轮机英语课文

LESSON 3

DIESEL ENGINE CONSTRUCTION I

Pistons and Piston Rods

A piston consists of a lower part--piston skirt of cast iron--and a upper part or crown made of a special heat-resistant steel. They are bolted together and fixed to the piston rod. The position of the piston parts in relation to each other is secured by means of machine-turned recesses and a dowel pin in the crown. Each piston is provided with five or six piston rings fitted in chromium plated grooves in the crown. The two or three topmost rings are narrow rings having diagonal cuts, while the next two are broad and are provided with overlapping end joints. The lowest ring is an oil distributor ring. All the rings are slightly rounded on the external top and bottom edges to keep the oil film on the cylinder liner during the running-in period for new piston rings.

In order to control thermal stresses, thin-wall intensively cooled pistons are used for some types of modern engines. In this case, the piston has an internal insert in the piston crown, which serves only to direct the cooling liquid flow and, thereby, to intensify the conventional 'cocktail shaker' effect.

The piston rods are bored from the top flange to a point opposite the centre of the crosshead. Through this bore, a long pipe is inserted which goes nearly to the bottom of the bore. The outside diameter of the pipe is less than the diameter of the bore, the result being that an annular space is formed between the piston rod and the pipe.

The lower end of the piston rod is reduced in diameter to fit a bore in the crosshead, this being secured to the piston rod by a nut. The piston rods are provided with dowel pins to ensure the correct assembly of the components.

On each of the two journals of the crosshead, crosshead shoes are mounted which are guided in the built-in crosshead guides of the engine frame. The position of the crosshead shoes on the crosshead is determined by dowel pins. The guide shoes are secured to the crosshead by means of tap-bolts.

The crosshead shoes are white metal lined, grooves being cut horizontally in the face of the white metal to ensure an adequate supply of lubricating oil.

The crosshead is short and rigid and the bearings are so constructed that the bearing pressure between the journal and bearing is distributed evenly over the entire length of the bearing. In order to improve the working conditions of the bearings, the bearing pressure is made smaller and the peripheral speed higher in later designs.

The pistons are cooled by oil supplied from the forced lubricating system. The cooling oil is admitted through the pipe, from which the oil is led to the cooling spaces of the piston through telescopic pipes or articulated pipes and round the internal pipes of the piston rods. The cooling oil is conducted from the pistons through the internal pipes of the piston rods and ducts in the crossheads, as well as the slotted pipes from which the oil is led to ‘control boxes’with sight glasses on the engine.

Sealing between the crankcase and scavenging air boxes is achieved by means of the piston rod stuffing boxes, located in the bottom of the scavenging air boxes. Each stuffing box is provided with two sealing rings and three scraper rings. The sealing rings, which are mounted