Common Rail - An Attractive Fuel Injection System for Passenger Car DI Diesel Engines

Transcription

1 SAE TECHNICAL PAPER SERIES Common Rail - An Attractive Fuel Injection System for Passenger Car DI Diesel Engines Gerhard Stumpp and Mario Ricco Robert Bosch GmbH Reprinted from: Fuel Spray Technology and Applications (SP-1132) SAE The Engineering Society For Advancing Mobility Land Sea Air and Space INTERNATIONAL International Congress & Exposition Detroit, Michigan February 26-29, Commonwealth Drive, Warrendale, PA U.S.A. Tel: (412) Fax:(412)

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3 Common Rail - An Attractive Fuel Injection System for Passenger Car DI Diesel Engines Copyright 1996 Society of Automotive Engineers, Inc. Gerhard Stumpp and Mario Ricco Robert Bosch GmbH ABSTRACT Passenger car DI Diesel engines need a flexible fuel injection system. Bosch develops a common rail system for this purpose. Besides variation of fuel quantity and start of injection, it permits to choosing freely injection pressure inthe rangeof 150 to 1400 barand injecting fuel in several portions. These new means will contribute to further improvements of DI engines concerning noise, exhaust emissions and engine torque. INTRODUCTION It has been a target for many years to achieve a flexible fuel injection system for DI-Diesel engines, permitting a free mapping of - start of injection - fuel quantity - injection pressure - rate of injection. Fig. 1: Common rail systems for passenger cars 183

4 The introduction into the market of the direct injection passenger car Diesel engines in 1988 intensified the demand for such a flexible fuel injection system. In 1978 Bosch reported in [1] about an electronically controlled accumulator injection system and its performance on a direct injection truck engine. The goals mentioned above had been reached to a large extent. This system reduced combustion noise and particulate emissions of the engine remarkably. The cost/benefit ratio, however, was unfavorable especially regarding the requirements at that time. In addition it appeared to be difficult to achieve the required safety and reliability. In 1991 a simpler Common Rail injection system was presented in [2]. This system operates with 1200 bar accumulator pressure and the accumulator pressure serves directly as injection pressure. In contrast to this approach, the system in [1] operates with an accumulator pressure of 200 bar and amplifies this pressure up to 2000 bar by means of a stepped piston in each injector. The system from [2] is simpler than that in [1]. In 1995 a further simplified concept of a Common Rail system was described [3]. In this system 2-way solenoid valves control the fuel injection whereas the systems [1] and [2] are operated by more complicated 3-way valves. This paper reports about the development of the 2-way solenoid valve concept towards a high volume production fuel injection system. Results are shown. DESIGN OF THE SYSTEM Figure 1 presents the system schematically and Figure 2 shows the system components. A supply pump draws fuel from the tank and feeds it to the high pressure pump. The high pressure pump is driven by the combustion engine and delivers fuel via the rail to the injectors of the engine cylinders. One part of this fuel is injected into the combustion chambers of the engine, a smaller part controls the injection nozzles of the injectors and then flows back to the tank. The fuel volume between the high pressure pump and the injectors serves as an accumulator. The fuel is compressible and dampens oscillations initiated by the pulsating delivery of the high pressure pump and especially by the abrupt extraction of fuel via the injectors. A pressure sensor measures the fuel pressure in the rail. Its signal is compared to a desired value stored in the ECU. If the measured value and the desired value are different, an overflow orifice in the pressure regulator on the high pressure side is opened or closed. The overflow fuel returns to the tank. The injectors are opened and closed by the ECU at defined times. The duration of injection, the fuel pressure in the rail, and the flow area of the injector determine the injected fuel quantity. Figure 2 shows the pump, the rail, the injectors, and the ECU. Fig. 2: Common rail system for passenger cars 184

5 COMPONENTS HIGH PRESSURE PUMP -The high pressure pump is designed as a radial piston pump (Fig. 3). An eccentric on its drive shaft displaces 3 pistons in succession. The pistons are held to the eccentric by springs and each piston draws fuel via a corresponding inlet valve. The drawn fuel is delivered via a check valve to the rail when the piston is lifted. The small eccentricity of the drive and the symmetric arrangement of the pistons contribute to a small pressure ripple in the rail. The inlet valve of one piston can be opened by a solenoid. In this way the delivered fuel quantity of the high pressure pump can be adapted to the demand. This results in low power absorption of the fuel injection system and in cool fuel. A safety valve is located in the fuel feed of the high pressure pump. The piston of this valve closes an orifice in the inlet to the high pressure pump when there is low fuel pressure. With a higher fuel pressure the orifice is open. By means of this valve the fuel flow to the engine can be shut down redundantly when the feed pump is switched off. Fig. 3: High pressure pump INJECTOR - The injector (Fig. 4) consists of: - a multi-hole-nozzle with a spring, pressing the nozzle needle to its sealing seat, - a control piston P - an orifice Z feeding fuel to the control piston, - an orifice A being opened or closed by a solenoid valve. Without fuel pressure on the spring side of the injector needle, the nozzle is opened at 45 bar. The deactivated solenoid valve closes the orifice A on top of the control piston. The fuel pressure from the rail works on the top side of the control piston via the throttle Z and on the bottom side of the smaller guide diameter of the injector needle. Therefore the fuel pressure exerts a force in addition to the nozzle spring, closing the nozzle. After the solenoid has been energized it opens the orifice A. As a consequence, the pressure to the control piston is reduced and the nozzle is opened. When the nozzle is completely open, the control piston almost covers the throttle A and thereby reduces the fuel quantity flowing to the tank. After the solenoid valve is deenergized, the pressure exerted on the control piston raises and the injector closes. 185 Fig. 4: Injector

