The Motor Vehicle 2010 Part 9 ppsx

584 The Motor Vehicle car operated under the LA4 US Federal Test Procedure, the supercharger would be operating for only 6% and, during the Highway Cycle, only 5.5% of the total time. Therefore, durability of the supercharger should not be a problem, though the same comment would not necessarily apply to the clutch. Fuel economies of the order of 13% relative to that of a naturally aspirated engine developing the same maximum power should be obtainable. 16.25 Screw-type compressors The first screw-type compressor was developed by Krigar in 1878. In the early nineteen-forties, two compressors of similar type were introduced for supercharging the internal combustion engine. One was the invention of A. J. R. Lysholm, of Ljungstrøms Angturbin, Sweden, and the other the unit produced by the Saurer company in Switzerland. In each, spiral lobes on a male rotor meshed with grooves in a female rotor, as shown in Figs 16.24 and 16.25. The spiral on one rotor is left- and that on the other right-handed, the two rotating in opposite directions. During the rotation, air is drawn through the inlet port, in one end of the casing, where it enters the spaces between, one after the other in turn, the spiral grooves on the female and their meshing lobes on the male rotor. This is the induction phase of the cycle, because the lobe that is beginning to uncover the port is also beginning to withdraw from its groove, so the space between each meshing pair is increasing, allowing the air at atmospheric pressure to enter. As rotation continues, this air is carried around by the grooves in the female rotor until the inlet port is covered by its leading edge. This is the beginning of the compression phase, because the lobe is beginning to mesh into the groove, at the inlet end, and thereafter progressively compresses and displaces the air towards the far end of the casing. Finally, the delivery 12 10 8 6 4 2 0 20 30 40 50 60 70 80 350 260 275 300 400 80% naturally aspirated torque 300 270 275 300 Bmep, bar 360 Engine speed, rev/s Fig. 16.23 Plots of estimated bsfc of a 1.7-litre diesel engine supercharged by a roots- type blower cut out by a clutch as the load falls below 80% of maximum torque (reported by Freese and Nightingale of Ricardo Consulting Engineers Ltd) 585Turbocharging and supercharging phase begins as the trailing edge of the groove uncovers the port in the opposite end, whence the air is delivered into the induction manifold. Because the air is compressed before delivery, there is no back-flow so the only significant noise is a high pitched whine due to meshing of the rotors. By virtue of the incorporation of several lobes and grooves, and progressive compression from one end to the other, delivery is practically pulse free. The Sprintex unit, which can operate at pressure ratios of up to about 2.2 :1, is a good example of a modern screw-type compressor. Its rotors are coated with PTFE and, by virtue of extremely close tolerances and a tip clearance of only 100 µ m, the lobes touch neither the grooves nor the casing. Their rotation is accurately synchronised because they are interlinked by a pair of gears in a housing at one end of the main casing. The range currently in production provides for maximum flow rates of 130 to 3301/s. In order to obtain good helical (or axial) sealing, a low addendum (distance between pitch circle and periphery of rotor) is necessary. To achieve this with male and female rotors having equal numbers of lobes and grooves, the female would have to very small. This condition has been avoided in the Sprintex units by having two more grooves than lobes, which provides a high displacement for any given speed. Sprintex machines operate at higher speeds than most other superchargers. Driving the female rotor limits the crankshaft to supercharger drive ratio and thus reduces the space needed for accommo- dating the crankshaft pulleys. Volumetric efficiencies of these units peak at a rotor tip speed of about 100 m/s and thereafter fall gradually, as compared with, for example, the roots type, which peak at about 50 m/s and then fall sharply. Matching to the engine is relatively simple since the output of the supercharger and air consumption of a four-stroke engine are both linearly proportional to speed. The first step therefore is to pick the size of supercharger the air throughput of which matches the consumption rate of the engine. Fig. 16.24 General section of Saurer rotors Fig. 16.25 Saurer