The Motor Vehicle 2010 Part 6 ppsx

374 The Motor Vehicle Solex unit had simply an on–off control, while later versions were progressive, having positive-feel two- or three-position rotary disc valve controls. A slightly different application of the multi-hole disc valve principle, can be seen in Fig. 11.27. In Fig. 10.19, the main view is of the early version, while the scrap view (left) shows the modification for the two-position version. In the original version, the orifice Ga metered the flow of air from immediately upstream of the venturi into the starter chamber. The disc valve opened and closed two ducts simultaneously. One of these was the small duct D, which drew fuel from the starter well integral with the float chamber shown in the sectioned view on the right. The other was the much larger duct leading from the bottom of the starter chamber to a point downstream of the throttle valve. When these two ducts were open, an extra supply of fuel was drawn from the starter well, mixed with the air in the starter chamber and delivered down through the large duct directly into the induction pipe. The strength of the mixture was determined by the sizes of the slow running jet Gs and the orifice D. This system, of course, can operate only when the throttle valve is closed. In the diagram, Sb is the idling mixture air bleed, Gs the slow running jet, G the main jet and g the air bleed orifice for the main jet emulsion system. In the modified version, scrap view on left, part of the upper portion of the disc is dished to embrace both the fuel inlet D and two ducts. One is from the slow running well and the other an extra air bleed Z, from immediately downstream of the venturi where, when the throttle is closed, the air pressure is atmospheric. In the crown of the dished section is the metering hole Hc, through which is drawn an emulsified mixture of fuel and air from these two ducts. This emulsion is then mixed with the air in the starter chamber and passes on, as before, down through the duct into the induction pipe. The outcome is that a slightly larger quantity of fuel and air, though better mixed, is fed to the engine induction system. As the pull type control is actuated to bring the system into and out of operation, the edges of the dished section progressively open and close the ports D and Z, but the hole over the delivery Fig. 10.19 Solex B32-PBI-5 carburettor Z D Hc g Gs G D Ga Sb Z Starter, main and idling systems 375Fundamentals of carburation P D A E port to the induction system is elongated so that it will continue to allow the mixture to pass into the induction system as long as the input ports are open. In a later variant, the disc valve was again flat, but had a series of fuel inlet holes in it. Also, the air orifice Ga was moved to the opposite side of the starter chamber, where it delivered its air through a port and an elongated hole in the disc at the outer end of the spring that loads the inner disc valve. In other words, there were two disc valves: that on one side for air and the other for fuel. 10.24 Idling systems and progression jets As was explained in Section 10.18, the idling mixture has to be discharged into the region of low pressure generated by the rapid air flow adjacent to the edge of the throttle valve. However, if only a single discharge orifice were to be placed there, it would become ineffective as soon as the throttle was opened, so the engine would hesitate, or even stall, before the depression over the main jet had risen sufficiently for it to take over. This problem is usually overcome by having two discharge orifices, one adjacent to the edge of the throttle valve when it is closed and the other a short distance downstream. Such an arrangement, for example that of the Zenith VE updraught carburettor shown in Fig. 10.20, and in the Stromberg DBV carburettor, Fig. 10.26, is termed the progression system. In Fig. 10.26, A is the adjustment screw for regulating the flow of air from above the venturi to emulsify the fuel entering below, D is the delivery duct for fuel passing from the idling jet into the passage that takes the emulsified mixture down to the progression holes H. When the engine is idling, fuel from the float chamber flows through the main and compensating jets into an idling well and on into the emulsion block E in Fig. 10.20. Under the influence of a depression, which is determined by the size of the hole O, this fuel is sprayed through the idling jet J into the large diameter duct D, which serves as an intermediate chamber. As the fuel issues from the jet, it is mixed with air bleeding through three Fig. 10.20 Zenith idling system 376 The Motor Vehicle separate orifices. One is P, and the second comprises a series of holes from the venturi where, because the throttle is closed, the pressure is atmospheric. These radial holes feed air into the fuel jet, to emulsify the mixture. Further emulsification is effected by air entering from the third bleed orifice, which is equipped with an adjustment screw A. The size of this orifice, relative to those of the others, is such as to enable the overall rate of bleed to be accurately adjusted. As the throttle is opened, and the depression over the hole O reduced, the resultant shortfall in fuel supply is made up by an additional flow of fuel through what was previously the air bleed orifice P. With further opening of the throttle, and a consequent significant increase in the air flow, the depression in D is reduced, and with it the quantity of fuel supplied through the idling system. This together with the progressive draining of the well, which ultimately starves the idling system of fuel, provides effective compensation right up to the point at which the main jet takes over. From this point on, extra air continues to bleed through the idling system into the emulsion block through the main jet, to contribute to compensation. There are various other ways of progressively increasing the supply of mixture and providing compensation during idling and warm-up. One is illustrated in Fig. 11.8 and another in Fig. 11.12. 