JP2013068148A - Spark ignition type direct injection engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a spark ignition type direct injection engine that can reduce the cooling loss.SOLUTION: A fuel injection valve 33 is arranged at the axis X of a cylinder 11 and injects the atomized fuel in such a way as spreading outward about the radial direction, while a piston 15 has a cavity 15a at its crown face, and the sidewall of the cavity 15a is inclined relative to the axial direction of the cylinder so that the diameter increases from the cavity bottom toward the cavity opening. By a controller 100, a period of injecting the fuel from the fuel injection valve is set within a range from the final stage of the compression stroke to the early stage of the expansion stroke when the operating condition of the engine body (engine 1) is in the high-load region, penetration of atomized fuel is decreased in the first half of the injecting period, and in the later half, the penetration of the atomized fuel is set larger.

Description

  The technology disclosed herein relates to a spark ignition direct injection engine.

  For example, in Patent Document 1, in order to increase the theoretical thermal efficiency of a spark-ignition gasoline engine, a cavity recessed in the lower surface of the cylinder head and a protrusion projecting from the piston crown surface divide the combustion chamber into the central combustion chamber and the main combustion chamber. The combustion chamber as a whole is set to a compression ratio as high as about 16, and the air-fuel mixture is relatively rich in the central combustion chamber, and the air-fuel mixture is relatively lean in the main combustion chamber. Thus, an engine having a lean air-fuel mixture is described for the entire combustion chamber.

  Further, for example, Patent Document 2 describes a technique in which a surface that defines a combustion chamber is formed of a heat insulating material including a large number of bubbles from the viewpoint of reducing cooling loss and improving thermal efficiency.

Japanese Patent Laid-Open No. 9-217627 JP 2009-243355 A

  By the way, in the Otto cycle, which is the theoretical cycle of a spark ignition engine, the theoretical thermal efficiency increases as the compression ratio increases and the specific heat ratio of the gas increases. For this reason, the combination of the high compression ratio and the leaning of the air-fuel mixture as described in Patent Document 1 is advantageous to some extent for improving the thermal efficiency (the illustrated thermal efficiency), but in this case, the compression ratio of 15 The illustrated thermal efficiency is maximized to some extent, and even if the compression ratio is increased further, the illustrated thermal efficiency does not increase (inversely, the higher the compression ratio is, the lower the illustrated thermal efficiency is). This is because, since the air-fuel mixture is lean, a relatively large amount of air is introduced into the cylinder. On the other hand, a large amount of air in the cylinder is greatly compressed as the compression ratio increases, and the combustion pressure and temperature are reduced. This is because it becomes significantly higher. That is, the amount of heat released through the cylinder wall and the like is increased by a high combustion pressure and combustion temperature, and the cooling loss is greatly increased. As a result, the illustrated thermal efficiency is lowered. For this reason, it is important to reduce the cooling loss in order to improve the thermal efficiency in the high compression ratio engine.

  Accordingly, the inventors of the present invention have formed a relatively small volume cavity in the crown surface of the piston in order to realize a high compression ratio with a geometric compression ratio of 18 or more, thereby increasing the squish area. After that, we studied to inject fuel spray into the cavity and burn it.

  Here, the cooling loss is proportional to the temperature of the combustion gas in contact with the partition wall of the combustion chamber and the temperature and temperature difference between the partition walls, so that the combustion gas itself is prevented from contacting the partition wall. We studied to reduce the fuel spray penetration and thereby prevent the fuel spray from reaching the side wall and bottom wall of the cavity. Such stratification in the cavity is advantageous in reducing the cooling loss by suppressing or avoiding the combustion gas from coming into contact with the side wall and the bottom wall of the cavity.

  When the operating state of the engine is in a low load region, the cooling loss can be effectively reduced by the stratification in the cavity by the fuel spray of the small penetration described above. However, when the engine operating state is in a high load region and high torque is required, the stratified combustion in the cavity causes a new problem that the desired torque cannot be obtained due to insufficient air. The present inventors have found out.

  The technology disclosed herein has been made in view of such a point, and the object is to achieve both reduction in cooling loss and generation of high torque in a spark-ignition direct injection engine with a high compression ratio. There is to make it.

  When the engine operating state is in a high load region, the inventors of the present application increase the fuel spray penetration to increase the air utilization rate by the air present in the squish area, and a portion of the injected fuel spray is squished. Made to reach the area.

  Specifically, the spark ignition type direct injection engine disclosed herein has an engine body having a cylinder in which a geometric compression ratio is set to 18 or more, and is fitted into the cylinder, and a cavity is formed in the crown surface thereof. According to the operating state of the engine body, a piston formed in a recess, a fuel injection valve configured to inject fuel spray into a combustion chamber defined by the cylinder and the piston of the engine body, And a controller configured to control a mode of fuel injection into the combustion chamber through the fuel injection valve.

