D EV E LO P M E N T
Injection Systems
2000 bar Diesel Common Rail by Bosch for Passenger Cars Modern combustion processes to reach actual and future emission limits request higher requirements from the fuel injection system. The 2000 bar Diesel Common Rail System from Bosch with piezo injectors and the high pressure pump CP4 gives the requisite for the development of new combustion processes. The engine manufacturer has the chance to reach the required emission levels as well as to reduce fuel consumption and therefore CO2 emissions.
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The Authors
Figure 1: Regime of conventional combustion processes and “oCCS“ combustion process
Dr.-Ing. Rolf Leonhard is Executive Vice President Engineering, Bosch Diesel Systems in Stuttgart (Germany).
Johann Warga is Senior Vice President Engineering, Bosch Diesel Systems in Stuttgart (Germany).
1 Introduction Since introduction of the first emission regulations and the following increase of engine emission demands, a permanent rise of the injection pressure is to be observed in the field of diesel technology. Milestones were the introduction of 1000 bar distribution pump approximately 20 years ago as well as the introduction of a Common Rail System with 1350 bar ten years ago. Five years ago Bosch has put to series production the first Common Rail System with piezo inline technology for passenger car applications. For future emission standards (e. g. Euro 5 and Euro 6) and on the back-
ground of the current discussed CO2 topic it is to be expected that the demands for combustion concepts are raised again. It is to be assumed that the demands for air- and fuel injection systems will rise considerably. A promising approach to reach future emission and CO2 targets is the “optimised Conventional Combustion System” (oCCS), Figure 1. This approach reaches nearly the particulate-matter and NOxemissions of homogeneous diesel fuel combustion processes (e. g. HCCI) however avoids the disadvantages of the HCCI combustion. With oCCS, downsizing concepts are supported due to the minor load dependency of NOx emissions, com-
pared to Euro 4 combustion processes. Necessary engine layout for oCCs is the reduction of the compression ratio, the reduction of swirl and the adaptation of the piston bowl. From special importance is the optimised injection concerning timing and quantity of the up to eight injections during the combustion process. To reach this task, the use of optimised multi-hole-nozzles for optimal spray preparation is mandatory. Precondition is the effective, cooled and switchable EGR in connection with optimised engine charging. Due to the reduced compression ratio, ceramic heater plugs support optimally the emission critical cold start phases.
Figure 2: Injection pressure map of typical Euro4 combustion process and “oCCS“ combustion process
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Figure 3: The Common Rail System CRS3 from Bosch
To use the potential of the higher EGRrates and the higher boost pressure for emission reduction it is necessary to increase the injection pressure up to 800 bar in part load conditions compared to conventional combustion processes, Figure 2.
2 The New 2000 Bar Common Rail System Bosch has developed for this combustion process an injection system, Figure 3, which is marked by flexible and very exact multiple injections with short dwell times. The demand on increased pressure during part load conditions is also fulfilled.
Major improvements were reached with the components injector, pump, and rail as well as with the whole system. With this system it is possible to reach the requested high injection pressures at low engine revolutions. The low weight and the high efficiency support the reaching of CO2 targets. Due to the fast pressure build up capability of the high pressure pump CP4 engine start stop systems can be realized. With the 2000 bar Common Rail System put into series production in 2007 the engine manufacturer has the chance to increase the thermodynamic efficiency by improved mixture formation.
