KR20160097375A - Engine system and control method - Google Patents

Abstract

An engine system according to the present invention includes an engine body, a fuel supply unit that generates water emulsion fuel to supply the fuel injection valve, an EGR unit that supplies EGR gas to the engine body, and a water emulsion fuel And controlling the flow rate of the EGR gas so that the discharge amount of NOx discharged from the engine body becomes a predetermined value or less.

Description

[0001] ENGINE SYSTEM AND CONTROL METHOD [0002]

The present invention relates to an engine system for performing exhaust gas recirculation (EGR) in which water emulsion fuel is used to reduce nitrogen oxide (NOx) emissions from an engine or part of an exhaust gas is returned to an engine. The present invention also relates to a control method of such an engine system.

 As one method of reducing NOx emissions from the engine, there is an EGR that returns a portion of the exhaust gas to the engine. By sending part of the exhaust gas to the engine, the combustion is performed in a state where the oxygen concentration is low, and as a result, the combustion temperature is lowered and the generation of NOx is suppressed. However, if EGR is performed, a sufficient NOx reduction effect can be obtained by increasing the EGR rate (the ratio of the EGR gas in the unburned gas), but depending on the conditions, the fuel efficiency may deteriorate or the amount of soot contained in the exhaust gas may decrease And the like.

On the other hand, as another method of reducing the amount of NOx discharged from the engine, there is a method of using a water emulsion fuel in which water is added to a pure fuel (a fuel to which no water is added). By using the water-emulsion fuel, the combustion temperature is lowered by the heat of vaporization of water, and the generation of NOx can be suppressed. Further, when water in the water emulsion fuel vaporizes / evaporates, the pure fuel surrounding the water particles is scattered, and the scattered pure fuel becomes small diameter particles. As a result, the surface area per volume of the pure water fuel of the water-emulsion fuel, that is, the area in contact with oxygen, becomes large, and local incomplete combustion is reduced. As a result, the combustion efficiency is increased and the soot generation amount is suppressed. Thus, when the water emulsion fuel is used, the amount of soot contained in the exhaust gas is suppressed although the NOx reduction effect is not large. In addition, there is also an advantage that fuel consumption is not substantially deteriorated up to a certain degree of water addition rate.

In the engine disclosed in Patent Document 1, the flow rate of the EGR gas is controlled to be constant, and when the EGR rate increases as the flow rate of the scavenge gas decreases, the fuel is converted from the pure fuel into the water emulsion fuel 2). Accordingly, the engine described in Patent Document 1 attempts to prevent excessive occurrence of soot when the EGR rate is high, specifically when the engine load is lower than 75%.

Japan Specification No. 2012-518748

However, the engine disclosed in Patent Document 1 has a plurality of fuel systems so that fuel can be switched (see Figs. 1 and 2). Therefore, the engine disclosed in Patent Document 1 has a complicated structure.

Further, in Patent Document 1, a configuration has been proposed in which the water addition rate of the water emulsion fuel is instantaneously changed by changing the ratio between the pure water and the water supplied to the engine (see Figs. 3 and 4) It is difficult to mix them uniformly.

The present invention has been made in view of the above circumstances. In carrying out the EGR control while using the water emulsion fuel, it is possible to stably reduce the NOx production amount while suppressing the fuel consumption deterioration and the soot generation in response to the change in the operating condition of the engine within a range not to cancel each characteristic, And it is an object of the present invention to provide an engine system which can realize such an engine.

According to an embodiment of the present invention, there is provided an engine system including an engine body having a cylinder and a fuel injection valve, a fuel supply unit for generating water emulsion fuel to supply the fuel injection valve, and an EGR unit And a control device configured to control the water emulsion fuel to be used in the entire region of the actual engine load and to control the flow rate of the EGR gas so that the amount of NOx discharged from the engine body is equal to or less than a predetermined value .

