Control system of engine

A control system of an engine including a cylinder and a fuel injector, the fuel mixing with air to form a mixture gas and combusting inside the cylinder is provided, which includes a water injector for injecting supercritical water or subcritical water into the cylinder, and a controller. Within a high engine speed operating range, the controller controls the water injector to inject the one of supercritical water and subcritical water into the cylinder for a given period including a top dead center of the cylinder on compression stroke, such that when an injection amount of the one of supercritical water and subcritical water reaches half of a given injection amount, a crank angle of the engine corresponds to a retarding side of the top dead center, the given injection amount being a total amount of the supercritical water or subcritical water injected for the given period.

BACKGROUND

The present invention relates to a control system of an engine, which includes a cylinder and a fuel injector for injecting fuel into the cylinder, and in which a mixture gas of the fuel and air combusts inside the cylinder.

Conventionally, in engines, it has been discussed to improve engine performance by injecting water into the cylinder.

For example, JP4335533B discloses an engine which injects supercritical water into a cylinder before or simultaneously with fuel injection, so as to reduce a combustion temperature inside the cylinder and reduce production of NOR.

Here, in engines, it is desired to improve exhaust emission performance and fuel efficiency. Especially within a high engine speed operating range where an engine speed is high, since mechanical resistance increases and exhaust gas is discharged from an engine while its temperature still remains comparatively high, thermal efficiency is low. Therefore, it is desired to improve thermal efficiency and fuel efficiency within this high engine speed operating range.

SUMMARY

The present invention is made in view of the above problems and aims to provide a control system of an engine, which is capable of improving fuel efficiency.

According to one aspect of the present invention, a control system of an engine including a cylinder and a fuel injector for injecting fuel into the cylinder, the fuel mixing with air to form a mixture gas and combusting inside the cylinder is provided. The system includes a water injector for injecting one of supercritical water and subcritical water into the cylinder, and a controller for controlling the fuel injector and the water injector. Within a high engine speed operating range where an engine speed is a given reference speed or above, the controller controls the water injector to inject the one of supercritical water and subcritical water into the cylinder for a given period including a top dead center of the cylinder on compression stroke, such that when an injection amount of the one of supercritical water and subcritical water reaches half of a given injection amount, a crank angle of the engine corresponds to a retarding side of the top dead center, the given injection amount being a total amount of the one of supercritical water and subcritical water injected for the given period.

With the above configuration, the one of supercritical water and subcritical water is injected into the cylinder within the high engine speed operating range where the engine speed is comparatively high and thermal efficiency and fuel efficiency easily degrade. Therefore, the water can progress expansion stroke and an amount of the fuel injected into the cylinder can be suppressed, which results in improving fuel efficiency.

Especially with the above configuration, the one of supercritical water and subcritical water as water is injected into the cylinder, the thermal efficiency and fuel efficiency of the engine can be improved more reliably. Specifically, by injecting the one of supercritical water and subcritical water with a higher density than water in a normal gas phase (water vapor), a large amount of water can efficiently be introduced into the cylinder compared to injecting the water in the gas phase. Therefore, the amount of gas which remains within the cylinder and progresses the stroke, and furthermore an output of the engine, is efficiently increased. Further, by injecting the one of supercritical water and subcritical water, which requires little to no latent heat at all, into the cylinder, compared to injecting water in a normal liquid phase, a significant temperature drop inside the cylinder and the thermal efficiency degradation which accompany the latent heat can be avoided. Therefore, the thermal efficiency can be improved.

Further, within a segment of the high engine speed operating range where an engine load is particularly high, a temperature of exhaust gas becomes extremely high and it is undesirable for an exhaust passage, etc. In this regard, with the above configuration, the water can progress the expansion stroke to suppress the fuel amount as described above. Therefore, a combustion temperature can be reduced to lower the exhaust gas temperature.

Additionally, with the above configuration, within the high engine speed operating range, the one of supercritical water and subcritical water is injected into the cylinder for the given period including the compression top dead center, such that when the injection amount of the one of supercritical water and subcritical water reaches half of the given injection amount, the crank angle of the engine corresponds to the retarding side of the top dead center. Thus, a large total amount of the water injected into the cylinder can be secured and the amount of water supplied on the expansion stroke can be increased. Therefore, the water can progress the expansion stroke even more and fuel efficiency can be improved more reliably.

In the above configuration, the engine may be a four-stroke engine. Within the high engine speed operating range, the controller may control the injection of the one of supercritical water and subcritical water such that the crank angle when the injection amount of the one of supercritical water and subcritical water reaches half of the given injection amount corresponds to a given timing between the top dead center and 15° CA (crank angle) after the top dead center on the compression stroke.

Thus, the water can progress the expansion stroke more effectively.

Further in the above configuration, the system may also include a temperature increasing device for increasing a temperature of the one of supercritical water and subcritical water supplied to the water injector, by using exhaust gas discharged from the engine. Within a low engine speed operating range where the engine speed is below the reference speed, the controller may control the water injector to inject the one of supercritical water and subcritical water for a given period that at least partially overlaps with a period in which a cool-flame reaction of the mixture gas occurs inside the cylinder.

Thus, while improving fuel efficiency within the high engine speed operating range as described above, within the low engine speed operating range where the exhaust gas temperature is comparatively low, an ignition delay time is extended and the combustion is performed at a further retarded timing and, thus, the exhaust gas temperature can be increased. Therefore, also within the low engine speed operating range, the temperature increasing device can suitably increase the temperature of the water supplied to the water injector, by using the exhaust gas, and the one of supercritical water and subcritical water can be supplied to the water injector and the cylinder more reliably.

