The Motor Vehicle 2010 Part 2 doc

62 The Motor Vehicle of the bronze bush, and thence through holes in the bush to lubricate its outer bearing surface. A sturdy piston and connecting rod assembly for a 10.35 litre high-speed compression ignition engine developing 97 kW at 1900 rev/min is illustrated in Fig. 3.13(a). The joint face between cap and the rod is inclined at 35° to the axis of the rod, to reduce the width of the assembly so that, after the bolts have been removed from below, the rod can be withdrawn upwards through the cylinder. Generally, access for the dismantling of the big end would be gained by removing the sump but, in large engines, for marine and industrial applications, there are usually inspection covers on the side of the crankcase. In this particular connecting rod arrangement, which was originally patented by Henry Meadows Ltd, there are four bolts, the upper pair being screwed into cylindrical nuts having diametral, instead of axial, threaded holes and which are housed in a transverse hole through the rod. This transverse hole is utilised for location during machining, as also is the extra hole between each pair of bolts. The latter can subsequently house location dowels, if required. Conventional stud or bolt-and-nut arrangements, however, are now widely used with inclined housing joint faces, in which case the unwaisted central portions of the bolts alone will take the shear component of the loading due to the inclination of the joint faces. In some instances, the bolts have rounded heads with flats on one side to locate against shoulders rising from the edges of the spot-faces on which they seat. An alternative method of taking the shear is to machine in the joint faces either shoulders or serrations parallel to the axis of the crankpin. The shear may be at a maximum under the tension arising from the inertial loading during the induction and exhaust strokes, since this loading reduces the frictional grip due to the clamping together of the joint faces. D D C C C A A B B A Fig. 3.12 Fig. 3.13 (a) (b) Constructional details of the engine 63 Other features of interest in Fig. 3.13(a) include the single rib around the bearing cap, the twin tab washers and the oil hole drilled axially from the big end, to take oil up to the small end. Twin ribs might have been used had the engine been more heavily loaded. On the other hand, had it been more lightly loaded, the small end would have been lubricated by splash thrown up from the big end. For diesel engines, in which the gas loading is generally more severe than the inertia loading, chamfered small end bosses are often employed, as in the Perkins T 6.3543, Fig. 3.13(b). This makes the bearing areas in the critical opposite halves of the bush in the rod and the bosses in the piston as large as practical for a given cylinder bore dimension. 3.16 Bearing bushes Except in certain applications, such as some motor-cycle engines, big end and main journal bearings are in halves – otherwise a fabricated crankshaft would have to be used so that they could be assembled on to it. One-piece cylindrical bushes are, however, used for camshaft, rocker and spiral gear bearings. In some instances these bushes are simply strips of bearing material, produced by wrapping them round a mandrel without actually joining their abutting ends. 3.17 Bearing materials Undoubtedly whitemetal has the best bearing properties. However, its fatigue strength is limited so other alloys have to be used for many modern highly- rated engines. Properties of bearing metals are set out in Table 3.2. Babbitt invented whitemetal in 1839. It contained 83% tin, 11% antimony and 6% copper. The hard copper-antimony particles suspended in a matrix of soft copper-tin alloy give good wear resistance plus the ability to embed solid abrasive particles that would otherwise wear the shaft. Additionally, whitemetal will conform readily to inaccuracies of the shaft profile and to accommodate deflections of the shaft. Because of its low melting point, high spots in the bearing interface cause this material to soften and flow slightly to relieve excessive local pressures, instead of seizing. Because of the increasing price of tin, there has been a tendency to use lead babbitts – which have properties similar to those of tin babbitts. Originally, whitemetal bearings were cast in their housings. Later, they were made in the form of thick shells, sometimes in a thick bronze backing, and could therefore be more easily replaced. In either case, it was necessary first to bore them in the engine and then to scrape them manually, using prussian blue marking, to obtain a good fit. 3.18 Thin-wall bearings In