  The fuel injection valve is disposed at the axial center of the cylinder and injects fuel spray so as to expand radially outward therefrom, and the cavity is directed from the bottom of the cavity toward the cavity opening. The side wall is configured to be inclined with respect to the axial direction of the cylinder so as to expand the diameter, and the controller is in a high load region where the operating state of the engine body is higher than a predetermined load. The period during which fuel is injected by the fuel injection valve is set within the range from the end of the compression stroke to the initial stage of the expansion stroke, the penetration of the fuel spray is reduced in the first half of the injection period, and the second half of the injection period Increase the fuel spray penetration.

  According to this configuration, the fuel injection valve is disposed at the axial position of the cylinder, and the fuel injection valve injects fuel spray from the axial position toward the outside in the radial direction.

  The controller sets the fuel injection period within the range from the end of the compression stroke to the initial stage of the expansion stroke when the operating state of the engine body is in a high load region higher than a predetermined load, and in the first half of the injection period, Reduce spray penetration. In the first half of the injection period, the piston is located near the compression top dead center, so that the injected fuel spray reaches the inside of the cavity formed in the crown of the piston, but back pressure (in other words, in the cylinder The atmospheric pressure of the fuel spray is high and the penetration of the fuel spray is small. Therefore, the fuel spray is prevented or suppressed from reaching the side wall of the cavity. As a result, the combustion gas from the fuel injected in the first half of the injection period is prevented from coming into contact with the side wall of the cavity and the like, which is advantageous for suppressing the cooling loss. In particular, since the fuel injected in the first half of the injection period burns in a state where the pressure and temperature in the cylinder near the compression top dead center are high, the temperature of the combustion gas becomes high and the cooling loss tends to increase. As described above, since the contact between the combustion gas and the sidewall of the cavity itself is avoided or suppressed, it is particularly effective for suppressing the cooling loss.

The controller increases the fuel spray penetration in the second half of the injection period.
The latter half of the injection period corresponds to the expansion stroke and the back pressure is low, so that the fuel spray injected into the cavity reaches the side wall of the cavity (in other words, collides). Here, since the side wall of the cavity is inclined with respect to the axial direction, a part of the fuel spray that collides with the side wall moves along the side wall, and from the cavity opening to the outside of the cavity, specifically, the cavity. Reach the squish area, which is the periphery of the opening. Thus, the fuel injected in the latter half of the injection period burns using the air present in the squish area, so that the air utilization rate is increased, which is advantageous for improving the torque. In addition, the fuel injected in the latter half of the injection period is combusted in an expansion stroke in which the temperature and pressure in the cylinder gradually decrease, and the temperature of the combustion gas becomes relatively low. Even if it contacts, cooling loss is suppressed.

  In this way, by switching the fuel spray penetration between the first half and the second half of the fuel injection period in the high load region, both reduction of cooling loss and securing of high torque are compatible.

  It should be noted that when the operating state of the engine body is in the low load region, it is preferable to set the fuel spray penetration small rather than switching the fuel spray penetration between the first half and the second half of the fuel injection period. By doing so, the combustion gas does not come into contact with the side wall of the cavity and the like, and the cooling loss is effectively suppressed, which is advantageous in improving fuel consumption.

  The fuel injection valve has an outer opening valve that opens and closes a nozzle opening, and the greater the lift amount of the outer opening valve, the greater the penetration of fuel spray injected from the nozzle opening into the combustion chamber. The controller sets the lift amount of the outer valve to a predetermined amount in the first half of the injection period, and sets the lift amount of the outer valve to a predetermined amount in the second half of the injection period. It may be set larger.

  In other words, the fuel injection valve of the outer opening type is such that the lift amount of the outer opening valve and the penetration of fuel spray are approximately proportional, and therefore the injection amount is controlled by controlling the lift amount of the fuel injection valve through the controller. Switching between the fuel spray penetration in the first half of the period and the fuel spray penetration in the second half of the injection period is easily realized.

  Here, the fuel injection may be performed at least twice, that is, divided injection, that is, the first stage injection performed in the first half of the injection period and the second stage injection performed in the second half of the injection period. Moreover, it is good also as not continuous injection but continuous injection which changes the lift amount of an outer opening valve in the middle of one injection which continues during an injection period.

  The cavity may have a bottom wall inclined with respect to a direction orthogonal to the axis of the cylinder so that the bottom of the cavity rises from the outside in the radial direction toward the center.

  In this way, a part of the fuel spray injected so as to expand radially outward from the axial center of the cylinder is guided along the inclined bottom wall and reaches the peripheral edge of the cavity bottom. From there, it moves along the inclined side wall and reaches the outside of the cavity through the cavity opening. As a result, part of the injected fuel spray reaches the squish area with certainty, and the air utilization rate and thus the torque are reliably improved.

  The engine body may be arranged such that an axis of the cylinder is deviated toward a crank angle advance side with respect to a rotation center of the crankshaft.

  Such an offset arrangement of the cylinders increases the descending speed of the piston after the compression top dead center, so that the squish area is quickly expanded. This is advantageous in improving the air utilization rate when the fuel injected in the latter half of the injection period from the end of the compression stroke to the beginning of the expansion stroke is combusted.

