DEVELOPMENT You will find

systems theInjection figures mentioned in this article in the German issue of MTZ 9/2002 beginning on page 696.

Neues Common-Rail-Einspritzsystem mit Piezo-Aktorik für Pkw-Dieselmotoren

New Common Rail Injection System with Piezo Actuation for Diesel Passenger Cars

As the market shares of diesel engines continue to grow, it is reasonable to assume that the diesel is indeed becoming the passenger car drive technology of the future. It continues to offer potential for optimisation with regard to fuel consumption, performance, torque and emissions. Injection technology and hence an even better control of the combustion process will have to make a particular contribution to this. Siemens VDO Automotive is helping to pave the way with its newly developed Piezo Common Rail System (PCR), heralding improvement in the critical area of particulate emission.

1 Introduction

By Klaus Egger, Johann Warga and Wendelin Klügl

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When the first generation of common rail systems for passenger cars arrived on the market from 1996 onwards, the situation for developing the combustion process changed. Since the late 1980s, the focus had been on increasing injection pressures via direct injection technology. Nowadays, common rail systems offer fuel at a permanent pressure of typically 1,200 to 1,500 bar. No doubt these pressures will continue to

increase. Yet, the present focus is also on making optimum use of this pressure potential. This includes improving the engine’s performance parameters, emissions and fuel economy without interfering with the smooth running of the engine. The latter is important, as high mean effective pressures and steep slopes of the injection rates are quite audible. Among the suitable measures to help this situation is pilot injection prior to the main injection, which results in a smoother pressure build-up in

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the cylinder and therefore less noise emission. The continuing increase in injection pressure plus even tougher emission legislation (EURO IV from 2005, EURO V from 2008) cause development engineers to move further along on this technological path of specifically optimising the combustion process with further injections before and after the main injection, Figure 1. This is particularly true for the lower and middle engine speed range, where flexibility is very important. It becomes apparent that the optimum would be up to five injections in the range below 2,500 rpm, double or triple injections in the middle speed range and single injections at rated speed. Present injection systems do not offer enough flexibility for such modification and refinement of the injection process for each operating condition of the performance map. This was the starting point for the development work of Siemens VDO. PCR technology replaces mechanical injection valves with mechatronic piezo actuation, allowing highly flexible multiple injections. 2 The Task for Development

Until the end of 2000, solenoid valves, used for starting and ending the injection process, were state of the art in common rail systems. However, due to the induction of the coil, solenoid valves have a rather long response time. Hence, injection systems with a promising future need to have more potential: ■ If injection pressures continue to increase, multiple injections will be needed to shape the combustion process constantly, causing a smooth pressure build-up (pressure and temperature in the combustion chamber) while lowering noise emission. Additional post injections will be necessary in order to burn soot particulates better. In any case, soot breakdown within the engine is a sensible development goal. ■ Exhaust gas treatment via a catalytic converter and increasingly also via a particulate filter has further implications. In order to regenerate both systems, additional fuel needs to be injected after the main injection to raise the temperature in the converter and/or the filter. Again, this applies particularly to the lower and middle speed range because otherwise there is no way of making sure that the particulate filter reaches its necessary temperature level to guarantee cyclical regeneration under all operating situations. ■ With regard to emission legislation and the ever higher demands for comfort, the metering accuracy of the injection has to be improved.

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All in all, the demand for higher speed and better precision of not just one but several injections threatens to become a bottleneck for further develop-ment. 3 Changeover to Piezo Technology

Siemens AG started very early to work on an injection system with piezo actuation. Experiments using the piezo effect were first carried out in 1980. By the mid-1990s, Siemens VDO in Regensburg had the first common rail systems with piezo injection running on the engine test beds. The results were highly encouraging. From the beginning, it was crucial to achieve a lower combustion noise and to fulfil future emissions legislation by exploiting the excellent metering accuracy of the injectors. A comprehensive evaluation of solenoid and piezo actuation plus that of various valve types have resulted in the present injector design, Figure 2. However, the technology of manufacturing piezo ceramics had not reached the necessary level for mass production at that time. Furthermore, the demand had not yet been driven by the market situation and by politics. Once common rail technology arose however, piezo actuation became relevant instantly. Positive engine test results caused Siemens VDO to enter the diesel market with the innovative piezo actuation, after having carried out a corresponding market study. Series development commenced in 1996. The first start of production took place on time in late 2000. New manufacturing sites were built, for example the injector production plant in Limbach-Oberfrohna (Saxony). 4 The Inverse Piezo Effect

In principle, piezo technology is not new in cars. The buzzer that reminds the driver to switch off the headlights is a typical application. The discovery of the basic principle goes back to the Curie brothers in 1880. They found that certain crystals produce a voltage when they are submitted to pressure or an impact, because, when under pressure, charged atoms were displaced in the polar crystals with a partly ionic bond. Hence, the Curie brothers christened their discovery the piezo effect, after the Greek verb “piezein” (to press). It was only in 1881 that the scientists realised that the effect also works the other way round. If a voltage is applied to a suitable crystal, it will show a distortion of the crystal lattice, which is nothing less than a linear movement. This inverse piezo effect provides the technical basis for the PCR system.

