A common rail system is known as a fuel injection system for an internal combustion engine. This common rail system is a system that includes an accumulator (common rail) for accumulating fuel in the state of a specified pressure and injects high-pressure fuel supplied by the accumulator into the cylinder of the internal combustion engine by an injector and has excellent performance of controlling an injection pressure and an injection quantity independently. In recent years, from the viewpoint of cleaning exhaust gas and decreasing fuel consumption, there has been a growing demand that such a common rail should be further enhanced in performance and an injection pressure needs to be increased. A publicly known technology capable of realizing this easily is proposed in U.S. Pat. No. 5,622,152.
A fuel injection system described in the patent document 1 has “a mechanism for hydraulically controlling an operation of opening and closing of an injection nozzle”, which is an advantage of the common rail, and a pressure intensifying mechanism for intensifying the pressure of fuel in the accumulator. With this pressure increasing mechanism, it is possible not only to inject the fuel at high pressure but also to control both of pressure intensification and injection. As a result, it is possible to change the injection pressure in one injection cycle and to realize small injection at low pressure and main injection at extremely high pressure. Moreover, the pattern of injection rate can be optimized and hence finer combustion can be optimized.
However, in the '152 patent, essentially, it is necessary to control two operations, that is, a pressure intensifying operation and an injecting operation independently from each other. Hence, there is presented a problem that the technology needs at least two actuators and hence makes the construction of the system complex and increases cost.
In contrast to this, another system, described in JP 2003-106235A. is capable of more easily realizing the same function as is provided by the '152 patent.
FIG. 10 is a hydraulic circuit diagram of a fuel injection system described in the patent document 2.
This fuel injection system includes one control valve 100 driven by an actuator, a first fuel passage 120 for connecting this control valve 100 and a control chamber 111 of a pressure intensifier 110, a second fuel passage 140 for connecting the control valve 100 and a backpressure chamber 131 of an injection nozzle 130, and a third fuel passage 160 for connecting an accumulator 150 and the control chamber 111 of the pressure intensifier 110, and the fuel passages 120, 140, and 160 are provided with restrictors 170, 180, and 190, respectively.
The control valve 100 has a hydraulic port 101 to which the first fuel passage 120 and the second fuel passage 140 are connected in common and a low pressure port 102 connecting with a low pressure side and a valve body 103 is driven between a valve closing position (state shown in FIG. 10) where the hydraulic port 101 is disconnected from the low pressure port 102 and a valve opening position where the hydraulic port 101 is connected to the low pressure port 102.
When the valve body 103 is driven to the valve closing position, the fuel pressure in the accumulator 150 is supplied to the control chamber 111 of the pressure intensifier 110 and the backpressure chamber 131 of the injection nozzle 130. At this time, in the pressure intensifier 110, the hydraulic pressures on both upper and lower sides of a hydraulic piston 112 balance with each other, so that the pressure of fuel supplied from the accumulator 150 to a pressurizing chamber 113 is not intensified. Moreover, in the injection nozzle 130, a needle (not shown) receives fuel pressure in the backpressure chamber 131 to keep a state where a valve is closed and hence the fuel is not injected.
Next, when the valve body 103 is driven to a valve opening position, the hydraulic port 101 and the low pressure port 102 of the control valve 100 are connected to each other to release the fuel pressure in the control chamber 111 and the backpressure chamber 131 via the control valve 100 to a low pressure side. With this, in the pressure intensifier 110, the hydraulic pressures on both upper and lower sides of the hydraulic piston 112 are thrown out of balance to move the hydraulic piston 112 downward in the drawing, whereby the fuel in the pressurizing chamber 113 is pressurized and is supplied to the injection nozzle 130. Moreover, in the injection nozzle 130, the fuel pressure in the backpressure chamber 131 is decreased to lift the needle, whereby the fuel of extremely high pressure, which is supplied from the pressure intensifier 110, is injected.
However, in a fuel injection system described in the patent document 2, the control chamber 111 of the pressure intensifier 110 and the backpressure chamber 131 of the injection nozzle 130 are always connected to the accumulator 150. In other words, the control chamber 111 of the pressure intensifier 110 and the backpressure chamber 131 of the injection nozzle 130 always communicate with the accumulator 150 irrespective of a state where the control valve 100 is opened or closed. For this reason, although the restrictors 170 to 190 are provided in the respective fuel passages 120, 140, and 160, even if these three restrictors 170 to 190 are used, the values of the respective restrictors 170 to 190 have effects on each other to make it difficult to optimally control the action of the pressure intensifier 110 and the action of the injection nozzle 130.
Moreover, when the control valve 100 is opened, the fuel pressure in the accumulator 150 is released to the low pressure side via the control valve 100 to bring about a state where the fuel freely flows out of the accumulator 150 to cause an energy loss, which results in reducing the fuel consumption of the internal combustion engine.