  As described above, the spark ignition direct injection engine is a high compression ratio engine in which a cavity is recessed in the crown surface of a piston, and fuel injection is performed when the operating state is in a high load region. In the first half of the period, the fuel spray penetration is reduced to reduce cooling loss, while in the second half of the injection period, the fuel spray penetration is increased to increase air utilization and improve torque. It is possible to achieve both reduction of loss and improvement of torque.

It is a figure showing roughly composition of a spark ignition type direct injection engine. It is sectional drawing which shows the structure of a fuel injection valve. It is a figure which illustrates the fuel injection characteristic of a fuel injection valve. It is a figure which compares the fuel-injection aspect (solid line) in a high load area | region with the fuel-injection aspect (one-dot chain line) in a low load area | region. It is an illustration of the state (upper figure) of the fuel spray when the outer opening valve is low lift, and the state (lower figure) of the fuel spray when the outer opening valve is high lift. It is a figure which illustrates the fuel-injection aspect different from FIG. It is the schematic which shows the engine of a structure different from FIG.

  Hereinafter, an embodiment of a spark ignition direct injection engine (hereinafter also simply referred to as an engine) will be described with reference to the drawings. The following description of the preferred embodiment is merely exemplary in nature. As shown in FIG. 1, the engine system includes an engine 1, various actuators associated with the engine 1, various sensors, and an engine controller 100 that controls the actuators based on signals from the sensors.

  The engine 1 is a spark ignition internal combustion engine, and has only a plurality of cylinders 11 although only one is shown in the figure. The engine 1 is mounted on a vehicle such as an automobile, and its output shaft is connected to drive wheels via a transmission, although not shown. The vehicle is propelled by the output of the engine 1 being transmitted to the drive wheels. The engine 1 includes a cylinder block 12 and a cylinder head 13 mounted thereon, and a cylinder 11 is formed inside the cylinder block 12.

  Although conceptually shown in FIG. 1, in this engine, the axis X of the cylinder 11 is offset by about 20 mm from the crankshaft rotation center C to the leading side of the crank angle indicated by the arrow in FIG. 1. Has been placed. With the offset arrangement of the cylinder 11, the lowering speed of the piston 15 after the compression top dead center is particularly increased in the engine 1.

  The piston 15 is slidably inserted into each cylinder 11, and defines a combustion chamber 17 together with the cylinder 11 and the cylinder head 13. In this embodiment, the combustion chamber 17 is configured such that the lower surface of the cylinder head 13 (the ceiling surface that defines the upper surface of the combustion chamber 17) and the crown surface of the piston 15 are surfaces that are perpendicular to the axis X of the cylinder 11. Has been. A cavity 15a having a relatively small volume is formed in the crown surface of the piston 15 so as to be recessed. In this example, as shown in an enlarged view in FIG. 5, the cavity 15 a has a part of the bottom wall orthogonal to the axis X of the cylinder 11 so that the bottom of the cavity 15 a protrudes from the outside in the radial direction toward the center. It is inclined with respect to the direction of Further, the side wall of the cavity 15 a is inclined with respect to the axis X of the cylinder 11 so that the diameter of the cavity 15 a increases from the bottom of the cavity toward the cavity opening. In this way, in the engine 1, a high geometric compression ratio is realized by the small cavity 15a and the squish area 15b enlarged accordingly.

  Although only one is shown in FIG. 1, two intake ports 18 are formed in the cylinder head 13 for each cylinder 11, and each communicates with the combustion chamber 17 by opening on the lower surface of the cylinder head 13. Similarly, two exhaust ports 19 are formed in the cylinder head 13 for each cylinder 11, and each communicates with the combustion chamber 17 by opening on the lower surface of the cylinder head 13. The intake port 18 is connected to an intake passage (not shown) through which fresh air introduced into the cylinder 11 flows. A throttle valve 20 that adjusts the intake air flow rate is provided upstream of the intake passage, and the throttle valve 20 receives a control signal from the engine controller 100 and its opening degree is adjusted. On the other hand, the exhaust port 19 is connected to an exhaust passage (not shown) through which burned gas (exhaust gas) from each cylinder 11 flows. An exhaust gas purification system having one or more catalytic converters is disposed in the exhaust passage. The catalytic converter includes, for example, a three-way catalyst.

  The intake valve 21 and the exhaust valve 22 are arranged so that the intake port 18 and the exhaust port 19 can be shut off (closed) from the combustion chamber 17, respectively. The intake valve 21 is driven by an intake valve drive mechanism, and the exhaust valve 22 is driven by an exhaust valve drive mechanism. The intake valve 21 and the exhaust valve 22 reciprocate at a predetermined timing to open and close the intake port 18 and the exhaust port 19 to exchange gas in the cylinder 11. Although not shown, the intake valve drive mechanism and the exhaust valve drive mechanism each have an intake camshaft and an exhaust camshaft that are drivingly connected to the crankshaft. These camshafts are synchronized with the rotation of the crankshaft. Rotate. Further, at least the intake valve drive mechanism includes a hydraulic or mechanical phase variable mechanism (Variable Valve Timing: VVT) 23 that can continuously change the phase of the intake camshaft within a predetermined angle range. ing. You may make it provide the lift variable mechanism (CVVL (Continuous Variable Valve Lift)) which can change a valve lift amount continuously with VVT23.