Here, the "practical engine load" refers to an engine load in a range excluding a low engine load such as an engine starting time, that is, a range used for normal operation, and corresponds to 25 to 100% of a maximum engine load. According to the above configuration, there is basically no conversion of the pure water fuel and the water emulsion fuel in the entire region of the practical engine load. Therefore, according to the engine system, a mechanism necessary for momentarily switching the fuel is not necessary, and the structure can be simplified. In addition, by using water emulsion fuel, it is possible to suppress fuel consumption deterioration and soot generation, and at the same time, it is possible to reduce NOx production amount by supplying EGR gas to the engine in accordance with the engine load. It is also possible to control the amount of NOx produced by changing the water addition rate of the water emulsion fuel, but it is very difficult to instantaneously change the water addition rate while uniformly mixing pure water with water. On the other hand, the flow rate of the EGR gas can be quickly changed by controlling the rotation speed of the EGR blower. Therefore, according to the above-described configuration, even if the situation is abruptly changed, the flow rate of the EGR gas can be controlled to maintain the suppression of the NOx production amount.

In addition, in the engine system, the control device may be configured to maintain the water addition rate of the water-based emulsion fuel constant in the entire region of the practical engine load. According to such a configuration, since the water addition rate is kept constant in the entire region of the practical engine load, soot generation can be stably suppressed. With this condition, it is easy to control the water addition rate.

In the engine system described above, the control device may be configured to increase the injection time per cycle as the injection amount per cycle of the water-emulsion fuel increases. According to such a configuration, even when the injection amount per one cycle largely fluctuates, the injection amount per unit time does not greatly fluctuate. Therefore, even if the fluctuation of the injection amount per cycle is somewhat large, the injection of the water emulsion fuel into the same fuel injection valve is possible.

Further, in the engine system, when increasing the injection time per one cycle, the control device is configured to advance the injection start timing of the water-emulsion fuel within a range in which the maximum pressure in the cylinder does not exceed the predetermined upper limit value . According to this configuration, it is possible to suppress the decrease of the maximum in-cylinder pressure due to the lengthening of the injection time, and the engine can be efficiently operated.

Further, in the engine system, the control device may be configured to determine the injection start timing according to one or both of the water addition rate and the EGR rate. According to such a configuration, since an appropriate injection start timing can be determined according to the water addition rate and the EGR rate, the engine can be quickly shifted to an efficient operation state.

A control method according to an embodiment of the present invention includes an engine main body having a cylinder and a fuel injection valve, a fuel supply unit for generating water emulsion fuel to supply the fuel injection valve, and an EGR unit Wherein the flow rate of the EGR gas is controlled so that the water emulsion fuel is used in the entire region of the load of the actual engine and the amount of NOx discharged from the engine body is equal to or less than a predetermined value.

In the control method, the water addition rate of the water emulsion fuel may be kept constant in the entire region of the actual engine load.

In the control method, the injection time per cycle may be increased as the injection amount per cycle of the water-emulsion fuel increases.

In the control method, when the injection time per one cycle is increased, the injection start timing of the water-emulsion fuel may be advanced within a range in which the maximum pressure in the cylinder does not exceed the predetermined upper limit value.

In the control method, the injection start timing may be determined based on one or both of the water addition rate and the EGR rate.

As described above, according to the engine system, the amount of NOx generated can be reduced while suppressing the fuel consumption deterioration and the soot emission by using the water emulsion fuel and performing the EGR, and furthermore, it can be realized with a simple structure.

1 is a schematic view of an entire engine system according to an embodiment.
2 is a block diagram of a control system of an engine system according to an embodiment.
3 is a diagram showing the relationship between the engine load, the target addition rate, and the target EGR rate.
FIG. 4 is a graph showing the relationship between the engine load and the target EGR rate when the target water addition ratio in FIG. 3 is lowered.
5 is a flowchart of fuel injection control.
6 is a conceptual diagram showing the relationship between the crank angle and the in-cylinder pressure.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description, the same or equivalent elements are denoted by the same reference numerals throughout the drawings, and redundant explanations are omitted.

≪ Overall configuration of engine system >

First, the entire configuration of the engine system 100 according to the present embodiment will be described. Fig. 1 is a block diagram of an engine system 100. Fig. 1, the engine system 100 includes an engine body 10, a supercharger 20, an EGR unit 30, and a fuel supply unit 40. [

The engine main body 10 of this embodiment is a large two-stroke diesel engine as a ship propulsion cycle. The engine main body 10 has a plurality of cylinders 11 (only one is shown in Fig. 1). Each cylinder 11 is provided with a small mechanism 12 at a lower portion thereof and an exhaust port 13 at an upper portion thereof. Each cylinder 11 is provided with a piston 14, a fuel injection valve 15, and an exhaust valve 16. The piston 14 slides in the cylinder 11 so as to cross the small mechanism 12 and the lower end portion is connected to a crankshaft (not shown). The fuel injection valve 15 is located in the upper portion of the cylinder 11, and fuel is supplied from the fuel supply unit 40. The cylinder 11 is provided with an in-cylinder pressure sensor 17 for measuring the pressure (in-cylinder pressure) in the cylinder 11. The engine body 10 is provided with an engine speed sensor An engine speed meter 18 (see FIG. On the other hand, the engine body 10 may be a four-stroke engine, a gas engine, or a gasoline engine.