the mid-nineteen-thirties the thin-wall, or shell-type, bearing was introduced for cars in the UK. This had been developed originally in the USA for aero- engines and then cars. The whitemetal was applied as a very thin lining on a steel backing about 1.5 mm thick. This had two main advantages: first, the steel backing gave good support to the whitemetal, and therefore the fatigue strength of the bearing was good; and secondly, the bearings could be made with such precision that, provided that their housings were equally precisely 64 Table 3.2—POPULAR GLA CIER BEARING MA TERIALS Lining Nominal composition Sapphire Fatigue Seizure Corrosion Embeddability Hardness Typical usage material fatigue strength resistance resistance and HV 5 (steel-backed) Al Cu Pb Sb Sn rating** conformability Tin babbitt – 3.5 – 7.5 89 4 500 2 10 10 9 2 7 Gas turbine, industrial gearbox, electric generators, slow-speed marine two-stroke engines, etc. Lead babbitt – – 84 10 6 4 500 2 9 10 10 1 6 Large industrial machinery, petrol engines, thrust-washers, wick-lubricated fractional hp motors (lower cost than tin) Tin-aluminium 60 – – – 40 8 500 4 9 10 9 2 7 Overlay plated in low-speed diesel cross-head, and unplated in medium-speed diesel engines and reciprocating compressors Tin-aluminium 79 1 – – 20 14 000 7 7 10 8 3 5 Overlay plated in medium- and high-speed diesel units, petrol engines Tin-aluminium 93 1 (+1% 6 16 000 8 5* 10 6 45 Overlay plated in medium- and high-speed nickel) diesel engines Aluminium-tin 83 2 (+4% Si) 12 14 000 8 9 10 7 6 0 Highly-rated petrol and high-speed diesel -silicon engines, especially with nodular iron crankshafts Copper-lead – 70 30 – – 17 000 8 5* 5 7 4 0 Overlay plated in medium-speed diesel and petrol engines Lead-bronze – 73.5 25 – 1.5 18 000 9 4* 5 5 5 0 Overlay plated in medium-speed diesel and turbo-blowers***, petrol and high-speed diesel engines Lead-bronze – 73.5 22 – 4.5 18 000 9 3* 5 4 5 5 Overlay plated in medium- and high-speed diesel Lead-bronze – 80 10 – 10 18 000 10 2 5 2 10 0 Overlay plated in small end and rocker bushes for petrol and medium- and high-speed diesel engines Aluminium-silicon 88.5 1 – (+ 10.5% 18 000 9 8* 10 4 56 Overlay plated in highly-rated high-speed silicon) diesel engines The Motor Vehicle * These ratings apply to the base material. Commonly, these alloys are supplied with a layer 0.0254 mm thick of electro-deposited lead–10% tin to improve seizure resistance to a rating of 10 initially. ** Sapphire rating is the dynamic load in lbf/in 2 which can be withstood by an oil-lubricated bearing with minimal misalignment in a 2-in diameter sapphire test rig for more than 3 × 10° load cycles; test temperatures 110 to 130° C, depending upon load. *** Turbo-blowers have cast lead-bronze lining, Glacier GL26 (Pb 26% Sn 2%) with properties similar to lead-bronze. Constructional details of the engine 65 machined, they could be assembled without the need for skilled manual fitting. Moreover, they were equally easy to replace in service. 3.19 Stronger materials In the meantime, engine speeds, and with them gas and inertia loadings, had been increasing. Additionally, diesel engines were becoming popular for commercial vehicles. Consequently, there was a demand for stronger bearing materials. Solid bronze shells had been used, and this entailed hardening the shafts to prevent rapid wear. For heavy-duty applications, the new thin-wall bearings, with copper-lead and lead-bronze bearing materials on steel backings, were used. Of these materials, the latter is the stronger, but its conformability is worst. In both, the lead is held within the matrix, so that it is immediately available to smear on the bearing surface. The difference between the two is that in one case the matrix is pure copper and in the other it is the stronger copper-tin alloy. With the harder bearing materials, the crankshafts must be hardened, usually within the range 350 to 900 Vickers. 3.20 Corrosion of bearings Unless engine oils are changed at fairly frequent intervals, copper-lead and lead-bronze bearings are liable to corrode. The lead phase is attacked by organic acids and peroxides that develop as a result of degradation of the oil at high temperatures. Weakening of the bearing structure and fatigue failure of the copper matrix ensue. To protect these bearings from corrosion, a lead-tin overlay is almost invariably applied to their surfaces, by electroplating to a nominal thickness of 0.025 mm. Such a plating also improves both seizure-resistance and bedding- in and, provided the environment is favourable, it can last the life of the engine. However, abrasive dirt can score the overlay and allow the corrosive elements to penetrate to the lining material. The lead-based overlay does not corrode because it is protected by a tin content, which is generally of the order of 10% – the minimum acceptable is 4%. 