The present invention relates to a method for injecting liquid fuel into a cylinder of a piston engine in accordance with the preamble of claim 1 . The invention also concerns a fuel injection system defined in the preamble of the other independent claim.

Background of the invention

Internal combustion engines, where diffusion combustion of fuel spray takes place, such as diesel engines, typically generate NOx and particulate emissions. NOx emission limits are becoming stricter especially in marine applications, where IMO Tier III limits are entering into force. In general, the different ways of reducing NOx emissions of engines can be divided into two categories: the NOx formation can be reduced by modifying the combustion process, or the emissions generated by the combustion can be reduced by after treatment of the exhaust gas, in practice by selective catalytic reduction (SCR). An SCR system is an effective way of reducing NOx emissions, but since a continuous supply of a reducing agent is needed, an SCR system increases both the investment and operating costs of an engine. The amount of NOx generated in the combustion chamber of an internal combustion engine depends mainly on the local temperatures inside the combustion chamber and the oxygen concentration. High temperatures and high oxygen concentration in the combustion chamber increase NOx emissions. NOx formation can thus be reduced by lowering the temperatures and oxygen con- centration. The combustion process can be modified for example by utilizing exhaust gas recirculation (EGR). By introducing exhaust gas into the combustion chamber, local temperatures and NOx formation can be reduced. Unfortunately, this often leads to increased fuel consumption. In addition, the higher density in the combustion chamber has an adverse effect on the mixing of the fuel, and the recirculated exhaust gas also reduces the combustion speed. The lower combustion speed and worse mixing of the fuel tend to increase particu- late matter. A further drawback of exhaust gas recirculation is the increased complexity of the technology and problems related to combustion residuals.

Another way to modify the combustion process for reducing NOx formation is to slow down the initial rate of fuel injection, thus reducing the combustion temperatures during the initial phase of combustion when oxygen is prevalent. Later during the combustion, when the oxygen concentration has been reduced and combustion products, such as CO ₂ , have been formed, the injection rate can be increased. However, this method has a drawback that the slow initial injection rate tends to increase the total duration of the injection. The dura- tion of the combustion (heat release) is thus increased, which reduces the total efficiency of the engine.

Summary of the invention

An object of the present invention is to provide an improved method for inject- ing liquid fuel into a cylinder of a piston engine. The characterizing features of the method according to the invention are given in the characterizing part of claim 1 . Another object of the invention is to provide an improved fuel injection system for a piston engine. The characterizing features of the fuel injection system are given in the characterizing part of the other independent claim. According to the invention, fuel is injected into the cylinder during each combustion cycle in a first phase at a first injection pressure and at a first flow rate, and in a second phase at a second injection pressure and at a second flow rate, the second phase following the first phase. The second injection pressure is higher than the first injection pressure and the first flow rate is at least 80 percent of the second flow rate.

The fuel injection system according to the invention comprises a first fuel injector and a second fuel injector for each cylinder of the engine and control means for controlling fuel injection through the first fuel injector and the second fuel injector. The control means and the first fuel injector and the second fuel injector are configured to implement the method defined above.

With the fuel injection method and fuel injection system according to the invention, NOx formation can be reduced while keeping the efficiency of the engine high. The lower injection pressure in the first phase increases the droplet size and helps to control NOx formation. By the higher pressure in the second phase and the constant or nearly constant flow rate of the fuel, the efficiency of the engine can be kept high. According to an embodiment of the invention, the first flow rate is 80-125 percent of the second flow rate, According to another embodiment of the invention, the first flow rate is at least 90 percent of the second flow rate. The amount of the fuel that is injected in the first phase is 15-60 %, preferably 25- 45 % of the fuel injected during each combustion cycle. According to an embodiment of the invention, the first injection pressure is 10- 50 percent of the second injection pressure, preferably 15-35 percent of the second injection pressure.

According to an embodiment of the invention, the second phase follows immediately the first phase without interruption of the fuel injection. The fuel can be injected in the first phase using a first fuel injector and in the second phase using a second fuel injector. According to an embodiment of the invention, the fuel is supplied to the first fuel injector from a first fuel rail and to the second fuel injector from a second fuel rail.

Brief description of the drawings

Embodiments of the invention are described below in more detail with reference to the accompanying drawings, in which

Fig. 1 shows schematically a simplified view of a fuel injection system of a piston engine, Fig. 2 shows the flow rate of fuel injection in one cylinder of the engine as a function of time, and

Fig. 3 shows the fuel injection pressure in one cylinder of the engine as a function of the time. Description of embodiments of the invention

