As a fuel injection device of a diesel engine, a mechanical fuel injection device and an electronically controlled fuel injection device are generally used.
The mechanical fuel injection device injects a high-pressure fuel, compressed by an injection pump, into a combustion chamber through a mechanical injector. In other words, in the injection pump, a fuel is compressed by driving a plunger with a fuel cam linked to a crank shaft, and in the mechanical injector, a nozzle hole is opened or closed by sliding of a needle (or pushrod) to inject the fuel or intercept the injection of fuel. In the mechanical fuel injection device, the injection condition (injection timing, injection pressure, or injection amount) is subordinate to an engine speed. This means that pressure increases in proportion to the engine speed. Therefore, a great load is applied to the pump at every revolution. In addition, since the pressure cannot be increased over a certain engine speed, high-pressure injection is not available at a low speed. Moreover, it is difficult to optimally control the injection condition according to an operating state of an engine or a traveling state of a ship, vehicle or power generator to which the engine is mounted. Further, it is impossible to inject a fuel in multi stages, and it is also impossible to inject dual fuels.
As an improvement overcoming the drawbacks of the general mechanical fuel injection device, an electronically controlled fuel injection device using a common rail has been proposed. The electronically controlled fuel injection device includes a low-pressure pump, a high-pressure pump, a common rail and a solenoid injector. The fuel is pressurized to an ultra-high pressure while passing through the low-pressure pump and the high-pressure pump in order, and compressed with a certain pressure at the common rail by an engine control unit (ECU). In addition, the solenoid injector is governed by the ECU to adjust injection timing and injection amount. The electronically controlled fuel injection device allows high-pressure injection at a low speed since pressurization and injection are separately performed. In addition, since the injection condition may be freely controlled according to an operating condition, engine performance (e.g., output) and fuel efficiency may be improved. Moreover, by controlling the solenoid injector, multi-stage injection such as pilot injection, main injection and post injection may be performed, which improves fuel efficiency and reduces discharge gas. However, the electronically controlled fuel injection device is not able to inject dual fuels and thus not useable for a dual fuel engine.
A duel or two-kind fuel engine has two combustion modes. For example, in a diesel fuel mode, before a main fuel (e.g., heavy fuel oil, marine diesel oil) is injected, an auxiliary fuel (e.g., marine diesel oil, marine gas oil) may be injected to improve a combustion environment of the combustion chamber, resulting in improvement of exhaust, NOx and combustion performance. In addition, in a gas fuel mode, only an auxiliary fuel may be injected by adjusting a fuel injection amount governor so that a gas fuel flowing in through a gas admission valve and an engine intake port may be stably ignited.
In order to inject dual fuels by using the mechanical fuel injection device or the electronically controlled fuel injection device, described above, another injector should be added, which complicates the system and greatly increases costs.
For example, a general fuel injection device shown in FIG. 17 uses a parallel twin injector to inject dual fuels. The twin injector is configured to inject different kinds of fuels by combining a single general mechanical injector and a single solenoid injector. However, since two fuel injection lines are arranged in a single injector body, the injection device has an increased size and occupies a greater space. In addition, two nozzle orifices and shafts corresponding to the dual fuels are installed in parallel at spaced points. For this reason, one of two kinds of fuels is inevitably injected at a location deviating from the center of the combustion chamber, which makes it difficult to optimize the fuel performance. Moreover, when a main fuel containing a large amount of particles (a heavy fuel oil) is injected, the auxiliary fuel nozzle orifice may be closed. Further, since a common rail, a high-pressure pump and a solenoid injector should be additionally installed for auxiliary injection, the system becomes more expensive and complicated.
In addition, as another example, for two-stage dual-fuel injection, in addition to an existing mechanical main injector disposed at the center of the combustion chamber, an electronically controlled auxiliary injector separately prepared may be installed with a slope around the main injector. However, in this case, the auxiliary injection is not performed at the center of the cylinder head but performed aside, and the injection direction (angle) is also inclined to one side, which results in bad fuel performance.