6 ACCUMULATOR -The fuel trapped between the checkvalves in the high pressure pump and the injector nozzle seats works as an accumulator. The trapped volume should contain cm 3 fuel for a 4cylinder passenger car engine with 2-liter-displacement. A smaller volume results in unacceptably large pulsation in fuel pressure; a larger volume increases the response time of the pressure during transient conditions. - The solenoid valve on the high pressure pump is switched according to engine operating information. - An electric feed pump can be switched on and off. A solenoid valve in the fuel feed can be opened and closed if an engine driven feed pump is used. PRESSURE REGULATOR - The pressure regulator (Fig. 5) varies the pressure in the accumulator. For this purpose a solenoid acts on a ball overflow valve. Increasing current in the coil of the solenoid increases the solenoid force, and thereby raises the fuel pressure. The overflowing fuel is returned to the tank. PRESSURE SENSOR - The pressure in the rail is measured by a silicon piezoresistive strain gauge (Fig. 6). ELECTRONIC CONTROL UNIT - The ECU (Fig. 7) contains all functions to control the fuel injection system: - The desired value for the fuel pressure is determined by operating information of the engine. If the measured fuel pressure deviates from the desired value, the electric current in the pressure regulator is varied until the measured rail pressure and the desired value are equal. - The solenoid valves of the injectors are controlled according to the accelerator pedal position and operating information of the engine. Fig. 6. Fuel pressure sensor Fig. 5: Pressure regulator 186 Fig. 7: Housing concept of the ECU

7 Fig. 8: Common rail system for passenger cars In addition, the ECU executes functions to control the engine and the vehicle and also provides information for the driver and diagnostic data (Fig. 8). 187

8 RESULTS RAIL PRESSURE MAP - Cam driven injection systems (inline pump, distributor pump, unit injector, unit pump) build up the injection pressure for each injection. Fuel metering and pressure buildup are linked. The injection pressure results from the metered fuel quantity, being pushed through the nozzle orifice by the injection piston with a velocity proportional to engine speed. Fig. 9a represents a typical map of the injection pressure from a VP 37 distributor pump. Advanced distributor pumps attain higher pressures. In contrast to this, the functions fuel metering and pressure buildup are independent with accumulator injection systems. The injection pressure may be chosen as a function of engine parameters, for example resulting from an optimization as in Fig. 10. The map in Fig. 9b results in optimal operating conditions of a specific engine. It can be realized with the Common Rail System. These pressures are higher than those reached by the VP37. In this way the particulate emissions at part load are reduced. The lower particulate emissions allow a higher EGR rate, so NOx-emissions can be reduced. In addition, the full load fuel quantity can be set to a higher level without exceeding the smoke values from the VP37. The fuel quantity of the engine tested is limited by the permissible cylinder pressure of 150 bar at engine speeds higher than 1500 rpm. The higher fuel quantity results in a substantially stronger torque from the engine. LAYOUT OF THE HIGH PRESSURE PUMP The displacement of the high pressure pump is designed to: - build up an injection pressure of 200 bar after 1,5 engine revolutions during engine start, - reach the correct injection pressure 200ms after a rapid acceleration, - deliver, at all engine speeds, at least the fuel quantity required for injection and for control of the injectors. Fig. 10: Emissions as a function of injection pressure Fig. 9: Injection pressure map 188

9 From these criteria, quick engine starting demands the largest pump displacement and determines its layout. A secondary criterion is the quick pressure buildup during acceleration. Fig.11 represents the realistic layout of a high pressure pump. It delivers more than 1,6 times the fuel quantity necessary for fuel injection and for controlling the injectors. This layout enables the injection pressure to rise within 200 ms by 600 bar at engine speeds higher than 1000 rpm. PULSATION OF THE PRESSURE IN THE RAIL The pressure in the rail oscillates by about 40 bar (peak/peak) resulting from the pulsating delivery of the high pressure pump and the abrupt extraction of fuel flowing to the injector. This value is valid for a delivered fuel quantity of 0,65 cm 3 /revolution, shared by 3 pistons. The pressure amplitude of the pulsation is nearly independent of the ratio of pump speed to engine speed in the range 1/2...2/3. The injection pressure in the injectors may oscillate in this range. This pressure oscillation mainly affects the fuel atomization. Fuel metering is influenced less because the actual fuel pressure is measured for calculation of the fuel quantity. DRIVING THE HIGH PRESSURE PUMP - The high pressure pump may be driven from the camshaft by a coupling, a chain, a toothed belt, or even a V-belt. The drive torque of the pump is relatively constant over the engine cycle due to the low piston velocity of the pump. The peak torque of a pump with 0,65 cm 3 /revolution displacement and 3 pistons is 17 Nm at 1300 bar. This value is only 1/9 of that required for a comparable distributor pump which has to build up the injection pressure for each injection during a short time. Such a low drive torque simplifies the pump drive and keeps the drive system noise at a low level. POWER ABSORPTION OF THE HIGH PRESSURE PUMP - The power to drive the pump increases proportionally with output pressure and with pump speed (Fig. 12). The pump absorbs up to about 2,5...3 KW when sized for a 4 cylinder engine with 2- liter - displacement and a max. injection pressure of 1300 bar. One of the 3 pistons can be switched off at small injection quantities. By this means the absorbed power can be reduced. In order to further reduce the absorbed power it is recommended to use a high pressure at part load only in those operating conditions where particulate emissions have to be reduced. These measures improve the fuel economy and contribute to a low fuel temperature. Fig. 11: Fuel delivery and rail pressure 189 Fig. 12: Power absorption of the high pressure pump Displacement 0.65ccm/revolution

