Despite the enormous variety in size and power of oil engines used to-day on land and sea, the manufacturing processes are generally the same, although they differ slightly in detail according to each maker’s standard practice
HEAVY MACHINE SHOP in a maker’s works where large types of oil engines are built. All the larger components of an oil engine, such as engine bed, crankcase or frame, cylinders, pistons and the like, are cast from wooden patterns and finished by machines such as lathes and planing machines. For smaller parts iron patterns are generally made.
IN that great group of prime movers known as internal combustion engines — a group embracing those using gas, petrol or oil as fuel — the diesel type in one form or another plays an increasingly dominant part. Diesel engines are made in many types and sizes for stationary power on land, or for mobile power, as on roads and railways. For marine purposes development has reached the stage where, out of the total machinery under construction in 1937 for the world’s shipping, nearly two-thirds were oil engines.
The applications of oil engines on land are almost innumerable. Although the horizontal, semi-enclosed type was formerly most common, and is still much favoured in some countries, the vertical enclosed diesel type is becoming increasingly popular. Sizes vary from one or two horse-power to about 1,000 horse-power, but the bulk of these engines do not exceed 300 brake horsepower. A fairly representative oil engine of medium size develops about 150 brake horse-power.
The manufacturing processes of oil engines vary in detail according to the particular maker’s standard practice. The first thing to be done in the works is to form the patterns from which the castings are made. All the larger parts of the engine, such as the engine bed, the crankcase or frame, the cylinders, liners, pistons and so on, are cast from wooden patterns.
When the parts are small, and there are many to be made of the same size, it is usual to make iron patterns, as they stand up much better to the more or less rough handling received during the processes of moulding and casting. Such parts are generally moulded in moulding machines ; the larger ones are individually moulded by hand. The craft of pattern making is a highly skilled one, and much of the success of a casting depends on the care with which the patterns and coreboxes are made. On the completion of the casting it is turned out of the moulding box, with hard-baked moulding sand adhering to it. In addition, bits of iron, which have been used to support the cores, have to be chipped off, or otherwise removed, and all the sand cleaned off. With the larger castings much of the first rough work is done by hand, and the finishing of the process is effected by sandblasting in a closed chamber.
The smaller castings are generally placed inside a rumbler, a machine consisting of a big iron barrel made in sections which can be easily removed for the insertion or removal of the castings. The machine is rotated at a slow speed, so that the castings, which are loosely packed, are turned over and over, knocking against one another. In this way the sand and bits of iron adhering to the casting are removed, and the rough surface of the casting is smoothed to a certain extent. There are, however, pockets and corners in most castings in which a little sand may lodge. The presence of the least particle of sand inside the parts of any engine would be detrimental, so the castings are sometimes “pickled” in a bath of dilute sulphuric acid.
PRINCIPAL PROCESSES IN THE MANUFACTURE OF OIL ENGINES
Having been thoroughly cleaned, the castings are then taken to the marking-off table to be marked off for the machining operations. If many engines are to be made of the same size much of this marking off is unnecessary as jigs and templets may be used. For the process of marking off larger parts, the casting is laid on a large flat cast-iron table with an absolutely flat machined surface. The casting is painted white, or merely chalked, on the places where centre lines, from which dimensions are to be measured, have to be scratched. The centres of holes to be drilled are marked with a centre punch, and similar work is done for the guidance of the machinist.
The engine bed, or bottom crankcase, as it is sometimes called, has to be of rigid box-section design, as it forms the foundation of the whole structure. It must also be stiff enough to prevent any distortion of the crankshaft while running. The housings for the crankshaft bearings form part of the casting, and the lower part is generally formed into a chamber or sump for the lubricating oil.
Having been marked off, the casting is then taken to be planed on the top and bottom surfaces. Nowadays planing is frequently done by a plano-miller. Instead of cutting off the metal by a single cutting tool, the machine carries one or more milling heads, which embody a large number of small cutting tools distributed over the surface of a thick disk plate. These are given a rotary motion and at the same time the feed is applied in a longitudinal direction.
The bearing caps are milled in the step, and then drilled and fitted to the engine bed, which had corresponding steps formed when the top was faced in the planing machine. The bearing housings are then bored and faced on a horizontal boring machine. The bearing shells are made separately and fitted when the engine is erected, the white metal lining being scraped to obtain the final bedding of the crankshaft. The top crankcase, or frame, may be in one piece or it may be m sections, according to the number of cranks used. Most makers adopt certain power unit sizes so that a frame section takes one, two, three or four cylinders. To get further cylinders and cranks up to eight, it is necessary only to combine the most suitable number of units. This method simplifies construction.