3.21 Aluminium-tin bearing alloys Although aluminium-tin bearings were introduced in the mid nineteen-thirties, no more than 6% tin could be used, otherwise fatigue strength was unacceptably reduced. Because of the hardness of this alloy, the shafts had to be hardened; this problem, however, was overcome by the end of the Second World War by overlay plating with alloys of lead with tin, copper or indium. The main incentive for using aluminium is its low cost relative to that of copper. Additionally, its melting point is low enough for easy casting and application in the manufacture of bearings. By 1950, the Glacier Metal Company Ltd, appreciating the fact that overlay plated copper-lead, lead-bronze and 6% tin-aluminium linings were expensive, intensified their efforts to develop a better material. The outcome was the introduction in 1951 of a reticular tin-aluminium alloy, containing 20% tin. This development, a joint project between Glacier and the Tin Research Institute, was a major advance. The problem of reduction of fatigue strength was overcome by preventing 66 The Motor Vehicle the tin from remaining as grain boundary films – its natural tendency – in the aluminium. Instead the tin forms continuous films along the edges of the grains of aluminium but not across their faces, thus forming a network structure – hence the term reticular tin. Essentially, this development was made possible by the perfection by Glacier of a cold roll-bonding process, instead of casting, for attaching the material to a steel backing. Increasing the tin content up to as much as 40% improves resistance to scuffing and seizure. However, additions beyond 20% reduce the mechanical strength of the alloy. 3.22 Aluminium-silicon and aluminium-tin-silicon alloys With the increasing use of turbocharging for diesel engines, even stronger aluminium alloy bearing materials have been developed by Glacier. One is their SA78 alloy, an 11% silicon-aluminium alloy, similar to that used for pistons, but with the hard silicon particles much more finely dispersed in the aluminium matrix. This fine dispersal makes the alloy very ductile, improves its fatigue strength and bearing properties, and renders it suitable for lining on to a steel backing. The material is normally overlay-plated to improve running-in and surface properties. Protection against corrosion is unnecessary, since neither aluminium nor silicon are affected. A 1% addition of copper in solution in the aluminium matrix helps to strengthen it. Currently, several aluminium-tin-silicon alloys are under investigation by Glacier, but at the time of writing only one automotive bearing has been developed to the production stage. It is their AS 124 which, because it needs neither corrosion protection nor enhancement of its surface properties, does not have to be overlay-plated. The figures 124 indicate 12% tin and 4% silicon. It is claimed to have excellent resistance to seizure, good strength at high temperature, and to be particularly suitable for use with nodular iron crankshafts, see Section 3.24. This alloy is continuously cast into strip and then roll-bonded to a steel backing, an aluminium foil being interposed between the two to facilitate the bonding. Subsequently, heat treatment further develops the bond strength and refines the nearing alloy. Its characteristic feature is a continuous reticular tin matrix in which are embedded the very fine particles of silicon. The copper, which serves as a solid solution hardener, is not visible in the microstructure at ×500 magnification. 3.23 The crankshaft Crankshafts in modern engines are carried in shell type bearings similar to those of big ends, as described in Sections 3.16 to 3.19. The bearing housings at each end are generally cast integrally in the lower edges of the front and rear walls of the crankcase, as described in Section 3.59, while transverse webs inside the crankcase support the intermediate bearing housings. Their caps are below, and each is usually secured by four bolts or studs, because they have to take the full force of combustion. A crankshaft may have as few as two main journal bearings, even in four- cylinder engines provided their rating is low. However, in highly-rated engines there is usually, in addition, one between each pair of crank throws, though some four-cylinder units have only three bearings. In the latter event, the shaft has to be very stiff, otherwise at certain speeds and loads it would Constructional