  The spark plug 31 is attached to the cylinder head 13 by a known structure such as a screw. In this embodiment, the spark plug 31 is attached to the axis X of the cylinder 11 so as to be inclined toward the exhaust side, and its tip (electrode) faces the ceiling of the combustion chamber 17. The arrangement of the spark plug 31 is not limited to this. The ignition system 32 receives a control signal from the engine controller 100 and energizes the spark plug 31 to generate a spark at a desired ignition timing. As an example, the ignition system 32 may include a plasma generation circuit, and the ignition plug may be a plasma ignition type plug.

  In this embodiment, the fuel injection valve 33 is disposed along the axis X of the cylinder 11 and is attached to the cylinder head 13 with a known structure such as a bracket. The tip of the fuel injection valve 33 faces the center of the ceiling of the combustion chamber 17.

  As shown in FIG. 2, the fuel injection valve 33 is an outer valve-opening injector having an outer valve 42 that opens and closes a nozzle port 41 that injects fuel into the cylinder 11. The nozzle port 41 is formed in a tapered shape whose diameter increases toward the distal end side at the distal end portion of the fuel pipe 43 extending along the axis X of the cylinder 11. The proximal end of the fuel pipe 43 is connected to a case 45 in which a piezo element 44 is disposed. The outer opening valve 42 includes a valve main body 42 a and a connecting portion 42 b that is connected from the valve main body 42 a through the fuel pipe 43 to the piezo element 44. A portion of the valve body 42a on the side of the connecting portion 42b has substantially the same shape as the nozzle port 41, and when the portion is in contact (sitting) with the nozzle port 41, the nozzle port 41 is closed. . At this time, the tip side portion of the valve main body 42 a is in a state of protruding to the outside of the fuel pipe 43.

  The piezoelectric element 44 presses the outer open valve 42 toward the combustion chamber 17 in the direction of the axis X of the cylinder 11 by deformation due to application of voltage, so that the outer open valve 42 is closed from the state in which the nozzle port 41 is closed. The nozzle port 41 is opened by lifting. At this time, as conceptually shown in FIG. 5, the fuel is injected from the nozzle port 41 into the cylinder 11 in a cone shape (specifically, a hollow cone shape) centering on the axis X of the cylinder 11. The taper angle of the cone is 90 ° to 100 ° in this embodiment (the taper angle of the inner hollow portion is about 70 °). When the application of voltage to the piezo element 44 is stopped, the piezo element 44 returns to the original state, and the outer opening valve 42 closes the nozzle port 41 again. At this time, the compression coil spring 46 disposed around the connecting portion 42 b in the case 45 facilitates the return of the piezo element 44.

  As the voltage applied to the piezo element 44 increases, the lift amount (hereinafter simply referred to as lift amount) of the outer open valve 42 from the state in which the nozzle port 41 is closed increases. As shown in FIG. 3, the larger the lift amount, the larger the opening of the nozzle port 41, the greater the penetration of fuel spray injected from the nozzle port 41 into the cylinder 11, and the longer the unit time. The amount of fuel injected per hit increases and the particle size of the fuel spray increases. The response of the piezo element 44 is fast, and the lift amount can be changed during fuel injection. The fuel injection time is, for example, about 2 ms at a light load, but the lift amount can be changed even during the injection time. Is possible.

  FIG. 3 exemplifies the fuel injection characteristics of the outer-open type fuel injection valve 33. According to this, as described above, the penetration increases as the lift amount of the outer valve increases. If the lift amount is the same, the penetration decreases as the back pressure (atmospheric pressure in the combustion chamber 17) increases.

  The fuel supply system 34 includes an electric circuit for driving the outer opening valve 42 (piezo element 44) and a fuel supply system for supplying fuel to the fuel injection valve 33. The engine controller 100 outputs an injection signal having a voltage corresponding to the lift amount to the electric circuit at a predetermined timing, thereby operating the piezo element 44 and the outer valve 42 via the electric circuit, A desired amount of fuel is injected into the cylinder 11. When the injection signal is not output (when the voltage of the injection signal is 0), the nozzle port 41 is closed by the outer opening valve 42. Thus, the operation of the piezo element 44 is controlled by the injection signal from the engine controller 100. Thus, the engine controller 100 controls the operation of the piezo element 44 to control the fuel injection from the nozzle port 41 of the injector and the lift amount during the fuel injection. Here, the fuel of the engine 1 is gasoline in this embodiment, but is not limited thereto, and may be various liquefied fuels containing gasoline, for example.

  The engine controller 100 is a controller based on a well-known microcomputer, and includes a central processing unit (CPU) that executes a program, a memory that is configured by, for example, RAM and ROM, and stores a program and data, And an input / output (I / O) bus for inputting and outputting signals.