Engine Stresses
The engine frame has also to be a casting combining rigidity and strength, and requires great care in its design, for it has several functions to fulfil. It must be strong enough to take the maximum stresses produced by the high cylinder pressures; and it must stand the side-to-side slogging action on the cylinder walls, caused by the combined action of the piston and connecting rods, which occurs to a greater extent in an internal combustion engine than in a steam engine. The frame, moreover, is subjected to high temperatures at one end, due to the hot cylinders, and to relatively cold ones in other parts. Apart from these requirements, it has to be of sufficiently open construction to provide accessibility to the interior and, being generally a casting — though welded, fabricated frames are sometimes made — the relatively different masses of metal of which it is made may be subject to latent casting stresses.
Some makers prefer the frame standards to be A-shaped and secured to the engine bed on both sides. Others use a C-shaped frame, the top bottom of the open gap being firmly tied together by cast-iron front doors of ample section to give rigidity.
The frame casting is first placed in a piano-milling machine, similar to the one which planed the engine bed, to have all its large flat surfaces machined. The top surface to which the cylinder heads will be attached is generally done by the vertical spindle of the machine; at the same time one of the side surfaces may be done by a horizontal head of the same machine.
The next big operation is the boring out of the housings or the cylinder or cylinder liners. This may be done on either a vertical-spindle or a horizontal-spindle machine. For a still finer finish, the holes are done by grinding if the cylinders are cast in a block, as in small engines.
Other operations on the frame include the boring out of camshaft bearing housings, facing up smaller flat surfaces, and so forth. These operations are carried out in suitable machines.
DIESEL ENGINE GENERATOR SET direct coupled to alternator with exciter and mounted on a baseplate common to both. Oil engines of this size, having three cylinders and developing 120 brake horse-power, are becoming increasingly popular.
Next comes the drilling and studding operation on the frame. Much of this work in standard sizes of engines is done with jigs, so that all engines of the same type and size will be interchangeable and similar parts may be used on different engines. This method reduces the cost of separate marking off and individual drilling. Drilling jigs and fixtures are now extensively used, being a development of the older and simpler templets. These, however, are used a good deal for such work as marking off large plates and sections in shipbuilding and all big steel structural work. In its simplest form, the drilling jig is a flat cast-iron plate, with holes drilled in it to the spacing required for the particular job to which it applies. These holes are fitted with hard steel bushes, in each of which is drilled a hole slightly larger than the diameter of the drill to be used. The bushes act as guides to the drills.
Holes of average size are generally bored in a vertical radial drilling machine with a single spindle head, but, where there are two or more moderate-sized holes to be drilled in line, multiple-spindle drilling machines are used. The position of the various spindles can sometimes be adjusted to suit the pitch of the holes. It is usual at the same time and with the same machine to tap all the holes where studs are to be used, and also to insert and drive home the studs in position.
Other parts, such as cylinder heads, crankcase doors, and other cast iron parts go through similar operations.
The size of engine determines whether the cylinders will be embodied in the frame, cast separately, or made in the form of separate liners. This last method is frequently adopted in engines of moderate size. The cylinder has to stand high explosion pressures up to about 600 lb. per sq. in., as well as high initial temperatures inside at the top end; its outer surface is surrounded by the water jacket. It has to maintain its true cylindrical form and yet allow a small clearance for the piston, so as to avoid leakage of the gases at high pressure or the passage of oil from the crankcase. Thus the cylinder should be of the simplest possible form, and free to expand uniformly. The liner is therefore made of the best close-grained cast iron — preferably a nickel iron.
Cylinder Heads Separately Cast
To provide the smoothest possible surface for the piston to work on, the cylinder liner bore is generally finely ground. In some instances it is finished by honing to an accuracy of 0·00025 in. in the diameter dimension. This operation is done by a set of hones mounted on a suitable frame with springs encircling the hones to maintain a constant light pressure on them.
While being driven round at fairly high speed the hones are given a longitudinal movement as well, so as not to wear even the most minute ridges. The outer surface of the cylinder liner is formed by turning in a lathe. The lathe used is generally of the combination turret type, which is semi-automatic in its functions, and capable of performing two or more operations at the same time.
