Land Rover Diesel Distributor Pumps Bosch Bosch Manual
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Injection timing In order to compensate for the injection lag and the ignition lag, as engine speed increases the timing device advances the distributor pump’s start of delivery referred to the engine’s crankshaft. Example (Fig. 1): Start of delivery (FB) takes place after the inlet port is closed. The high pres- sure then builds up in the pump which, as soon as the nozzle-opening pres- sure has been reached leads to the start of injection (SB). The period between FB and SB is referred to as the injection lag (SV). The increasing compression of the air-fuel mixture in the combustion chamber then initiates the ignition (VB). The period between SB and VB is the ignition lag (ZV). As soon as the cutoff port is opened again the pump pressure collapses (end of pump delivery), and the nozzle needle closes again (end of injection, SE). This is followed by the end of combustion (VE). Assignment During the fuel-delivery process, the injection nozzle is opened by a pressure wave which propagates in the high- pressure line at the speed of sound. Basically speaking, the time required for this process is independent of engine speed, although with increasing engine speed the crankshaft angle between start of delivery and start of injection also increases. This must be compensated for by advancing the start of delivery. The pressure wave’s propagation time is determined by the length of the high-pressure line and the speed of sound which is approx. 1,500 m/s in diesel fuel. The interval represented by this propagation time is termed the injection lag. In other words, the start of injection lags behind the start of delivery. This phenomena is the reason for the injector opening later (referred to the engine’s piston position) at higher engine speeds than at low engine speeds. Following injection, the injected fuel needs a certain time in Injection timing 29Curve of a working stroke at full load and at low speed (not drawn to scale). FB Start of delivery, SB Start of injection, SV Injection lag, VB Start of combustion, ZV Ignition lag, SE End of injection, VE End of combustion. 1Combustion pressure, 2Compression pressure, UT BDC, OT TDC. BDC TDC BDC TDC2841216-4-2-16-12-8 SV 0 100 200 300 400bar TDC TDC ZV SV FB SBSE VE Pump high pressure p Nozzle-needle lift n D Rate of injection Q 0 0.1 0.2 0.3 mm 0 2 4 6 mm 3 °cms °cms ATDC °cms BTDC Degrees camshaft Combustion-chamber pressure bar 1 2 VB FB SB SE Plunger position h Fig. 1 UMK0357E
order to atomize and mix with the air to form an ignitable mixture. This is termed the air-fuel mixture preparation time and is independent of engine speed. In a diesel engine, the time required between start of injection and start of combustion is termed the ignition lag. The ignition lag is influenced by the diesel fuel’s ignition quality (defined by the Cetane Number), the compression ratio, the intake-air temperature, and the quality of fuel atomization. As a rule, the ignition lag is in the order of 1 millisecond. This means that pre- suming a constant start of injection, the crankshaft angle between start of injection and start of combustion increases along with increasing engine speed. The result is that combustion can no longer start at the correct point (referred to the engine-piston position). Being as the diesel engine’s most efficient combustion and power can only be developed at a given crankshaft or piston position, this means that the in- jection pump’s start of delivery must be advanced along with increasing engine speed in order to compensate for the overall delay caused by ignition lag and injection lag. This start-of-delivery advance is carried out by the engine- speed-dependent timing device. Timing device Design and construction The hydraulically controlled timing de- vice is located in the bottom of the distributor pump’s housing, at right angles to the pump’s longitudinal axis (Fig. 2), whereby its piston is free to move in the pump housing. The housing is closed with a cover on each side. There is a passage in one end of the timing device plunger through which the fuel can enter, while at the other end the plunger is held by a compression spring. The piston is connected to the roller ring Axial-piston distributor pumps 30 Distributor injection pump with timing device 1Roller ring, 2Roller-ring rollers, 3Sliding block, 4Pin, 5Timing-device piston, 6Cam plate, 7Distributor plunger. 12345 67 Fig. 2 UMK0354Y
