COVER STORY INJECTION

SOLENOID COMMON-RAIL INJECTOR FOR 1800 BAR The use of solenoid technology for common-rail injectors is well proven and used in millions of systems. The technology shows potential for further increase of injection pressure and optimization of multiple injection capability. Bosch presents the injector CRI2.5 for 1800 bar with a new innovative solenoid valve.

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AUTHORS

DR. ROLF LEONHARD

is Executive Vice President Engineering, Diesel Systems at Robert Bosch GmbH in Stuttgart (Germany).

JOHANN WARGA

is Senior Vice President Engineering Passenger Cars, Diesel Systems at Robert Bosch GmbH in Stuttgart (Germany).

DR. THOMAS PAUER

is Vice President Platform Engineering Common-rail Injector Passenger Cars and Engineering Nozzle, Diesel Systems at Robert Bosch GmbH in Stuttgart (Germany).

MARKUS RÜCKLE

is Director for the Development of Passenger Car Solenoid Injectors, Diesel Systems at Robert Bosch GmbH in Stuttgart (Germany).

DR. MATTHIAS SCHNELL

is Senior Project Manager for the Development of the CRI2.5 Solenoid Injector, Diesel Systems at Robert Bosch GmbH in Stuttgart (Germany).

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INTRODUCTION AND MOTIVATION

Since the introduction of direct-injection turbocharged engines, the Diesel engine has great popularity. This engine is characterized by the high torque resulting in good driving performance and low fuel consumption which allow for a high cruising range and low operating costs. The continuing development of fuel injection systems made a significant contribution to fulfill the emission requirements which have been tightened several times during the last years. Important steps have been the introduction of the distributor pump with 1000 bar about 20 years ago and the launch of the first common-rail systems with a system pressure of 1350 bar in 1997. The number of over 50 million Bosch common-rail systems produced demonstrates the success of the Diesel engine. The need to reduce CO2 emission is currently in focus and the Diesel engine is in a good position due to the favourable fuel consumption. But now new direct-injecting gasoline engines, hybrid concepts, electric drive with batteries or fuel cells are entering the market. Previously, the drivers for the development of fuel injection systems were based on emission reductions and specific power output. In addition to the above factors, increased demand regarding cost reduction is currently in focus. Bosch has consequently improved its solenoid injector technology and now presents a new generation of injectors to meet this challenge. Furthermore, this new injector generation is part of a sustainable modular concept. INJECTOR ROADMAP

Bosch solenoid injectors have been in series production since 1997 and have proven robustness in many different applications all over the world. The technique has been improved continually to meet the increasing demands of emission regulation and power output. The solenoid system CRS2.2 has been successfully in series production since 2003. Since the same year a 1600 bar injector with a piezo actuator has been available with an enhanced performance for challenging applications. The piezo system with injection pressure of 2000 bar has been produced since 2007 to comply with highest targets for specific power output [1-3]. The production of a piezo actuator is more complex and hence more expensive than a solenoid actuator. The increasing cost pressure was the motivation to start an advanced development of the solenoid injector technology towards higher injection pressure and enhanced multiple injection capability. The new injector types CRI2.5 for 1800 bar and CRI2.6 for 2000 bar extend the solenoid injector roadmap, . As a first step a basically new valve concept was added to the well established high pressure hydraulics of the CRI2.2 injector. The new valve of the 1800 bar injector CRI2.5 should meet the requirements of increased system pressure and enhanced multiple injections. The same valve can be used for the next pressure step towards 2000 bar (CRI2.6) and still has potential for a further increase of injection pressure. Thus, a modular injector concept is generated which can comply with a broad range of applications.

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COVER STORY INJECTION

 Solenoid injector development

DESIGN OF THE NEW SOLENOID VALVE INJECTOR

The basic hydraulic concept of the injector is a servo driven actuation of the nozzle needle as known from the CRI2.2 1600 bar injector. The relevant parameters of the high pressure hydraulics (A- and Zthrottle, guidance diameters) have been optimized for the 1800 bar application regarding injection rate and permanent leakage. The nozzle needle is operated in the fully ballistic range to achieve a linear fuelling map. The performance of a servo driven injector is mainly ruled by the control valve. Therefore, the main focus was put on the development of a new solenoid valve for the injector.  shows the cross