rotors 586 The Motor Vehicle M d C T 2 D L m P e C T 2 D L m N e C T 2 D Lm p e C T 2 D b e C T 2 D L m b e C T 2 D N e C T 2 D M d CT2D However, some adjustment may then have to be made because the volumetric efficiencies of the engine and supercharger vary differently with speed and pressure. The two-stroke engine calls for a slightly different approach, since pressures and flows have to be related to the additional air requirements of scavenging. With internal compression, a by-pass valve does not offer the same advantage as with the displacement type, since the work done in compressing the air would be wasted. However, the efficiency of compression is higher, and some economy could be obtained by incorporating a clutch in the drive. Figures quoted by FTD, who developed the Sprintex compressor, indicate that the adiabatic compression efficiencies of the roots type, Sprintex unit and a turbocharger approximate to respectively 30 to 50%, 70 to 75% and 60 to 65%. An adiabatic efficiency of 80% has been claimed for the Saurer unit operating at a pressure ratio of 1.5 :1 at 5000 rev/min, and pressure ratios of up to 2.0 :1 are said to be obtainable at up to 10 000 rev/min. Some performance curves for this compressor are shown in Fig. 16.26. 16.26 Other methods of supercharging A method that, prior to the Second World War, was fairly widely used for two-stroke engines was to have double the number of cylinders needed for generating the power required. The additional cylinders were used for compressing the air for scavenging and pressure charging those adjacent to them. Well-known examples were the Trojan engines, Section 9.5. This method, however, considerably adds to the bulk, weight and cost of the engine. 80 70 60 50 11 10 9 8 7 190 180 170 160 800 1000 1200 1400 1600 1800 2000 170 160 150 140 130 120 110 100 90 80 70 60 R.p.m. b e –g/PSh P e –kg/cm 2 Md–m-kg Fig. 16.26 Performance curves for the Saurer supercharger N e –PS 587Turbocharging and supercharging In the nineteen-eighties, KKK developed their Ro-Charger, Fig. 16.27. It has two rotors, one inside the other. The inner one is of a figure-of-eight, or dumb-bell, section and is mounted on the driven shaft. This shaft is carried in a pair of sealed-for-life bearings at the driven end and a single bearing at the opposite end. The outer rotor, mounted eccentrically relative to the inner one, is of cylindrical form but with internal lobes meshing with those of the inner rotor. It rotates in two bearings, carried one at each end on eccentric spigots projecting inwards from the end covers. At one end, a multi-V belt pulley drives the shaft on which, immediately inboard of the bearing at the driven end, is a pinion driving a ring gear mounted in the adjacent end of the rotor. The gearing is such that the ratio of the speeds of the inner to the outer rotor is 3 : 2, the maximum speed of the outer one being 10 000 rev/min. ZF introduced at about the same time their mechanically driven Turmat centrifugal compressor for engines up to about 3.5 litres swept volume. Relative to the turbocharger, it has the advantage of the absence of a hot and costly turbine. On the other hand, it is certainly no simpler, since it has a Variator V-belt drive (operating on the principle of the Van Doorne transmission, Section 26.12, which is driven from the crankshaft, either directly or through gears, Fig. 16.28. The Variator, the ratio of which can be varied between about 1 : 2 and 1 :1.108, drives a 1 :15 ratio planetary gear system which, in turn drives the centrifugal compressor. An optional feature is a clutch interposed between the Variator and the planetary gear system. The transmission ratio of the Variator is controlled automatically by a flyweight actuated mechanism, which draws the flanges of the secondary pulley closer together as the speed increases and allows them to be forced apart by the V-belt as the speed decreases. This control can be supplemented or supplanted by a pressure- or depression-actuated control which similarly moves the flanges of the primary pulley. An alternative is electronic control. 