Further with the above configuration, the controller may control the water injector to inject the one of supercritical water and subcritical water into the cylinder within a high-load, high-speed segment that is a part of the high engine speed operating range where an engine load is a reference load or above, and the controller may control the water injector to stop the injection of the one of supercritical water and subcritical water into the cylinder within a low-load, high-speed segment that is a part of the high engine speed operating range where the engine load is below the reference load.

Thus, within the high-load, high-speed segment where the thermally efficiency particularly easily degrades and the exhaust gas temperature easily becomes high, by injecting the one of supercritical water and subcritical water, the thermal efficiency can effectively be improved and the exhaust gas temperature can effectively be reduced. Whereas within the low-load, high-speed segment, by stopping the injection of the one of supercritical water and subcritical water, the energy required for this injection can be conserved. In other words, the energy required for the generation of the one of supercritical water and subcritical water can be conserved.

DETAILED DESCRIPTION OF EMBODIMENT

FIG. 1is a view illustrating a structure of an engine system according to one embodiment of the present invention. The engine system of this embodiment includes an engine1of a four stroke type, an intake passage30for introducing air for combustion into the engine1, an exhaust passage40for discharging exhaust gas generated in the engine1, and a water circulation device60.

The engine1is, for example, a four-cylinder engine having four cylinders2. In this embodiment, the engine1is driven by receiving a fuel containing gasoline. The engine system of this embodiment is mounted on a vehicle, and the engine1is used as a drive source of the vehicle.

FIG. 2is a schematic cross-sectional view of the engine1.

The engine1includes a cylinder block3formed therein with the cylinders2, a cylinder head4formed on the cylinder block3, and pistons5fitted into the cylinders2to be reciprocatable (in up-and-down directions), respectively.

A combustion chamber6is formed above each piston5. The combustion chamber6is a so-called pent-roof type, and a roof surface of the combustion chamber6(a bottom surface of the cylinder head4) has a triangular roof shape formed by two inclining surfaces on an intake side and an exhaust side.

A crown surface5aof the piston5has a cavity10formed by denting, to an opposite side from the cylinder head4(downward), an area including a center of the crown surface5a. The cavity10is formed to have a volume corresponding to a major part of the combustion chamber6when the piston5is at a top dead center (TDC).

The cylinder head4is formed with intake ports16for introducing air supplied from the intake passage30into the cylinders2(combustion chambers6), and exhaust ports17for guiding out the exhaust gas generated inside the cylinders2to the exhaust passage40, respectively. In this embodiment, each cylinder2is formed with two intake ports16and two exhaust ports17. The cylinder head4is further provided with intake valves18for opening and closing the intake ports16on the cylinder2side, and exhaust valves19for opening and closing the exhaust ports17on the cylinder2side, respectively.

Each exhaust port17is attached with a heat pipe (temperature increasing device)70. In this embodiment, each exhaust port17is provided with one heat pipe70, i.e., each cylinder2is provided with two heat pipes70. The heat pipes70constitute a part of the water circulation device60, and a detailed description thereof is given later.

Further, fuel injectors (fuel supplier)21for injecting the fuel into the cylinders2, respectively, are provided to the cylinder head4. Each fuel injector21is arranged so that its tip part is located near a center axis of the corresponding cylinder2and oriented toward a substantially center portion of the crown surface of the piston5.

The fuel injector21injects fuel pumped from a fuel pump (not illustrated) into the cylinder2. In this embodiment, a premixed charge compression self-ignition combustion is performed, in which the fuel and air are premixed to form a mixture gas and the mixture gas is compressed to self-ignite near the TDC on compression stroke (CTDC) throughout all operating ranges of the engine body. Accordingly, in the example ofFIG. 2, ignition plugs for igniting the gas inside the cylinders2are not provided to the engine1; however, in a case where an additional ignition power is required for suitable combustion of the mixture gas in a cold start etc., the ignition plugs may suitably be provided to the engine1.

The fuel injector21injects into the cylinder2the fuel by an amount corresponding to an operating state of the engine1at a corresponding timing thereto. In this embodiment, the fuel is injected into the cylinder2before the CTDC, so as to cause the mixture gas to self-ignite near the CTDC.

The cylinder head4is further provided with water injectors22for injecting supercritical water or subcritical water into the cylinders2, respectively. As illustrated inFIG. 2, each water injector22is attached to the cylinder head4so as to inject the water into the corresponding combustion chamber6from a side thereof, and is arranged so that its tip part is oriented toward an inside of the combustion chamber6from an inner circumferential surface of the combustion chamber6.

Further, the water injector22is disposed adjacent to the exhaust port17. In this embodiment, the water injector22is disposed immediately below the exhaust port17. As the water injector22, for example, an injector for injecting fuel into the cylinder2, which is used in conventional engines, may be applied, for which description of a specific structure is omitted here. Note that the water injector22injects the supercritical water into the cylinder2at about 20 MPa, for example.

The supercritical/subcritical water injection into the cylinders2is mainly for improving fuel efficiency.

Specifically, by injecting the water into the cylinders2, the water is able to function as operation gas to progress expansion stroke. Thus, an amount of fuel supplied to the cylinders2is reduced while maintaining the same work amount, i.e., engine output, and fuel efficiency is improved.