The supercharger 20 is a device for boosting the air and supplying it to the engine main body 10. The exhaust gas generated in each cylinder 11 is supplied to the turbine section 21 of the turbocharger 20 through the exhaust port 13, the exhaust pipe 24 and the exhaust passage 25. The turbine portion 21 is rotated by the energy of the supplied exhaust gas. The turbine portion 21 and the compressor portion 22 are connected by a connecting shaft 23 and the compressor portion 22 is also rotated in accordance with the rotation of the turbine portion 21. [ When the compressor 22 rotates, the air (incoming air) flowing in from the outside is stepped up and the raised air is supplied into each of the cylinders 11 through the baffle 26, the scaffold 27 and the baffle 12 .

The EGR unit 30 is provided in the EGR passage 28 connecting the exhaust passage 25 and the baffle passage 26 to extract a part of the exhaust gas from the exhaust passage 25 ) Is referred to as "EGR gas"), and supplies the EGR gas to the scavenge flow path 26. The EGR gas supplied to the scavenging passage 26 is mixed with the fresh air raised in the turbocharger 20 and supplied to the engine main body 10 (each cylinder 11) as a scavenging gas. Accordingly, the oxygen concentration of the scavenging gas supplied to each cylinder 11 decreases, and the amount of NOx discharged from the engine body 10 can be reduced. The EGR unit 30 includes a scrubber 31 for cleaning the EGR gas, a cooling device 32 for cooling the EGR gas, and an EGR blower 33 for boosting the EGR gas to supply the EGR gas to the scavenging flow path 26. The flow rate of the EGR gas and the EGR rate can be changed by adjusting the number of revolutions of the EGR blower 33. [ However, the flow rate of the EGR gas (and hence the EGR rate) may be changed by a flow control valve (not shown) provided in the EGR flow passage 28.

The fuel supply unit 40 is a unit that generates and supplies water emulsion fuel to the fuel injection valve 15. [ The fuel supply unit 40 has a fuel generating unit 41 and a fuel supply unit 42. The fuel generating portion 41 is a portion that mixes the pure fuel and water introduced into the mixing vessel 43 with a mixer or the like to produce a water emulsion fuel having a predetermined water addition ratio. 3 and 4 in Patent Document 1), it is difficult to uniformly mix the pure fuel and water. In this embodiment, however, pure fuel and pure water are mixed in the fuel injection valve 15, Water can be uniformly mixed. In addition, the actual water addition rate can be calculated according to the ratio of the pure fuel and water supplied. The fuel supply unit 42 is a unit that supplies the water emulsion fuel generated by the fuel generation unit 41 to the fuel injection valve 15 of the engine body 10. [ The fuel supply unit 42 can change the injection time of the water-emulsion fuel injected from the fuel injection valve 15 and the injection start timing.

<Configuration of control system>

Next, the configuration of the control system of the engine system 100 will be described. Fig. 2 is a block diagram of the control system of the engine system 100. Fig. As shown in FIG. 2, the engine system 100 includes a control device 50 that controls the entire engine system 100. The control device 50 is composed of, for example, a CPU, a ROM, a RAM, and the like.

The control device 50 is electrically connected to the in-cylinder pressure sensor 17, the engine revolving system 18, and the input setting unit 51. The control device 50 acquires the cylinder in-cylinder pressure, the engine speed, and the set water addition rate in accordance with signals received from these devices. On the other hand, the input setting unit 51 is installed in a cabin of the hull where the engine system 100 is mounted. Further, the input setting unit 51 is configured to be able to input the set water addition rate by the driver, and to save the input set water addition rate.

