The Motor Vehicle 2010 Part 4 docx

234 The Motor Vehicle Inputs Outputs Throttle position pressure Electronic control module Command pulse Injectors Feedback EDU Diagnostic data link (DDL) Stop engine light Check engine light reference Oil temperature Oil pressure Coolant level PROM Synch reference – + Turbo-boost Timing Battery Fig. 6.50 This is the first generation DDEC electronic control system for the GM unit injectors. It differs from the second generation system in that the command pulse and feedback are directed to and from the injectors through an EDU instead of directly. The EDU (electronic distributor unit) functions as a high current switching unit for energising the solenoids remains open. The layout of the system is illustrated diagrammatically in Figs 6.51 and 6.52. Fuel is drawn from the tank, through a filter to a gear type pump and thence into the governor, whence it passes through a throttle valve and a shut-down valve, to the pipeline that delivers it to the injectors. Of these components, all between the pipelines from the tank and to the injectors are actually grouped in a single unit, Fig. 6.53, into which both the spin-on filter may be screwed and the drive taken, either directly or in tandem with another auxiliary such as the compressor, from the engine to the gear type pump. Delivery pressure from the fuel pump will be subsequently boosted to the injection pressure by the cam and rocker mechanism, so it does not have to be more than 1750 kN/m 2 as compared with well over 70 000 kN/m 2 for injectors in which the valves have to be opened by hydraulic pressure supplied from an external pump. The governor, which is of the rotating twin bob-weight type, regulates only maximum and idling speeds. It does this by moving a spool valve axially between stops to limit the rate of supply of fuel at its two extreme positions. From zero load up to maximum speed at any load, the driver effects control through the accelerator pedal, which actuates the throttle in the fuel delivery line. When maximum speed is attained at full load (maximum power output), the throttle valve lever is in the maximum fuel position, so the pressure, and therefore quantity of fuel delivered, is at its maximum. If the load is then increased, the engine speed and, with it, the fuel pressure from the gear type pump will fall. This fall in speed causes the mechanical governor to relax its axial pressure on its return spring, called the torque spring, thus 235 Diesel injection equipment and systems 6 7 4 3 5 1 2 for actuating the injector 1 Fuel from tank 2 Gear type pump 3 Governor/pressure regulator 4 Hydraulic throttle 5 Shut-down valve 6 Injector 7 Cam, roller follower and pushrod Fig. 6.51 Diagram of the Cummins PT injection system hydraulics allowing the spool valve to move to the left, in Fig. 6.51, to reduce the quantity of fuel recirculating back to the induction side of the pump. Consequently, more fuel is delivered through the driver-controlled throttle in the delivery line to the injectors. Another, but natural, consequence of a fall in engine speed is that the duration of opening of the injector orifice increases, so more fuel can enter the injector cup. Both effects increase the engine torque as the speed and power fall off. The shut-down valve simply cuts off the fuel supply. It is actuated either electrically, pneumatically or manually. For turbocharged engines, an air–fuel control (AFC) valve is introduced into the main control unit, Fig. 6.53. This is a spool valve actuated by a 236 The Motor Vehicle Fig. 6.52 Diagram showing layout of Cummins PT system diaphragm exposed to the boost pressure, and it is interposed between the throttle and shut-down valves. If the accelerator pedal is suddenly depressed, and throttle valve in the fuel supply system thus opened, the passage on to the injectors is restricted by the AFC valve which, progressively opening, limits the rate of increase of flow to match that of the boost pressure. This avoids the emission of black smoke while the turbocharger is accelerating to catch up to supply enough air for combustion for coping with the extra load. Other components in the main control unit include a magnetic screen between the gear type pump and the governor, to take out any particles of metal that might damage or impair the operation of the unit injectors; a pulsation damper to smooth out the delivery from the pump; and a spiral gear for driving a tachometer. A screw on the end remote from the bob- weights on the governor shaft limits the axial movement of the governor sleeve away from it, for setting the idling speed. The injectors are illustrated in Fig. 6.54. At the beginning of the upstroke, in preparation for the next injection, fuel from the low pressure manifold enters at A, passes through the inlet orifice B, and on down through a series of drilled holes, turns up to pass through a check valve F, and then down again to an annular groove in the top end of the injector cup, whence it flows up yet again through passage D into the waisted portion of the stem of the injector. From there it flows out and up through passage E on its way back to the tank. This fuel flow cools the injector and tends to warm the fuel in the 237 1 Shut-down valve 2 Fuel to injectors 3 Pulsation damper 4 Tachometer shaft 5 Filter screen 6 Fuel inlet 7 Gear pump 8 Air-fuel control barrel 9 Main shaft 10 Drive coupling 11 Throttle shaft 12 Idle speed adjusting screw 13 By-pass ‘button’ 14 Governor plunger 15 Torque spring 16 Idle spring pack 17 Governor weights Fig. 6.53 The combined control, governor