1. Field of the Invention
The present invention relates to a common-rail fuel-injection system and more particularly a common-rail fuel-injection system in which, on fuel injection into a combustion chamber, an auxiliary injection of a small amount of fuel is provided at any timing of soon before or after a main injection of major amount of fuel.
2. Description of the Prior Art
Among various types of fuel-injection systems for engines is conventionally well-known a common-rail fuel-injection system in which a high-injection pressure is maintained in the fuel discharge lines, and fuel-injection conditions such as a timing of fuel injection and an amount of fuel to be injected or injected are controlled depending on operating requirements of the engine. In accordance with the common-rail fuel-injection system, the fuel intensified in pressure by a fuel pump is stored in a common rail at desired pressures, and is injected into the combustion chambers through the injectors, which are arranged to the engine cylinders, each to each cylinder, with the fuel-injection conditions such as an amount of fuel to be delivered and a timing of fuel injection, which are regulated by a controller at the most suitable situations for the engine operating requirements.
A fuel pressure to ensure the injection pressure is maintained constantly in the fuel discharge lines extending from the common rail through injection lines to discharge orifices, which are formed at the distal ends of the injectors that are each provided with a solenoid valve to block or allow the discharge of fuel applied via the injection lines. The controller is to regulate both the fuel pressure in the common rail and the solenoid valves in the injectors to spray the high-pressure fuel out of the injectors under the desired fuel-injection conditions, which are the most suitable for the engine operating requirements. Of these common-rail fuel-injection systems, there is a type in which the actuation of the solenoid valves makes use of a part of the high-pressure fuel as a working fluid to operate the injectors.
An example of a conventional common-rail fuel-injection system will be explained below with reference to FIG. 5. A fuel feed pump 6 draws fuel from a fuel tank 4 through a fuel filter 5 and forces it under a preselected intake pressure to a high-pressure, fuel-supply pump 8 of, for example, a variable stroke plunger type, through a fuel line 7. The high-pressure, fuel-supply pump 8 is driven by the output of the engine, which intensifies the fuel to a high pressure desired depending on the engine operating requirements, and supplies the pressurized fuel into the common rail 2 through another fuel line 9. The fuel-supply pump 8 is also provided with a fuel flow-rate control valve 14, where the fuel pressure in the common rail 2 is maintained at the preselected high-pressure level. The fuel relieved from the fuel-supply pump 8 is allowed to flow back the fuel tank 4 through a fuel-return line 10. The fuel, thus supplied, is stored in the common rail 2 at the preselected high pressure and forced to the injectors 1 through injection lines 3. The unconsumed fuel remaining in each injector 1 out of the fuel fed through the injection lines 3 into the injectors 1 may return to the fuel tank 4 through a fuel-recovery line 11.
The controller 12 of an electronically-controlled unit is applied with various signals of sensors monitoring the engine operating conditions, such as a cylinder-identification sensor, a crankshaft position sensor for detecting the engine rpm Ne and top dead center TDC, a throttle-position sensor depending on the depression Ac of an accelerator pedal, an engine coolant temperature sensor, an intake manifold pressure sensor and the like. The controller 12 is also applied with a detected single as to a fuel pressure in a common-rail 2, or a common-rail pressure, which is reported from a pressure sensor 13 installed in the common rail 2. The controller unit 12 may regulate the fuel injection conditions on the injectors 1 such as the injection timing (the instant the injection starts and the duration of injection) and the amount of fuel injected or the others, depending on the signals issued from the sensors, to thereby operate the engine with the engine output, which is the most suitable for engine operating requirements. Although the injection of fuel out of the injectors 1 consumes the fuel in the common rail 2, resulting in lowering the fuel pressure in the common rail 2, the controller unit 12 actuates the fuel flow-rate control valve 14 of the fuel supply-pump 8, which in turn regulates the quantity of delivery of the fuel from the high-pressure, fuel-supply pump 8 to the common rail 2 whereby the common-rail pressure recovers the preselected fuel pressure or ensures the fuel pressure that might be desired in accordance with the engine operating requirements.