  As shown in FIG. 1, the engine controller 100 detects at least a signal related to the intake air flow from the air flow sensor 71, a crank angle pulse signal from the crank angle sensor 72, and an accelerator opening sensor 73 that detects the amount of depression of the accelerator pedal. The accelerator opening signal from the vehicle and the vehicle speed signal from the vehicle speed sensor 74 are received. The engine controller 100 calculates the following control parameters of the engine 1 based on these input signals. For example, a desired throttle opening signal, fuel injection pulse, ignition signal, valve phase angle signal, etc. The engine controller 100 outputs these signals to the throttle valve 20 (throttle actuator that moves the throttle valve 20), the fuel supply system 34, the ignition system 32, the VVT 23, and the like.

  The characteristic feature of the engine 1 is that the geometrical compression ratio ε of the engine 1 is 18 or more and 40 or less from the viewpoint of improving the illustrated thermal efficiency of the engine and greatly improving the fuel consumption performance compared to the conventional one. In addition to setting the excess air ratio λ to 2 or more and 8 or less in at least a partial load operation region to make the air-fuel mixture lean, the heat insulating structure of the combustion chamber 17 is further combined is there.

  Here, the engine 1 is also an engine 1 having a relatively high expansion ratio at the same time as the high compression ratio because the compression ratio = expansion ratio. In addition, you may employ | adopt the structure (for example, Atkinson cycle and a mirror cycle) used as compression ratio <expansion ratio.

  As shown in FIG. 1, the combustion chamber 17 includes a wall surface of the cylinder 11, a crown surface of the piston 15, a lower surface (ceiling surface) of the cylinder head 13, and valve heads of the intake valve 21 and the exhaust valve 22. The combustion chamber 17 is thermally insulated by providing heat insulation layers 61, 62, 63, 64, and 65 having a configuration described later on each of these surfaces. In addition, below, when these heat insulation layers 61-65 are named generically, a code | symbol "6" may be attached | subjected to a heat insulation layer. The heat insulation layer 6 may be provided on all of these section screens, or may be provided on a part of these section screens. Further, in the illustrated example, the heat insulating layer 61 on the cylinder wall surface is provided at a position above the piston ring 14 in a state where the piston 15 is located at the top dead center. 14 is configured not to slide. However, the heat insulating layer 61 on the cylinder wall surface is not limited to this configuration, and the heat insulating layer 61 may be provided over the entire stroke or a part of the stroke of the piston 15 by extending the heat insulating layer 61 downward. In addition, the thickness of each heat insulation layer 61-65 illustrated in FIG. 1 does not show actual thickness, but is only an illustration, and does not show the magnitude relationship of the thickness of the heat insulation layer in each surface.

In the lean burn engine 1, the geometric compression ratio ε is set to 18 ≦ ε ≦ 40 as described above. The theoretical thermal efficiency η _th in the Otto cycle, which is the theoretical cycle, is η _th = 1−1 / (ε ^κ−1 ), and the theoretical thermal efficiency η _th increases as the compression ratio ε increases. Further, the higher the specific heat ratio κ of gas, in other words, the higher the excess air ratio λ, the higher the theoretical thermal efficiency η _th .

  However, the illustrated thermal efficiency of the engine (more precisely, the engine having no combustion chamber insulation structure) peaks at a predetermined geometric compression ratio ε (for example, about 15), and the geometric compression ratio ε is more than that. However, the illustrated thermal efficiency does not increase, and conversely, the illustrated thermal efficiency decreases. This is because, when the geometric compression ratio is increased while the fuel amount and the intake air amount are kept constant, the higher the compression ratio, the higher the combustion pressure and the combustion temperature. That is, the cooling loss due to heat released through the surface defining the combustion chamber 17 is determined by cooling loss = heat transfer coefficient × heat transfer area × (gas temperature−zone screen temperature), and the combustion gas pressure and The higher the temperature, the higher the heat transfer rate. Therefore, the higher the combustion pressure and the combustion temperature will increase the cooling loss accordingly. As a result, in the lean burn engine, the higher the geometric compression ratio, the lower the illustrated thermal efficiency. In this way, even if it is attempted to increase the indicated thermal efficiency of the engine by increasing the geometric compression ratio while making the air-fuel mixture lean, the increase in cooling loss results in a peak in the indicated thermal efficiency that is significantly lower than the theoretical thermal efficiency. End up.

  On the other hand, in the lean burn engine 1, the heat insulating structure of the combustion chamber 17 is combined so that the illustrated thermal efficiency is increased at a high geometric compression ratio ε. That is, the heat loss of the combustion chamber 17 is reduced to reduce the cooling loss, thereby increasing the indicated thermal efficiency.

  On the other hand, merely reducing the cooling loss by insulating the combustion chamber 17 converts the reduced cooling loss into the exhaust loss and does not contribute much to the improvement in the illustrated thermal efficiency. As described above, the high expansion ratio accompanying the high compression ratio efficiently converts the combustion gas energy corresponding to the reduced cooling loss into mechanical work. That is, it can be said that the lean burn engine 1 greatly improves the illustrated thermal efficiency by adopting a configuration that reduces both the cooling loss and the exhaust loss.