5 The Piezo Actuator

The crystal lattice of a piezo element shows such a small distortion that using it as an actuator is a truly great challenge. The experience with thin-film technology that Siemens Corporate Development had amassed from the 1970s onwards was important for developing the PCR actuator. In order to achieve sufficient distortion (lift) of the actuator, numerous thin ceramic layers are sintered to a single block. Due to this structure, the actuator with a length of 30 mm consists of more than 300 layers of 80 μm each, Figure 3. This multi-layer element is mounted in the injector head, safely contained in a pre-assembled actuator module and optimised for automotive applications (i.e. temperatures of between -40 ° Celsius and +140 ° Celsius plus strong vibrations). It produces a lift of 40 μm. Over the several years of development, a special ceramic mixture for the actuator has been formulated, which solves one problem in particular: high temperatures cause a polarisation of the actuator’s crystal lattice, thereby reducing the distortion of the piezo element, which in turn diminishes the work lift. For exactly this reason, the injector application necessitates a ceramic mix with a high Curie point. Unfortunately, ceramics with that property show only a weak piezo effect. The actuator now used is a multi-layer PZT ceramic material. The ceramic material consists of doped lead-zirconate-titanate. The electrodes, which are installed during the sintering process, are made of a silverpalladium alloy. Developing this piezo element required an inter-disciplinary combination of chemical, electrical engineering and physical expertise. Achieving a tightly controllable manufacturing process which avoids possible diffusion of the contacts between the individual ceramic films was a further challenge. 6 Specific Advantages of Piezo Technology

Compared to solenoid valves, the piezo actuator’s extremely fast reaction time is its most outstanding characteristic. Since piezo ceramics are mechatronic elements, they show the same behaviour as a multilayer ceramic capacitor, or that of a capacitance which is immediately charged under voltage. The distortion of the crystal lattice occurs within 0.1 ms. That is faster than any other applicable physical effect known to date. Compared to solenoid valves, the piezo actuation in the injector has the following advantages: ■ the piezo actuator has practically no reaction time

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DEVELOPMENT

Injection Systems

■ it switches very rapidly and with high precision ■ it has excellent reproducibility ■ there are no design-related tolerances, e.g. air gaps etc. ■ the actuator shows a constant behaviour over its whole life cycle ■ the piezo module can be delivered as a pre-manufactured and tested module. There were special challenges during the development process: ■ temperature compensation of the different materials and packaging in a pre-assembled module ■ design to minimise energy absorption and to optimise energy recovery ■ the electronic control unit, control strategy and the safety concept had to be optimised for the piezo actuator.

1,000 bar and an engine speed of 500 rpm, is more than 86 % (more than 90 % at 1,000 rpm). The mechanical pre-feed pump is integrated into the pump as a vane cell pump. In order to achieve the best efficiency, the pump has a volume flow control valve. The pressure control valve is also mounted on the pump. This results in a very compact design. The modular design of the pump permits different combinations of these main modules, according to customer requirements. 7.3 Rail and Pipes

The rail is manufactured as a welded component with an appropriate pipe layout. The high-pressure sensor is also located on the rail, Figure 8. 7.4 Electronic Control Unit

7 The PCR System 7.1 Design and Operation of the Injector

The PCR injector consists of just over 30 individual parts in total, with the actuator module being treated as one part. Figure 4 shows a cross-section of the injector. The rail pressure occurs at the nozzle and in the plenum chamber via the feed aperture. When there is no voltage across the piezo element, the 2/2-valve is closed, the piston valve pushes the injector needle into the seat and the injector is closed. When a voltage is applied to the piezo element, it expands by about 40 μm. The 2/2 valve is opened by the small lever with a transmission ratio of 1:1.5. Consequently, the pressure in the plenum chamber drops, and the injector needle opens. The piezo element only absorbs charge when switched on, and emits charge again when switched off, resulting in energy recovery (see Section 7.4). The piezo actuator does not consume any energy between switching on and switching off, i.e. while kept in open status. Figure 5 shows typical injection rates with varying pilot injection positions. Even for large rail pressures, the minimum pilot quantity can be set with 1.5 mm3 per stroke. A minimum interval of approx. 100 microseconds can be set between pilot and main injection. Multiple injections in accordance with Figure 6 are applicable for the future system, in which case the individual quantities can merge. 7.2 Design and Mode of Operation of the High-Pressure Pump

The high-pressure pump is a radial piston pump with three cylinders, Figure 7. The volumetric efficiency of the pump, at a supply temperature of 40° C, a rail pressure of