At the lower end of the liner some form of joint or packing is necessary to serve the dual purpose of allowing freedom of expansion of the liner in the housing of the frame at that point, and of preventing leakage of water from the cylinder jacket to the crankcase below. This joint may take the form of rubber rings pressed into grooves cut in the liner.
The cylinder heads, except in the smallest sizes, are made separately and fixed to the top of the frame. This practice has the advantages of forming better castings, as well as fitting in with the practice of subdivision of parts to facilitate the production of engines with any number of cranks up to the maximum. A cylinder head has generally to combine several important parts in an oil engine, such as the housing of the air and exhaust valve mechanisms, the fuel injection fittings, and nowadays the swirl chamber designed to give a rapid turbulent effect to the incoming air.
SECTIONS THROUGH THREE-CYLINDER VERTICAL ENCLOSED DIESEL ENGINE
1. Control Handwheel; 2. Governor; 3. Fuel and Oil Filters; 4. Oil Cooler; 5. Air Filter; 6. Water Outlet Manifold; 7. Exhaust Thermometer; 8. Exhaust Valve; 9. Water Inlet Manifold; 10. Fuel Injector; 11. Air Valve; 12. Fuel Pump.
The casting may be fairly complicated, as it has to be efficiently water jacketed and able to withstand high temperatures. It is therefore made of close-grained cast iron, again preferably nickel iron. Provision has to be made for the easy withdrawal of the air and exhaust valves and valve boxes so that inspection can be easily and quickly made.
The pistons of internal combustion engines of the sizes under consideration have to serve the purpose not only of transmitting the full force of the combustion pressures to the connecting rod and crankshaft, but also of acting as the crosshead, which is generally separate in steam engines. The pistons also have to stand higher initial pressures and temperatures than steam engines, and the reciprocating speeds are quite as high for corresponding sizes. Thus their design and construction require great care if these features are to be satisfactorily combined.
The pistons are made of close-grained cast iron, except those for smaller engines or those having to run at piston speeds of, say, over 1,000 feet a minute. They also must be light and strong. After having been turned, bored, grooved for the rings and faced, they are given the necessary finish by having the outer surface ground. Two or three piston rings are provided to prevent escape of the gases, and another to act as an oil scraper ring. At the lower end is yet another ring to prevent oil from passing up from the crankcase and mixing with the products of combustion.
Sometimes the top end of the piston is formed, either in the casting or by separate means, to assist the swirling action of the combustion chamber. The gudgeon pins may be of the fully floating type, and are ground and hardened. They are lubricated from a hole or pipe led from the connecting rod big end.
Flywheels for internal combustion engines in these moderate sizes are often of the solid disk type and are bolted securely to a flange on the end of the crankshaft.
These are most of the cast-iron parts of the engine. The most important of the mild-steel stampings or forgings is the crankshaft. Whatever the number of cranks, they are all combined in a single crankshaft, though for large marine engines the crankshaft is sometimes made in sections bolted together. The arrangement of cranks depends on their number and the predetermined sequence of firing as well as on its suitability from the point of view of balancing. The shaft must be of sufficient rigidity to withstand, with ample margin, all bending and torsional stresses.
Machined All Over
It is generally made from a solid forging, or stamping, of alloy steel and is heat-treated to give a tensile strength of up to 50 tons per sq. in. The forging is first turned to approximately the finished dimensions, and then ground to perfect accuracy with a finely finished surface for the crank pins and main bearing journals. Even the crank webs are machined all over, so that every spot of the surface is open to inspection of the slightest flaw. In most instances, the crankshaft is fitted with balance weights to counteract the reciprocating masses of piston and connecting rods. The balance weights, when not combined in the shaft forging, are of cast iron and are securely fixed to the crank webs by high-tensile steel bolts.
For the lubrication of the crank pins and the connecting-rod big end journals, small holes are drilled diagonally through the crank webs connecting the crank pins with the main journals, which get their supply from oil pipes led to the main bearings. At the flywheel end a flange which is forged solid with the shaft forms a half coupling to which the flywheel is bolted. At one end may be fitted a chain, or gearwheel, from which is driven the camshaft. At the other end a similar form of drive is arranged to take the governor mechanism.