through a sliding block and a pin so that piston movement can be converted to rotational movement of the roller ring. Method of operation The timing-device piston is held in its initial position by the timing-device spring (Fig. 3a). During operation, the pressure- control valve regulates the fuel pressure inside the pump so that it is proportional to engine speed. As a result, the engine- speed-dependent fuel pressure is ap- plied to the end of the timing-device piston opposite to the spring. As from about 300 min –1, the fuel pressure inside the pump overcomes the spring preload and shifts the timing- device piston to the left and with it the sliding block and the pin which engages in the roller ring (Fig. 3b). The roller ring is rotated by movement of the pin, and the relative position of the roller ring to the cam plate changes with the result that the rollers lift the rotating cam plate at an earlier moment in time. In other words, the roller ring has been rotated through a defined angle with respect to the cam plate and the distributor plunger. Normally, the maximum angle is 12 degrees camshaft (24 degrees crankshaft). Injection timing 31Timing device, method of operation aInitial position, bOperating position. 1Pump housing, 2Roller ring, 3Roller-ring rollers, 4Pin, 5Passage in timing-device piston, 6Cover, 7Timing-device piston, 8Sliding block, 9Timing-device spring. 1 2 3 4 5 6 7 8 9 a b Fig. 3 UMK0355Y
Add-on modules and shutoff devices Application The distributor injection pump is built according to modular construction principles, and can be equipped with a variety of supplementary (add-on) units (Fig. 1). These enable the implemen- tation of a wide range of adaptation possibilities with regard to optimization of engine torque, power output, fuel economy, and exhaust-gas composition. The overview provides a summary of the add-on modules and their effects upon the diesel engine. The schematic (Fig. 2) shows the interaction of the basic distributor pump and the various add-on modules.Torque control Torque control is defined as varying fuel delivery as a function of engine speed in order to match it to the engine’s fuel-requirement characteristic. If there are special stipulations with regard to the full-load characteristic (optimization of exhaust-gas compo- sition, of torque characteristic curve, and of fuel economy), it may be necessary Axial-piston distributor pumps 32 Distributor injection pump with add-on modules 1Cold-start accelerator, 2Manifold-pressure compensator. 12 Fig. 1 UMK0358Y
Add-on modules and shutoff devices 33Schematic of the VE distributor pump with mechanical/hydraulic full-load torque control LDA Manifold-pressure compensator. Controls the delivery quantity as a function of the charge-air pressure. HBA Hydraulically controlled torque control. Controls the delivery quantity as a function of the engine speed (not for pressure-charged engines with LDA). LFB Load-dependent start of delivery. Adaptation of pump delivery to load. For reduction of noise and exhaust-gas emissions. ADA Altitude-pressure compensator. Controls the delivery quantity as a function of atmospheric pressure. KSB Cold-start accelerator. Improves cold-start behavior by changing the start of delivery. GST Graded (or variable) start quantity. Prevents excessive start quantity during warm start. TLA Temperature-controlled idle-speed increase. Improves engine warm-up and smooth running when the engine is cold. ELAB Electrical shutoff device. ACutoff port, n actualActual engine speed (controlled variable), nsetpointDesired engine speed (reference variable), Q FDelivery quantity, tMEngine temperature, tLUAmbient-air temperature, pLCharge-air pressure, p AAtmospheric pressure, pi Pump interior pressure. 1Full-load torque control with governor lever assembly, 2Hydraulic full-load torque control. TLA GST Control of injected fuel quantity Engine-speed controlLDA ADA ELAB HBA High-pressure pump with distributor Vane-type fuel- supply pumpDelivery-valve assembly Timing deviceLFB12 A KSB t LU/tMnsetpointUon /UoffpL /pA p pi nactualDrive FuelInjection nozzles QF Add-on module Basic pump tM Fig. 2 UMK0359E
to install torque control. In other words, the engine should receive precisely the amount of fuel it needs. The engine’s fuel requirement first of all climbs as a function of engine speed and then levels off somewhat at higher speeds. The fuel-delivery curve of an injection pump without torque control is shown in Fig. 3. As can be seen, with the same setting of the control collar on the distributor plunger, the injection pump delivers slightly more fuel at high speeds than it does at lower speeds. This is due to the throttling effect at the distributor plunger’s cutoff port. This means that if the injection pump’s delivery quantity is specified so that maximum-possible torque is developed at low engine speeds, this would lead to the engine being unable to completely combust the excess fuel injected at higher speeds and smoke would be the result together with engine overheat. On the other hand, if the maximum delivery quantity is specified so that it corresponds to the engine’s requirements at maximum speed and full-load, the engine will not be able to develop full power at low engine speeds due to the delivery quantity dropping along with reductions in engine speed. Performance would be below optimum. The injected fuel quantity must therefore be adjusted to the engine’sactual fuel requirements. This is known as “torque control”, and in the case of the distributor injection pump can be implemented using the delivery valve, the cutoff port, or an extended governor- lever assembly, or the hydraulically controlled torque control (HBA). Full-load torque control