10 MULTIPLE INJECTION - The solenoid valve of an injector can be energized several times during one working cycle of the engine. In this way a pilot injection, a main injection and a post injection are feasible. PILOT INJECTION - A small injection quantity of 1-2mm 3 /stroke before the main injection is suitable to reduce the combustion noise [4]. For this purpose, the pilot quantity has to be controlled precisely and must take place at the right time interval before the main injection. A pilot injection too small and too early raises the combustion noise. A pilot injection too large increases the particulate emission. In the best case, the pilot fuel quantity decreases with increasing engine speed, and its interval - in crank angles - to the main injection increases with raising engine speed. Such a variable pilot injection is feasible with the Common Rail System (Fig. 13). A precise control of the pilot injection quantity requires precisely manufactured injectors. The effective flow areas of the following throttles are important: - the inlet throttle Z (Fig. 4) to the operating chamber for the nozzle actuating piston, - the outlet throttle A from the operating chamber, - the variable throttle D at the nozzle seat, formed by the variable lift of the nozzle needle. For a precise control, it is important that the solenoid valve opens and closes the orifice A rapidly and fully. The pilot injection lasts 300 µs in Fig. 13. The solenoid valve needs about 270 µs in order to open and close completely. POST INJECTION - The solenoid valve can be opened again far after the main injection. Fuel can therefore be injected during the expansion stroke and can serve as a reducing agent for a lean-nox-catalyst. SPECIFIC PROPERTIES OF A COMMON RAIL SYSTEM Fuel is accumulated under high pressure and the sealing seat of the injector nozzle is permanently exposed to the high pressure. In order to avoid engine damage special measures are necessary. EXTERNAL LEAKAGE-High pressure leakage can happen with conventional injection systems, for example by a crack in a high pressure line. The leaked fuel represents the metered fuel quantity in this case. The fuel leakage is reduced with low acceleration pedal position and low engine speed. Due to the permanent high pressure, the Common Rail System will loose more fuel than conventional systems. This fuel quantity may be nearly the delivery volume of the high pressure pump. Very extensive leaks are monitored by the control unit and stop the engine. Fig. 13: Pilot injection 190 INTERNAL LEAKAGE - In the case of a leaky solenoid valve seat or a leaky nozzle seat in the injector, as could be caused by dirt, fuel can flow continuously into a combustion chamber of the engine for a short period of time until detection. In this way fuel can be injected into the engine far before top dead center and could bum. This defect would cause high pressure and high temperature in the corresponding engine cylinder. It is recommended to ensure the tolerance of the engine to the high pressure for a short time - for example 1 second. During this time the situation can be detected and the fuel flow stopped.

11 SUMMARY The Common Rail System offers; a number of favorable properties: - Injection pressure can be chosen freely within the limits of the pump characteristics. - Injection pressure up to 1400 bar can be used. This results in low particulate emissions and in a large maximum engine torque even at low speed. - A flexible pilot injection can be obtained, which enables reduced combustion noise. - A post injection can be applied, which, in combination with a lean-nox-catalyst, could reduce NOx-emissions. - The even drive torque of the high pressure pump contributes to a low mechanical noise of the engine. REFERENCES [1] E. Eblen, G. Stumpp Beitrag des Einspritzsystems zur Verbesserung des Dieselmotors. Bosch Techn. Berichte Band 6 (1978) Heft 2 [2] Masahiko Miyaki, Hideya Fujisawa, Akira Masuda, and Yoshihisa Yamamoto Development of a New Electronically Controlled Fuel Injection System ECD-U2 for Diesel Engines SAE [3] Dr. R. Rinolfi, Dr. R. Imarisio and Dr. R. Buratti The Potentials of a New Common Rail Diesel Fuel Injection System for the next Generation of DI Diesel Engines 16. Internationales Wiener Motorensymposium, VDI-Verlag Reihe 12, Nr. 239 [4] Manfred Dümholz, Helmut Endres, and Peter Frisse Preinjection A Measure to Optimize the Emission Behavior of DI-Diesel Engines SAE