 New solenoid injector CRI2.5 for 1800 bar

section of the injector and a detail view of the valve area. The concept of the new valve has been chosen in the area of conflict between pressure increase and valve dynamics. Starting with a solenoid valve for 1350 or 1600 bar, an increase of system pressure can be achieved by reducing the valve seat diameter while maintaining the same actuating forces. Either, this results in a reduced valve opening area for the control of the high pressure hydraulics when using the same valve lift. On the other hand, the valve lift could be increased for compensation but at the expense of the valve dynamics. Both possibilities deteriorate the hydraulic properties of the injector regarding injection rate and multiple injection performance and hence, are not useful for future applications. For increase of system pressure and injection performance at the same time, a pressure-balanced valve was developed where the hydraulic pressure forces inside the valve seat diameter are compensated by a fixed element. Such a

valve has a three times larger valve opening area at the same valve lift. Therefore, it can be operated with a smaller valve lift than current valves, . The large valve opening area can be used for a layout of the high pressure hydraulics with bestsuited injection rates. The small valve lift allows for fast switching times and hence improved multiple injection capability. The number of injections over vehicle lifetime had increased continually during the last years. For optimized emissions and reduced noise, up to 3 pilot injections are used. The exhaust gas treatment needs up to 4 post injections. Therefore, the engineering target for the valve was increased by about 50 % to 1.5·109 actuation over lifetime. Furthermore, the valve has to be robust against different fuel qualities regarding lubricity and particle contamination. When opened, the valve spills the control quantity coming from the A-throttle into the injector backflow. Due to the pressure relief from system pressure to backflow pressure the fuel heats up by approx. 40 K per 1000 bar pressure difference. With increasing system pressure this effect leads to an increase of temperature in the valve area. Additionally, a pressurebalanced valve has a permanent leakage at the guidance of the valve which results in an additional heat input. High temperatures enhance the wear rate at the seat and had been taken into account when designing the valve. A pressure-balanced valve has another requirement, : the pressure force is fully compensated only when there is a line of contact at the seat. Due to the high contact pressure and wear, an alignment of armature and valve piece in the seat area has to be anticipated. Inside this ring surface, the system pressure can exert an opening force Fp on the armature. If this pressurized area increases due to seat

 Valve flow area for pressure balanced valve and ball valve

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wear, the opening force grows and the valve begins to leak at high system pressure. To avoid this effect, pressure-balanced valves usually get a seat limitation to restrict the maximum area of alignment. But if this chosen seat limitation is too small, the higher contact pressure can result in a higher wear rate and drift of valve lift. Using FEM simulation and systematic tests, an optimized geometry was found which remains leak-proof against system pressure and shows a low drift of valve lift over lifetime. 3 shows the design of the new solenoid valve. The valve seat is placed concentric above the A-throttle. The moving valve element (named armature) is designed in the form of a bushing and simultaneously serves as an armature for the magnetic circuit. The armature has a C-coating in the seat area for wear reduction. The inner bore of the armature is sealed by a cylindrical bolt, whose outer diameter is matched with a small clearance to the bore. This design with two parts allows for a high variability when configuring the geometry of the seat. This could be used, to enforce the armature in the seat area inside the guidance diameter. Thus, a precone is generated just ahead of the intrinsic seat edge. This new design helps for an easier coating and better adhesion of the coating. And it enhances the robustness of the seat against particles which is shown by the testing results. The above mentioned research for the valve seat shows that using a higher valve spring force is a definite advantage, not only for reducing the seat wear but simultaneously allows for increased sealing areas. For higher dynamics of the actuating valve, a surplus force of the magnet actuator is required.  describes the requirements. On the left hand side, the graph indicates the hydraulic opening force Fopen as a function of valve seat diameter dseat. The typical operating range of a ball valve is plotted for d = 0.5 … 0.75 mm and F > 100 N depending on system pressure. A pressure-balance valve based on its operating principle required a lower hydraulic opening force. In the right section of 4, the possible magnet force is plotted as a function of the armature diameter. The armature diameter mainly defines the pole area and hence is a key influencing parameter to determine the magnet force. The combination of a pres02I2010

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 Force Fp at seat due to pressure penetration, details seat area

 Surplus force ΔF and valve concept

sure balanced valve together with a bigger armature diameter allows for the maximum surplus force ΔF. Besides the advantages in valve dynamics, a surplus magnetic force also helps in increasing robust-

ness in actuating function due to other obstructing factors, e.g.: particles, bad fuel causing deposits on moving surfaces In order to utilize the potential of the concept completely, a detailed parameter

 Optimization of the magnetic circuit: the CRI2.5 design (red) was chosen out of 14.000 variants

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COVER STORY INJECTION

 Normalized valve lift

 Injection rate

the first criteria for evaluation of the valve dynamics. In the figure values of around 14,000 simulated variants are shown (blue dots). Out of these, the gray dots were selected for complete simulation of the valve dynamics considering two important criteria: minimal value and minimum gradient of magnet force. For the selected variants the calculated course of force over time is shown in 5, wherein the simulation model was combined with the complete electric circuit including supply and output stages in the control unit. From the evaluated variants, the variant marked in red was selected for CRI2.5 which shows a good valve dynamics together with the maximum magnet force.