Further, using the supercritical water or the subcritical water as the water injected into the cylinders2is to reliably improve the thermal efficiency and fuel efficiency of the engine1. Specifically, by injecting the supercritical water or the subcritical water with a higher density than water in a normal gas phase (water vapor), a large amount of water is efficiently introduced into the cylinders compared to injecting the water in the gas phase. Therefore, the amount of gas which remains within the cylinders and progresses the stroke, furthermore an output of the engine body, is efficiently increased. Further, by injecting one of the supercritical water and the subcritical water, which requires little to no latent heat at all, into the cylinders2, compared to injecting water in a normal liquid phase, a significant temperature drop inside the cylinders and a thermal efficiency degradation which accompany the latent heat are avoided. Therefore, the thermal efficiency is improved.

A specific description regarding this matter is given with reference toFIG. 3which is a pressure enthalpy diagram for water, of which a horizontal axis indicates enthalpy and a vertical axis indicates pressure. InFIG. 3, an area Z2is an area of liquid, an area Z3is an area of gas, and an area Z4is an area of coexisting liquid and gas. Lines LT350, LT400, . . . , LT1000indicated by solid lines are isothermal lines, each formed by connecting points of the same temperature. The numerals of the lines indicate temperatures (K). For example, LT350is an isothermal line of 350 K, and LT1000is an isothermal line of 1,000 K. Further, a point X1is the critical point and an area Z1is an area where a temperature and pressure are higher than the critical point X1, and the supercritical water belongs to this area Z1. Specifically, while the critical point of water is at the temperature of 647.3 K and the pressure of 22.12 MPa, the temperature and pressure of the supercritical water are the same or above, in other words, the temperature is 647.3 K or above and the pressure is 22.12 MPa or above.

InFIG. 3, lines LR0.01, LR0.1, . . . , LR500indicated by dashed lines are isopycnic lines, each formed by connecting points of the same density. The numerals of the lines indicate densities (kg/m3). For example, LR0.01is an isopycnic line of 0.01 kg/m3, and LR500is an isopycnic line of 500 kg/m3.

As is apparent from comparisons of these isopycnic lines LR with the areas Z1and Z3, the density of the water within the area Z1, in other words, the supercritical water, is about from 50 kg/m3to 500 kg/m3, which is close to that of water in the liquid phase and much higher than the density of gas.

Therefore, by injecting the supercritical water with the high density into the cylinders2, a large amount of water is efficiently introduced into the cylinders compared to injecting the water in the gas phase.

Note that the supercritical water generated by the engine system and injected into the cylinders2preferably has a density of 250 kg/m3or above.

Further, as indicated by the arrow Y1inFIG. 3, water in a normal liquid phase requires a high enthalpy to change into gas. In other words, the water in the normal liquid phase requires comparatively high latent heat to change into gas. In this regard, as indicated by the arrow Y2, the supercritical water requires almost no enthalpy, in other words, latent heat, to change into water in a normal gas phase.

Therefore, by injecting the supercritical water which does not require latent heat into the cylinders2, compared to injecting the water in the normal liquid phase, a significant temperature drop inside the cylinders and the thermal efficiency degradation which are accompanied by the latent heat are avoided.

Here, as is apparent fromFIG. 3, water which belongs to an area near the area Z1has a high density and requires low latent heat to change into gas, which are properties similar to the supercritical water. Therefore, although the supercritical water is injected into the cylinders2in this embodiment as described above, instead of the supercritical water, subcritical water which is water belonging to the area near the area Z1may be generated and injected into the cylinders2. For example, subcritical water within an area Z10where the temperature is 600 K or above and the density is 250 kg/m3or above (seeFIG. 4) may be generated and injected.

The intake passage30is provided with an air cleaner31and a throttle valve32located in this order from an upstream side. The air passes through the air cleaner31and the throttle valve32and then is introduced into the engine1.

The throttle valve32opens and closes the intake passage30. Note that, in this embodiment, while the engine is in operation, the throttle valve32is basically kept fully opened or nearly fully opened, and only in a limited operation condition (e.g., the engine is stopped) is the throttle valve32closed to block the intake passage30.

The exhaust passage40is provided with, in the following order from its upstream side, a purifying system41for purifying the exhaust gas, a heat exchanger (temperature increasing device)42, a condenser43, and an exhaust shutter valve44. The heat exchanger42and the condenser43constitute a part of the water circulation device60. The purifying system41comprises, for example, a three-way catalyst.

In this embodiment, as illustrated inFIG. 1etc., the purifying system41and the heat exchanger42are accommodated inside a heat accumulation case49for heat retention of the purifying system41and the heat exchanger42. The heat accumulation case49has a dual tube structure, of which an outer circumferential wall forms a space49atherein. The space49ais filled with a heat accumulation agent, and by this heat accumulation agent, the purifying system41and the heat exchanger42retain heat. In other words, when the exhaust gas at a high temperature flows into the purifying system41etc. located inside the heat accumulation case49, the heat accumulation agent within the space49ais warmed up by the exhaust gas, and then the purifying system41and the heat exchanger42retains heat by the heat accumulation agent. Examples of the heat accumulation agent include a latent heat accumulation agent that melts by being heated and accumulates thermal energy, such as erythritol, and a chemical heat accumulation agent that causes a chemical reaction by being heated and accumulates thermal energy, such as calcium chloride. By the heat retention with the heat accumulation agent as above, water inside a heat exchanging passage61a(described later) is effectively increased in temperature by the heat exchanger42, and the purifying system41is kept at a more suitable temperature.

The exhaust shutter valve44stimulates a recirculation of exhaust gas recirculation (EGR) gas to the intake passage30.