and pump unit of the Cummins PT system Diesel injection equipment and systems 238 The Motor Vehicle E A B F D E F D C D C Start upstroke Upstroke complete Downstroke (fuel circulates) (fuel enters injector cup) (fuel injection) Fuel at low pressure enters As injector plunger moves As plunger moves down injector at (A) and flows upward, metering orifice and closes metering orifice, through inlet orifice (B), (C) is uncovered and fuel fuel entry into cup is cut internal drillings, around enters injector cup. Amount off. As plunger continues annular groove in injector is determined by fuel pressure. down, it forces fuel out of cup and up passage (D) to Passage (D) is blocked, cup through tiny holes at return to fuel tank. Amount momentarily stopping high pressure as fine spray. of fuel flowing through circulation of fuel and This assures complete injector is determined by isolating metering orifice combustion of fuel in fuel pressure before inlet from pressure pulsations. cylinder. When fuel passage orifice (B). Fuel pressure (D) is uncovered by plunger in turn is determined by undercut, fuel again begins engine speed, governor to flow through return and throttle. passage (E) to fuel tank. Fig. 6.54 Sequence of operations of Cummins unit injector: (left) start; (centre) upstroke; (right) downstroke tank, thus helping to prevent wax formation in very cold weather. The quantity of fuel flowing is a function of its pressure which, in turn, is primarily a function of engine speed but modified by the restrictions imposed by the governor, throttle valve and, in the case of a turbocharged engine, the AFC valve. As the upstroke is completed, the metering orifice C is uncovered, and the circulation back to the tank is interrupted by the closure of the passage D. Pulsations in the supply from the fuel pump are absorbed by the pulsation damper in the control unit so, with the closure of passage D, the flow through orifice C is steady. Therefore the quantity of fuel passing through this orifice into the injector cup is a function of its pressure. Any back-flow will close the check valve F. On the next injection stroke the downwardly moving plunger first shuts off the fuel supply coming through the metering orifice C and thus traps the metered quantity of fuel in the injector cup. Since no more fuel can subsequently pass in from the metering orifice, there is no possibility of dribbling through the injector holes after the injection stroke has been completed. Continuing down, the plunger pressurises the fuel in the cup and forces it Diesel injection equipment and systems 239 out through tiny holes in the nozzle, spraying it into the combustion chamber. Toward the end of the stroke, the passage D is once more uncovered, and the cooling flow of fuel back to the tank resumed. On completion of injection, the tapered end of the plunger momentarily remains on its seat, in the bottom of the cup, until the next metering and injection sequence begins. 6.45 The GM unit injection system In basic concept, the GM unit injection, Fig. 6.55, bears some similarity to the Cummins PT system just described, but it differs in many respects. First, there is no separate unit housing all the control functions: instead, each injector, Fig. 6.56, houses what is virtually a single element of a jerk pump, such as that illustrated in Fig. 6.27, and injection is controlled by a multi- segment toothed rack that extends the full length of the head from the foremost to the rearmost injectors. From the tank, the fuel is lifted by a transfer pump, through first a strainer and then a fine filter, up to the gallery and on into branch pipes connecting it to the unit injectors. As the fuel enters each injector, at A, Fig. 6.56, it passes through an additional, small, filter from which ducts take it down through B into a sleeve in the casting around the injector barrel and plunger. Thence it flows through the radial port F in the barrel, into the chamber Fig. 6.55 Diagram showing layout of the General Motors unit injection system 240 The Motor Vehicle Fig. 6.56 GM unit injector below the end of the plunger. As the plunger descends, the fuel beneath it is forced up the axial hole in it and out through a radial hole into the spill groove. From the spill groove, it flows through the radial port E, on the left of the barrel, out into the sleeve in the housing. The return passage from the housing, delivering to the outlet H, is behind that for the inlet. It is of smaller diameter than the inlet, so that the fuel in the housing remains always under pressure. The function of the surplus fuel flow is to cool the unit during its passage through the barrel. As the plunger is lifted by the return spring at its upper end, it shuts off Diesel injection equipment and systems 241 the spill port on the left in Fig. 6.56, and then draws fuel through the radial hole on the right, in the barrel, into the chamber beneath it. Incidentally, higher up on the right, there is another hole C sloping upwards, to allow fuel to run into an annular groove in the bore of the barrel, for its lubrication. When the cam actuates the rocker mechanism, it pushes the plunger down again, so that its lower end D first shuts off the inlet hole, after which the upper edge of its spill groove shuts off the spill port E. The closure of the latter traps a metered quantity of fuel beneath the plunger which, continuing down, forces this fuel, at increasing pressure, through hole G in the wall of the cylindrical housing for the needle return spring, whence it passes into the nozzle. On the pressure