The control device 50 controls various parts of the engine system 100 by performing various operations according to input signals from the respective devices. The controller 50 is electrically connected to the EGR blower 33, the fuel generator 41 and the fuel supplier 42 and transmits a control signal to these devices in accordance with the results of various calculations and the like . The controller 50 not only transmits the control signal to the fuel generator 41 but also obtains the actual water addition ratio according to the signal transmitted from the fuel generator 41. [ However, the actual water addition rate may be obtained by providing a sensor for measuring the water addition rate in the flow path from the mixing vessel 43 to the injection valve 15 of the fuel production section 41, and acquiring it in accordance with a signal received from the sensor good.

The control device 50 has a water addition rate control section 52, an EGR rate control section 53, and a fuel injection control section 54 as a functional configuration.

The water addition rate control unit 52 controls the water addition rate of the water emulsion fuel. The water addition rate control section 52 first sets the target water addition rate according to the set water addition rate input by the driver. In the present embodiment, the target water addition rate is set as indicated by the broken line in Fig. More specifically, the target water addition rate is set to 0% when the engine load is 0 to 15%, and the target water addition rate when the engine load is 15 to 100%. The water addition rate control unit 52 sends a control signal to the fuel generation unit 41 to control the actual water addition rate to be the target addition rate.

As described above, in this embodiment, pure water is not used and only the water emulsion fuel is used in the whole volume region of the practical engine load (engine load is 25 to 100%). In addition, the water addition rate is constant in the real engine load range. Therefore, according to the present embodiment, there is no need for a mechanism for momentarily changing the pure water fuel and the water emulsion fuel, and a mechanism for instantly changing the water addition rate. In addition, in the above-mentioned Patent Document 1, the fuel injection valve to be used is switched when switching between the pure water fuel and the water emulsion fuel, but according to the present embodiment, a mechanism for switching the fuel injection valve is also unnecessary. As described above, according to the present embodiment, the configuration of the engine system 100 can be simplified.

The EGR rate control unit 53 controls the EGR rate by controlling the flow rate of the EGR gas. The EGR rate control unit 53 first sets the target EGR rate. 3, the target EGR rate is set to 0% when the engine load is 0 to 15%, and the target EGR rate is set to 0% when the engine load is 15 to 100% . The target EGR rate is a value derived from a test conducted in advance and is an EGR rate at which the NOx emission amount regulation can be cleared, that is, an EGR rate at which the NOx emission amount becomes a predetermined value or less. The target EGR rate is stored in the EGR rate control unit 53. [

FIG. 4 is a graph showing the relationship between the engine load and the target EGR rate when the target water addition ratio in FIG. 3 is lowered. The two dotted lines in FIG. 4 correspond to the target water addition rate and the target EGR rate in FIG. 3, respectively. As shown in Fig. 4, it is assumed that the target water addition rate is lowered when the set water addition rate is decreased and the engine load is 15% or more. This increases the target EGR rate when the engine load is greater than 15%. This is because if the water addition rate is lowered, the effect of reducing the emission of NOx by the use of the water emulsion fuel is lowered. Therefore, it is necessary to increase the EGR rate in order to clear the NOx emission regulation.

Then, the EGR rate control unit 53 sends a control signal to the EGR blower 33 so that the actual EGR rate is calculated based on the engine load at that time (in this embodiment, the engine load is calculated in accordance with the engine speed and the fuel injection amount, For example, it may be calculated from the number of revolutions of the turbocharger 20, or may be calculated from the illustrated work obtained by the in-cylinder pressure. The EGR gas flow rate is adjusted so that the target EGR rate becomes equal to the target EGR rate according to the target EGR rate. Thus, the NOx emission regulation can be cleared. In this embodiment, the control of the flow rate of the EGR gas is performed by controlling the number of revolutions of the EGR blower 33. However, the flow rate of the EGR gas may be controlled by a flow control valve or other method. The EGR gas flow rate can be easily controlled regardless of the method, and the response of the EGR gas flow rate change to the situation change is very high.

The fuel injection control section 54 controls the fuel injection (injection amount, injection time and injection start timing) of the water emulsion fuel. 5 is a flowchart showing a fuel injection control method of the water emulsion fuel. The calculation and control described below are performed by the fuel injection control unit 54. [

First, when the process is started, the fuel injection control unit 54 reads signals transmitted from the in-cylinder pressure sensor 17, the engine revolving system 18, and the fuel generating unit 41, Water and water addition rate (step S1).