2 3 Fig. 16.27 The Ro-Charger has been developed under licence from Felix Wankel. Turning in the same direction about different axes, the two rotors are geared together in the ratio 3 : 2, inner : outer. During rotation, the lobe of the inner rotor in chamber 1 retreats, drawing air in while rotating past the inlet port. At the same time, the lobe in chamber 3, is advancing to discharge compressed air into the delivery port. Next, the lobe in chamber 2 discharges into the delivery port while that in chamber 3 is inducting air 588 The Motor Vehicle Compressor Planetary gear Magnetic clutch Variator secondary pulley Variator primary pulley Air out Air in Fig. 16.28 The ZF Turmat supercharger comprises a centrifugal compressor driven by a Variator infinitely variable pulley-type transmission and a step-up planetary gear set. Its magnetic clutch is optional 16.27 The pressure-wave supercharger The pressure-wave, or Comprex, supercharger has been developed by Brown Boverie. Basically it takes energy, in form of pressure pulses, directly from the exhaust gas, in contrast to the turbocharger, which does so mechanically. Consequently, the losses are small and pressure ratios of up to 3 : 1 are said to be attainable. The principle is illustrated in Fig. 16.29. Energy interchange between the exhaust and inlet gases occurs in a set of straight tubular cells of approximately trapezoidal section within the drum B. These cells, the ends of which are open, are arranged around and parallel to the shaft on which the drum rotates. The shaft is driven by a belt from the crankshaft. Because the unit does not have to compress the gas it absorbs no more than between 1 and 2% of the power output of the engine. Moreover, with such a large number of cells, the compression process is virtually continuous, so synchronisation of rotation with that of the crankshaft is unnecessary. The sequence of operations is as follows. During each revolution of the drum, one end of each cell in turn passes the end of the exhaust passage A. This allows the exhaust gas, at the pressure in that passage, to flow along the cell, compressing the air that it already contains against the closed far end. Further rotation opens a port at the latter end, which allows the air thus compressed to flow into the inlet passage F, which is then closed again when the cell passes it. Closure of the port reflects a pressure pulse back to the other end, which is now open to the exhaust downpipe through E. Consequently, the exhaust gas is discharged to atmosphere, at the same time generating a suction wave which travels along the cell. When this reaches the opposite end, the port to the inlet pipe D is open, so a fresh charge of air is drawn into the cell, and the cycle begins again. 589Turbocharging and supercharging A E B D F Fig. 16.29 The Comprex pressure-wave supercharger With spark ignition engines, unless the Comprex machine is installed upstream of the carburettor, the exhaust gases will come into direct contact with the fuel–air mixture, so it is principally of value only for diesel engines. Since the exhaust gas pressure pulses travel at the speed of sound the Comprex machine has the advantage of virtually instantaneous response. Another advantage is that it is equally effective over the whole of the useful operating range of the engine. Moreover, since the energy utilised is taken from the exhaust gas, significant fuel economy is obtainable. The timing can be arranged to provide a degree of exhaust gas-recirculation during the valve overlap period, and this is claimed to reduce emissions of NO x by between 20 and 30%. 590 Chapter 17 Fuels and their combustion Because the needs of spark and compression ignition engines differ widely we shall deal with them separately in this chapter, taking spark ignition engines first. Petroleum, more widely termed crude oil, is found in natural reservoirs underground. It is the fossilised remains of minute fauna, as opposed to the flora matter, mainly trees, from which coal is derived. This crude oil is distilled in what is called a fractionating tower, to separate out its many constituents, or fractions, among which are those for blending into petrol. These boil off at temperatures ranging from about 25 to 220°C, Fig. 17.1. They comprise mainly organic compounds, among which three chemical groups predominate. One group is the alkanes, alternatively known as paraffins. Typical arrange- ments of the straight-chain normal molecules characteristic of alkanes can be seen from Fig. 17.2. The longer the chain, the heavier is the molecule and the higher the boiling point of the liquid. Variants termed alkenes and alkynes, under the generic name of olefins, can be produced from the alkanes by cracking processes, Section 17.2. Their molecules are like those of the alkanes, but with some of the hydrogen atoms removed. Alternative arrangements of