using the governor lever assembly is applied in those cases in which the positive full-load torque control with the delivery valve no longer suffices, or a negative full-load torque control has become necessary. Positive torque control Positive torque control is required on those injection pumps which deliver too much fuel at higher engine revs. The delivery quantity must be reduced as engine speed increases. Positive torque control using the delivery valve Within certain limits, positive torque control can be achieved by means of the delivery valve, for instance by fitting a softer delivery-valve spring. Positive torque control using the cutoff port Optimization of the cutoff port’s dimen- sions and shape permit its throttling effect to be utilized for reducing the delivery quantity at higher engine speeds. Positive torque control using the governor lever assembly (Fig. 4a) The decisive engine speed for start of torque control is set by preloading the torque-control springs. When this speed is reached, the sliding-sleeve force ( FM) and the spring preload must be in equilibrium, whereby the torque-control lever (6) abuts against the stop lug (5) of the tensioning lever (4). The free end of the torque-control lever (6) abuts against the torque-control pin (7). If engine speed now increases, the sliding-sleeve force acting against the starting lever (1) increases and the common pivot point (M 4) of starting lever and torque-control lever (6) changes its position. At the same time, Axial-piston distributor pumps 34 Fuel-delivery characteristics, with and without torque control aNegative, bPositive torque control. 1Excess injected fuel, 2Engine fuel requirement, 3Full-load delivery with torque control, Shaded area: Full-load delivery without torque control. Engine speed n Delivery quantity Q F min–1 mm3 stroke ab 123 Fig. 3 UMK0360E
the torque-control lever tilts around the stop pin (5) and forces the torque- control pin (7) in the direction of the stop, while the starting lever (1) swivels around the pivot point (M 2) and forces the control collar (8) in the direction of re- duced fuel delivery. Torque control ceases as soon as the torque-control-pin collar (10) abuts against the starting lever (1). Negative torque control Negative torque control may be necessary in the case of engines which have black-smoke problems in the lower speed range, or which must generate specific torque characteristics. Similarly, turbocharged engines also need negative torque control when the manifold-pressure compensator (LDA) has ceased to be effective. In this case, the fuel delivery is increased along with engine speed (Fig. 3). Negative torque control using the governor lever assembly (Fig. 4b) Once the starting spring (9) has been compressed, the torque-control lever (6) applies pressure to the tensioning lever (4) through the stop lug (5). The torque-control pin (7) also abuts against the tensioning lever (4). If the sliding- sleeve force ( FM) increases due to rising engine speed, the torque-control leverpresses against the preloaded torque- control spring. As soon as the slid- ing-sleeve force exceeds the torque- control spring force, the torque-control lever (6) is forced in the direction of the torque-control-pin collar. As a result, the common pivot point (M 4) of the starting lever and torque-control lever changes its position. At the same time the starting lever swivels around its pivot point (M 2) and pushes the control collar (8) in the direction of increased delivery. Torque control ceases as soon as the torque-control lever abuts against the pin collar. Negative torque control using hydrauli- cally controlled torque control HBA In the case of naturally aspirated diesel engines, in order to give a special shape to the full-load delivery characteristic as a function of engine speed, a form of torque control can be applied which is similar to the LDA (manifold-pressure compensator). Here, the shift force developed by the hydraulic piston is generated by the pressure in the pump interior, which in turn depends upon pump speed. In contrast to spring-type torque control, within limits the shape of the full-load characteristic can be determined by a cam on a sliding pin. Add-on modules and shutoff devices 35 Torque control using the governor-lever assembly aPositive torque control, bNegative torque control. 1Starting lever, 2Torque-control spring, 3Governor spring, 4Tensioning lever, 5Stop lug, 6Torque-control lever, 7Torque-control pin, 8Control collar, 9Starting spring, 10Pin collar, 11Stop point, M 2Pivot point for 1 and 4, M 4Pivot point for 1 and 6, F MSliding-sleeve force, Ds Control-collar travel. 4 M 4 2 6 7 b 34 2 59 1110 1M2 8 F M 1 M45 6 7 M2 8 a FM D sD s Fig. 4 UMK0362Y