Subsequently, the fuel injection control section 54 determines the injection amount of the water-emulsion fuel injected per cycle (step S2). In this embodiment, the injection amount is determined so that the engine speed can be maintained at a constant number of revolutions (100% number of revolutions). However, even if the injection amount is the same, if the water addition rate is different, the heat quantity is different, and if the EGR rate is different, the combustion efficiency is different. Therefore, for example, when the water addition rate or the EGR rate is increased, the injection amount is increased.

Subsequently, the fuel injection control section 54 determines the injection time of the water-emulsion fuel injected per cycle (step S3). In this embodiment, the injection time is increased as the fuel injection amount per one cycle is increased. Accordingly, the injection amount per unit time does not increase even if the injection amount per one cycle increases. Therefore, even if the injection amount per cycle is largely changed to some extent, normal injection can be performed by using the same fuel injection valve 15. [

Subsequently, the fuel injection control section 54 judges whether there is a change in the injection time (step S4). As will be described later, each step is repeated, but it is determined whether the injection time determined in step S3 has changed from the injection time set in the previous cycle. When the fuel injection control unit 54 determines that there is a change in the injection time (YES in step S4), the fuel injection control unit 54 moves to step S5. On the other hand, if it is determined that there is no change in the injection time (NO in step S4), the flow proceeds to step S6.

Subsequently, when the routine proceeds to step S5, the fuel injection control section 54 determines the injection start timing (the crank angle at which the injection starts). More specifically, based on the water addition rate, the EGR rate, and the engine load, values read out from the map data stored in advance are set as the injection timing timing. On the other hand, in this embodiment, the injection start timing is determined based on the water addition rate, the EGR rate, and the engine load, but the injection start timing may be determined based on the parameters corresponding to the water addition rate, the EGR rate, and the engine load. For example, the value of the oxygen concentration in the scavenge 27 may be used instead of the EGR rate.

6 is a conceptual diagram showing the relationship between the crank angle and the in-cylinder pressure. In the figure, the solid line indicates the case where pure fuel is used, the broken line indicates the case of using the water emulsion fuel, and the dotted line indicates the case where the injection start timing is advanced by using the water emulsion fuel.

When pure fuel is used as the fuel, as shown by the solid line in Fig. 6, when the piston 14 rises and reaches the top dead center (crank angle = 0), the in-cylinder pressure reaches the first peak. At this time, when the injection of the fuel is started, the internal pressure of the cylinder is once lowered, then raised again by the combustion to reach the second peak. At this second peak, the in-cylinder pressure is the maximum in-cylinder pressure Pmax. This maximum cylinder internal pressure can be adjusted by varying the injection start timing of the fuel. The higher the maximum cylinder internal pressure, the higher the efficiency of the engine. However, if the in-cylinder pressure becomes too high, the cylinder 11 may be damaged. Therefore, in general, the injection start timing is determined such that the maximum in-cylinder pressure becomes a predetermined target value (to be a maximum value in a range not exceeding the upper limit value). On the other hand, the fuel injection control unit 54 may perform control to correct the injection start timing when there is a concern that the maximum in-cylinder pressure exceeds the upper limit value.

On the other hand, it is assumed that the water injection is started when the water emulsion fuel is used as the fuel and the crank angle is 0 ° as in the pure fuel. Then, the water emulsion fuel has a calorie volume smaller than that of the pure fuel and is injected for a comparatively long period of time, so that the curve toward the second peak of the in-cylinder pressure becomes gentle as shown by the broken line in Fig. As a result, the maximum in-cylinder pressure becomes lower than a predetermined target value.

On the other hand, water emulsion fuel is used as the fuel, but the injection start time is earlier than that of the pure fuel. Then, as indicated by the dotted line in Fig. 6, the rise of the curve toward the second peak of the in-cylinder pressure starts quickly, and as a result, the maximum in-cylinder pressure can be set to a predetermined target value or a value close thereto. That is, as the injection time per one cycle becomes longer, the injection start timing is made faster, and the maximum in-cylinder pressure can be set to a predetermined target value or a value close to the predetermined target value.

In this embodiment, the injection start timing at which the water addition rate, the EGR rate, and the maximum cylinder internal pressure for each engine load become a predetermined target value or a value close thereto is calculated by a test conducted in advance, and the injection start timing is stored as map data . Therefore, when the injection of the water emulsion fuel is started at the injection start timing determined in step S5, the maximum in-cylinder pressure becomes a predetermined target value or a value close to the predetermined target value.