Specifically, with the engine system of this embodiment, an EGR passage51communicating a part of the intake passage30downstream of the throttle valve32with a part of the exhaust passage40upstream of the purifying system41is formed, and a portion of the exhaust gas is recirculated to the intake passage30. Further, the exhaust shutter valve44opens and closes the exhaust passage40. When the EGR is performed and pressure inside the exhaust passage40is low, an opening of the exhaust shutter valve44is narrowed to increase pressure inside an upstream part of the EGR passage51and stimulate the EGR gas recirculation.

The EGR passage51is provided with an EGR valve52for opening and closing the EGR passage51, and an amount of the EGR gas recirculated to the intake passage30is controlled by adjusting an opening of the EGR valve52. Further in this embodiment, the EGR passage51is provided with an EGR cooler53for cooling the EGR gas passing therethrough, and the EGR gas is recirculated to the intake passage30after being cooled by the EGR cooler53.

The EGR valve52is, for example, opened within an engine operating range where an engine load is comparatively low, and the EGR gas is introduced into the cylinders2within this range.

(4) Water Circulation Device

The water circulation device60generates supercritical water by using thermal energy of the exhaust gas.

The water circulation device60includes, in addition to the heat pipes70, the heat exchanger42and the condenser43, a water supply passage61connecting the water injectors22with the condenser43, a water tank62, a low-pressure pump63, and a high-pressure pump64.

The condenser43condenses water (water vapor) within the exhaust gas passing through the exhaust passage40, and the water condensed by the condenser43is supplied to the water injectors22. Thus in this embodiment, the water within the exhaust gas is used as the water injected into the cylinders2. The water tank62stores the condensed water therein. The condensed water generated by the condenser43is introduced into the water tank62through the water supply passage61and stored in the water tank62.

The low-pressure pump63is disposed at a position of the water supply passage61between the water tank62and the heat exchanger42, and pumps the condensed water inside the water tank62to the heat exchanger42. The condensed water in the water tank62is pumped to the heat exchanger42by the low-pressure pump63.

The heat exchanger42exchanges heat between the condensed water pumped by the low-pressure pump63and the exhaust gas passing through the exhaust passage40. The heat exchanger42is disposed at a position of the exhaust passage40downstream of the purifying system41, to be adjacent to the purifying system41.

In this embodiment, the part61aof the water supply passage61is inserted into the exhaust passage40to be in contact therewith, and thus, the heat exchanger42is formed. In other words, the heat exchanger42is formed by the part61aof the water supply passage61and the exhaust passage40so that the exhaust gas passes outside the part61a. Hereinafter, the part of the water supply passage61inserted into the exhaust passage40is referred to as the heat exchanging passage61a.

The heat exchanging passage61ais inserted into a part of the exhaust passage40immediately downstream of the purifying system41. Specifically, the heat exchanging passage61ais inserted into the exhaust passage40located inside the heat accumulation case49. Therefore in this embodiment, the heat exchanger42and the heat exchanging passage61a, as well as the purifying system41, are heat retained by the heat accumulation case49.

The condensed water inside the heat exchanging passage61ais increased in temperature by the exhaust gas passing through the part of the exhaust passage40where the heat exchanging passage61ais inserted. Specifically, if the temperature of the exhaust gas passing through the part of the exhaust passage40where the heat exchanging passage61ais inserted is higher than the temperature of the condensed water inside the heat exchanging passage61a, the thermal energy is applied to the condensed water from the exhaust gas and the condensed water is increased in temperature. Note that the temperature of the exhaust gas discharged from the engine1is constantly at 100° C. or above, which is constantly higher than the temperature of the condensed water in the liquid phase. Therefore, the condensed water inside the heat exchanging passage61ais always increased in temperature by the exhaust gas.

In this embodiment, since the heat exchanging passage61ais disposed immediately downstream of the purifying system41as described above, the condensed water inside the heat exchanging passage61aalso receives reaction heat at the purifying system41. Thus the condensed water is effectively increased in temperature. Additionally, the condensed water is effectively increased in temperature by the heat retention of the heat exchanging passage61aby the heat accumulation case49.

The high-pressure pump64pumps the condensed water from the heat exchanger42to the water injectors22. The high-pressure pump64is disposed in the water supply passage61, between the heat exchanger42(i.e., the heat exchanging passage61a) and the heat pipes70. The high-pressure pump64pumps the supercritical water to the water injectors22by pressurizing the condensed water increased in temperature by the heat exchanger42.

Here, the supercritical water after being highly pressurized by the high-pressure pump64, flows through a part of the water supply passage61downstream of the high-pressure pump64. Therefore, this part is piped for high-pressure use.

As described above, in this embodiment, the supercritical water is basically generated by increasing the condensed water in temperature and pressure by the heat exchanger42and the high-pressure pump64, and is supplied to the water injectors22.

Note that if the temperature of the exhaust gas discharged from the cylinders2is comparatively high, the water inside the water supply passage61is increased in temperature by this high-temperature exhaust gas, through the heat pipes70.

Specifically, each heat pipe70exchanges heat between the water pumped from the high-pressure pump64and the exhaust gas passing through the exhaust passage40. The heat pipe70increases the temperature of the condensed water only when the temperature of the exhaust gas is above a reference temperature.