The injector 1 constructed as shown in FIG. 6 is installed through a hermetic sealing member in a port bored in a basement such as a cylinder head. The injector 1 is communicated at an upper section thereof with the associated injection line 3 through a high-pressure fuel inlet coupling 30. The injector 1 is made therein with fuel passages 21, 22 comprising a fuel-flow line in combination with the associated injection line 3. The high-pressure fuel applied via the fuel passages 21, 22 is allowed to reach a discharge orifice 25 through a fuel sac 23 and a clearance around the needle valve 24. Thus, the instant the needle valve 24 is lifted to open the discharge orifice 25, the fuel is injected out of the discharge orifice 25 into the combustion chamber.
The injector 1 is provided with a needle-valve lift mechanism of pressure-control chamber type in order to adjust the lift of the needle valve 24. The injector 1 has at the head section thereof a solenoid-operated actuator 26 comprising a solenoid valve. A solenoid 28 of the solenoid-operated actuator 26 is applied with a control current through a signal line 27, depending on a command pulse issued from the controller 12. Upon energizing the solenoid 28, an armature 29 is lifted to open a valve 32 arranged at one end of a fuel path 31, so that the fuel pressure of fuel fed in a pressure-control chamber 30 via the fuel passages is relieved through the fuel path 31. A control piston 34 is arranged for axial linear movement in an axial recess 33 formed in the injector 1. As the resultant force of the reduced fuel pressure in the pressure-control chamber 30 with the spring force of a return spring 35, acting on the control piston 34 to push it downward, is made less than the upward force exerted on the control piston 34 owing to the fuel pressure acting on a tapered surface 36 exposed to the fuel sac 23, the control piston 34 moves upwards. As a result, the needle valve 24 lifts to allow the fuel to spray out of the discharge orifice 25. It will be understood that a timing of fuel injection is defined by the instant the needle valve 24 starts lifting while the amount of fuel injected per cycle is defined dependent on the fuel pressure in the fuel passages and both the amount and duration of lift of the needle valve 24.
It is commonly found that the relation of the amount of fuel injected out of the injector 1 with the pulse width of command pulse issued from the controller 12 is mapped or plotted for various choices of a parameter: the common-rail pressure Pr. With the common-rail pressure Pr being at constant, the amount of fuel injected increases in proportion to the command pulse becoming large in pulse width. In contrast, on the pulse width being kept at constant, the higher the common-rail pressure Pr is, the greater is the amount of fuel injected per cycle. On the other hand, the fuel injection usually starts or ceases with a fixed time lag after the fall time and rise time of the command pulse. Thus, the injection timing may be controlled by regulating the timing the command pulse is turned on or off. The amount of fuel injected per combustion cycle may be found or calculated, depending on the engine operating requirements, on the basis of a characteristic map of the fundamental amount of fuel injected, in which a fixed relationship of the fundamental amount of fuel injected per cycle with the rpm of the engine has been previously mapped for various choices of a parameter: an amount of depression of the accelerator pedal.
In conventional diesel engines, a fuel injection system commonly referred to as pilot injection control is employed, in which a small amount of fuel is injected prior to the major injection to previously heat up the combustion chamber, thereby keeping the combustion chamber against too sudden firing of the charged fuel, which might otherwise trend to cause what is called the diesel knock as well as to increase the amount of nitrogen oxides contained in the exhaust gases. If considering the significance of the pilot injection system, the minor amount of fuel in the pilot injection should be determined with taking into account an exhaust amount of nitrogen oxides, which has been found experimentally. However, the minor amount of fuel in pilot injection determined as described just above makes possibly the combustion so slow as to cause the adverse influence such as surging or the like. Although but the pilot injection is usually desired when the engine is operated in an operating range of partial load or idling, the minor amount of fuel in pilot injection commonly is determined at a fixed proportion to the whole amount of fuel injected or is an absolute amount that is determined indiscriminately. Thus, the minor amount of fuel in pilot injection is in general too small in the proportion to the whole amount of fuel injected as well as in absolute value.