details of the engine 67 whip, heavily loading the central bearing as it does so, possibly causing it to fail. This tendency can be completely eliminated, or at least reduced, by balance weight on the crank webs each side of that bearing. A major risk associated with inadequate stiffness is that bending of the shaft will transfer a high proportion of the load to the edges of the plain bearing at each end. The larger the number of bearings to support the shaft uniformly along its length, the more slender, and therefore lighter, can it be without risk of whip due to bending. More bearings, however, will entail increased cost and, since the accuracy of their alignment is critical, the crankcase structure supporting them must be very stable under variations of temperature and load. Frictional drag of the bearings on the shaft increases as the square of the diameter but only linearly with length. Where only two bearings, one at each end, are required, they may be of the rolling element type, though these tend to be noisy, heavy, of large diameter, more difficult to seal, and costly. Additionally, because they impose little encastré effect on the ends of the crankshaft, the latter has to be stiffer than if two plain bearings were employed. The only advantages of rolling element bearings are axial compactness and, especially if they are of the ball type, low friction. On the other hand, roller types are better for withstanding impact loading due, for example, to detonation in the cylinders or lugging at low speed. A major factor governing the dimensions of the shaft is the torsional stiffness needed to raise its natural frequencies of vibration, together with their harmonics, above the rotational speed of the engine. For any given length of shaft both the bending and torsional stiffness depend on the diameters and overlap of the main and big end journals and the thicknesses and widths of the webs. The lengths of the journals are a function of their loadings and the strengths of the bearing materials. In the interests of compactness and stiffness, however, the aim is always at keeping the lengths of the bearings and thicknesses of webs as small as practicable, though the shorter the bearing shells, the more difficult it is to keep the lubricating oil from being squeezed out before it can spread right round their working surfaces. The strength of the shaft depends primarily on that of the material from which it is made. Measures such as the incorporation of generous fillet radii between the webs and journals, and perhaps rolling these fillets to induce in them residual compressive stresses, can improve fatigue strength, which is also affected by heat and hardening treatments. 3.24 Crankshaft materials Crankshafts are generally steel forgings, though high carbon, high copper, chromium silicon iron has been used and nodular, or spheroidal graphite (SG), cast iron is becoming increasingly popular. Best of the steels for crankshafts, but the most costly, are the nitrogen-hardened types, Section 3.26. Less costly, though also inferior to a significant degree, are the high carbon or alloy steels, surface hardened by the flame or induction methods, Section 3.28. Last in order of durability come the heat treated high carbon or alloy steels that have not been surface hardened, for which only the soft whitemetal bearings are suitable. Factors favouring cast iron crankshafts are the low cost of this material, which also has a high hysteresis for damping out vibrations; shafts can be 68 The Motor Vehicle produced in more complex shapes without need for costly tooling or machining; and the larger sections required for heavily loaded shafts on account of the lower tensile strength of the material would, in any case, tend to be necessary also with steel ones, to obtain adequate stiffness. The significance of being able to produce more complex shapes is that balance weights can be cast integrally, instead of having to be bolted on during the balancing operation and, if hollow journals are called for, to reduce weight, they can be cored in the casting instead of having to be machined subsequently at a high cost. Bosses usually have to be left in the hollow sections, so that oilways from the main to the big end journals can be drilled through them. SG irons used for crankshafts have both better tensile strength and fatigue resistance and better bearing qualities than the cast irons containing graphite in flake form. They include the following grades: 600/3, 650/2, 700/2, and 800/2 where the larger figure represents its ultimate tensile strength in N/ mm 2 and the smaller one its percentage elongation. The graphite is converted to SG form by