In this embodiment, the heat pipe70has a substantially circular-column-shaped contour extending in a given direction.FIG. 5is a schematic cross-sectional view illustrating an operation of the heat pipe70. As illustrated in thisFIG. 5and alsoFIG. 2, the heat pipe70is arranged so that an end part71thereof in its longitudinal directions is inserted into the exhaust port17so as to be in contact with the exhaust gas, and the other end part72thereof is inserted into the water supply passage61so as to be in contact with the water in the water supply passage61.

As described above with reference toFIGS. 1 and 2, in this embodiment, the heat pipe70is inserted into each exhaust port17. Specifically, a pressure accumulation part65extending in the aligned direction of the cylinders2is provided to a part of the water supply passage61near its downstream end, and independent passages61bextend toward the water injectors22, respectively, from the pressure accumulation part65. Further each exhaust port17is provided with one heat pipe70, and the end parts71and72of each heat pipe70are inserted into the exhaust port17and the pressure accumulation part65, respectively.

In this embodiment, as illustrated inFIG. 2, the pressure accumulation part65is disposed near the cylinder head4, and the heat pipe70is built in the cylinder head4. Specifically, the pressure accumulation part65is located above the exhaust ports17, and the heat pipe70extends upward from an inner space of the exhaust port17and is inserted into the pressure accumulation part65. In this embodiment, the end part71of the heat pipe70on the exhaust port17side is provided with a stacked fin73formed by stacking metallic plate members in the up-and-down directions, so as to increase the transmitted amount of heat of the exhaust gas in the exhaust port17to the end part71.

As illustrated inFIG. 5, the heat pipe70is a pipe member made of a material with high thermal conductivity (e.g., metal), and includes liquid working medium S therein in a vacuum state. A porous member70a(e.g., metallic mesh) is provided in an inner wall of the heat pipe70to form a capillary structure, a so-called wick.

In the heat pipe70, the end part71inserted into the exhaust port17(hereinafter, suitably referred to as “the heat-reception-side end part71”) is warmed up by the exhaust gas and, after exceeding a given temperature, the working medium S vaporizes, and as indicated by the arrow Y10ofFIG. 5, it spreads toward the other end part72inserted into the water supply passage61(hereinafter, suitably referred to as “the heat-release-side end part72”). Here, the temperature of the exhaust gas in the exhaust port17decreases by applying its thermal energy to the heat pipe70, i.e., the working medium S. Further the vapor of the working medium S condenses by releasing heat from the heat-release-side end part72to the water supply passage61, and then reverts back to being liquid. Here, the water inside the water supply passage61is increased in temperature by receiving the thermal energy from the working medium S. As indicated by the arrow Y20ofFIG. 5, the working medium S, after reverting back to the liquid phase, returns to the heat-reception-side end part71due to the capillary action in the porous member70a, and then retrieves thermal energy from the exhaust gas to change into vapor again. This thermal energy is applied to the water inside the water supply passage61.

In this embodiment, the temperature of the exhaust gas within which the heat transfer occurs (reference temperature) is set to about 650 K, and the working medium S corresponding thereto is included in the heat pipe70. For example, the working medium S is cesium.

Thus in this embodiment, when the temperature of the exhaust gas becomes high by exceeding the given temperature and the working medium S exceeds its boiling point due to the heat pipe70, the thermal energy of the exhaust gas in the exhaust port17is applied to the water supply passage61and the temperature of the water therein is increased. Therefore, the temperature of the water inside the water supply passage61is substantially constantly increased by the heat exchanger42, and when the temperature of the exhaust gas is high, which is above the reference temperature, the heat pipe70increases the temperature of the water inside the water supply passage61even higher by the exhaust gas, and the supercritical water is generated by effectively using the energy of the exhaust gas. Especially since the heat pipe70is disposed near the cylinder2, the temperature of the water inside the water supply passage61is effectively increased by the heat pipe70, using the high thermal energy of the exhaust gas. Further, when the exhaust gas temperature is excessively high, the temperature of the exhaust gas which flows into the purifying system41is lowered by the heat pipe70, and when the exhaust gas temperature is low, the exhaust gas is flown into the purifying system41as it is so as to keep the purifying system41at a high temperature. Thus, the temperature of the purifying system41is kept within a suitable range.

(5) Control System

(5-1) System Configuration

FIG. 6is a block diagram illustrating a control system of the engine body. As illustrated inFIG. 6, the engine system of this embodiment is controlled by a Powertrain Control Module (PCM, may be referred to as the controller)100as a whole. The PCM100is, as well-known, comprised of a microprocessor including a CPU, a ROM, and a RAM.

The PCM100is electrically connected with various sensors for detecting an operating state of the engine.

For example, the cylinder block3is provided with a crank angle sensor SN1for detecting a rotational angle and speed of a crankshaft, in other words, an engine speed. Further, an airflow sensor SN2for detecting an air amount (fresh air amount) to be sucked into the cylinder2through the air cleaner31is provided in the intake passage30, between the air cleaner31and the throttle valve32. Moreover, an accelerator opening sensor SN3for detecting a position of an accelerator pedal (accelerator opening) which is disposed out of the range of the drawings and controlled by a driver of the vehicle is provided to the vehicle.

The PCM100controls respective parts of the engine while performing various determinations, operations etc. based on input signals from the various sensors. Specifically, the PCM100is electrically connected with the fuel injectors21, the water injectors22, the throttle valve32, the exhaust shutter valve44, the EGR valve52, the low-pressure pump63, the high-pressure pump64etc., and outputs control signals to these components based on results of the operations, etc.

For example, as described above, the throttle valve32is closed only in a limited operation condition, such as when the engine is stopped. The PCM100closes the throttle valve32in such an operation condition.