10.25 Requirements for acceleration If, after a period of operation at low speed and light throttle, the accelerator pedal is suddenly depressed, the mixture suddenly becomes very weak. This is partly because, although the depression that previously existed in the induction pipe is momentarily applied to the venturi, the sudden rush of air that this induces is too short lived to overcome the inertia and drag of the fuel in the jets. In any case, the inertia of the fuel in the delivery system from the jets will cause delivery to lag behind the increase in depression. Furthermore, the opening of the throttle may have cut the idling system out of operation. Since the pistons will have had neither the combustion pressures nor the time required for them to accelerate, the rate of flow of air through the venturi will rise relatively slowly so, temporarily, the depression over the main jet will not be high enough to atomise the fuel adequately. Moreover, the sudden collapse of the depression in the manifold will reduce the rate of vaporisation and, if the engine is cold, some that has already evaporated may condense out on the manifold walls. In Fig. 10.21, the air : fuel ratio requirements and levels of manifold depression experienced as the throttle is opened progressively are plotted against air consumption. Also, the plot at A shows the air : fuel ratio required for producing the acceleration, and that at B shows what the air : fuel ratio is if the mixture is not enriched. 10.26 Provision for acceleration The simplest method of enrichment is to insert a well between the discharge end of the spray tube and the main jet, so that the fuel in it is instantly available for acceleration. However, this measure is rarely, if ever, adequate to provide the enrichment needed during the initial snap acceleration period in automotive applications. On the other hand, as already explained, it is 377Fundamentals of carburation used almost universally in association with an emulsion tube, for general mixture compensation. If, however, the fuel supply for an acceleration pump is taken from a point between the main jet and the well of the compensating system, the contents of that well are available for complementing the flow from that pump. This flow is dependent on the acceleration pump spring rate, as explained in the next paragraph but one, whereas that from the compensating well is at least partly dependent on the value of the depression over the main jet. Most carburettors have an acceleration pump. This is a simple plunger- or diaphragm-type pump the control linkage of which is interconnected with that of the throttle. As the throttle is opened the pump plunger or diaphragm is depressed, spraying a small dose of fuel directly into the induction system, usually just above the venturi, the low pressure in which assists evaporation. To prolong the spraying process during the acceleration, the piston rod generally incorporates a lost motion device, so that the control does not instantly move it but first compresses a spring around the rod. This spring pushes the piston down its cylinder to discharge the fuel progressively through the acceleration jet. To avoid over-enrichment and waste of fuel during slow movements of the accelerator pedal, there may be a controlled leak-back, usually through a small clearance between the piston and its cylinder walls, though sometimes through a restricted orifice or a by-pass duct. This leak- back may be adequate to avoid supplying fuel in excess of the requirement when the throttle is opened only very slowly. 10.27 Mechanically actuated acceleration pumps Two examples of acceleration pump mechanisms are that in the Zenith IV, Fig. 10.22 and the Stromberg DBV carburettor, Fig. 10.23. In the Zenith unit, there are two concentric springs over the pump. Both are compressed 500 400 300 200 100 0 10 12 14 16 18 Air: fuel ratio Intake manifold depression (mm Hg) mm Hg Part load Full-power Part load mm Hg A B 0 50 100 150 200 250 Air consumed (kg/h) Fig. 10.21 Air : fuel ratio requirements and typical levels of manifold depression as the throttle is opened incrementally 378 The Motor Vehicle C D A B H J K G F E I L M by the lever connected to the throttle control. However, the inner one, by pushing the piston down after the piston rod has been slid through the hole in its crown by the actuation lever, performs the delaying function; the other returns the piston after the throttle has been closed again. The throttle control is linked to the acceleration pump actuation lever, so when it is closed the piston B is lifted, drawing fuel up through the inlet valve in the base of its cylinder. As the throttle pedal is depressed, for acceleration, so also is the pump piston rod A which, while compressing the delay action spring, slides down through the hole in the piston. This pressurises the fuel below, closes the inlet valve, and opens the non-return valve D, through which it delivers the initial charge of fuel for acceleration through the spray jet C. During subsequent closure of the throttle, valve D closes to prevent reverse flow as the piston rises. The vent above this valve prevents fuel in the pump from being siphoned out through the spray jet. The Stromberg accelerator pump in Fig. 10.23 functions in a similar manner but, because there is a direct link connection R between the pump P and throttle control, there is only a single spring S: the return spring having been omitted. A plate valve V is used instead of a ball-type inlet valve and the delivery valve is in the base of the cylinder. Another difference is the interposition of a discharge reducer D between the delivery valve and the spray jet J. Again, a clearance between the piston and cylinder obviates wastage of fuel when the throttle is opened only slowly. 