the injection of magnesium inoculants during smelting and by controlling the cooling. Without further treatment, the safe working stresses of these grades under fatigue loading are, respectively, ±64, 68, 72 and 80 N/mm 2 . Salt bath nitriding to a depth of about 0.5 mm will increase the safe working stress by about 15% and more than double the wear resistance, induction hardening of the fillets to a depth of about 3 mm increases the working stress by about 53%, and rolling the hardened fillets at a load of between 1 and 2 tonnes for about ten revolutions increases it by approximately 80% relative to that of the untreated iron. Prior to fillet rolling, the radii are slightly undercut but the finish grinding operations are left until aterwards to ensure that the journals are truly cylindrical. Because the strength of cast iron, in either flake or SG form, is lower than that of steel, the sections have to be larger. The critical dimension is the overlap between the main and big end journals. If the distance between the centres of adjacent main journals is taken as 1 unit and their diameter as 0.64, the proportions of a typical SG iron shaft would be approximately as follows: main journal length 0.32, crankpin journal diameter 0.52 and length 0.28, crank web thickness 0.2. If failure occurs owing to bending, the most likely fracture path is between the nearest points on adjacent fillets around the main and crankpin journals. The bending stress is given by a formula developed by MIRA from the Kerr Wilson formula: Stress = 0.75 × Load on crankpin × Main bearing span/bt 2 where b and t are, respectively, the breadth and thickness of the fracture path. Some shafts have an integral collar on each web, around the main big end journals, the faces of these collars being ground to form thrust rings between which the bearings float, as in Fig. 3.80. If the webs are thick, the paths between the adjacent points on the outer peripheries of these collars might not be longer than the previously described fracture path. In a well- designed shaft the square of the length of this path should be greater than twice that between adjacent fillets, otherwise, owing to stress concentration at the edges of the collar, it might become the primary fracture path. As regards bearing properties, cast irons are almost equal to nitrided steel, except in that they demand bearing materials of greater seizure resistance. 69 Constructional details of the engine AlSn20Cu1 alloys (aluminium with 20% tin and 1% copper) are not suitable, but AlSn10Si4Cu1 and AlSn10Si4Cu2 are. The hardness of these alloys increases with copper content. During the processing of the SG iron the graphite nodules breaking out to the surface may migrate, leaving sharp edges around the crater that had held them, while also improving the oil- retaining properties of the surface. The sharp edges, however, can adversely affect the rate of wear of the soft bearings in which they run, so the journals are first ground in a direction opposite to that in which they will be wiped by the bearing, and then lapped or polished in the other direction. With steel shafts, all grinding or lapping is done in the same direction as that in which the shaft will be wiped by the bearing. Examples of forged crankshafts for four-cylinder engines are illustrated in Figs 3.14 and 3.15, while a cast iron crankshaft is shown in Fig. 3.16. In the three-bearing crankshaft in Fig. 3.14 the crank webs are extended to form masses to balance the revolving couples individually for each half of the shaft (see Section 2.6). Without these masses, although the shaft would still be in balance owing to its mirror symmetry, the revolving couples would load the bearings more heavily, because they would have to be reacted ultimately through engine structure. The reason for extending the webs in a fan shape is not only to increase their masses within the radius limitation imposed by the need to clear the bottom of the piston skirts during rotation of the masses past them but also in order that adjustments to the balance can be made accurately by drilling holes radially into it, from the appropriate direction over the whole range of angles. There are no balance weights in the stiffly designed five-bearing shaft in Fig. 3.15 because a primary requirement was to keep weight to a minimum and, since this is for a compression ignition engine, the structure is in any Fig. 3.14 Forged crankshaft with balanced webs, by Laystall Engineering Fig. 3.15 Five-bearing shaft for ci engine 70 The Motor Vehicle case stiff enough to react the opposed revolving couples of each half of the shaft. Figure 3.16 shows how casting facilitates the production of a shaft of complex form. The purpose of the balance weights B 1 and B 2 is explained in Section 4.20, where the general arrangement of V-eight engines is described. Machining is normally confined to the journals and crankpins and the drilling of the oil ways, the balance being correct by the drilling of lightening holes and rough grinding webs as required. For a comprehensive treatise on nodular iron for crankshafts the reader is referred to a paper presented in 1954 by S. B. Bailey, Proc. I. Mech. E., Vol. 168. 