Figure 4: Bosch Common Rail high pressure pump CP4 28
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3 Common Rail Pump Family CP4 The new pump family CP4 differs radically in new design features from three cylinder pumps currently available at the market, e. g. the CP1H from Bosch. The CP4 basic layout is a radial plunger pump with a roller on a cam drive featuring a double cam, Figure 4. Due to the physical operating principle of the double cam the CP4 has one cylinderhead module less compared to classical eccentric shaft pumps. This is one reason why a CP4 needs less installation space and has significantly less weight than e. g. a CP1H. The CP4 is designed for high pump speed (up to 5500 pump revolutions were tested). So the CP4 supports engines with high speed and a gear ratio of 1. Due to the combination of double cam and high pump speed the delivery frequency is much higher than the delivery frequency of a normal eccentric shaft pump. Therefore the leakage at the plunger is considerable reduced and the efficiency of the CP4 reaches best in class values, Figure 5. To keep the number of variants as low as possible, the high pressure area of the pump was limited to the compact cylinderhead, Figure 6. This has advantages for the management of pump variants as well as the high pressure capability and the pump weight. Therefore it is possible to use aluminium for the pump housing. This leads beside the already mentioned design features to further weight reduc-
Figure 5: Efficiency of Bosch high pressure pump CP4 compared to conventional pump
value that the number of pumping events match the number of engine working cycles. With its high efficiency the CP4 reaches high pressure levels at low engine speeds which allows the use of new combustion concepts, e. g. oCCS. The described features of the CP4 support emission quality, fuel consumption and engine smoothness [4]. For the use of e. g. US-fuel it is possible to qualify the CP4 with already proved additional measures.
4 Piezo Inline Injectors
Figure 6: Cylinderhead of Bosch high pressure pump CP4
Figure 7: Weightpressure ratio of different Common Rail pumps
tion. E. g. a CP with one high pressure element has only 60 % of the weight of a well known CP1H. For clarification Figure 7 shows a comparison of the specific weight (normed to 1000 bar pressure) of different pumps available at the market. The modular (one or two plunger) layout of the CP4 allows delivery quantities
which up to now are only reached by large three plunger pumps. The performance of a fuel injection system depends also from the metering accuracy. Due to its physical principle the CP4 has big advantages. It is possible to reach injection synchronous delivery with setting the pump/engine ratio to a
The realization of an optimal mixture formation to reach above mentioned emission- and CO2-targets is significantly influenced by the injection quality of the fuel injector. Therefore all Common Rail injectors have the same basic functional groups: – nozzle (the interface between fuel injection system and combustion chamber) – servo valve – electromechanical actuator. During realization of this functional groups certain disturbances occur, e. g. unwanted leakage, friction due to shear forces or small stiffness. With increasing injection pressure requirements to tightness and strength, e. g. at critical bore intersections, are also increasing. During further development of the third generation Bosch Common Rail injector all this requirements were taken into account. Without increase of weight or installation space a piezo injector was developed which reaches for the first time for passenger car applications 2000 bar injection pressure. Based on the basic layout, Figure 8, of the Bosch piezo injector family the piezo injector was successive qualified for higher pressures since market introduction in 2003. This basic layout consists of injector body, nozzle and a so called actuator chain consisting of control valve, hydraulic coupler and actor module. This actuator chain can be used for all customers. Therefore the variance is limited basically to injector body, plug and nozzle definition. Advantages of the above mentioned basic layout are a high overall stiffness within the actuator chain and the avoidMTZ 10I2008 Volume 69
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The key element of this development are smallest spray holes approximately 100 μm in diameter which are manufactured with high accuracy concerning position and diameter but also surface roughness. Therefore Bosch developed an innovative machining technique which ensures with an optimised eroding process the completion of the high requirements. Together with an optimized process for hydro erosive machining of spray hole surfaces it is now possible to produce nozzles which allow high specific engine power with very good emission values in part and high load [2, 3]. Figure 8: Basic layout of Bosch piezo inline injector CRI3
6 Rail Development Focuses ance of mechanical forces on the nozzle needle. These forces could occur in injectors with pressure pin which could lead to higher wear. With this basic layout moving masses and friction were significantly reduced, which has a positive impact on injector stability and drift. Summarizing the piezo inline design is characterised through: – reduction of seat throttling losses due to very high needle dynamics – up to 8 injections per combustion cycle – stable small quantity ability over rail pressure, means high quantity accuracy with smallest injection quantities – optimised bore intersections to increase the high pressure stability – minimisation of pressure losses – precise and reproducible injection quantities over lifetime within whole engine map incl. compensation functions from ECU. The key issues to reach the targets, mainly the significantly increased injection pressure were identified with a functional analysis, Figure 9. Due to the very detailed knowledge of load and load capacity of the single injector elements the Bosch engineers achieved their target to stretch the limits of the injector design. Together with optimised and further developed manufacturing processes this design was successfully transferred to series production. From the start of series production a very good quality level was achieved. Until today Bosch manufactured over 20 million piezo injectors. 30
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5 Nozzle Development The nozzle is the interface between combustion chamber and fuel injection system. Therefore the spray preparation through the nozzle is very important today as well as for the future. For modern fuel injection systems Bosch developed a nozzle concept with the following advantages: – high hydraulic efficiency at high injection pressure – good robustness concerning nozzle cooking – small spread of functional geometries, e. g. tolerances of elevation angle of the nozzle holes was reduced with factor 2.
