Раздел IV. Практика устной речи

4.1 DIESEL ENGINE

The diesel engine is a type of internal combustion engine, which ignites the fuel by injecting it into hot, high-pressure air in a combustion chamber. In common with all internal combustion engines, the diesel engine operates with a fixed sequence of events, which may be achieved either in four strokes or in two, a stroke being the travel of the piston between its extreme points. Each stroke is accomplished in half a revolution of the crankshaft.

The fundamental requirements for the operation of a diesel engine are a supply of fuel, the necessary air for combustion of the fuel and some means to get the air and fuel into the cylinders and the products of combustion out.

The stages in the operation of a diesel engine are as follows:

6. Supply of air.

7. Compression of the air to raise its temperature high enough to initiate combustion of the fuel.

8. Supply of fuel.

9. Expansion of the hot high-pressure gas, which forces out the piston against the resistance of the load on the crankshaft.

10. Removal of the products of combustion.

These stages may be performed in two or four strokes of the piston (one or two revolutions of the engine crank).

The diesel engine has no ignition system. The injected fuel is ignited by the temperature rise associated with compressing air to a high pressure.

4.2 CYCLES OF OPEATION

The cycle may be defined as the complete sequence of events that occur in the cylinder of an engine for each power stroke or impulse delivered to the crankshaft. Those events always occur in the same order each time the cycle is repeated.

Each cycle of operation is closely related to piston position and movement in the cylinder. Regardless of the number of piston strokes involved in a cycle, there are four definite events or phases that must occur in the cylinders.

1. Either air or a mixture of air and fuel must be taken into the cylinder and compressed.

2. The fuel and air mixture must be ignited, or fuel must be injected into the hot compressed air to cause ignition.

3. The heat and expansion of gases resulting from combustion must perform work on the piston to produce motion.

4. The residual or exhaust gases must be discharged from the cylinder when expansion work is completed.

In the 4-stroke diesel cycle, the piston makes four strokes to complete the cycle. There is one power stroke or power impulse for every four piston strokes, or two complete revolutions of the crankshaft.

In 2-stroke diesel cycle the piston makes two strokes to complete the cycle. There is one power stroke for every two piston strokes or for each revolution of the crankshaft.

4.3 Moving Parts of a Diesel Engine

The moving parts may be divided into three groups:

(1) the parts which have only reciprocating motion (pistons), (2) the parts which have both reciprocating and rotating motion (connecting rods), and (3) the parts which have only rotating motion (crankshafts and camshafts).

The piston and rod assemblies include a piston, piston rings, piston pin, connecting rod, and related bearings. The piston must be so designed and must be made of such materials that it can withstand the extreme heat and pressure of combustion. The piston aids in the sealing of the cylinder to prevent the escape of gas and transmits some of the heat to the cylinder wall .A piston serves as a valve in opening and closing the ports of a two-stroke cycle engine.

Piston rings perform three functions: seal the cylinder, distribute and control lubricating oil on the cylinder wall, and transfer heat from the piston to the cylinder wall. In trunk-type piston assemblies, the connection between the piston and the connecting rod is usually the piston pin.

The connecting link between the piston and crankshaft or the crankshaft and the crosshead of an engine is the connecting rod.

The camshaft is a shaft with eccentric projections, called cams, designed to control the operation of valves, usually through various intermediate parts. The cams are arranged on the shaft to provide the proper firing order of the cylinders. The shape of the cam determines the point of opening and closing, the speed of opening and closing, and the amount of the valve lift. All cylinders will be affected if there is a change in timing. The camshaft is driven by the crankshaft by various means: the most common are gears or a chain and sprocket:

One of the principal engine parts, which has only rotating motion, is the crankshaft. The crankshaft changes the movement of the piston and the connecting rod into the rotating motion required to drive such items as reduction gears, propeller shafts generators, pumps, etc. The parts of a crankshaft are main bearing journals, connecting rod journals, counterweights, timing gear bolting flange, lubricating oil holes and flywheel dowel. The flywheel is a means, which stabilizes shaft rotation.

An important group of engine parts consists of the bearings. Some bearings remain stationary in performing their function while others move. Main bearings which support the crankshaft are of the sliding contact, or plain type, consisting of two halves or shells. The bearings used in connection with the piston pins are of three types: the integral bearing, the sleeve bearing or bushing, and the needle type roller bearing. The bearings in the bosses (hubs) of most pistons are of the sleeve bushing type.

4.4 Structural Parts of a Diesel Engine

The diesel engine in its main features is composed of the following parts:

• bedplate

• frame or column

• engine or cylinder block

• cylinder liners

• cylinder head or cover

The bedplate must act as the main strength part of the engine section, providing rigid support for the main bearings and crankshaft. It is also a platform on which the other components may accurately be mounted to support engine cylinders and all working parts.

Frame is a load-carrying part of an engine; it may include parts as the cylinder block, base, sump and end plates.

The engine frame may form the main strength member with a comparatively lighter bedplate construction. In that case main bearings may be suspended from the frame and the crankshaft is said to be underslung.

Cylinder Block is a part of the engine frame that supports the engine cylinder liners, heads and crankshafts, it is a cast iron casing which encloses / houses cylinder liners and supports heads.

Cylinders are of cast iron. They are clamped to the frame by means of long through-bolts which transmit the combustion pressure in the cylinders direct to the frame without causing tensile stresses.

Cylinder covers (heads) close the top end of the cylinders. Theу are tightly secured by studs and nuts to the cylinder block.