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New Generation Bosch Gasoline Direct-injection Systems Due to future exhaust gas emission legislations, vehicle manufacturers need to implement additional emissions reductions. Based on three factors – market development, legislation and end customers – Bosch has developed a new generation of gasoline direct-injection systems. According to the engineers, approximately 80 % lower PN emissions can be achieved under RDE conditions.
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© Bosch
A UT H O R S
Dr. Thomas Pauer is Executive Vice President, Division Gasoline Systems, and responsible for Engineering and Combustion Engines at Robert Bosch GmbH in Schwieberdingen (Germany).
Hakan Yilmaz, M. Sc. is Vice President System Engineering Powertrain, Division Gasoline Systems at Robert Bosch GmbH in Schwieberdingen (Germany).
REQUIREMENTS ON NEW GASOLINE DI SYSTEMS
Today, gasoline engines are the preferred solution for passenger car powertrains due to favourable unit costs, high power density, potential for low exhaust gas emissions and high robustness against worldwide fuels. Compared to 70 million gasoline engine cars in 2015, 78 million gasoline engine powered vehicles are forecasted for 2025, FIGURE 1. While gasoline engine volumes with port fuel injection (PFI) are expected to decline, gasoline DI engines are predicted to double during this time frame. Consequently, an estimated 39 million PFI engines as well as 39 million DI engines can be expected in 2025. Due to future exhaust gas emission legislations, vehicle manufacturers need to implement additional reductions of
Dr. Joachim Zumbrägel is Vice President Engineering G asoline Direct Injection of the Business Unit Direct Injection (DI), Division Gasoline Systems, at Robert Bosch GmbH in Schwieberdingen (Germany).
gaseous and particulate emissions (particulate mass PM and particulate number PN). Exceptionally stringent requirements regarding gaseous emissions are demanded by the CARB LEVIII legislation, FIGURE 2. The reduction of fleet limits for NMOG and NOx given by this legislation requires the fulfilment of SULEV 30 limits as a fleet average. Other important automotive markets, for example China (CN 6a/b and Beijing 6) or India (BS 6) are converging quickly to the European (Euro 6d) and US (LEV III/Tier 3) legislations respectively. Furthermore, Europe, China and India will ask for testing the real driving emissions (RDE) during vehicle operation on public roads. Likewise, requirements on the manufacturers regarding useful life and evaporative emissions (EVAP) will become more severe. Worldwide CO2 legislations are calling for further reduction of tank-to-
Dr. Erik Schünemann is Department Manager Engineering Combustion Systems, Division Gasoline Systems, at Robert Bosch GmbH in Schwieberdingen (Germany).
wheel CO2 emissions. End customers expect functional benefits with every new car, for example with respect to engine and driving performance, fuel consumption and comfort or NVH. Gasoline engines with direct-injection and turbocharging can make a contribution to the fulfilment of these demands by the manufacturers. Direct-injection is permitting both high specific power and an increased compression ratio. It provides potential for optimised catalyst heating strategies that can compensate for the additional heat sink given by the turbine. Today’s gasoline DI engines predominantly apply multihole injectors with solenoid actuation that are installed into the cylinder head in side-mounted or central-mounted position and operate with system pressures of up to 25 MPa. Using laser drilling of fuel holes is allowing for large flexibility with respect to spray lay-
FIGURE 1 Market development of p assenger cars and light-duty vehicle powertrains (© Bosch)
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FIGURE 2 Exhaust gas emission legislation in the US (LEV III), Europe, India and China (top); LEV III legislation and fleet limits (bottom) (© Bosch)
out. Central-mounted injector position close to the spark plug enables highly efficient catalyst heating operation using the injection of a small fuel quantity close to the ignition for fastest catalyst light-off and reduced exhaust gas emissions [1]. For this purpose, in addition to both appropriate spray layout and respective injection strategy, a software functionality is needed (Bosch CVO: Controlled Valve Operation) that guarantees stable injector operation with a small fuel quantity injection inside the ballistic area of the characteristic line over lifetime [2]. Incomplete fuel-mixture preparation of liquid fuel films formed at the combustion chamber walls during injection is the major source of particulate formation inside gasoline DI engines [3]. Optimising the spray layout in combination with the charge motion of the engine as well as using multi-injection strategies can already reduce piston and cylinder liner wetting considerably and hence, lower particulate emissions. Further optimisation of fuel metering and fuel-mixture preparation by an advanced DI fuel system can make a strong contribution to additional substantial improvements regarding particulate emissions [4].