Further, as described above, the PCM100injects the fuel into the cylinder2before the CTDC.

Further, as described above, the PCM100opens the EGR valve52within the engine operating range where an engine load is comparatively low, and further when the engine load is particularly low and the pressure in the exhaust passage40is low, the PCM100narrows the opening of the exhaust shutter valve44. Moreover, the PCM100closes the EGR valve52and stops the recirculation of the EGR gas within the engine operating range where an engine load is comparatively high.

Further, the PCM100controls the water injector22based on a control map ofFIG. 7.FIG. 7is the control map of which a horizontal axis indicates the engine speed and a vertical axis indicates the engine load. As illustrated inFIG. 7, in this embodiment, a low speed range A1where the engine speed is below a reference speed N1and a high speed range A2where the engine speed is the reference speed N1or above are designed as a control range of the engine body. The high speed range A2is designed to have a high-load, high-speed segment A2_1where the engine load is a reference load T1or above, and a low-load, high-speed segment A2_2where the engine load is below the reference load T1. Hereinafter, controls relating to the water injection within the range and segments A1, A2_1and A2_2are respectively described.

(5-2) Low Speed Range

Within the low speed range A1, in order to improve fuel efficiency, the low-pressure pump63and the high-pressure pump64are driven to inject the supercritical water into the cylinder2from the water injector22.

Note that within the low speed range A1, the exhaust gas temperature easily becomes low compared to the high speed range A2due to a cooling loss being high, etc., and the supercritical water may not suitably be generated by the energy of the exhaust gas. Specifically within the low speed range A1, since the temperature of the exhaust gas is low, the heat pipe70does not operate and the condensed water may not suitably be increased in temperature with the exhaust gas by the heat exchanger42.

Therefore, within the low speed range A1, the supercritical water is injected to extend an ignition delay time, so that the supercritical water is generated more reliably. In other words, by extending the ignition delay time so as to retard the timing of combustion, the temperature of the exhaust gas is increased and the temperature of the condensed water is also suitably increased by the heat exchanger42, and the opportunity for the heat pipe70to operate is increased so that the supercritical water is generated more reliably.

Here, the ignition time delay is a period of time from the fuel injection until the mixture gas ignites, and here, the ignition timing is a timing at which a cool-flame reaction of the mixture gas ends and a hot-flame reaction starts.

A specific description regarding this matter is given with reference toFIG. 8which shows charts schematically illustrating one example of a heat release rate, a fuel injection ratio, and a water injection ratio when the premixed charge compression self-ignition combustion is performed within the low speed range A1.

As illustrated inFIG. 8, in the premixed charge compression self-ignition combustion, the fuel is injected at a timing t11(injection Q1), then the mixture gas starts releasing heat (oxidation reaction) when the temperature and pressure reach given values, and accordingly, the heat release rate gradually increases or first gradually increases and then drops. Then, at a timing t13, the heat release rate sharply increases. Here, a low-temperature heat release which is the reaction occurring between the timings t11and t13, and causes a slight heat generation to the extent that the cooling loss, etc. do not occur, is referred to as the cool-flame reaction. A main combustion which occurs after the cool-flame reaction is referred to as the hot-flame reaction. Further, the timing at which the hot-flame reaction starts (the timing at which the heat release rate sharply rises, corresponding to the timing t13inFIG. 8) is referred to as the ignition timing. A period of time from the fuel injection timing (the timing t11inFIG. 8) to the ignition timing defined as described above is referred to as the ignition delay time.

Note that, inFIG. 8, although the horizontal axis indicates the crank angle, the ignition delay time is a parameter defined by time, not the crank angle. Moreover, in the hot-flame reaction, the temperature of the mixture gas is known to be about 1,500 K or above. Therefore, a timing at which the temperature of the mixture gas reaches or exceeds 1,500 K may be the ignition timing and a period of time until this timing may be the ignition delay time. Further inFIG. 8, the case where the fuel injection is performed only once is illustrated; however, in a case where the fuel injection is performed by being divided into a plurality of injections, a period of time from the last fuel injection among the injections which cause the hot-flame reaction, until the hot-flame reaction starts is the ignition delay time.

Within the low speed range A1, as illustrated inFIG. 8, the injection of the supercritical water by the water injector22is started between the completion of a fuel injection Q1and ignition of the mixture gas, so that the ignition delay time becomes longer. In other words, by leaving in the cylinder2water which is a substance which does not contribute in combustion (hereinafter, may be referred to as “the inactive substance”) before the mixture gas ignition, the ratio of the fuel and air with respect to the entire gas inside the cylinder2is reduced and the increase of the gas temperature inside the cylinder2is suppressed. Thus, a reaction speed of the fuel and air is kept low and the ignition delay time is extended. Note that in the example ofFIG. 8, the fuel is injected into the cylinder2by the fuel injector21to start the combustion in a latter half of the compression stroke (between 90° CA before the CTDC and the CTDC), and an injection W1of the supercritical water into the cylinder2is performed in a given period from the completion timing t11of the fuel injection Q1until the CTDC.

Further within the low speed range A1, as illustrated inFIG. 8, the supercritical water is injected so that the cool-flame reaction period and a water injection period overlap. In the example ofFIG. 8, a start timing t21of the water injection W1is close to a start timing of the cool-flame reaction.

Further, for the start timing of the water injection, it may be considered to inject the supercritical water into the cylinder2before the fuel injection completes. However, the present inventors found through their extensive research that the ignition delay time can be extended longer if the start timing of the water injection is after the fuel injection completion, rather than before the fuel injection completion.

