Lesson 12 第 12课

MARINE MEDIUM SPEED DIESEL

ENGINE Ⅰ

The term medium speed diesel engines is currently taken to mean an engine with an operating rotational speed of between 300 to 1200 r/min.

This group of engines is now entirely of the trunk piston type, mainly operating on the four-stroke cycle and the majority is now fitted with exhaust gas driven turbochargers and charge coolers.

*Rationalization(合理配置 ) of cylinder layout(布置 ) and numbers of cylinders has been carried out in the power range of 300 to 750 kW by most manufacturers, resulting in the basic configuration( 结构 , 外形 ) of either six or eight cylinders in line, or alternatively( 或者 ) six or eight cylinders in a Vee form.

For power requirements greater than, say, 900kW, 8, 12 and 16 cylinders Vee or twin bank engines are used.

Cylinder and crankcase

The cylinders of medium speed engines are water cooled. The bore of each cylinder is formed in a liner which can be replaced when worn out.

The cooling water is in direct contact with the outer surface of the liner as a result of which it is termed a ‘wet’ liner. A number of cylinder liners are enclosed in one cast iron casing to form a cylinder block.

The cooling water jacket is common to all the cylinders but there are often dividing walls to ensure that each cylinder receives the right amount and flow of water.

Doors are provided on the cylinders casing, through which the water spaces may be cleaned and inspected when overhauling the engine.

In an alternative construction each cylinder is enclosed by a separate water jacket.

＊ On a bedplate is mounted a casing termed the column(机架 ) which forms the crankcase and supports the cylinder block.

In many designs the force produced by the reaction 　 of the cylinder heads to the gas pressure in the cylinders is transmitted from the top of the cylinder block directly to the crankshaft main bearing by through bolts(long-tie bolt贯穿螺栓 ).

Smaller engines can be made more rigid than large ones and may not have through bolts. In addition, because the casing are not so big and heavy, the column and cylinder block may be made in one piece.

For Vee-form engines the top of the column is shaped to give two sloping　 surfaces on which the cylinder blocks are placed at the correct relative angle to each other.

Doors are formed in the column of the both in line and Vee engines to give maintenance access to the crankshaft main and connecting rod large end bearings.

Crankshafts

Crankshafts of the medium speed engines are almost invariably solid forged from a single piece of steel. The cranks of a multi-throw(多拐的 ) shaft are set at angles to each other giving a ‘firing order’ for the engine.

Although the crankshafts of medium diesel engines appear to be robust(坚固的 ), they rely on the support of the main bearings to develop their full strength.

When a crankshaft has to be handle outside the engine, it should be carefully supported in a manner which will avoid imposing( 施加 ) high bending moments on it.

In the engine it is essential to ensure the bearings carrying it are in good alignment(对中 ).

Connecting rods

With a few exceptions medium speed diesel engines have trunk pistons with the result that piston and connection rods have to be fitted together before being assembled into the cylinder.

For one or two types of engine, the piston and rod can be withdrawn downwards from the cylinder into the crankcase and then out through the crankcase door.

However, this design tends to result in a high engine and therefore the piston and rod is more usually withdrawn upwards passing through the bore of the cylinder.

In case of engines have separate water jackets, the whole cylinder assembly with the piston and rod inside may be removed as a single unit;

The aperture(孔 ) in the frame for the water jacket being much larger than the cylinder bore gives much more room for the connecting rod large end to pass.

Connecting rod large ends are either of fixed centre(固定中心式 ) or ‘marine’ type( 船用式 , 中心可调式 ) design. In the latter design, the large end bearing is separate from the rod.

A distance piece, with ground faces, known as a compression plate may be interposed between the rod and the bearing housing, its thickness being chosen to ensure the correct compression ratio(压缩比 ).

Cylinder liners

Cylinder liners are made from close-grained cast iron( 细晶粒铸铁 ). *They are simple cylindrical shapes flanged at the top end to provide location and a means of securing them in the cylinder block or to the water jacket.

Immediately below this flange there is often a joint ring which may be of copper( 铜 ) or in some designs of a heat resisting rubber( 耐热橡胶 ).

The lower end is fitted with rubber rings to form a seal for the bottom of the water space. As well as stopping water leaks into the crankcase these rubber rings may be arranged also to prevent oil from the crankcase entering the water jacket.

The upper part of the liner bore where the top piston ring reaches the top of its travel suffers greatest wear.

This is because at this point the ring comes to rest and reverses its direction of motion and it is difficult to maintain an adequate film of oil between the surface of the ring and liner.

Also the gas pressure is highest when the ring is in this position, forces it hard against the liner and, of course, the top of liner is hot from the repeated combustion cycles which tends to dry off (弄干 ) any oil there is.

Additional, tiny particles of carbonaceous matter are formed by the combustion processes, some of them may be abrasive and over a period of time an accumulation builds up in the groove around the ring leading to wear promoting(促进 ) condition.

Obviously, wear will be reduced if the top part of the liner is kept reasonably cool. Liners are sometimes specially shaped to fit the jacket in a way that promotes cooling without sacrificing(牺牲 ) strength.

The bores of cylinder liners when new have a specially prepared surface designed to aid the running in of the piston rings and the liner. It is slightly rough in order to retain the oil and to promote rapid wear in.

Some designs of liners have a deposit of chromium plate on their bores. The chromium plate is of a porous(多孔的 ) nature to provide an oil retaining surface.