In figure 1 is shown schematically a simplified view of a fuel injection system of a piston engine. The engine comprises a plurality of cylinders 1 , of which only one is shown. The engine is a large internal combustion engine, such as a main or an auxiliary engine of a ship or an engine that is used at a power plant for producing electricity. Each cylinder 1 of the engine is provided with a reciprocating piston 2. The engine is a compression ignition engine, where the fuel is ignited by compression by the piston 2. The engine is operated on liquid fuel. The fuel can be, for instance, light fuel oil (LFO), heavy fuel oil (HFO) or marine diesel oil (MDO). The engine could be a dual-fuel or a multi-fuel engine that can be operated on different liquid fuels and/or also on gaseous fuel, but in the method according to the invention, the engine is operated using only one type of liquid fuel. For introducing the fuel into the cylinders 1 , each cylinder 1 of the engine is provided with a first fuel injector 4 and a second fuel injector 5. The fuel injectors 4, 5 are arranged to inject the fuel directly into the combustion chamber 3. The fuel injectors 4, 5 can be separate injectors, or both fuel injectors 4, 5 can be integrated into the same fuel injector unit. Each fuel injector 4, 5 comprises one or more nozzle holes for injecting the fuel into the combustion chamber 3 and a valve member, such as an injector needle for selec- tively allowing and preventing flow to the nozzle holes. If the first and the second fuel injector 4, 5 are arranged in a common fuel injector unit, the injector needles can be arranged in parallel or coaxially.

The engine comprises a first fuel rail 8 and a second fuel rail 9. The first fuel injector 4 is connected to the first fuel rail 8 and the second fuel injector 5 is connected to the second fuel rail 9. The engine comprises a first high-pressure pump 10 for supplying fuel at a high-pressure into the first fuel rail 8 and a second high-pressure pump 1 1 for supplying fuel at a high-pressure into the second fuel rail 9. The high-pressure pumps 10, 1 1 can be driven by the crankshaft or a camshaft of the engine. Alternatively, they can be for example elec- trically or hydraulically driven. The engine further comprises one or more low- pressure pumps (not shown) for supplying fuel from a fuel tank to the first and the second high-pressure pumps 10, 1 1 . The engine can comprise more than two fuel rails 8, 9. For instance, in a V-engine each bank of the engine can be provided with a first fuel rail 8 and a second fuel rail 9. It is also possible that the engine is not provided with fuel rails 4, 5, but each cylinder 1 of the engine is provided with individual pressure accumulators for storing fuel. In that case, each cylinder 1 is provided with a first pressure accumulator and a second pressure accumulator. One high-pressure pump may be provided for each pressure accumulator, or the first pressure accumulators of a group of cylinders 1 may be connected to a first common high-pressure pump, and the se- cond pressure accumulators of the group of cylinders may be connected to a second common high-pressure pump. A further possibility is that the engine is not provided with storages for high-pressure fuel, but each cylinder 1 of the engine is provided with a first fuel injection pump and a second fuel injection pump, the first fuel injection pump being connected to the first fuel injector 4 and the second fuel injection pump being connected to the second fuel injector 5.

Fuel injection through each fuel injector 4, 5 can be individually controlled. For this purpose, the fuel injection system is provided with control means 6, 7, 12. In the embodiment of figure 1 , the control means comprise an actuator 6, 7 for each fuel injector 4, 5 and a control unit 12 for controlling the fuel injectors 4, 5 through the actuators 6, 7. The actuators 6, 7 can be, for instance, electrically controlled. If the fuel injection system is not a common rail system, but comprises conventional fuel injection pumps, the fuel injection system is not provided with separate actuators, but the opening and closing of the fuel injectors 4, 5 is determined by the pressure created by the fuel injection pumps. The control means are thus partly integrated into the fuel injection pumps.

During each combustion cycle, i.e. in a four-stroke engine once during four consecutive strokes and in a two-stroke engine once during two consecutive strokes, fuel is injected into the cylinder 1 in a first phase and a second phase. The second phase follows the first phase. In the first phase, the fuel is injected at a first injection pressure and at a first flow rate. In the second phase, the fuel is injected at a second injection pressure and at a second flow rate. The second injection pressure is higher than the first injection pressure and the first flow rate is 80-125 percent of the second flow rate. Preferably the first flow rate is at least 90 percent of the second flow rate. Since the same liquid fuel, which is nearly incompressible, is used in the both phases, the flow rate can be determined either as a volume flow rate or as a mass flow rate. The expression "flow rate" refers here to the average flow rate during the constant flow rate phase of each injection phase, i.e. when the injector needles are fully lifted. The opening and closing phases of the injector needles are thus excluded. The flow rate is not completely constant over the whole injection event, but some oscillations usually occur also when the injector needles are in fully lifted positions.

The first injection pressure is 10-50 percent of the second injection pressure, preferably 15-35 percent. Suitable pressure ranges depend on the engine. In a four-stroke engine, the first injection pressure can be, for instance, 200-700 bar and the second injection pressure 800-2600 bar. In a two-stroke engine, the pressures may be slightly lower. The first injection pressure can be, for instance, 150-400 bar and the second injection pressure 500-1800 bar. The expression "injection pressure" refers here to the average pressure during the constant pressure phase of each injection phase, i.e. when the injector needles are fully lifted. The opening and closing phases of the injector needles are thus excluded. The injection pressure is not completely constant over the whole injection event, but some pressure oscillations usually occur also when the injector needles are in fully lifted positions. The amount of fuel injected dur- ing the first phase is between 15 and 60 percent of the total amount of the injected fuel, preferably between 25 and 45 percent. Each fuel injection event thus comprises a first phase and a second phase, which have different pressure levels, but the same or almost the same flow rate. Due to the smaller pressure during the first phase, the droplet size of the fuel spray is greater at the beginning of the fuel injection. The big droplet size reduces the effective diffusion combustion area, which reduces NOx formation. The higher injection pressure during the second phase results in smaller droplet size. The effective diffusion combustion area is thus increased and particulate matter is burned more effectively. Since the oxygen concentration during the second phase of the injection is lower and the combustion products, such as CO2 are present in the combustion chamber, the smaller droplet size does not lead to as high NOx formation as it would lead during the first phase of the fuel injection. Since the flow rate is kept nearly constant over the whole injection event, the duration of the fuel injection can be kept relatively short. With the fuel injection method according to the invention, the NOx formation can be reduced without an increase in the specific fuel oil consumption.