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According to the requirements of market development, worldwide legislation and end customers, the following development guidelines have been set for the new generation of Bosch DI systems: –– optimise fuel-mixture formation for potential of lowered particulate
FIGURE 3 Variant frame and NVH performance of the high-pressure pump HDP6 (© Bosch)
emissions by increasing DI system pressure to 35 MPa together with improved NVH performance –– extend injector operation in combination with CVO software functionality –– advance multi-injection capabilities by reduced pause time
–– improve injector tip geometry for contribution to reduced particulate emissions due to injector tip wetting –– provide compact system design, high modularity and flexibility for compliance with customer specific requirements –– worldwide utilisability with respect to EVAP, useful life and fuel qualities. In the following, the technical solutions on component level including software functionalities are described and the functional results realised on system level are presented. SYSTEM PRESSURE GENERATION WITH HIGH-PRESSURE PUMP HDP6
Increasing the DI system pressure to 35 MPa required the development of a new generation of high-pressure pumps, FIGURE 3. Bosch high-pressure pump HDP6 offers an enhanced variant frame with higher flexibility regarding fuel connections, angular positions and installation on the engine. The modular concept permits a choice of two piston diameters. While the small piston diameter variant minimises required driving power at volume flows of up to 0.9 ccm/rev camshaft, the large piston diameter variant allows volume flows of up to 1.3 ccm/rev camshaft. The NVH performance of the new HDP6 under idle conditions represents “best in class”.
FIGURE 4 Variant frame of the multihole injector HDEV6 (© Bosch)
VARIANT FRAME OF MULTIHOLE INJECTOR HDEV6
The new HDEV6 is designed for DI system pressures of up to 35 MPa. A highly flexible variant frame, FIGURE 4, comprises amongst others short and long injector versions, different electrical connectors as well as antirotating fixations and permits an installation into sidemounted and central-mounted position. A compact injector design with an injec-
tor tip diameter having been decreased from 7.5 to 6.0 mm and an O-ring diameter of the fuel connection which has been lowered from 11.6 to 9.4 mm is reducing required installation space. Injector weight has been cut by 20 %. HDEV6 FUEL METERING AND SOFTWARE FUNCTIONALITIES
The HDEV6 qdyn characteristic line shown in FIGURE 5, exhibits a mono
FIGURE 5 HDEV6 qdyn characteristic line with ballistic area and transition area (left); reduction of HDEV6 qdyn tolerance using Bosch CVO3 and Qstat-adaptation (right) (© Bosch)
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FIGURE 6 PN sources at gasoline DI engines in homogeneous operation (Lambda_global = 1) (© Bosch)
tonically increasing dependence of injected fuel mass qdyn on the injector actuation time ti over the complete operating range. Bosch CVO software functionality was further improved for operation with HDEV6 and is in the 3rd generation (CVO3) now working over the complete injector operating range (ballistic area, transition area, linear area), FIGURE 5. CVO3 reduces deviations of the actual injected fuel mass from the tar-
get value over the complete injector operating area. This feature significantly contributes to fulfilment of future useful life requirements. Additional improvements can be expected by a software functionality for the adaptation of static flow addressing relevant injector parameters, which is currently under devel opment (SOP 2018). HDEV6’s multiinjection capability was considerably
improved by internal injector measures, thereby providing injector pause times smaller than 1 ms. The reduction of valve seat leakage over injector lifetime is an important target for further improvement regarding evaporative emissions (EVAP). HDEV6 was further optimised with respect to the relevant valve seat properties and provides an improved valve seat leakage after durability run, enabling a robust
FIGURE 7 Particulate formation at the DI injector tip and opti misation measure as well as injector tip-wetting and stationary PN emission after optimisation compared to baseline injector and clean level (© Bosch)
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compliance with the specified valve seat leakage target of 1.5 mm³/min at 10 MPa. MIXTURE PREPARATION AND REDUCTION OF PARTICULATE EMISSIONS