10.28 Depression actuated acceleration pumps Acceleration pumps can also be actuated by manifold depression. The device illustrated in Fig.10.24 was fitted to the Solex AIP carburettor. When the engine starts, the high depression in the manifold is communicated through the hole C, to pull the double diaphragm to the left and compress its return spring. This draws fuel through the inlet valve D into the pump chamber P. Subsequently, when the throttle is opened suddenly, causing the depression in the manifold to collapse, the diaphragm is pushed to the right by its return spring. This forces the fuel past the delivery valve and through the acceleration Fig. 10.22 Zenith IV carburettor, showing accelerator pump 379Fundamentals of carburation jet J, which sprays it into the air flow upstream of the venturi. If one of the two diaphragms leaks, there is still no possibility of fuel being drawn continuously through the device into the induction manifold. Hopefully, the consequent deterioration of the functioning of the pump would be noticed before the second diaphragm leaked. For adjusting the stroke of the pump there is a screw on the left. The level of manifold depression at which the pump will begin to draw fuel into the chamber P is determined by the pre-load of the return spring which, in turn, sets the degree of throttle opening beyond which the pump ceases to become effective. At large throttle openings, the depression over the jet J is high enough to draw fuel continuously through the device and thus enrich the mixture for maximum power. 10.29 Enrichment for maximum power In some instances, the air flow over the discharge orifice when the throttle is wide open generates a depression sufficient to draw fuel through it for producing at least some of the enrichment needed for developing maximum power. However, more is needed. The earliest devices for automatic power enrichment were mechanically actuated, by means of linkages connected to the throttle control. Two such mechanisms are illustrated in Fig. 10.25. That at (a) is from the early Claudel–Hobson carburettor of Fig. 10.8, in which a lever connected to the throttle valve mechanism opens the power enrichment valve F over the last few degrees of throttle opening. This allows fuel to pass from the float chamber, through the power jet into the emulsion system. An air Fig. 10.23 Stromberg mechanical pump Fig. 10.24 Solex membrane type acceleration pump P V D S R J C D J P 380 The Motor Vehicle bleed hole in the plug in the end of the passage delivering the fuel to the emulsion tube not only helps to emulsify the extra fuel supplied and to regulate the flow according to the degree of depression in the venturi, but also serves as the air bleed for economy when the power jet is not in operation. The section at (b) is a Solex device, in which F is again the power enrichment valve and is actuated through a lost motion device by the throttle control. The next development was of manifold depression actuated devices for bringing the power jet into operation. In Fig. 10.26, R is the rod that actuates the acceleration pump, which is not shown in this illustration. To the left of it is the idling system previously described, while to the right is the power- enrichment device. Manifold depression is taken through the external pipe to a connection above the piston P, within which is its return spring. When the depression is high, it lifts the piston against its return spring and the conical valve V closes. As the throttle approaches the fully-open position, the depression largely disappears, so the spring pushes the piston down. This opens the valve and thus allows fuel from the float chamber to pass through it and the power jet J, whence it flows up again to pass through a duct to the left of J, ultimately supplementing the flow through the spray tube into the venturi. Other systems such as a mechanical connection with the throttle control, incorporating either a lost-motion device or a cam to actuate an enrichment valve have been used. Also needle valves, tapered to provide the required fuel-flow characteristics, have been linked to the throttle control. Another method is simply to place the enrichment discharge orifice upstream of the venturi, where the depression to which it is subjected is calculated to be sufficient for drawing fuel from it only when the throttle is wide open. 10.30 Static power enrichment Mechanisms are potential sources of unreliability and wear and therefore are ( a )( b ) Fig. 10.25 F F To throttle 381Fundamentals of carburation Air in Enrichment spray tube Fuel in Enrichment jet Air in D V H J R P A Fig. 10.26 Stromberg DBV carburettor by-pass valve and jet Fig. 10.27 In this static power enrichment system, the extra fuel is drawn from the float chamber and discharged at a point well above the twin venturi undesirable. Static devices, such as that in Fig. 10.27, tend to be more attractive. Extra fuel is delivered through a duct in the top of the spray tube into the venturi. This fuel is drawn directly from the float chamber and discharged well upstream of the twin venturis so that it is evaporating in the air stream for as long as is practicable. Evaporation is further increased by both its passage through the low-pressure regions in the venturis and the turbulence generated around the end of the spray tube. To ensure that this device comes into operation only when the throttle is wide open and the depression upstream of the venturi therefore large, the position of the discharge orifice is such that the head of fuel against which the depression must lift the fuel is fairly large. 