3.25 Built-up crankshafts Two examples of the built-up type of crankshaft are shown in Figs 3.17 and 3.18. In Fig. 3.17 is shown one throw of a crankshaft in which the crank webs are permanently shrunk on to the journals; the case-hardened crankpins being secured in the split webs by the clamping bolts shown. The other example, Fig. 3.18, is the crankshaft of a special racing engine. The webs of the shaft are formed of circular discs A and B, the A discs 2 B 1 B 2 3 4 2 3 4 3 4 4 U U B 2 B 1 2 3 S 1 1 S Section on UU Section on SS Fig. 3.16 Cast crankshaft for Ford V-eight A B K D E F B A H C A Fig. 3.17 Built-up crankshaft Fig. 3.18 Built-up crankshaft 1 Constructional details of the engine 71 having as an integral portion the journals C while the crankpins D are integral with the B discs. The B discs are a tight fit on the journals C and are secured thereon by means of plugs E. The large ends of the latter are slightly tapered and are forced into the correspondingly tapered holes of the journals, thereby expanding the latter firmly inside the B discs. Dowel pins F fitting in holes drilled half in the journals and half in the B discs give added security against relative motion of those parts. Similar tapered plugs are used to secure the crankpins in the A discs but no dowel pins are used. When taking the shaft to pieces, plugs are screwed into the holes in the journals, thus forcing out the plugs E. To ensure correct alignment of the journals an accurately-ground rod is passed through holes H formed in the discs, during assembly of the shaft. 3.26 Surface-hardening of shafts The term case-hardening, though also applicable to the nitriding and chill casting processes, is normally used for the time-honoured process of carburising the surface layer of a suitable low carbon steel to obtain a high carbon case. The carburising or carbon supplying agent may be solid, liquid, or gaseous. Subsequent quenching and heat treatment is applied with the object of producing a high degree of hardness in the case while maintaining strength and toughness in the core. Case-hardening steels are low carbon steels containing alloys which assist both the carburising process and the requirements of the core. The depth of case is dependent on time, temperature, and composition of the steel. The time required ranges from a minimum of about a quarter of an hour in the cyanide bath to several hours in box hardening with solid carburising agents. In complicated forms such as crankshafts, though selective hardening of pins and journals is possible, the high temperatures involved and the subsequent quenching are liable to lead to quite unmanageable distortion, since the temperature of the whole component must be raised above the critical change point. Nitriding is a similar process in that the chemical composition of the surface layer is altered, but by the production of very hard nitrides of iron and certain alloying metals, of which aluminium and chromium are effective in producing extreme hardness in the case, while molybdenum increases the toughness and depth of penetration. The nitriding agent is ammonia gas, which decomposes into hydrogen and nitrogen at the furnace temperature of about 500°C. The process occupies from one to two days, and is thus a slow one compared with the rapid production methods described below. No quench is required. The great advantage compared with carburising is the exceptional degree of hardness obtainable and the relatively low temperature necessary, this being below the change point of the parent steel. This has the double merit of reducing or preventing distortion, and permitting the normal annealing and heat treatment processes of alloy steels to be carried out beforehand, without risk of subsequent interference. Another surface hardening process is termed New Tufftriding. The best results are obtained with low alloy steels containing aluminium and, perhaps, chromium, tungsten, molybdenum, vanadium or titanium. Treatment of a crankshaft generally takes about two hours, during which it is immersed in a bath of molten sodium cyanate at a temperature of 570°C. [...]