in a single housing, are known as the engine block. Usually, the engine block is manufactured from cast iron 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 . 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.
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.
Most modern engines have specific 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.