On the other hand, when the process goes to step S6, the fuel injection control unit 54 corrects the injection start timing. Specifically, the fuel injection control section 54 corrects the injection start timing so that the difference between the actual maximum in-cylinder pressure and the predetermined target value becomes small. As described above, in this embodiment, immediately after the injection time is changed, the injection start timing to be the initial value is determined based on the water addition rate and the EGR rate (step S5), and thereafter, the injection start timing is set to the actual maximum cylinder internal pressure (Step S6). That is, in the present embodiment, the control combining the feedforward control and the feedback control is performed.

Subsequently, after step S5 or step S6, the fuel injection control section 54 sends a control signal to the fuel supply section 42 to inject the water emulsion fuel at the determined injection amount, injection time, and injection start timing (step S7). When the step S7 is completed, the fuel injection control unit 54 returns to the step S1 and repeats the steps S1 to S7. In this embodiment, since the water emulsion fuel is injected at an appropriate injection start timing for performing the fuel injection control as described above, the engine main body 10 can be efficiently operated.

The foregoing is a description of embodiments according to the present invention. In the above description, the case where the feedback control and the feedforward control are combined in determining the injection start timing of the fuel has been described. However, the injection start timing of the fuel may be determined only by controlling the portion corresponding to the feedforward control. That is, the injection start timing may be determined based on the water addition rate, the EGR rate, and the engine load without providing the in-cylinder pressure sensor 17 in the cylinder 11, and the injection start timing may be maintained.

In the above description, the case where the water addition rate of the water emulsion fuel is constant in the entire region of the practical engine load has been described, but the water addition rate may be changed gently. Even in such an engine system, a simple structure can exhibit an action effect of reducing NOx production while suppressing fuel consumption deterioration and soot generation.

According to the engine system according to the present invention, by performing the EGR with the use of the water emulsion fuel, the NOx production amount can be reduced while suppressing the fuel consumption deterioration and the soot generation, and furthermore, it can be realized with a simple structure. Moreover, since the main control variable is the EGR rate which is easy to control the follow-up with respect to the load fluctuation, the control can be surely performed without the control delay for the NOx reduction. It can be realized with a simple structure, which is advantageous as a widely versatile NOx reduction system.

10: engine body
11: Cylinder
15: Fuel injection valve
30: EGR unit
40: fuel supply unit
50: Control device
100: Engine system

Claims (10)

An engine body having a cylinder and a fuel injection valve,
A fuel supply unit for generating and supplying a water-emulsion fuel to the fuel injection valve,
An EGR unit that supplies EGR gas to the engine body;
And controls the flow rate of the EGR gas so as to use the water emulsion fuel in the entire region of the practical engine load and to control the amount of NOx discharged from the engine body to be equal to or less than a predetermined value Wherein the engine system comprises:
The method according to claim 1,
Wherein the control device is configured to maintain the water addition rate of the water emulsion fuel constant in the entire region of the load of the practical engine.
3. The method according to claim 1 or 2,
Wherein the control device is configured to increase the injection time per cycle as the injection amount per cycle of the water emulsion fuel increases.
The method of claim 3,
Wherein the controller is configured to advance the injection start timing of the water emulsion fuel within a range in which the maximum pressure in the cylinder does not exceed a predetermined upper limit value when the injection time per one cycle is increased. .
5. The method of claim 4,
Wherein the control device is configured to determine the injection start timing according to one or both of the water addition rate and the EGR rate.
An engine body having a cylinder and a fuel injection valve,
A fuel supply unit for generating and supplying a water-emulsion fuel to the fuel injection valve,
A control method for an engine system including an EGR unit for supplying EGR gas to the engine body,
Wherein the water emulsion fuel is used in the entire region of the practical engine load and the flow rate of the EGR gas is controlled so that the amount of NOx discharged from the engine body becomes a predetermined value or less.
The method according to claim 6,
And the water addition rate of the water emulsion fuel is kept constant in the entire region of the practical engine load.
8. The method according to claim 6 or 7,
Wherein the injection time per cycle is increased as the injection amount per cycle of the water-emulsion fuel increases.
9. The method of claim 8,
Wherein when the injection time per one cycle is increased, the injection start timing of the water emulsion fuel is advanced within a range in which the maximum pressure in the cylinder does not exceed a predetermined upper limit value.
10. The method of claim 9,
Wherein the injection start timing is determined based on one or both of the water addition rate and the EGR rate.

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