Soot particle formation inside combustion engines can occur if both high temperatures and very rich conditions are present simultaneously during the combustion process. For gasoline directinjection and premixed gasoline engine combustion with globally stoichiometric mixture, this can appear, if due to the interaction of liquid fuel and combustion chamber walls a fuel wall film is formed. Regular stoichiometric premixed combustion is largely finished, when the flame front reaches the respective combustion chamber walls. Then, both high gas temperature and low oxygen content are existent. Soot particles are formed if the aforementioned fuel wall film is not evaporated and homogenised at the time when the flame front arrives at the affected combustion chamber wall. Particulate formation can be visualised easily by detection of the typical soot luminance using optical measurement techniques. Particulate formation due to wetting of intake or exhaust valves, combustion chamber roof and spark plug can be avoided by positioning DI injector and spark plug accordingly and by a thoroughly designed spray of the injector. Although contact of liquid fuel with piston and cylinder liner can be minimised by proper spray layout and injection strategy, typically it cannot be completely avoided under all boundary conditions.
Consequently, particles are formed especially during accelerations under cold engine conditions as well as with a warm engine, FIGURE 6. Here, the particulate formation at the injector tip itself is contributing to the overall particulate emission level. Due to injector internal fuel flow, spray breakup and interaction of fuel spray and fuel-hole/pre-hole as well as interaction with the surrounding air inside the cylinder during the injection process, fluctuations of the spray jets in radial direction can occur. These fluctuations can lead to a liquid fuel film build-up at the injector tip. At the arrival of the regular flame front with high gas temperatures and low oxygen content, remaining fuel at the injector tip can form soot particles. A portion of these soot particles remains at the injector tip and can lead to deposit formation. These deposits can absorb more liquid fuel and consequently are self-reinforcing the process while strongly increasing PN emissions (“PN drift”) until a balance of deposit formation and deposit reduction mechanisms is realised at the injector tip (“stabilised injector”). Therefore, minimising injector tip-wetting is one of possible measures to reduce PN drift. Internal injector flow and geometry of the injector tip can be improved in a way that injector tip-wetting is reduced considerably, resulting in a significantly increased robustness against deposit formation, FIGURE 7. For this purpose, several development tools have been used intensively, for example CFD simulation, near-field and far-field spray measurements with high temporal and spacial resolution and engine tests. By application of the optimisation measures, PN
emissions resulting from the injector tip are lowered considerably. FIGURE 7 depicts the current status compared with baseline injector and clean injector tip free of deposits at 20 MPa. A further increase of the DI system pressure to 35 MPa, which is feasible with HDEV6, results in an additional improvement of robustness against PN drift [1]. Furthermore, an injection pressure increase results in a SMD (Sauter Mean Diameter) reduction by improved primary breakup, leading to a raising liquid surface area provided for evaporation of the injected fuel, FIGURE 8 (left). At the same time, the entrainment of ambient air into the spray is elevated significantly (airentrainment), FIGURE 8 (middle). Thus, enthalpy supporting fuel evaporation is provided. Consequently, the fuel droplets are following the air-motion much better, resulting in a strong decrease of liquid fuel film at the respective piston and cylinder liner area that is responsible for particulate formation. The mentioned effect of increased injection pressure on fuel wall film formation is shown by fundamental investigations, FIGURE 8 (right) [5]. By combining all hardware measures with an advanced calibration, PN emissions under RDE conditions – and for engine start temperatures and ambient temperatures of 20 °C – can be reduced by about 80 %, FIGURE 9. Due to delayed fuel evaporation because of cold combustion chamber walls and low in-cylinder gas temperatures, low temperature conditions present a particular challenge regarding PN emissions. Here, in addition to the available engine internal measures and an advanced calibration as well as quickest readiness of the
FIGURE 8 Influence of DI system pressure on SMD, air-entrainment and fuel film build-up (© Bosch)
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FIGURE 9 Status of engine internal PN reduction measures in the RTS aggressive cycle at 20 °C and status of PN emission reduction in Bosch internal RDE cycle at -7 °C (© Bosch)
lambda control, a gasoline particulate filter (GPF) is necessary to support manufacturers in achieving the future RDE In order to maximise the functional benefits of the DI injector, an optimal integration into the customer-specific engine concept and combustion system is necessary. Therefore, an overarching project organisation has been established within Bosch Gasoline Systems. A combined application of development tools for injector, spray and combustion system reaching from component level to vehicle level, FIGURE 10, as well as a close cooperation with the customer starting in the early engine development phase are crucial preconditions for achieving optimal overall results.
