Cast iron has been successfully used for monolithic blocks. This is because the casting process can mass-produce large complex shapes. The cylinder bore must have high dimensional accuracy. Honing is the finishing process used to give accurate roundness and straightness. It is performed after the fine boring process. Figure 2.10 shows a micrograph of a honed bore surface.
The honing whetstone carved the crosshatch pattern. During engine operation, the groove of the crosshatch holds lubricating oil. The resulting oil film generates hydrodynamic lubrication Figure 2.11 is a picture of a honing machine. The tool shaft installs a honing head with a segmented whetstone at its end (Fig. 2.12). The whetstone grinds the bore by exerting an expanding pressure. The vertical motion of the head together with revolution generates the crosshatch pattern. The sharpness of the whetstone determines the profile of the crosshatch. A dull honing
stone with an excess pressure gives an unwanted over-smeared surface. Ideally the finished surface exposes graphite without burr. The quality of the honing is measured by surface roughness value.
The graphite in the cast iron block works as a solid lubricant during machining as well as in engine operation. A solid lubricant gives a low frictional force without hydrodynamic lubrication. Graphite, MoS2, WS2,
Sn, and Pb are all well known as solid lubricants. The low frictional force of graphite comes from the fact that the crystal structure has a very low frictional coefficient during slip at the basal plane. Figure 2.13 shows a schematic representation of the mechanism. The crystal slides easily along its basal plane even with a low shear force. The graphite decreases friction for tools during machining. The brittle nature of graphite makes chips discontinuous.The resultant high machinability gives high dimensional accuracy to cast iron parts. The graphite also works as a solid lubricant to prevent seizure of the piston or piston ring even under less oily conditions.
The micro-burr of the crosshatch disrupts the oil film to obstruct hydrodynamic lubrication. Additional Mn-phosphate conversion coating (refer to Appendix H) chemically removes the micro-burr to increase oil retention.This prevents seizure during the running-in stage. As well as dimensional accuracy, the surface profile also determines oil retention which, in turn, greatly influences wear resistance. An appropriate profile should be established. One pass finish with the whetstone usually shapes the surface profile to the normal type shown in Fig. 2.14(a)2. An additional finish, scraping off the peak, generates the trapezoid pattern shown in Fig. 2.14(b). This finishing is called plateau honing.
A customer does not want to change engine oil frequently. Less oil consumption is therefore required. Figure 2.152 compares the oil consumption of a 1.9L car engine, measured by the final oil consumption value (FOC). For a normal type profile, the low Ra = 0.12 μm, middle Ra = 0.4 μm and high Ra = 0.62 μm. For the plateau type, the low Ra = 0.14 μm, middle Ra = 0.32 μm and high Ra = 0.88 μm. The oil consumption is least in the normal type of low Ra. However, it is worth mentioning that the scuffing resistance of the low Ra surface is poor. When the bore wall temperature is high, the plateau surface shows excellent resistance to scuffing although oil consumption is high. This feature comes from the fact that the plateau shape can maintain
more lubricating oil without disrupting the oil film.
The science and technology of materials in automotive engines
Hiroshi Yamagata
Woodhead Publishing and Maney Publishing
on behalf of
The Institute of Materials, Minerals & Mining
CRC Press
Boca Raton Boston New York Washington, DC
WOODHEAD PUBLISHING LIMITED
Cambridge England
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