In the first phase of the fuel injection, fuel is delivered from the first fuel rail 8, where fuel is stored at a first pressure level. In the second phase, fuel is delivered from the second fuel rail 9, where fuel is stored at a second pressure lev- el. The second pressure level is significantly higher than the first pressure level. The pressures in the first fuel rail 8 and in the second fuel rail are selected so that the suitable injection pressures can be achieved. The relation of the injection pressure and the fuel rail pressure depends on several factors. Typically, the average injection pressure is 75-85 percent of the pressure in the fuel rail, if the flow rate is 70-100 percent of the maximum flow rate. At lower flow rates, the difference between the pressure in the fuel rail and the average injection pressure is greater.

The method of injecting fuel is now described in more detail by referring to the figures. The first phase of the fuel injection starts at time tO, which corresponds to a certain crank angle. At that moment, the actuator 6 of the first fuel injector 4 is arranged to lift the injector needle of the first fuel injector 4 and allow fuel injection through the first fuel injector 4. Figure 2 shows with a dashed line the flow rate through the first fuel injector 4 and figure 3 shows with a dashed line the injection pressure in the first fuel injector 4. The injection pressure refers here to the pressure in the sac volume of the fuel injectors 4, 5 just before the nozzle holes. Because of pressure losses in the fuel injection system, the injection pressure is slightly lower than the pressure in the fuel rails 8, 9. When the first fuel injector 4 is activated, the injection pressure rises almost instantly to a pressure level p1 , which is the first pressure level that is used in the first phase of the injection. In practice, the pressure is not completely constant, but some pressure oscillations occur in the system. Since a certain amount of time is needed for the full opening of the injector needle of the first fuel injector 4, a slightly longer period of time is required before the flow rate is raised to a level q1 , which is the first flow rate used in the first phase of the fuel injection. Also the flow rate is slightly oscillating. The second phase of the fuel injection starts at time t1 . At that moment, the actuator 7 of the second fuel injector 5 is arranged to lift the injector needle of the second fuel injector 5 and allow fuel injection through the second fuel injector 5. Figure 2 shows with a solid line the flow rate through the second fuel injector 5 and figure 3 shows with a solid line the injection pressure in the second fuel injector 5. At the same time, the actuator 6 of the first fuel injector 4 is arranged to close the injector needle of the first fuel injector 4 and to terminate the fuel injection through the first fuel injector 4. A certain amount of time is needed for returning the injector needle of the first fuel injector 4 to the closed position, and therefore fuel injection through the first fuel injector 5 continues for a short period of time after time t1 . Both the flow rate and the injection pressure of the first fuel injector 4 drop over that period of time. At time t2, the fuel injection through the first fuel injector 4 is terminated. When the second fuel injector 5 is activated, the injection pressure rises almost instantly to a pressure level p2, which is the second pressure level that is used in the second phase of the injection. Since a certain amount of time is needed for the full opening of the in- jector needle of the second fuel injector 5, a slightly longer period of time is required before the flow rate is raised to a level q2, which is the second flow rate used in the second phase of the fuel injection. The second fuel injector 5 reaches the second flow rate q2 at approximately the same time as the fuel injection through the first fuel injector 4 is completely terminated, i.e. at time t2. Between time t1 and time t2, part of the fuel is injected through the second fuel injector 5 at the second pressure level p2, and part of the fuel is injected through the first fuel injector at a pressure that equals first the first pressure level p1 and decreases over the period between time t1 and time t2. After t2, all the fuel is injected at the second pressure level p2 through the second fuel injector 5.

The first flow rate q1 equals the second flow rate q2. Since the closing of the first fuel injector 4 and the opening of the second fuel injector 5 overlap, except for the beginning and end of the fuel injection, the flow rate is essentially constant over the whole fuel injection, despite of the opening and closing delays of the fuel injectors 4, 5. The second phase of the fuel injection thus follows immediately the first phase without interruption of the fuel injection.

It will be appreciated by a person skilled in the art that the invention is not limited to the embodiments described above, but may vary within the scope of the appended claims. For instance, although the fuel injection method has been described by referring to a common rail system, the method could also be implement by using conventional fuel injection pumps. It could also be possible to implement the method by using a single fuel injector in each cylinder of the engine and injecting the fuel in the first phase and in the second phase through the same injector.