SUMMARY
Based on the requirements of market development, legislation and end customers, Bosch has developed a new generation of gasoline direct-injection systems. Essential features are the optimised mixture formation for lowered particulate emissions by increasing the DI system pressure to 35 MPa while at the same time improving the NVH performance, an enhanced injector operating range with narrowed tolerances using software functionalities CVO3 and Qstat-adaptation, improved multi-injection capabilities. An optimised injector tip geometry contributes to the PN emission reduction due to an injector tip-wetting. The new genera-
FIGURE 10 Development of injector, spray and gasoline combustion systems at Bosch (© Bosch)
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tion is providing compactness, high modularity and flexibility for the fulfilment of customer-specific requirements. It is designed for worldwide operation regarding EVAP, useful life and fuel qualities. Approximately 80 % lower PN emissions can be achieved under RDE con ditions by optimising DI injector and calibration. For particularly challenging low temperature conditions, in addition to internal engine measures, an advanced calibration and quickest readiness of the lambda control, a gasoline particulate filter (GPF) is necessary to support vehicle manufacturers in reaching the future RDE conformity factors of 1.0. To ensure optimal integration of Bosch DI injectors into the indi-
vidual engine concept and combustion system together with the customer, an overarching project organisation has been established within Bosch Gasoline Systems. This organisation is applying a range of Bosch development tools starting from the component to the vehicle. The new generation of gasoline direct-injection systems is currently launched into series production and will be rolled out into the worldwide production network step by step. The introduction is supported by implementation of an integrated Industry 4.0 value stream. REFERENCES [1] Kufferath, A.; Wiese, W.; Samenfink, W.; Dageförde, H.; Knorsch, T.; Jochmann, P.: Assessment of Feasible System Solutions for Future Par ticle Emission Requirements. Conference IMECHE – Fuel Systems for IC Engines, London, 2015 [2] Schlüter, R.; Kümpel, J.; Okuyama, H.: Mechatronic Component Packages within Gasoline Direct Injection Systems and their Impact on OEM-Supplier-Cooperation. 7 th IFAC Symposium on Advances in Automotive Control, Tokyo, 2013 [3] Kufferath, A.; Berns, S.; Hammer, J.; Busch, R.; Frank, M.; Storch, A.: EU 6 als Herausforderung für die Benzindirekteinspritzung – Eine Bewertung zukunftsfähiger Systemansätze. 33. International Vienna Motor Symposium, Vienna, 2012 [4] Wiese, W.; Kufferath, A.; Storch, A.; Rogler, P.: FIE requirements in GDI engines to meet Future emissions legislation. 2. International Engine Congress, Baden-Baden, 2015 [5] Kufferath, A.; Samenfink, W.; Hammer, J.; Schulz, F.; Könnig, M.; Schmidt, J.: Charak terisierung des Wandfilms relevanter Betriebs bedingungen für einen direkteinspritzenden Ottomotor als Grundlage zur Schadstoffmini mierung. Conference 10. Tagung Motorische Verbrennung, Munich, 2011
THANKS The authors would like to thank Dr.-Ing. Wolfram Wiese, Dr.-Ing. Philipp Rogler and Dr.-Ing. Martin Schmitt for their valuable support in the preparation of this publication. MTZ worldwide 07-08|2017
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