Wednesday, June 1, 2011

Cylinder surface-finish & Crankshafts

Fig 4.8
The surface of the cylinder consists of fine scratches that form a cross-hatch pattern (Figure 4.8). This finish is produced with a hone that uses abrasive stones.
Cylinder walls must be smooth enough to enable the piston rings to seal and for the pistons to operate without friction. At the same time, they must have a surface that is able to retain a film of oil.
If the surface of the cylinder (the cylinder wall) is too smooth, oil will be wiped from the cylinder by the piston rings. Oil can also be ‘pumped’ upwards by the pistons and rings into the combustion chamber where it will burn. This will produce smoke and carbon.
The ideal finish for a cylinder is a series of plateaus, or very small flat surfaces between the fine grooves that are left after honing. The grooves in the surface retain oil.
A surface with sharp peaks and grooves will also retain oil, but the peaks will have to wear away to allow the piston rings to ‘bed in’ before a satisfactory working surface is obtained. During this time, there could be some blowby and the engine could use oil.

•     The term blowby refers to combustion gases that escape past the piston rings.

Cylinder wear
Fig 4.9
Figure 4.9 shows, how an engine cylinder wears (the wear is shown exaggerated). Most wear takes place at the top of the cylinder and so the cylinder becomes tapered. The maximum wear is at the top, just below the ring ridge. This is where combustion pressure and temperature are greatest. As the piston moves down the cylinder on the power stroke, the pressure and temperature both decrease so that less wear takes place.
The cylinder also tends to wear oval-shaped. This is due to the side thrust of the piston as it moves down the cylinder on the power stroke. The side thrust is caused by the angle of the connecting rod

Another possible cause of wear, with carburettor engines, is the washing action of the fuel. If the air and fuel are not correctly mixed, small droplets of fuel tend to wash the oil film from the cylinder walls. This does not occur with EFI engines.

Worn cylinders can be reconditioned to a certain extent by honing, but badly worn cylinders have to be re-bored. The worn cylinder shown could be bored oversize to the diameter A, or bored out to diameter B and a dry sleeve fitted.

·      The ring ridges at the top and bottom of the cylinder are caused by the limits of the
    piston-ring travel.

Fig 4.10
Figure 4.10 shows a basic crankshaft with its parts named. This is for a four-cylinder engine, so it has four cranks. Each crank, also called a throw, consists of a crank web and a journal (crank-pin) that has a finely- finished surface for the connecting-rod bearing. This crankshaft is supported in the crankcase by three main bearings and so is referred to as a three-bearing crankshaft.
Fig 4.11
The type of crankshaft normally used is shown in Figure 4.11. This is a five-bearing crankshaft, with a main bearing located between each crank. It also has balance weights. The extra main bearings reduce shaft distortion, and the balance weights reduce vibration.
Crankshafts are a one-piece casting or forging of heat-treated alloy steel with high mechanical strength. The crankshaft must be strong enough to take the downward thrusts of the pistons during the power strokes without distortion, as well as being well balanced to offset the effects of the cranks.

Parts label Figure 4.11 Crankshaft for a four-cylinder engine
1—5 main-bearing journals, 6—9 crank-pin
journals, 10—12 balance weights, 13 mounting flange for flywheel or drive plate.

See balance weights (for crankshafts)>>>

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