382 The Motor Vehicle P N U L H E D C F J K B Q M G A T S R Fig. 10.28 Zenith IZ carburettor 383Fundamentals of carburation L J C A F M N H G D P E K B Petrol level Fig. 10.29 Zenith IV carburettor [...]... further, the increasing depression in the venturi brings the main system into operation From the main jet G, the fuel passes into the well C Air, metered through the orifice L, passes down the emulsion tube and, passing through radial holes in it, mixes with the fuel before it enters the main discharge orifice D in the narrowest part of the venturi As the engine speed increases, the fuel level in the main... set the idling speed, the volume adjustment screw regulates the quantity of emulsified idling mixture supplied for mixing with the air passing the throttle Smooth transfer from the idle to main circuits is obtained by the two progression holes K, which come in turn under the influence of the local venturi effect caused by the proximity of the edge of the throttle to them As the throttle is opened further,... overcomes the load in the return spring N and moves the diaphragm to the left, as viewed in Fig 10.28, allowing the chamber between the diaphragm and the main body of the device to fill with fuel, and the spring-loaded valve P to close Since closure of this valve puts the jet Q out of action, fuel can now be drawn only from the main jet As the throttle is opened further, and the depression in the manifold... ignition advance, and the small hole M, through which it communicates with the throttle bore, is carefully calibrated With further opening of the throttle, and the consequent increase in the depression in the waist of the choke tube, fuel is drawn from the outlet from the emulsion block This fuel comes from the main jet G and compensating jet H As the level of the fuel in the channels above these jets falls,... throttle opening In both the WI and the WIA, the acceleration pump is mechanically actuated but there is no progressive delivery spring, the piston having only a return spring Seasonal adjustment of the pump stroke can be made by transferring the pin in the end of the interconnecting link into the appropriate hole of the three in the end of the pump actuation lever Two of these holes can just be seen... When the engine is switched on, the solenoid is energised, to lift the plunger off the seating, thus opening the slow running supply system On the IZE, instead of the drilled hole and dust cap on the float chamber, there is either a two-way venting system or a simpler internal vent The simpler system is a vent channel running within the float chamber cover casting and breaking out into the upper part. .. conditions in the manifold is high enough to assist evaporation of the fuel and thus to optimise the torque characteristics The carburettor has twin barrels, the primary barrel having a bore of 25 mm and the other 64 mm Over the first 20 mm of pedal travel, the primary throttle valve opens 27° Up to this point, because there is a lostmotion mechanism in the linkage interconnecting the two throttles, the secondary... barrels are the primary ones 404 The Motor Vehicle The output from the ECM is a stream of timed-pulse electric signals, issued at a rate of ten per second, which modulate the action of a single, mixture-control solenoid Movement of the core of this solenoid is transmitted by a yoke to the upper ends of the two metering needles, or rods, in the jets serving only the primary barrels The lower ends of these...384 10.31 The Motor Vehicle Economiser devices Actually, there are two different approaches to providing for maximum power operation: one is the provision of extra fuel, as just described, over the last few degrees of throttle opening, while the other is to reduce the strength of the mixture throughout most of the range, leaving the fuel to flow more freely over the last few degrees of... of the primary choke, while the other is the jet G, into which air is bled through orifice H from both J, in the primary air intake, and K, just below the waist of the primary choke The relative positions of all these inlets and outlets are such that, when the throttles in their chokes are open, the pressures in the mixture passages are not low enough to draw any fuel from the jets Jets D1 and D2 are . under the influence of the local venturi effect caused by the proximity of the edge of the throttle to them. As the throttle is opened further, the increasing depression in the venturi brings the. of these was the small duct D, which drew fuel from the starter well integral with the float chamber shown in the sectioned view on the right. The other was the much larger duct leading from the bottom. opened incrementally 378 The Motor Vehicle C D A B H J K G F E I L M by the lever connected to the throttle control. However, the inner one, by pushing the piston down after the piston rod has been slid through the