hydrocarbon molecules are possible, and are called isomers. Of these, perhaps the most widely known is iso-octane. For assessing the octane number of a fuel that is being tested, a mixture of iso- octane, defined as having an octane number of 100, and normal- or n-heptane, defined as having an octane number of zero, is employed. The octane number is the percentage of iso-octane in a mixture which has precisely the same tendency to detonate as the fuel being tested. Incidentally, heptane (C 7 H 16 ) has nine isomers. Note that the general formula for alkanes is C n H nx+2 , where n is the number of hydrogen atoms. The molecular arrangements of n-octane and iso-octane are illustrated in Fig. 17.3. Another of the three main groups referred to in the opening paragraph of this chapter comprises the cyclo-alkanes, so called because of their ring-like molecular structures. Their general formula is C n H 2n . They are present mainly as cyclo-pentane and cyclo-hexane, Fig. 17.4. An alternative name for these products is naphthenes, not to be confused with naphtha, which is a rather loose term for a mixture of the light hydrocarbons. The third series, the aromatics, also have ring like structures but, as can be seen from Fig. 17.5, the arrangement differs slightly in that each carbon 591Fuels and their combustion 220 200 180 160 140 120 100 80 60 40 20 0 Cleanliness and lube oil dilution Cold driveability Cold start Hot fuel handling Temperature, °C 0 20 40 60 80 100 % volume evaporated (standard test) Economy Fig. 17.1 Ease of cold starting is dependent on the percentage of fuel evaporating below 70°C; too high a percentage, however, can lead to vapour formation in the fuel system when the engine is hot Fig. 17.2 The two smallest and lightest alkane molecules are those of methane (top) and ethane (bottom). Both are gases at atmospheric temperature C CC H H H H H H H H H H 592 The Motor Vehicle H CC C C CCCC H CCCC CCC H H H HH H HH H H H H H H HH H H H H HHH H H H H H H H HH Fig. 17.3 Examples of the long chain heavier alkanes are iso-octane (top) and n-octane (below) C C C C C C CC C C CC C Fig. 17.4 Molecules of methyl cyclo-pentane and ethyl cyclo-pentane. This is a simplified diagram, in which only the carbon molecules are shown. Such simplifications are often used and are justified because we can take it that, at least in hydrocarbon fuels, each carbon atom has four arms to which either carbon or hydrogen atoms must be attached atom is associated with only one hydrogen atom. The aromatics have high octane numbers and therefore have been blended into unleaded petrol for improving octane numbers. Some of the heavier among these compounds are suspected of being carcinogens, but this has not been actually proved. 17.1 Distillation and blending As can be seen from Table. 17.1, the contents of crude oils from different oil fields differ widely. The fractions condense out of the distillation tower 593Fuels and their combustion C C C H H H H H C H C C Fig. 17.5 This is a molecule of an aromatic, C 6 H 6 benzene. The inner arms of the carbon atoms are linked in pairs instead of to hydrogen atoms Table 17.1 PERCENTAGE PRODUCTS BY WEIGHT IN CRUDE OIL FROM VARIOUS SOURCES N. Africa N. Sea Mid. East N. America S. America Sulphur 0.1 0.3 2.5 1.0 5.5 Wax 3 9 6 7 2 Light gasoline, 0–70°C 8.9 5.8 4.7 2.4 0.1 Octane No. 73 76 72 75 70 Naphtha, 70–140°C 16.0 11.0 7.9 6.5 1.1 Kerosine,180–250°C 26.3 18.6 16.4 15.6 4.4 Diesel oil: 250–350°C 18.2 19.1 15.3 19.6 9.6 Cetane No. 55 53 58 45 30 Residue ≥ 350°C 27.5 36.2 47.2 47.9 76.9 progressively into a series of trays, one above the other in the tower, Fig. 17.6: the heaviest fuels (those having the lowest boiling point) condense into the lowest and the others into each tray in turn, until the lightest come out into the topmost tray. A gaseous residue, mainly propane with small quantities of iso-butane and n-butane issues from the top, and the heavy residue, containing mainly bitumen, is drawn off from the base of the tower. For two main reasons the distilled fractions have to be refined and blended to render them suitable for use as petrol. First, they may contain impurities such as sulphur that have to be removed and, secondly, there may be high proportions of some constituents and shortages of others needed for producing [...]