... steel thimble over the end of the stem, seating its rim on the retainer washer for the spring, so that there is a small clearance between the tip of the stem and the other end of the thimble When the cam or rocker bears down on top of the thimble it first compresses the spring, momentarily freeing the valve while the clearance between the tip of the stem and the end of thimble closes and the valve begins... pressure to push the three-piece hydraulic plunger to the right, locking all three together In this condition, the central cam, because it is higher than the other two, actuates the valves In the graph (b) the central curve shows how the ECU varies the change-over point with speed The upper lines in the two other pairs of curves represent operation with and the lower ones without change-over 3.44 The Mechadyne–Mitchell... dispense with the thimble and have no clearance between the abutting ends of the halves of the collet, so that it does not actually grip the valve stem At the same time, there has to be a clearance between the ends of the internal collar in the collet and the groove in which it registers in the stem, so that as soon as the valve is lifted from its seat it floats in the collet and is therefore free... (b), its instantaneous speed of rotation is multiplied in the ratio 2. 4 :2. 9 = 1 .26 3 : 1 Therefore, as the shaft rotates, the ratio reduces progressively first to 1:1 at 90°, and then on to the inverse of 2. 4 : 1.9, at 180°, and finally back again to complete the 360° As the control is moved to increase the eccentricity of the drive shaft, the duration of valve lift is progressively reduced because... temperature in the cylinder is reduced, with a consequent diminution of the NOx content of the exhaust (Chapter 14) Because of the large diameters of two valves relative to the bore of the cylinder, the gas flow past the portions of their edges adjacent to the cylinder wall tends to be masked by it With four valves, on the other hand, there need be little or no masking and moreover the interaction of the two... between the rocker and cam, the pad on the end of the rocker is curved This, however, tends to increase the velocities of both opening and closing of the valve and therefore has to be taken into account in the design of the cam profile The sparking plug is very close to the centre of the top of the combustion chamber, for efficient combustion As a result, it has not been possible to make the diameter of the. .. the cylinder, where it is evaporated by the heat generated during the subsequent compression stroke Mixing is further assisted, at TDC, by the squish effect between the flat area surrounding the slightly dished portion of the crown of the piston and the flat lower face of the casting, each side of the pairs of valves Because of the steepness of the slope of the inlet ports, this fuel runs down positively... side of the axis of the cylinder and the inlets 19° to the other side Their seats are in a penthouse-type combustion chamber Since the engine, when installed, is tilted 45° towards the exhaust side, the inlet valve ports then slope steeply downwards This facilitates cold starting, in the following manner As the crankshaft is turned, any fuel remaining unevaporated in the manifold runs down into the cylinder,... TDC BDC Fig 3 .25 This diagram of mass flow of air plotted against crank angle shows the effect, with fixed valve timing, of back-flow into the induction manifold at low speed 82 The Motor Vehicle idling, the overlap should be almost zero Under all other conditions, larger overlaps are needed to increase the breathing potential and reduce the output of NOx Going further, by utilising the valve control... of the 96 The Motor Vehicle exhausts but, to compensate for this, their lifts are greater – 8.7 12 mm as compared with 7.798 mm To span the distance between the camshaft and the exhaust valves, the rockers have to be long Consequently forged En8 steel, instead of the usual cast iron, rockers are used Because of the relatively poor bearing characteristics of steel, however, oil from a radial hole in the . spring, so that there is a small clearance between the tip of the stem and the other end of the thimble. When the cam or rocker bears down on top of the thimble it first compresses the spring,. between the abutting ends of the halves of the collet, so that it does not actually grip the valve stem. At the same time, there has to be a clearance between the ends of the internal collar in the. include the following grades: 600/3, 650 /2, 700 /2, and 800 /2 where the larger figure represents its ultimate tensile strength in N/ mm 2 and the smaller one its percentage elongation. The graphite