The connecting rods are almost invariably made from stampings of a high-grade nickel steel and of H-section, which combines lightness with strength and rigidity. The small end is generally solid, having a chilled phosphor-bronze bearing. The small end is bored to take the gudgeon pin, but the big end is generally of the flat palm-ended type, fitted -with steel bearings lined with anti-friction white metal and secured to the palm by heat-treated nickel-chrome steel bolts. A thin packing strip between the palm and the bearings provides for suitable adjustment of the compression in the cylinder.
The camshaft is, in the smallest sizes, made in one solid piece from a steel stamping, the cams forming part of it; but,in the larger sizes the cams are made separate from the shaft and securely fixed to it. The cams are hardened and ground to the guidance of corresponding “formers” on special grinding machines. The whole shaft, as well as the tappets and valve-operating mechanism, should be easily accessible and removable for inspection or adjustment. The drive to the camshafts may be either by chains of ample strength, with provision for adjustment, or by gearwheels.
COMPONENT PARTS of an oil engine are accurately made of carefully tested materials. The engine bed is a single casting of rigid box-section design. The crank-shaft is machined from a solid forging of nickel steel. The piston is of close-grained cast iron, its connecting rod being a nickel-steel stamping of H section.
The valves are made of a special heat-resisting steel; for horizontal engines the valve heads sometimes are of cast iron and cast on to a mild-steel spindle. As the fuel oil used in diesel engines has to be sprayed into the cylinders at high pressures the pumps for that purpose must be of the best possible design and construction. To secure uniformity of feed to each cylinder it is usual to provide a separate fuel pump to each cylinder and to keep the fuel pipes as short as possible. Automatic fuel injectors are fitted to the cylinder heads, and provision is made for adjusting the injection pressure to that which gives the best performance from the engine. The function of the injectors is to deliver the fuel oil in a finely atomized spray, thereby assisting the rapid mixture and instantaneous combustion of the air in the cylinder and the fuel oil.
Governors are generally of the four-weight high-speed centrifugal type and regulate the fuel supply in accordance with the load demand on the engine. The limits of speed variations specified by the British Standards Institution are a momentary increase or decrease of 10 per cent of the normal when the full load is suddenly thrown off or on respectively, and a permanent or settled variation of 4 per cent between the corresponding speeds of no load and full load.
Provision is generally made on the governor for varying the speed while running to the extent of 5 per cent above normal or to 10 per cent below it. This range of control fulfils all requirements for the satisfactory driving of electrical generators and alternators running in parallel.
The lubricating system of most high-speed diesel engines is provided under pressure, generally about 25 lb. per sq. in., and supplied from a valveless gearwheel pump. The oil is first passed through a suitable strainer, then, in all but the smallest engines, through an oil cooler, from which it is distributed to a manifold which feeds the oil to the various bearings through pipes or oil holes drilled in certain parts. The pressure can be adjusted as required and any excess of pressure is relieved by a by-pass safety valve.
Lubrication Under Pressure
The water-cooling system may be one of several types, according to the size of the engine, the local water supplies available and other conditions. The temperature of the water leaving the engine should not exceed about 150° Fahrenheit. To avoid wear in the cylinders due to dust in the air supply, an effective air filter is provided, with means for replacing or cleaning the filtering material used as it becomes choked.
In most instances, what might be termed the auxiliaries of the main engine itself, such as the valve gears, the governor gear, the lubricating system and so forth, are virtually complete units. The parts are individually small, manufactured in special departments, and finally assembled in more or less complete units before assembling on the engine.
During the whole period of manufacture every part of the engine has to undergo strict inspection and scrutiny in the inspecting departments, so that any defect in material or construction will be revealed and corrected or replaced at once.
After complete assembly of the engine it is put on to a testing bed in the test shop. This department is fitted out with all kinds of appliances, such as for the application of the necessary load to the engine, whether it is a dynamometer brake or an electrical test; means for accurately measuring the quantity of fuel oil and lubricating oil used, for making governor tests and for making efficiency tests in conditions as nearly as possible resembling those in which the engine will finally work.
After the test the engine is opened out and thoroughly examined in all parts for the purpose of noting any signs of failure to comply with the specified requirements, or defects of any kind. It is then, after having been reassembled in its perfect condition, cleaned, painted and packed for its final destination.
LARGE MARINE DIESEL ENGINE SET in the maker's works. The engines were built tor the Ciudad de Asuncion, a triple-screw Argentine vessel of 2,550 tons gross. The cylinders have a bore of 15·75 in. and a stroke of 29·12 in.