In the meantime, the injection characteristics such as the injection timing, the amount of fuel injected or the like are inevitably somewhat different among the injectors and also vary on even the same injector owing to aging. It is thus common that the injectors show some scatter in the amount of fuel injected at every injector even if the injectors are kept identical with each other in the common-rail pressure and the duration of command pulse made turned-on to actuate the associated injector. If the amount of fuel to be injected per cycle were too much, somewhat variations in the amount of fuel at every injection would not become a major problem. Nevertheless, a minute amount of fuel injected as in the pilot injection experiences the considerable influence of variations at every injector and/or due to aging. Eventually, there is the possibility of too much amount of fuel injected or failure of fuel injection. Under partial-load operation or non-load idling, where the pilot injection is desired, if the injectors having succeeded in pilot injection intermingle with other injectors having failed in pilot injection, it makes the combustion unstable and causes an uncomfortable ride.
To reduce the variations in the amount of fuel injected, which might occur among the injectors, a control system according to a senior invention disclosed in Japanese Patent Laid-Open No. 2000-18068 has been proposed, in which an actual amount of fuel injected is calculated on the basis of a pressure drop in the common rail at the time of fuel injection, and a controlled variable is corrected so as to coincide with a desired amount of fuel to be injected, which is found depending on the engine operating requirements. In accordance with the controlling system of the senior invention, the actual amount of fuel injected may be derived from the mapped data correspondingly to both of the common-rail pressure just before the fuel injection and the pressure difference between the common-rail pressure just before the fuel injection and that just after the fuel injection. That is to say, a common-rail fuel-injection system has been proposed, in which an actual amount of fuel injected is found by making use of a signal issued from the pressure sensor, which is installed to monitor the common-rail pressure. In the control system of the senior co-pending application, a dynamic fuel leakage out of a pressure-control chamber of the injector to carry out the fuel injection is considered when finding the actual amount of fuel injected per cycle.
In the map explaining the correlation between the pressure drop in the common rail and the amount of fuel injected, there is an area where the actual amount of fuel injected can not be obtained with accuracy in accordance with only the pressure drop in the common rail. As apparent from a graphic representation of FIG. 2 showing the correlation between the amount of fuel injected and the pressure drop in the common rail, which is plotted for the various choices of a parameter: the common-rail pressure, the relationship between the amount Q of fuel injected and the common-rail pressure Pr becomes indefinite at the pressure drop below a threshold value .DELTA.Ps, for example, 1 Mpa in the common rail. Thus, it is substantially impossible to calculate the actual amount of fuel injected in compliance with the pressure drop below the threshold value .DELTA.Ps in the common rail. That is to say, no actual amount of fuel injected is obtained accurately corresponding to the pressure drop below the threshold value .DELTA.Ps shown in FIG. 2 in the common rail and, therefore, it is very tough to accomplish the feedback control with accuracy to make the actual amount of fuel injected coincide with a desired amount of fuel to be injected.
In the common-rail fuel-injection system in which an auxiliary fuel injection of a small amount of fuel is provided at any timing either soon before or after a main injection of major amount of fuel, and a controller calculates a desired minor amount of fuel to be injected at the auxiliary fuel injection out of an injector into a combustion chamber while the auxiliary fuel injection is feedback controlled such that the actual minor amount of fuel injected really in the auxiliary fuel injection comes in accord with the desired minor amount of fuel to be injected, therefore, it has been necessary to eliminate such adverse situation that a control variable for the amount of fuel injected becomes unstable when the desired minor amount of fuel to be injected at the auxiliary fuel injection is less than a critical amount of fuel injected, which is obtainable on the basis of the pressure drop in the common-rail pressure.