RESULTS  indicates the normalized lift of the

 Fuelling map CRI2.5

study was conducted for optimizing the solenoid actuator. By computer simulations the coil parameters (coil diameter, number of windings) as well as geometric parameters of the armature, magnet core and coil were varied. The installation space was an

 Results from testing for reducing the wear rate

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important boundary condition which depends on the position of solenoid on the injector body. The insert in  describes the magnetic force over the effective death time t11 (begin activation – begin valve movement) being

actuating valve over time. The CRI2.5 valve technology shows a significantly higher dynamics in comparison to the valve used in CRI2.2 for 1600 bar. The closing time of the valve (defined as end of actuation to actual closing time) is approx. 115 μs at 1800 bar system pressure. The injection rate for the CRI2.5 at the 1800 bar full load test point can be seen in . In comparison to a CRI2.2 with 1600 bar, a clear advantage of higher pressure in the region of maximum injection rate can be seen with CRI2.5. The decrease of the injection rate during closing of the needle is also significantly faster. Hence, the injection rate map is comparable to that of an 1800 bar CRI3.2 piezo injector.

 Valve seat before and after durability test with particle contamination; left hand: new seat design; right hand: valve seat without measures

 shows a typical injection map of CRI2.5. The injection quantities are plotted over energising time for different system pressures. The linearity up to higher injection quantities is very good. For 1800 bar the measured valve closing time is plotted additionally which confirms the fast turnoff time of 115 μs. The new seat geometry is optimized for low wear based on extensive testing and engineering. Previous variants of the seat geometry showed seat wear higher than the permissible limits despite of robust measures with wear resistant coating, . A detailed analysis of the mechanism causing wear helped us to identify the influencing variables. Secondly the variables were quantified to help in design of reduced seat wear geometry. The actual design for series production shows a seat wear rate which is well below the design target, 9 (right view). Particles in the fuel can damage the seat edges of the pressure balanced valve

and thus lead to leakage. The new valve is designed with a precone which significantly increases the mechanical stability at the seat area. This has been validated in a special endurance test wherein a high number of metallic particles with critical diameters were introduced at the inlet of the injectors. In the left view of  it can be seen that the seat with actual geometric design is almost free of damages at the end of this test. The actuating and sealing function is proved without limitations. Without this precone as seen in right view of : the particle contamination results in massive damage at the seat edge leading to unwanted leakage of the valve. The new 1800 bar solenoid valve injector is comparable to the piezo injector in view of injection performance with respect to emission and specific power output targets. This has been demonstrated in an actual four-cylinder engine with approx. 0.5 l cc/cylinder. Emission results for a

typical part load test point are shown in . By keeping the noise levels constant the fuel consumption and soot/NOx tradeoff is comparable for both 1800 bar solenoid valve and piezo actuated systems. A favourable tendence for HC and CO emissions can be seen for the solenoid injector. These results show that the injection rate shape at part load solenoid injector has positive influence on the HC and CO emissions, which will become more critical when lowering NOx emission levels. At full load the 1800 bar solenoid injector meets the same power targets as that of an 1800 bar piezo injector. SUMMARY

The new 1800 bar solenoid injector combines reliable solenoid valve technology with a new innovative solenoid valve. This new design of a pressure balanced valve allows us to achieve extreme low rate of wear, enabling a robust valve function. The performance of this solenoid injector system design is comparable with a piezo actuated system. This valve concept has potential for further increased system pressures and thus is a solid basis in the Bosch solenoid valve injector roadmap for future generations. REFERENCES

Emission test results part load

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[1] Leonhard, R.; Warga, J.; Pauer, T.; Boecking, F.; Straub, D.: 2000 bar Common-Rail-System von Bosch für Pkw und leichte Nutzfahrzeuge, 29. Wiener Motorensymposium. VDI Verlag, Düsseldorf, 2008 [2] Dohle, U.; Leonhard, R.: Diesel Technology Development Trends, 8. Internationales Stuttgarter Symposium. Stuttgart, 2008 [3] Leonhard, R.; Warga, J.: Common-Rail-System von Bosch mit 2000 bar Einspritzdruck für Pkw. In: MTZ 10/2008, Jahrgang 69

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