Manifold-pressure compensation Exhaust-gas turbocharging Because it increases the mass of air inducted by the engine, exhaust turbo- charging boosts a diesel engine’s power output considerably over that of a nat- urally aspirated diesel engine, with little increase in dimensions and engine speeds. This means that the brake horsepower can be increased corre- sponding to the increase in air mass (Figure 6). In addition, it is often possible to also reduce the specific fuel con- sumption. An exhaust-gas turbocharger is used to pressure-charge the diesel engine (Fig. 5).With an exhaust turbocharger, the engine’s exhaust gas, instead of simply being discharged into the atmosphere, is used to drive the turbocharger’s turbine at speeds which can exceed 100,000 min –1. Turbine and turbocharger compressor are connected through a shaft. The compressor draws in air, compresses it, and supplies it to the engine’s combustion chambers under pressure, whereby not only the air pressure rises but also the air temperature. If temperatures become excessive, some form of air cooling (intercooling) is needed between the turbocharger and the engine intake. Axial-piston distributor pumps 36 UMK0365Y Fig. 5: Diesel engine with exhaust-gas turbo- charger
Manifold-pressure compensator (LDA) The manifold-pressure compensator (LDA) reacts to the charge-air pressure generated by the exhaust-gas turbo- charger, or the (mechanical) super- charger, and adapts the full-load deliv- ery to the charge-air pressure (Figs. 6 and 7). Assignment The manifold-pressure compensator (LDA) is used on pressure-charged diesel engines. On these engines the injected fuel quantity is adapted to the engine’s increased air charge (due to pressure-charging). If the pressure- charged diesel engine operates with a reduced cylinder air charge, the in- Add-on modules and shutoff devices 37Power and torque comparison, naturally aspi- rated and pressure-charged engines kW min–1 Nm Pe Md Engine speed n Torque M d Power Pe Naturally aspirated engine Pressure - charged engine Distributor injection pump with manifold-pressure compensator (LDA) 1Governor spring, 2Governor cover, 3Reverse lever, 4Guide pin, 5Adjusting nut, 6Diaphragm, 7 Compression spring, 8Sliding pin, 9Control cone, 10Full-load adjusting screw, 11Adjusting lever, 12Tensioning lever, 13Starting lever, 14Connection for the charge-air, 15Vent bore. M 1pivot for 3. 6 8 7 9 10 14 15 11 12 13 5 4 M1 123 Fig. 6Fig. 7 UMK0367E UMK0364Y
jected fuel quantity must be adapted to the lower air mass. This is performed by the manifold-pressure compensator which, below a given (selectable) charge-air pressure, reduces the full-load quantity. Design and construction The LDA is mounted on the top of the distributor pump (Fig. 7). In turn, the top of the LDA incorporates the connection for the charge-air and the vent bore. The interior of the LDA is divided into two separate airtight chambers by a dia- phragm to which pressure is applied by a spring. At its opposite end, the spring is held by an adjusting nut with which the spring’s preload is set. This serves to match the LDA’s response point to the charge pressure of the exhaust turbocharger. The diaphragm is con- nected to the LDA’s sliding pin which has a taper in the form of a control cone. This is contacted by a guide pin which transfers the sliding-pin movements to the reverse lever which in turn changes the setting of the full-load stop. The initial setting of the diaphragm and the sliding pin is set by the adjusting screw in the top of the LDA. Method of operation In the lower engine-speed range the charge-air pressure generated by the exhaust turbocharger and applied to the diaphragm is insufficient to overcome the pressure of the spring. The diaphragm remains in its initial position. As soon as the charge-air pressure applied to the diaphragm becomes effective, the dia- phragm, and with it the sliding pin and control cone, shift against the force of the spring. The guide pin changes its position as a result of the control cone’s vertical movement and causes the reverse lever to swivel around its pivot point M 1(Fig. 7). Due to the force exerted by the governor spring, there is a non- positive connection between tensioning lever, reverse lever, guide pin, and sliding-pin control cone. As a result, the tensioning lever follows the reverse lever’s swivelling movement, causing thestarting lever and tensioning lever to swivel around their common pivot point thus shifting the control collar in the direction of increased fuel delivery. Fuel delivery is adapted in response to the increased air mass in the combustion chamber (Fig. 8). On the other hand, when the charge-air pressure drops, the spring underneath the diaphragm pushes the diaphragm upwards, and with it the sliding pin. The compensation action of the governor lever mechanism now takes place in the reverse direction and the injected fuel quantity is adapted to the change in charge pressure. Should the turbocharger fail, the LDA reverts to its initial position and the engine operates normally without developing smoke. The full-load delivery with charge-air pressure is adjusted by the full-load stop screw fitted in the governor cover. Axial-piston distributor pumps 38 Charge-air pressure: Operative range aTurbocharger operation, bNormally aspirated operation. p 1Lower charge-air pressure, p 2Upper charge-air pressure. Charge-air pressure p p 1p2mbar a b mm3/ strokeLDA operative range Injected fuel quantity Qe Fig. 8 UMK0368E