For the 2000 bar system a laser welded rail as well as a hot forged rail with the necessary and for the high system pressure adapted components pressure sensor and pressure regulation valve is available. The welded rail has the possibility to use standardised parts whereas the forged rail has the higher potential for weight reduction.
7 System Functions Bosch as system supplier has beside the components for high pressure hydraulics also developed system functions for its Common Rail System. With these system functions a further optimisation of
Figure 9: Focus of development for pressure enhancement of Bosch 2000 bar piezo inline injector
the potential to increase pressures. The target is to reach pressures above 2000 bar with compatible installation space and without additional weight, Figure 11. Beside the pressure increase improvements in dynamic and accuracy of quantity as well as spray preparation have to be made.
9 Summary
Figure 10: Overview system functions of Bosch Common Rail System
injection- and air system is possible. Such system functions recognize very early possible deviations and are able to minimize them, Figure 10. So called learning functions are very important for the system. They allow the best metering accuracy in new conditions as well as over lifetime. This functions often need no additional sensors which reduces cost and increases also the robustness of the system. One example is the ZFC (zero fuel calibration) which was developed 2003 from Bosch [1] and has proven itself since then. This function identifies and corrects without additional sensors continuous during driving conditions the injection quantities at pre-injection conditions.
8 Outlook To reach further emission and fuel consumption targets the EGR-rate and charging pressure will increase. With downsizing concepts the emission relevant operating ranges will be shifted to higher loads. Not only for high performance engines the trend to higher injection pressures will continue. Therefore Bosch will offer in the future passenger car Common Rail Systems with pressures above 2000 bar. The best preconditions are given with the piezo injector which is already in series production with 2000 bar. The high pressure pump CP4 presented in this article and the hot forged rail have also
Modern combustion processes request higher requirements from the fuel injection system. Important is the optimization of the injection into the combustion chamber in terms of position, quantity accuracy and precision. To make use of the full potential it is necessary to increase injection pressure during part load of the engine. Bosch has developed a fuel injection system which is marked by very accurate multiple injections with short dwell times in the whole engine map. Therefore the piezo injector proved at pressures of 1800 bar with millions of pieces in the field was further developed up to a system pressure of 2000 bar. With the new developed and efficiency and weight optimised high pressure pump CP4 the requirement of higher pressures during part load can be fulfilled. All components of the presented 2000 bar Common Rail System from Bosch have the potential for further pressure increases.
References [1] Kampmann, S.; Dohle, U.; Hammer, J.; Boecking, F: Common Rail Systeme zur Erreichung künftiger EUEmissionsstandards. 27. Internationales Wiener Motorensymposium 2005 [2] Mattes, W.; Nefischer, P.; Praschak, N.; Steinparzer, F.: Neuer zweistufig aufgeladener 4-Zylinder Dieselmotor von BMW. 16. Aachener Kolloquium Fahrzeug- und Motorentechnik, 2007 [3] Bauder, R.; Schiffgens, H.-J.; Streng, C.; Zitzler, G: Weiterentwicklungspotenziale des Audi V6 TDIMotors. 7. Internationales Stuttgarter Symposium 2007 [4] Hadler, J.; Rudolph, F.; Dorenkamp, R.; Stehr, H.; Hilzendeger, J.; Kranzusch, S.: Der neue 2,0-l-TDIMotor von Volkswagen für niedrigste Abgasgrenzwerte – Teil1. In: MTZ 69 (2008) Nr. 5
Figure 11: Roadmap Bosch Common Rail System for passenger cars and light duty vehicles MTZ 10I2008 Volume 69
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