FIG. 9Aillustrates a temperature of the mixture gas when a start timing tW of the water injection is before a fuel injection period tQ, specifically, before the fuel injection start timing, andFIG. 9Billustrates a temperature of the mixture gas when the start timing tW of the water injection is after the fuel injection period tQ, specifically, after the fuel injection completion timing. InFIG. 9A, LW1, LW2and LW3indicate mixture gas temperatures when the ratio of the injection amount of the supercritical water to the fuel injection amount is 1, 3, and 4 times, respectively. Similarly inFIG. 9B, LW11, LW12and LW13indicate mixture gas temperatures when the ratio of the injection amount of the supercritical water to the fuel injection amount is 1, 3, and 4 times, respectively.

As is apparent from a result of a comparison betweenFIGS. 9A and 9B, the ignition delay time becomes longer when the water injection is started after the fuel injection completion timing.

Moreover, the present inventors found that even after the fuel injection, the ignition delay time can be extended longer when the supercritical water is injected during the cool-flame reaction, rather than before the cool-flame reaction.

It is assumed that the ignition delay time can be extended longer because when the water is injected during the cool-flame reaction and the mixture gas combines with the water, a generation amount of formaldehyde during the cool-flame reaction increases and the heat release (oxidation reaction) becomes slower. Note thatFIG. 9Bis an example of a case where the supercritical water is injected during the cool-flame reaction.

Furthermore, the present inventors found that if the water is injected particularly in an initial stage of the cool-flame reaction, the heat release becomes even slower. Therefore, in this embodiment, the first water injection W1is started during the cool-flame reaction, particularly in the initial stage thereof.

Here, in order to suitably obtain the engine torque, it is preferred that, as illustrated inFIG. 8, the hot-flame reaction which is the main combustion of the mixture gas is performed at a timing after the CTDC and comparatively close to the CTDC, and accordingly, the cool-flame reaction starts before the CTDC. Therefore in this embodiment, by controlling the timing of the water injection W1as described above, a centroid α of the injection ratio of the water injection W1becomes before the CTDC within the low speed range A1. In other words, in order to obtain more efficiently a work amount of the water injected into the cylinder2, although it is preferable to inject a larger amount of water on expansion stroke, in this embodiment, within the low speed range A1, the ignition delay time is extended to generate and inject the supercritical water more reliably as described above so as to improve fuel efficiency. Note that the term “centroid” corresponds to a crank angle when the injection amount of the water reaches half of a total amount of the water injected for the water injection period.

Additionally, when the ignition delay time becomes longer as described above, effects are obtained, in which a more suitable premixed charge compression self-ignition combustion is realized and increases in combustion noises and smoke production are suppressed.

Specifically, in the premixed charge compression self-ignition combustion, if the ignition delay time which is the period of time from injection of the fuel into the cylinder2until the ignition of the fuel is short, the combustion starts in a state where the injected fuel is not sufficiently mixed with air, and the suitable premixed charge compression self-ignition combustion is not realized. As a result, an issue that pressure inside the cylinder2, i.e., in-cylinder pressure, sharply rises and combustion noises become louder, and an issue that the smoke production increases, occurs. In this regard, when the ignition delay time becomes longer as described above, the combustion is generated in the state where the fuel is sufficiently mixed with the air, the suitable premixed charge compression self-ignition combustion is realized, and the increases in combustion noises and the smoke production are suppressed.

Within the low-load, high-speed segment A2_2, in order to improve the energy efficiency of the entire system, the low-pressure and high-pressure pumps63and64are stopped so that the injection of the supercritical water into the cylinder2from the water injector22is stopped.

Specifically, within the high speed range A2where the engine speed is high, the mechanical resistance increases. However, within the low-load, high-speed segment A2_2, the thermal efficiency is comparatively high and the degradation of fuel efficiency is comparatively small. Therefore, within the low-load, high-speed segment A2_2, consumption of energy, such as power, which accompanies the driving of the low-pressure and high-pressure pumps63and64, may exceed an amount of the fuel consumption reduced by a reduction effect which is obtained by injecting the supercritical water into the cylinder2from the water injector22. For this reason, in this embodiment, in order to improve the energy efficiency of the entire system, within the low-load, high-speed segment A2_2, the low-pressure and high-pressure pumps63and64are stopped so that the injection of the supercritical water into the cylinder2from the water injector22is stopped as described above.

On the other hand, within the high-load, high-speed segment A2_1, in addition to the increase of the mechanical resistance, the thermal efficiency is low. Therefore, the degradation level of fuel efficiency easily becomes worse. Further within the high-load, high-speed segment A2_1, the temperature of the exhaust gas easily becomes extremely high, and the exhaust passage40and the purifying system41may receive a thermal damage. For this reason, within the high-load, high-speed segment A2_1, in order to improve fuel efficiency and reduce the thermal damage of the purifying system41, etc., the low-pressure and high-pressure pumps63and64are driven to inject the supercritical water into the cylinder2from the water injector22.

Here, within the high-load, high-speed segment A2_1, a period of time corresponding to 1° CA (one crank angle) is short, and in order to secure a sufficient amount of water injected into the cylinder2, the water injection period needs to be longer in terms of crank angle compared to the low speed range A1. Therefore, within the high-load, high-speed segment A2_1, as illustrated inFIG. 10, a water injection W10is performed for a given period to continue over the CTDC. In this embodiment, the water injection W10is performed to continue from a timing t31(after a given period from a fuel injection Q10completion) to a timing t32which is after the CTDC.