Reading material

INSPECTION OF MAIN PARTS

Piston, rings and cylinder liners

The piston should be inspected to see that it is free from cracks and that there are no ridges(隆起 ) or grooves on bearing surfaces other than the ring grooves.

If scores( 划痕 ) cover more than 5 per cent of the contact area with the liner the piston is unacceptable.

The axial clearance of each piston ring should be measured at about four place round its circumference(圆周 ), this is done by pressing the ring down on the bottom land of the groove.

This axial clearance should be between 0.00045 and 0.0015 of the piston diameter.

The rings should be inspected to see that they are free from ridges, cracks and grooves.

The gap clearance of the ring should be measured by inserting each ring in turn in a ring gauge or by placing it in the bottom unworn portion of the cylinder liner and using a depth gauge or a piston crown to make sure that it is positioned symmetrically(对称地 ).

The gap is measured by feeler gauges between the ends of the rings. The top compression ring gap should be between 0.006 and 0.009 of the cylinder bore, the other rings between 0.004 and 0.006.

If the gap clearance of a piston ring is too large the thickness of the ring should be checked and the ring replaced if it is apparent that it is badly worn.

The new ring should also be checked for gap clearance and filed(锉 , 锉平 ) on the butts(端面 ) to the correct minimum dimension.

If the axial clearance( 轴向间隙，天地间隙 ) of the piston rings is unacceptable, the ring should be measured to see whether it is the ring or the grooves that is worn.

If it is the grooves, and rings of oversize axial thickness are available, the grooves can be re-machined to suit such a new ring.

The bore of the cylinder liners should be inspected and measured. They should be free from scores, cracks and ridges.

If the liner is chromium plated then patches( 斑点 ) where the chromium has worn off should be carefully examined to see if gas blow past is occurring.

*A cylinder dial gauge(千分表，量缸表 ) should be used to measure the wear of liner by taking readings parallel to and at right angles to the center line of the crankshaft at the top of the top piston ring travel, half way down the liner bore, and at the bottom of the liner bore where it is probably not worn.

Any liner which is badly cracked or scored causing gas blow past should be replaced. The chromium plate on the bore of a liner is beginning to wear through, the liner should be replaced.

The wear of a cylinder liner should not exceed 0.004 times the bore diameter. If it is a chromium plated bore liner the permissible wear will be depend on the thickness of the chromium plate.

A cylinder liner will have to be replaced if it is cracked or if there is excessive wear or scoring of the bore, or pitting and corrosion on the outside, a ridge will have formed in the liner bore at the top of the piston ring travel, this should be removed by stoning if the top ring is renewed.

Crankshaft and bearings

Only one main bearing half shell should be removed at a time, the other being left in place to give the crankshaft adequate support.

Each journal and pin should be examined for ridging and grooving. Ridging are formed in the way of oil grooves in the bearing shells, if they do not exceed 0.0015 inches in height they are acceptable unless a new bearing shell is to be fitted.

Grooves are formed by abrasive particles entering the bearing and scoring the shaft, if the effective bearing area has been reduced by more than 10 percent remedial action should be taken.

The diameter of the crankpins should be measures using a micrometer(千分表 ). Measurement should be taken at two positions along the length of each pin at eight or twelve positions round the pin(i.e. at 45 °or 30° intervals).

The difference between the maximum and minimum diameters of a crank pin should not exceed ¼ of the maximum permissible bearing clearance.

It is usually not possible to use a micrometer in order to measure the diameter of the main journal with the crank in position in the engine.

However, some measurements can be taken by a special bridge gauge(桥规 ) and feelers(塞尺 ) as shown in Fig.5.

The measurements are always taken in the horizontal plane and the crankshaft turned through 30º or 45º intervals for each successive(连续的 ) measurement.

Bearing shells should be rejected( 报废 ) if:

a) The bearing metal is cracked or broken up;

b) The bearing surface is grooved over more than 10 percent of its area;

c) The arc(弧 ) of contact between the journal or pin and its bearing is less than 140º and the area of contact is less than 90 percent within the arc;

d) The steel back has been fretting (磨损 ) in the housing;

e) There is less than 90 percent contact area between the back of the bearing and the housing.

The ‘nip’(压隙 ) of each bearing should be measured. With the shell and cap assembled as for running the nuts should be tightened the full amount and a check made that the clearance between cap and housing is zero.

The nuts should then be slackened back to finger tightness and the clearance between the cap and the housing measured at each side.

The sum of the reading is the ‘nip’. For thin shells it should usually be between 0.0008 and 0.0018 of the journal diameter.

After ensuring that the ‘nip’ is correct the bearing clearance should be checked. The bearing cap and its associated half shell are removed leaving the other half in position.

Two pieces of pure lead wire are placed across the journal at position 1/3 r and 2/3 r along its length.

The cap and shell are replaced and tightened up until the ‘nip’ is zero. The cap and shell are then removed and the thickness of the lead wire measured by a micrometer.

It is important to use only pure lead wire which is very soft and to choose a thickness of wire which is greater than the normal allowable clearance but not greater than twice this amount.

The clearances should be within the manufacturer’s limits.

As a guide these will be approximately 0.1mm to 0.2mm for white metal bearings and approximately 0.25mm to 0.4mm for copper lead or tin-aluminium bearings with crankshaft journal diameters in the range from about 200mm to 400mm.