  Here, the excess air ratio λ will be examined. When the excess air ratio λ is lower than 2, the maximum combustion temperature in the combustion chamber 17 becomes high, and RawNOx can be discharged from the combustion chamber 17. As described above, since the lean burn engine 1 aims to reduce exhaust loss as well as cooling loss, the exhaust temperature is relatively low, which is disadvantageous for catalyst activation. Therefore, it is desirable to avoid or suppress the discharge of RawNOx from the combustion chamber 17, and for that purpose, the excess air ratio λ is preferably set to 2 or more. More preferably, it is 2.5 or more. In other words, it is desirable to set the excess air ratio λ in a range where the maximum combustion temperature in the combustion chamber 17 is a predetermined temperature (for example, 1800 K (Kelvin) as a temperature at which RawNOx can be generated) or less. The engine controller 100, for example, within the operation region of the partial load of the engine 1 is accompanied by an increase in the load (in other words, as the excess air ratio λ decreases due to an increase in the fuel injection amount), and the maximum combustion temperature becomes a predetermined temperature. When exceeding the above, it is desirable to operate the engine 1 by increasing the excess air ratio λ.

  On the other hand, according to the study by the inventors of the present application, the illustrated thermal efficiency peaks when the excess air ratio λ = 8. Therefore, the range of the excess air ratio λ is preferably 2 ≦ λ ≦ 8. In the high load operation region including the full load of the engine 1, the excess air ratio λ may be further reduced so that, for example, λ = 1 or λ ≦ 1 by torque priority. The numerical range of the excess air ratio λ is a preferable range of the engine 1 in a medium load and low load operation region. Note that the lean air-fuel mixture sets the throttle valve 20 on the open side, which can contribute to the improvement of the indicated thermal efficiency by reducing the gas exchange loss (pumping loss).

  Next, the heat insulation structure of the combustion chamber 17 will be described in more detail. As described above, the heat insulating structure of the combustion chamber 17 is configured by the heat insulating layers 61 to 65 provided on the respective screens that define the combustion chamber 17, and these heat insulating layers 61 to 65 are the combustion in the combustion chamber 17. In order to suppress the release of the heat of the gas through the section screen, the thermal conductivity is set lower than that of the metal base material constituting the combustion chamber 17. Here, for the heat insulating layer 61 provided on the wall surface of the cylinder 11, the cylinder block 12 is the base material, and for the heat insulating layer 62 provided on the crown surface of the piston 15, the piston 15 is the base material. For the heat insulating layer 63 provided on the ceiling surface, the cylinder head 13 is a base material, and for the heat insulating layers 64 and 65 provided on the valve head surfaces of the intake valve 21 and the exhaust valve 22, respectively, the intake valve 21 and the exhaust valve 22 are provided. Are the base materials. Therefore, the material of the base material is aluminum alloy or cast iron for the cylinder block 12, the cylinder head 13 and the piston 15, and the heat-resisting steel or cast iron for the intake valve 21 and the exhaust valve 22. However, as described above, since the lean burn engine 1 has reduced exhaust loss, the exhaust gas temperature has been greatly reduced. It is also possible to use a material that could not be used or was difficult to use (for example, an aluminum alloy).

  In addition, the heat insulating layer 6 preferably has a volumetric specific heat smaller than that of the base material in order to reduce cooling loss. That is, the gas temperature in the combustion chamber 17 varies with the progress of the combustion cycle, but in a conventional engine that does not have a heat insulation structure of the combustion chamber, cooling water flows in a water jacket formed in the cylinder head or cylinder block. Thus, the temperature of the surface defining the combustion chamber 17 is maintained substantially constant regardless of the progress of the combustion cycle.

  On the other hand, as described above, the cooling loss is determined by cooling loss = heat transfer coefficient × heat transfer area × (gas temperature−temperature of the section screen), so that the difference between the gas temperature and the wall surface temperature is large. The higher the loss, the greater the cooling loss. In order to suppress the cooling loss, it is desirable to reduce the difference temperature between the gas temperature and the temperature of the section screen. However, as described above, when the temperature of the section screen of the combustion chamber 17 is maintained substantially constant, It is inevitable that the temperature difference will increase as the temperature changes.

  Therefore, it is preferable that the heat insulating layer 6 has a small heat capacity, and the temperature of the section screen of the combustion chamber 17 changes following the fluctuation of the gas temperature in the combustion chamber 17.

  Further, reducing the heat capacity of the heat insulating layer 6 is advantageous for reducing exhaust loss. That is, if the heat capacity of the heat insulating layer is large, the temperature of the section screen does not decrease even when the temperature in the combustion chamber 17 decreases, but the combustion chamber 17 has a heat insulating structure. The temperature inside is kept high. This results in an increase in exhaust loss and hinders improvement in the thermal efficiency of the engine 1.

  On the other hand, reducing the heat capacity of the heat insulating layer 6 reduces the temperature of the section screen following that when the temperature in the combustion chamber 17 decreases. Therefore, it is possible to avoid maintaining the temperature in the combustion chamber 17 at a high temperature, which can be advantageous in suppressing exhaust loss in addition to the above-described suppression of cooling loss due to temperature followability.