... after the vehicle has stopped, especially after a slow climb to the top of a steep hill in hot weather In these circumstances, both the forward speed of the vehicle and the rotational speed of the engine are generally low, so also therefore are the rates of flow from both a mechanically driven fan and the water pump, and the engine therefore overheats This heat is then conducted out to the surrounding parts,... filter becomes partially blocked, the rate of flow could be such that the return flow of warm fuel to the tank is reduced, and therefore also the rate at which the temperature of the fuel in the tank is raised 17.27 Cold weather additives All additives for cold weather operation modify the shapes of the wax crystals, which otherwise tend to adhere to each other They therefore come under the general heading... which the wax crystals become visible in a swirling sample of fuel Then there is the pour point, which is the temperature at which the quantity of wax in the fuel is such as to cause it to begin to gel (ASTM D97) To establish the pour point, checks on the condition of the fuel are made a 3°C intervals by removing the test vessel from the cooling bath and tilting it to see if the fuel flows Another criterion,... pumps become over-heated The low pressure both inside the pump and in the pipeline between it and the tank 598 The Motor Vehicle reduces the boiling point of the fuel, so heat transmitted to these components by, for example, the exhaust manifold causes it to vaporise Because pumps are designed to deliver liquids, they cannot cope with vapour; consequently, the supply of fuel to the engine is interrupted,... or reduce the efficiency of carburettor jets and injector nozzles Sulphur is present in all crude oils and therefore has to be removed during the refining of the fuel, otherwise it will not only cause corrosion but also reduce the effectiveness of 596 The Motor Vehicle the catalytic converters in exhaust systems Other substances may leave deposits of ash in the combustion chambers after they have burnt... term used mainly for describing the smoothness of the response of the engine to movements of the accelerator pedal By the late 196 0s and early 197 0s, the introduction first of positive crankcase ventilation and then exhaust gas recirculation led to the appearance of deposits in all the passages from air filter to inlet valves and even on the valves themselves Again, the result was poor driveability... unattractive, except where the cloud point is included as part of a diesel-fuel specification and the blender therefore wishes to lower it 17.28 Dispersants and corrosion inhibitors The primary function of dispersant additives is to restrict the size of the particles formed within the fuel at the high temperatures in the engine and, additionally, to remove them from the metal surfaces However, they must be used... remain by the time it reaches the vehicle Therefore, if vehicle fuel-system protection is required too, the treatment must be heavier As in the case of the detergents, the polar heads attach themselves to the metal, but the water repellent tails form an oily coating over the surface to protect it against corrosive attack In Fig 17.18, test samples that have been left over a long period in the base fuel... more rapid, and the engine therefore noisy Also, because the fuel has less time to burn before the exhaust valve opens, the hydrocarbon emissions are increased On the other hand, if the cetane number is higher than that for which the injection system is timed, power will be lost because a high proportion of the pressure rise will occur when the piston is at or near TDC Furthermore, the Fig 17.13 Combustion... low wear and a smaller particulate content in the exhaust Density The higher the density the greater is the energy content of the fuel Waxing tendency Wax precipitation can render cold starting difficult and subsequently stop the engine As in the case of petrol, properties of a diesel fuel depend in the first instance on the source of the crude oil from which it is distilled These vary as follows— . use. One is the research octane number (RON) and the other the motor octane number (MON). There is also a third, applicable to the vehicle rather than the fuel, and this is termed the road octane. over-heated. The low pressure both inside the pump and in the pipeline between it and the tank 598 The Motor Vehicle reduces the boiling point of the fuel, so heat transmitted to these components by,. it enters the spaces between, one after the other in turn, the spiral grooves on the female and their meshing lobes on the male rotor. This is the induction phase of the cycle, because the lobe