Pistons are generally manufactured from an
aluminium alloy, which reduces weight and increases heat dissipation. There are numerous designs to accommodate thermal expansion according to engine type and application (Fig. 2.61).
In order to supply pressurized oil to the engine moving surfaces, an oil gallery is formed along the length of the cylinder block (Fig. 2.60). This has drillings to supply oil directly to the bearings in the block,
Figure 2.60 Oilways
Figure 2.61 Piston features: 1, fi re land; 2, crown;
3, piston pin bore; 4, skirt diameter; 5, skirt; 6, piston rings
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Figure 2.62 Piston dimensions: 1, crown diameter;
2, skirt diameter; 3, diameter in piston pin direction;
4, diameter at right angles to the pin
Figure 2.63 Piston design features: 1, piston;
2, offset; 3, centre line; 4, block thrust face
slap’. Note that pistons are marked so that they can be installed correctly and this should be carefully observed.
Around the upper portion of the piston, grooves are cut to accommodate sealing rings, known as piston rings (Fig. 2.64). In general, there are three or four grooves and rings, the lowest of which is known as the oil control ring and is used to control the amount of lubricant remaining on the cylinder bore surface to lubricate the piston. The upper rings are known as compression rings and these provide the gas-tight seal, maintaining the cylinder pressures that create force to move the piston.
Piston rings seal the combustion chamber to prevent the escape of combustion gases and loss of cylinder pressure; these are known as ‘compression rings’ (Fig.
2.65). In addition, the piston rings must control the oil Figure 2.64 Piston rings: 1, rectangular; 2, internally chamfered; 3, taper faced; 4, trapezoidal; 5, L shaped;
6, stepped; 7, slotted oil control; 8, oil ring with expander; 9, oil ring with spiral expander
Figure 2.65 Compression and oil control rings Aluminium has greater thermal expansion than
cast iron used for the block and cylinder liners. This means that the piston expands more than the block as the engine temperature increases. When the engine is cold, the working tolerances are greater to allow for expansion. The piston has design features to allow for expansion and correct tolerances at running temperatures; for example, a cold piston is slightly oval and tapered inwards towards the crown (Fig. 2.62).
The piston pin or gudgeon pin has an offset by a small amount toward the thrust face of the cylinder bore, which allows the thrust forces at the piston crown to maintain the piston against the cylinder wall (Fig. 2.63). This has an effect when the engine is cold by reducing piston movement due to excessive clearance, which creates a noise known as ‘piston
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Figure 2.66 Cross-section of a piston
Figure 2.67 Piston pin (gudgeon pin)
Figure 2.68 Con rod and pin
fi lm on the cylinder bore surface, and these are known as ‘oil control rings’. Combustion pressure is allowed to act on the back of the cylinder-sealing compression rings to help maintain a gas-tight seal of the piston assembly.
Key fact
Piston rings seal the combustion chamber to prevent the escape of combustion gases.
Compression rings are manufactured from cast iron, with a surface coating to promote fast bedding in. This means that the rings quickly wear in to give a gas-tight seal against the cylinder pressures. It is important not to damage this coating during fi tting. Note that rings have different cross-sections according to their mounting position on the piston (Fig. 2.66).
Oil control rings can be one of two designs. A multipart ring consists of two thin alloy rings used in conjunction with an expander between them. A cast iron ring has a groove and slot arrangement to allow oil fl ow back to the sump via the ring and piston.
Figure 2.69 Con rod features: 1, front of engine;
2, identifi cation marks; 3, big end cap; 4, oil spray hole for cylinder wall lubrication
The piston pin or gudgeon pin bore is machined into the piston to accept the piston pin, also known as the gudgeon pin (Fig. 2.67). The fi xing mechanism of the piston pin to the piston and the connecting rod can vary (Fig. 2.68). It can be an interference fi t in the connecting rod, or a push fi t in both the piston and connecting rod end. If the piston pin is clamped in the connecting rod, the piston pin bore is smooth (Fig. 2.69). Circlip grooves are formed in the piston pin bore when a push fi t piston pin is used.
The piston crown forms part of the combustion chamber and experiences the full cylinder pressure applied by the expanding gases. Many different designs are available depending on engine type.
Complex shapes can be formed in the piston crown to allow for valve movements and to create an effective combustion chamber space, promoting the correct charge motion for effi cient combustion.
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The piston or gudgeon pin provides the mechanical link between the piston and connecting rod. The pin locates in the piston body and the little end of the connecting rod (Fig. 2.70). The pin can be a clearance fi t into the little-end bearing or bush, and hence a corresponding interference fi t, or located via circlips, in the piston.
An alternative to circlips is that the pin is an
interference fi t in the little end, or is clamped by the connecting rod. In this case, the piston pin bore is the bearing surface and there are appropriate drillings in the piston to allow for lubrication.
The main purpose of the connecting rod is to transfer the linear force from the piston and apply it to the rotating crankshaft (Fig. 2.71). It is generally manufactured from carbon steel in a process known as drop forging to form the required shape and profi le.
The connecting rod is designed specifically with a high resistance to bending, compressive and tensile forces via an I-section profile. The piston
end, known as the little end, has an appropriate bush, bearing or clamping arrangement for the piston pin. The crankshaft end of the connecting rod is known as the big end (Fig. 2.72). This consists of a split bearing with a removable bearing cap. The bearing cap is attached to the connecting rod via bolts or nuts.
It is important to note that the connecting rod and bearing cap are machined as one unit, and hence, the parts are matched. Therefore, they must always be reassembled as a pair and fi tted correctly oriented.
The crankshaft receives the linear force of the pistons, via the connecting rods, and converts this force into a rotating torque (Fig. 2.73). The crankshaft is generally manufactured from cast iron or steel alloy via a forging or casting process.
Key fact
The crankshaft of a four-cylinder engine generally has fi ve main bearings.
Figure 2.70 Piston pin in position
Figure 2.71 Force transfer
Figure 2.72 Big end bearing: 1, front of engine;
2, identifi cation marks; 3, big end cap; 4, oil spray hole for cylinder wall lubrication
Figure 2.73 Conversion of linear to rotary motion
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Figure 2.74 Crankshaft pulleys
Figure 2.75 Crankshaft: 1–5 main journals
Figure 2.76 Journals in position in the engine block