As an example of the heat insulating layer 6, the heat insulating layer 6 includes the wall surface of the cylinder 11, the crown surface of the piston 15, the ceiling surface of the cylinder head 13, and the valve head surfaces of the intake valve 21 and the exhaust valve 22, that is, the combustion chamber. 17 to the partition face defining the, for example, formed by plasma spraying, zirconia (ZrO _2), or may be formed by coating of partially stabilized zirconia (PSZ). Zirconia or partially stabilized zirconia has a relatively low thermal conductivity and a relatively low volumetric specific heat. Layer 6 is constructed.

  Further, in the present embodiment, as shown in FIG. 1, a port liner 181 made of aluminum titanate having an extremely low thermal conductivity, excellent heat insulation, and excellent heat resistance is integrated with the cylinder head 13. A heat insulating layer is provided in the intake port 18 by casting. This configuration suppresses or avoids an increase in temperature due to heat received from the cylinder head 13 when fresh air passes through the intake port 18. As a result, the temperature of the fresh air introduced into the cylinder 11 (initial gas temperature) is lowered, so that the gas temperature at the time of combustion is lowered and the temperature difference between the gas temperature and the section screen of the combustion chamber 17 is reduced. Become advantageous. Lowering the gas temperature at the time of combustion lowers the heat transfer coefficient, which is advantageous for reducing cooling loss. In addition, the structure of the heat insulation layer provided in the intake port 18 is not limited to the casting of the port liner 181.

  Thus, the engine 1 further suppresses the contact of the combustion gas with the wall surface defining the combustion chamber for the purpose of reducing the cooling loss. Specifically, when the operating state of the engine 1 is in the low load region, the engine controller 100 determines the fuel injection period by the fuel injection valve 33, for example, the compression top dead center, as shown by a one-dot chain line in FIG. To ATDC 20 ° CA, and the lift amount of the outer opening valve 42 is set to be relatively small, thereby reducing the penetration of the fuel spray. Thus, for example, as shown in the upper diagram of FIG. 5, the fuel spray injected into the cavity 15a does not reach (i.e., collides with) the side wall of the cavity 15a, so that the side wall of the cavity 15a is within the cavity 15a. Further, a layer that does not contribute to combustion, specifically, an air layer or a lean layer is interposed between the bottom wall and the combustion gas. By interposing this layer, the combustion gas is prevented from coming into contact with the side wall and the bottom wall of the cavity 15a, which is advantageous in reducing the cooling loss.

  On the other hand, when the operating state of the engine 1 is in the high load region, an improvement in torque is required, but if stratification occurs in the cavity 15a as in combustion in the low load region, the air becomes insufficient. It is disadvantageous for torque improvement. Therefore, when the operating state of the engine 1 is in the high load region, the engine controller 100 sets the fuel injection period by the fuel injection valve 33 from, for example, the compression top dead center to ATDC 20 ° CA as shown by the solid line in FIG. In the first stage injection from the compression top dead center to ATDC 10 ° CA, which is the first half of the injection period, the lift amount of the outer opening valve 42 is set to be relatively small, while in the second half of the injection period. In the latter stage injection from ATDC 10 ° CA to ATDC 20 ° CA, it is set relatively large. As a result, the fuel spray injected in the first half of the injection period is near the compression top dead center, has a high back pressure, and has a small lift amount, resulting in a small penetration (see FIG. 3). As shown in the upper figure, collision with the side wall and bottom wall of the cavity 15a is avoided. This is advantageous for reducing the cooling loss as described above. In particular, the fuel injected in the first half of the combustion period burns at a relatively high temperature and pressure in the cylinder 11 near the compression top dead center, so the temperature of the combustion gas becomes high. For this reason, it is disadvantageous for the increase in the cooling loss, but it is possible to effectively avoid the increase in the cooling loss by avoiding or suppressing the contact between the combustion gas and the side wall or the bottom wall of the cavity 15a itself. .

  On the other hand, since the fuel spray injected in the latter half of the injection period is an expansion stroke, the back pressure is low and the lift amount is large, so that the penetration becomes large (see FIG. 3). As shown in FIG. 4, a part of the impact collides with the side wall or bottom wall of the cavity 15a. Thereafter, part of the fuel spray is guided along the side wall and the bottom wall, and reaches the outside of the cavity 15a, that is, the squish area 15b through the cavity opening (see the arrow in the figure). Thus, the fuel injected in the latter half of the injection period is combusted using the air present in the squish area 15b, and the air utilization rate is increased, which is advantageous for increasing torque.

  Here, by inclining the bottom wall of the cavity 15a in a direction orthogonal to the axis X of the cylinder 11 and by inclining the side wall with respect to the axis X of the cylinder 11, the diameter from the axial center position of the cylinder 11 is increased. A part of the fuel spray injected outward in the direction can be reliably guided to the squish area 15b by using the bottom wall and the side wall as a guide. As a result, it is advantageous for securing a high torque in a high load region.