of this fuel reaching a predetermined value, it lifts the piston on which the needle return spring seats and, with it, the needle from its conical seat, whereupon the fuel sprays out through the holes in the nozzle into the combustion chamber. As the plunger returns, the spiral upper edge of the spill groove in the plunger uncovers the spill port in the barrel, suddenly releasing any pressure in the fuel remaining in the nozzle so that, subsequently, there can be no dribble through its spray holes. The surplus fuel flows back through the axial and radial holes in the plunger into the spill groove, whence it passes out through the radial hole, on the left in the illustration, back into the main housing. On completion of the injection cycle, the plunger comes back up to its original position, with both the inlet and spill ports open, for resumption of the cooling flow. The upper edge of the spill groove around the plunger is of spiral form, so that the spill timing, and thus the metering of the quantity of fuel injected, can be regulated by rotation of the plunger, This is done by means of the previously mentioned rack. To stop the engine, the rack is moved to the right-hand extreme of its travel, rotating the plunger clockwise to the position where, as can be seen in the illustration, the spill port is at no point shut off by any vertical displacement of the plunger between the limits of its operation. 6.46 Common rail injection systems With the current demand for high injection pressures for satisfying the regulations on exhaust emissions, interest in the common rail system of injection has intensified. The basic principle stemmed from a Vickers Patent of 1913, and a practical system first went into production in the USA by the Atlas Imperial Diesel Engine Company. However, for meeting the requirements prior to the introduction of legal limits on emissions and noise, the in-line and, later, the distributor type pumps were more economical to produce and posed fewer design problems. In the late 1980s and early 1990s, Fiat and its subsidiaries in Italy developed a workable system. However, because specialist suppliers could supply a wide range of manufacturers, and therefore in much larger quantities and at a lower cost, Fiat decided to drop their own version. The first major producer in the field for light high speed diesel engines therefore was Bosch. In this system the common rail serves as the hydraulic accumulator, the compressibility of the fuel in it catering for injection without significant interference by pulsation. Several other common rail schemes have been proposed. For example, the pressure in the rail can be multiplied by a conventional plunger type unit 242 The Motor Vehicle injection pump the spill valve of which is controlled electronically by the ECU. With such a system it is still possible to boost the injection pressure up to perhaps 2000 bar or more, but it is less compact than the Bosch system described in the next section. For large engines, a conventional hydraulic accumulator can be included to supplement the capacity of the common rail. 6.47 The Bosch system As can be seen from Fig. 6.57, the fuel is lifted by the low pressure pump in the tank, through a filter to the roller cell type high pressure pump, which transfers it to the forged steel common rail. This rail, extends approximately the full length of the cylinder head. Generally about 10 mm diameter and from 280 to 600 mm long, it serves as a pressure accumulator. For minimum pressure fluctuation, the rail needs to be as long a practicable but, if too long, engine starting may be slow. In a well-designed installation, the pressure in the rail remains virtually constant throughout the injection process, and injection pressures ranging from 1350 to 1600 bar can be obtained. From the common rail, a separate pipe takes the fuel to the injector for each cylinder. The injectors are solenoid controlled, the injection pressure being nominally that in the common rail. A number of advantages arise out of this separation of the injection and pressurising functions. First, the injector in the cylinder head is much more compact than one combining a pump and injection valve, so there is more room around it for the inlet and exhaust valves and cooling passages. Second, the injection pressure can be more easily regulated. Third, two-stage injection is readily effected, simply by causing the ECU to send signals to the high speed solenoid to open and close the injection valve twice in rapid succession. In addition to the simplicity Fuel tank Pre-supply pump Pressure control valve Rail pressure sensor Common rail Four injectors Air mass sensor Filters ECU A B C D E F Sensors Pressure control valve High pressure pump Fig. 6.57 Principal components of the Bosch common rail injection system. The sensors A to F are as follows: A Crankshaft position; B Camshaft position; C Accelerator pedal; D Boost pressure; E Air temperature; F Coolant temperature Diesel injection equipment and systems 243 and compactness of this system, it has the advantage that, if required, injection into each cylinder can be varied individually by the ECU to compensate for slight variations in compression ratio due, for example, to wear. Finally, there are several ways in which the injection characteristic curve can be shaped, see the penultimate paragraph of Section 6.49. 