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The control unit is based on existing control units for spark ignition engine controls, Figure 9. It is equipped with a 16-bit microcontroller and an 8-bit safety controller, as well as 4 MB Flash Memory. In addition, it has 8 analogue inputs, 6 digital inputs and 10 digital outputs, as well as a diagnostic interface and a CAN data bus. The end stage is essential for piezo actuators with energy recovery. No current flows during the dwell period of the open valve, Figure 10. 7.5 Functions and Software

The diesel engine control located in the control unit of the Piezo Common Rail System has a logical, hierarchical and modular design, Figure 11. The use of standard basic structures – such as the pi section, for example – guarantees excellent and efficient maintainability, as well as a standard procedure for calibrating the controls. At the top level, the control strategies are essentially divided according to the subsystems physically present on the engine. A torque structure is already adapted in the PCR control. With the torque structure, all requests coming from the driver and the drive train groups and almost all requests coming from the engine control are co-ordinated on a torque basis, before being converted into the fuel apportioning for the individual injections in the downstream module. The injection parameters, such as quantity proportioning, injection times and fuel pressure, are freely applicable within very broad limits. As the only module still working in parallel with the torque structure, the idle-running control in the PCR system undertakes not only the idle-running stabilisation, but also the function of starting the engine. With this design, Siemens VDO is able not only to offer a complete engine control and regulation strategy, but also to respond

flexibly to customer requirements and to adapt customer-specific strategies to the existing basic structure with relative ease. 8 Manufacturing

Siemens VDO has built a new production site for the production of the PCR system. The Limbach-Oberfrohna site near Chemnitz is gradually being expanded into a specialist centre for diesel technology. The mechanical characteristics of PCR injectors also meet extremely high requirements with regard to surface quality and fine geometry. The smallest injection hole diameters of 0.12 mm can be produced with a precise conical shape and with hydro-erosive rounding of the hole entrance, Figure 12. An automatic pairing machine, which measures all nozzles pneumatically, allocates the injector needle to the nozzle body, so that pairing clearances of approx. 2 μm can be achieved. Due to the pressure of 1,500 bar at which the common rail system operates, the nozzle body, needle and injector must be matched to each other within these small tolerances. The surface of the borehole in the nozzle body, for instance, is processed to an Rz of 0.6 μm. Processing of the blanks (impact extruded parts) is performed in a temperature-controlled workshop at 23 °C. Continuous tests by laser interference ensure that the fine geometry of the needle and nozzle is correct, so that the needle can move freely. The pressuresealed surfaces of the injector parts are fine-machined concavely or convexly, depending on the seal point. Inspection of the spray pattern, amongst other things, serves to confirm consistent production quality. Final assembly of the injectors is performed under dust-free conditions. Due to the extremely small tolerances plus the required functional safety, particulates from 50 μm could interfere with the operation. Particulates of 200 μm and over must not get into the injectors at all. Aspects of operation and safety generally dictate higher standards of cleanliness for the high-pressure components of common rail systems as compared to older injection systems. In addition to the final assembly of the injectors, the final testing is mostly automated as well. This creates the foundations for consistent quality. The high-pressure pumps are manufactured according to analogous quality criteria. 9 Current Status and Outlook

Considering its advantages, it is to be expected that piezo technology will supersede solenoid valves in common rail systems and potentially in other injection sys-

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Titanium

MATERIALS

tems as well. The beginning of a technological changeover has been rung in. Initially, the system replaces present technology in its functionality, which implies high-pressure direct injection with a single pilot injection. However, with the piezo application, there is now the option of beginning with map-controlled multiple injections. Common rail systems are not the only likely field of application. In principle, the new type of actuator with its minimal switching time lends itself to use in other injection technologies as well. A modulation of the valve lift could be one of the next steps in development in order to achieve an even more flexible control over the start, end and rate of the injection. At the same time, development work goes on to increase the injection pressure first to 1,600 bar and in the medium term to 1,800 bar with a view to further optimising the specific, engine capacity related performance. The number of injections per working cycle may well go up to five. The potential of piezo technology can equally well be used for gasoline direct injection. Again, initial tests are being carried out. Here, the piezo actuator’s fast-switching operation is valuable in particular for the complex combustion control in stratified-charge engines. 10 Conclusion

With the same weight and with the same dimensions, the PCR system offers higher performance data and better control of the combustion process. Piezo actuation offers a potential primarily for internally optimising the engine and improving particulate combustion. Considering that engine technology will have to continue to contribute the most to reducing consumption and emission, this is of particular relevance. Clearly, the PCR helps to further optimise the diesel engine’s politically and economically weakest spot. Furthermore, considered from a realistic viewpoint, the desired CO2 reduction implies a higher share of diesel cars world-wide. New approaches to the internal optimisation of the engine are a must in this light. In the case of PCR, they also help to improve the smooth running of this popular engine technology, thus further increasing its acceptance.

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