Pistons, connecting rods and bearings

Fig 6.1

Pistons are designed to resist high combustion temperatures and their piston rings have to seal against high combustion pressures. Connecting rods have to withstand high stresses as they transfer the reciprocating motion of the pistons to the cranks of the crankshaft.
The piston, piston rings, piston pin, connecting rod and the bearing form the piston and connecting- rod assembly. The various parts are identified in Figure 6.1.

Pistons
Figure 6.2 shows a piston with its parts identified. This is a short-skirt piston with grooves for three piston rings. There are a number of variations in piston design, which include shape, mass, provision for expansion, and type of material.
Pistons are made of cast iron or aluminium alloy. Aluminium alloy pistons are used in petrol and smaller diesel engines because they arc much lighter than cast iron pistons. However, aluminium has a greater rate of expansion than the cast iron cylinders in which most pistons operate. Because of this, aluminium alloy pistons are specially designed to control expansion. Some larger diesel engines, which operate at lower rpm than petrol engines, have cast iron pistons.

Fig 6.2

Piston clearance
A piston is slightly smaller in diameter than the cylinder in which it operates. This gives it a working clearance of about 0.02 mm to 0.05 mm. Figure 6.3 illustrates the clearance of a piston in its bore, but there are particular ways in which it is measured.

·       See Figures 7. 7 to 7.9 in the coming posts which show how piston clearance is measured.

If a piston has insufficient clearance, there will be no room for the piston to expand when it is hot and it will tend to seize on the cylinder wall. This would cap score both the piston and the cylinder. If there is too much clearance, piston slap will occur, particularly when the engine is cold. Piston slap is caused by the sudden tilting of the piston in the cylinder as it starts down its power stroke. This is done with such force that it produces a distinct noise.

Fig 6.3

·     Piston slap can occur in older engines that have excessive piston clearance due to worn cylinders or worn or collapsed piston skirts.

Piston temperature
The piston is subjected to the full effects of combustion and so it has to be resistant to heat. Typical operating temperatures are shown in Figure 6.4. Under heavy operating conditions, these temperatures can become even higher.
There is a big difference between the temperature of the head of the piston and the skirt of the piston. The heat causes expansion, and this is greatest at the top of the piston. For this reason, the lands at the top of the piston are given extra clearance. They could be as much as 0.75 mm smaller in diameter than the skirt.

Fig 6.4

Control of piston temperature
There are several methods used to prevent pistons from expanding excessively. These include cam grinding, using steel struts, controlling the heat path, and oil cooling. All pistons have some means of controlling their temperature and some pistons have more than one.

Fig 6.5

Cam-ground pistons
A piston that is cam-ground is slightly oval in shape. The shape is obtained by relieving some metal in the area of the piston-pin bosses (Figure 6.5). This makes the diameter of the piston slightly smaller at the piston- pin bosses than at the thrust faces. This allows the piston to be fitted to the cylinder with minimum clearance at the thrust sides. The extra clearance provided at the bosses by cam grinding allows for piston expansion.

·    For an explanation of piston thrusts, see the coming post ‘Piston thrusts’.

Cam-ground pistons get their name from the process by which pistons were finished to size. The machine uses a cam to move the piston backwards and forwards as it is being ground. When cam-ground pistons warm up, they become more round in shape, so the area of contact with the cylinder wall increases (Figure 6.6). Grinding generally refers to cast iron pistons, aluminium pistons can be turned, rather than ground.

Fig 6.6

Steel-strut pistons
Special alloy-steel struts are cast into the piston during manufacture. A strut is fitted to each side of thc piston (Figure 6.7). The steel from which the struts are made has a very low rate of expansion when it is heated. On the other hand, aluminium alloy has a fairly high rate of expansion. As the temperature of the piston increases, the aluminium alloy tries to expand, but it is restrained by the steel struts. This holds the piston at the thrust face to its specified size.

Fig 6.7

Heat path in pistons
As well as controlling piston expansion by struts, heat is kept away from the thrust faces of the piston as much as possible. This also reduces expansion.

One way of doing this is with horizontal slots in the thrust sides of the piston, either in or below the oil-ring groove (Figure 6.8). The slots are used to break the heat path between the head of the piston and the skirt at the thrust faces. The heat is directed to other parts of the piston, such as the piston-pin bosses, where there is more clearance and also more metal to absorb the heat. This reduces expansion at the thrust faces.

Fig 6.8

Piston cooling jets

Normal lubrication helps to cool the pistons and cylinder walls, but as well as this, some engines have cooling jets. These are small nozzles, or jets, which spray a jet of oil up into the piston. The oil strikes the piston head, absorbs heat and then drops back into the oil pan. The oil also lubricates the piston pin and cylinder walls.
Figure 6.9 shows a cooling jet and its action. The jet is supplied with oil from the main oil gallery of the engine. It has a ball cheek valve that closes when the oil pressure in the gallery is low, or when the engine is stopped. At normal operating pressure, the ball valve is forced off its seat, so that the jet sprays oil into the piston. 

Fig 6.9

 Continued

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