6.48 Components of the Bosch system The ECU is served by sensors as follows: temperature and mass flow of the air passing through the intake filter; pressure of the fuel in the rail; engine speed and crank angle, which can be sensed from teeth on the rim of the flywheel; a sensor in the throttle pedal unit transmits signals indicting throttle position and rate of change of position; and another senses the temperature of the coolant in the engine. Illustrated in Fig. 6.58 is the fuel lift pump, which Bosch term the pre- supply pump. It is of the roller cell type, although gear type pumps can be employed. For cars, the pressure of fuel delivered from the lift pump is boosted to that required for injection by the radial plunger type high pressure (a) (b) Fig. 6.58 (a) A characteristic of the roller cell type pre-supply, or fuel lift, pump is an output with a lower level of pulsation than the principal alternatives. It is generally installed in the fuel tank. (b) Diagrammatic representation of the cross-section of the roller cell assembly, illustrating the progress of the fuel from inlet to outlet [...]... through the metering valve In principle, the operation of the metering valve is the same as that in the DPA system, described in Section 7.3, paragraphs 4 and 5 Also the same as in the DPA system are the holes in the rotor, the flow of fuel first through the metering valve and then at metered pressure to the injection pump, distributor and injectors The limiting of the maximum pressure in the cam box by the. .. through two passages: one to the injection pump and the other to the latch valve 13 in Fig 7.9 When the engine is turned by the starter motor, the fuel from the transfer pump is also delivered to these two passages, but the latch valve then closes so the fuel is delivered solely to the transfer pump The reason for closing the latch valve is to prevent transfer pressure from reaching the injection advance... drive of the governor assembly To transmit the drive from the drive shaft to the rotor, a tongue in the end of the latter registers in a slot in the end of the drive shaft The arrangement of the ducting in the rotor, distributor and hydraulic head for filling the chamber and the delivery and distribution of the fuel to the injectors is similar to that of the DPA pump Spigoted into a counterbore in the end... systems 247 Fig 6.63 When the injector is inoperative, the valve spring and the hydraulic pressure in the control chamber hold the ball valve down When the solenoid is energised, it overcomes the valve spring force Consequently, the pressure in the control chamber drops, while that in the nozzle chamber, acting on the area of the lower end of the needle valve, including that of the chamfer, lifts the needle... adjust the pressure to suit each particular engine application When the manual priming pump is used, the passage of the fuel to the injection pump is blocked by the stationary transfer pump The priming pressure therefore forces the piston down, compressing the priming spring beneath it and uncovering the priming ports, through which the fuel passes to the delivery side of the transfer pump It then goes... against the anti-stall stop 1, the cam ring is in its retarded position Also, the excess fuel lever 3, bearing against the lug 4, pulls the link plate in the opposite direction to that of rotation of the pump This loads the torsion spring 12 and rotates the scroll plates clockwise, as viewed in the illustration When the rotor filling port opens, the fuel under metering pressure begins 266 The Motor Vehicle. .. enter the plunger chambers, it forces the plungers outwards towards the point at which they will ultimately be stopped by the cam profiles 6 of the scroll plates at (a) in Fig 7.17 This point is further outwards than the normal running maximum The filling port closes at (b) When the plungers reach the point (c) the cam moves them inwards again to deliver the excess fuel which, in the meantime, the plungers... in the end cap 13, in which the other end of the second stage spring seats Spindle spring 18 is compressed between a collar on the spindle and the end cap The function of this spring is to load the three ball detents 15 for the manual control lever 17 If the engine is stationary and the latch valve therefore closed, the piston, solely under the influence of the two springs, is pushed up against the. .. plungers have drawn in As the engine fires, the governor rotates the metering valve, to reduce the rate of fuelling to that required for idling Opening the throttle moves the lever 5 (Fig 7.13) away from the lug 4, so that the link plate can return to the maximum fuel stop 10 This rotates the scroll plates in the same direction as that of the pump until the cam profiles are in the normal maximum fuel... by the setting of a pressure limiting valve in the pump end plate As the rotor turns, it opens each inlet port in the distributor sleeve one after the other Fuel from the transfer pump is delivered into a radial hole in the rotor, and then through an axial hole towards the end of the rotor, where it enters the space between the plungers Transfer pump pressure forces the plungers outwards while, at the . through the solenoid. When the current to the solenoid is cut, the valve is closed by its return spring and the pressure in the tiny volume of the valve control chamber rises 248 The Motor Vehicle. valve for the common rail injection system 246 The Motor Vehicle Fig. 6.62 The pressure regulator valve the upper end of the injector body, Fig. 6.63. By virtue of the facts that the injector. Shear between the abrasive particles and the edges of the delivery and spill ports and the edges of the plungers as they sweep over them can also cause wear. The trapping of debris on the seats