Further within the high-load, high-speed segment A2_1, since the temperature of the exhaust gas is sufficiently high as described above, there is no need to extend the ignition delay time to increase the temperature of the exhaust gas in order to generate the supercritical water as the low speed range A1. Therefore, within the high-load, high-speed segment A2_1, as illustrated inFIG. 10, a centroid β of the injection ratio of the water injection W10is brought to after the CTDC so as to increase the work amount of the water. In this embodiment, the injection ratio of the water injection W10is fixed and first and second water injection periods t_a and t_b are designed so that the second water injection period t_b from the CTDC to the completion timing t32of the water injection is longer than the first water injection period t_a from the start timing t31of the water injection to the CTDC, thus, the centroid β of the injection ratio of the water injection W10is brought to after the CTDC.

Further, it is already known that injected water functions effectively by bringing the centroid β of the injection ratio of the water injection W10to a given timing between the CTDC and 15° CA after the CTDC. Therefore, in this embodiment, the supercritical water is injected into the cylinder2so that the centroid β of the injection ratio of the water injection W10is at a given timing between the CTDC and 15° CA after the CTDC, for example, 10° CA after the CTDC.

As described above, in this embodiment, within the high-load, high-speed segment A2_1where the degradation of fuel efficiency easily becomes comparatively large and the temperature of the exhaust gas easily becomes extremely high, the supercritical water is injected into the cylinder2for the given period including the CTDC, and the centroid β of the injection ratio of the supercritical water is brought to be on the retarding side of the CTDC.

Therefore, within the high-load, high-speed segment A2_1, a large total amount of the supercritical water injected into the cylinder2is secured and the amount of supercritical water supplied on the expansion stroke is increased, and thus the progression of expansion stroke with the supercritical water is stimulated and fuel efficiency is improved. Moreover, the temperature of the exhaust gas is lowered and the thermal damage of the purifying system41, etc. is reduced. Especially within the high-load, high-speed segment A2_1, in order to lower the temperature of the exhaust gas so as to reduce the thermal damage of the purifying system41, etc., so-called enrichment is conventionally performed, in which an excessive amount of fuel is injected into the cylinder2, whereas in this embodiment, the temperature of the exhaust gas is reduced without the enrichment or while reducing the excessive fuel amount, and thus, fuel efficiency is effectively improved.

Further in this embodiment, since the supercritical water which has a higher density than normal water and requires no latent heat is injected into the cylinder2, a large amount of water is efficiently injected into the cylinder2, and the thermal efficiency and fuel efficiency of the engine are improved more reliably as described above.

Further in this embodiment, the water within the exhaust gas in the exhaust passage40is utilized and this water is increased in temperature with the energy of the exhaust gas by the heat exchanger42and the heat pipe70so as to generate the supercritical water. Therefore, the energy efficiency of the entire system is improved.

Further, within the low speed range A1where the temperature of the exhaust gas easily becomes low, at least a portion of the supercritical water is injected during a cool-flame reaction period, thus, the ignition delay time of the mixture gas is extended and the combustion timing is retarded. Therefore, also within the low speed range A1, the supercritical water is suitably generated by the energy of the exhaust gas and fuel efficiency is improved by this supercritical water.

In this embodiment, the case where the supercritical water (i.e., water) is injected into the cylinder2is described; however, as described above, instead of the supercritical water, subcritical water which has properties similar to the supercritical water may be injected into the cylinder2. Also in this case, since the density is higher than normal water and the required latent heat is extremely low, the large amount of water is efficiently injected into the cylinder2, and the thermal efficiency and fuel efficiency of the engine are improved more reliably.

Further in this embodiment, the case where the injection of the supercritical water into the cylinder2is stopped within the low-load, high-speed segment A2_2which is the part of the high speed range A2where the engine load is below the reference load T1is described; however, the supercritical water may be injected into the cylinder2within the low-load, high-speed segment A2_2.

Note that, as described above, within the low-load, high-speed segment A2_2, the consumption of energy, such as power, which accompanies the driving of the low-pressure and high-pressure pumps63and64, may exceed the reduced amount of the fuel consumption amount in the reduction effect which is obtained by injecting the supercritical water into the cylinder2from the water injector22. For this reason, by stopping the low-pressure and high-pressure pumps63and64to stop the injection of the supercritical water into the cylinder2from the water injector22within the low-load, high-speed segment A2_2, the energy efficiency of the entire system is improved more reliably.

Further, the water circulation device60may be omitted and, for example, a heater may separately be provided to generate the supercritical water. Note that by providing the water circulation device60as described above, it is possible to improve the energy efficiency of the entire system.

Further in this embodiment, within the high-load, high-speed segment A2_1, the case is described, in which the centroid β of the injection ratio of the water injection W10is brought to after the CTDC by adjusting the water injection timing, specifically, by fixing the water injection ratio and designing the second water injection period t_b from the CTDC to the completion timing t32of the water injection to be longer than the first water injection period t_a from the start timing t31of the water injection to the CTDC; however, the centroid β may be brought to after the CTDC by changing the injection ratio of the water injection W10over time (crank angle). For example, the centroid β may be brought to after the CTDC by increasing the water injection ratio after the CTDC while designing the time length from the start timing of the water injection to the CTDC to be the same as the time length from the CTDC to the completion timing of the water injection, so that the total amount of water injected into the cylinder2after the CTDC becomes larger than the amount before the CTDC.

LIST OF REFERENCE CHARACTERS