  Further, since the cylinder 11 is disposed offset, the descending speed of the piston 15 after the compression top dead center is increased. This quickly expands the squish area 15b in the expansion stroke, and improves the air utilization rate when the fuel injected in the second half of the injection period burns.

  On the other hand, a part of the fuel spray injected in the second half of the injection period collides with the bottom wall and side wall of the cavity 15a, but the second half of the injection period corresponds to the expansion stroke, and the temperature and pressure in the cylinder 11 gradually decrease. As a result, the temperature of the combustion gas is relatively low. As a result, even if the combustion gas comes into contact with the bottom wall or the side wall of the cavity 15a, it is avoided that the cooling loss greatly increases.

  Thus, when the operating state of the engine 1 is in the high load region, both reduction of cooling loss and securing of high torque are compatible.

  Here, in the engine 1, the air-fuel mixture may be forcibly ignited by the spark plug 31 at an appropriate timing, or self-ignition (compression ignition) is performed because it is a high compression ratio engine. You may do it.

  In FIG. 4, as a fuel injection mode in the high load region, split injection is performed in which injection is performed twice, that is, upstream injection with a relatively small lift amount and subsequent injection with a relatively large lift amount. Yes. However, as shown in FIG. 6, for example, fuel injection is continued during the injection period from compression top dead center to ATDC 20 ° CA, and from the compression top dead center, which is the first half of the injection period, to ATDC 10 ° CA. Alternatively, the lift amount may be relatively small, and the lift amount may be switched during the injection so that the lift amount is relatively large from ATDC 10 ° CA to ATDC 20 ° CA in the second half of the injection period.

  In addition, the fuel injection period is set in the range from the compression top dead center to ATDC 20 ° CA in the illustrated example, but the fuel injection period can be appropriately set, for example, within the range from the end of the compression stroke to the initial stage of the expansion stroke. Good.

  Furthermore, the shape of the combustion chamber 17 is not limited to the shape shown in FIG. 1, and the shape of the combustion chamber 17 may be a so-called pent roof shape, for example, as shown in FIG. That is, in the engine 10 shown in FIG. 7, the lower surface of the cylinder head 13 has a triangular roof shape including two inclined surfaces on the intake side and the exhaust side. The crown surface of the piston 15 has a convex shape corresponding to the inclined ceiling surface, and a concave cavity 15a is formed at the center of the crown surface. Even in such an engine 10, when the operating state is in a high load region, the fuel spray injected in the second half of the injection period is changed by switching the fuel spray penetration between the first half and the second half of the fuel injection period. The air existing in the area can be used for combustion, and the torque can be improved.

  Further, the technology disclosed herein is not limited to the application to the high compression ratio lean burn engine 1 having the heat insulation structure of the combustion chamber 17 as described above. For example, the heat insulation structure of the combustion chamber 17 is omitted. May be.

1 Engine (Engine body)
10 Engine (Engine body)
11 cylinders
15 Piston 15a Cavity 17 Combustion chamber 100 Engine controller 33 Fuel injection valve 41 Nozzle port 42a Outer valve C Crankshaft rotation center X Cylinder axis

Claims (4)

An engine body having a cylinder with a geometric compression ratio set to 18 or more;
A piston inserted into the cylinder and formed with a cavity recessed in the crown surface;
A fuel injection valve configured to inject fuel spray into a combustion chamber defined by the cylinder and the piston of the engine body;
A controller configured to control a fuel injection mode into the combustion chamber through the fuel injection valve according to an operating state of the engine body,
The fuel injection valve is disposed at the axial center position of the cylinder, and injects fuel spray so as to expand outward in the radial direction therefrom,
The cavity is configured such that its side wall is inclined with respect to the axial direction of the cylinder so as to expand from the cavity bottom toward the cavity opening,
The controller sets a period during which fuel is injected by the fuel injection valve within a range from the end of the compression stroke to the initial stage of the expansion stroke when the operating state of the engine body is in a high load region higher than a predetermined load. The spark ignition direct injection engine in which the fuel spray penetration is reduced in the first half of the injection period and the fuel spray penetration is set higher in the second half of the injection period. In the spark ignition direct injection engine according to claim 1,
The fuel injection valve has an outer opening valve that opens and closes a nozzle opening, and the greater the lift amount of the outer opening valve, the greater the penetration of fuel spray injected from the nozzle opening into the combustion chamber. Configured,
The controller sets the lift amount of the outer valve to a predetermined amount in the first half of the injection period, and sets the lift amount of the outer valve to be larger than the predetermined amount in the second half of the injection period. Direct injection engine. In the spark ignition direct injection engine according to claim 1 or 2,
The cavity is a spark ignition direct injection engine in which a bottom wall is inclined with respect to a direction orthogonal to the axis of the cylinder so that the bottom of the cavity rises from the outside in the radial direction toward the center. The spark ignition direct injection engine according to any one of claims 1 to 3,
The engine main body is a spark ignition direct injection engine in which the axis of the cylinder is arranged so as to be shifted toward the crank angle advance side with respect to the rotation center of the crankshaft.

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