JP2012072749A - Lean burn engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lean burn engine capable of largely improving fuel economy performance, by largely improving heat efficiency.SOLUTION: This lean burn engine 1 includes an engine body having a combustion chamber 17, and a control means 100 for controlling operation of the engine body. At least a part of a surface for partitioning the combustion chamber 17 is constituted of combustion chamber heat insulation layers 61-65 arranged on the surface side of a base material. The engine body is set to 20≤ε≤50 in the geometric compression ratio ε. The control means 100 is set to 2.5≤λ≤6 in the air excessive ratio λ in combustion when the engine body exists in an operation area of at least a partial load.

Description

  The technology disclosed herein relates to a lean burn engine.

  For example, Patent Document 1 describes an engine in which the compression ratio is increased and the air-fuel mixture is made lean to increase the theoretical thermal efficiency of a spark ignition gasoline engine.

  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 gasoline 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. Therefore, it is impossible to expect to improve the thermal efficiency of the engine and greatly improve the fuel consumption performance only by combining the high compression ratio and the lean air-fuel mixture as described in Patent Document 1.

  On the other hand, since the reduction of the cooling loss due to the heat insulation of the combustion chamber as described in Patent Document 2 raises the exhaust gas temperature, the reduced amount of the cooling loss is converted into the increase of the exhaust loss. Therefore, even if only the cooling loss is suppressed by heat insulation of the combustion chamber, it does not contribute much to the improvement of the thermal efficiency of the engine.

  On the other hand, for example, from the viewpoint of environmental conservation, there is a demand for a technology that greatly improves the thermal efficiency of the engine and greatly improves its fuel efficiency.

  The technology disclosed herein has been made in view of the above points, and an object thereof is to provide a lean burn engine that can improve thermal efficiency and greatly improve fuel efficiency.

  As described above, the inventors of the present application have assumed a combination of a high compression ratio of the engine and leaning of the air-fuel mixture, so that the illustrated thermal efficiency is reduced due to an increase in cooling loss. In order to reduce the cooling loss in the stoichiometric compression ratio, at least a part of the combustion chamber is adiabatic, while the engine expansion ratio (= high compression ratio) is further combined to reduce the cooling loss. Therefore, the illustrated thermal efficiency of the engine is greatly improved by effectively converting to mechanical work without increasing exhaust loss.

  Specifically, the lean burn engine disclosed herein is formed in a cylinder block in which a cylinder is formed, a piston that is fitted into the cylinder, a cylinder head that is placed on the cylinder block, and the cylinder head. An engine body having a combustion chamber surrounded by an intake valve and an exhaust valve for opening and closing the intake port and the exhaust port; and a control means for controlling the operation of the engine body through control of at least the excess air ratio λ.

  The combustion chamber includes a wall surface of the cylinder, a crown surface of the piston, a ceiling surface that is formed in the cylinder head and that opens the intake port and the exhaust port, and closes the upper end opening of the cylinder, The cylinder block, the piston, the cylinder head, the cylinder head, the cylinder block, the cylinder head, The combustion chamber heat insulating layer is provided on the surface side of the base material constituting the intake valve and the exhaust valve and suppresses the heat of the gas in the combustion chamber from being released to the outside through the partition wall. Here, the combustion chamber heat insulating layer may be configured of, for example, a material having a thermal conductivity lower than that of the base material, thereby suppressing heat from being released to the outside through the partition wall.

  Thus, the engine body has a geometric compression ratio ε set to 20 ≦ ε ≦ 50, and the control means has the air excess during combustion when the engine body is at least in a partial load operating region. The rate λ is set to 2.5 ≦ λ ≦ 6.

  According to this configuration, when the engine body is at least in the partial load operation region, the excess air ratio λ during combustion is set to 2.5 ≦ λ ≦ 6. That is, this engine is a so-called lean burn engine that can be operated at an excess air ratio higher than the excess air ratio λ = 1. Since this engine body is an ultra-high compression ratio engine in which the geometric compression ratio ε is set to 20 ≦ ε ≦ 50, the theoretical thermal efficiency is obtained by combining the high compression ratio and the lean air-fuel mixture. It can improve.

  As described above, a high compression ratio of 20 or more can significantly increase the combustion pressure and temperature in the combustion chamber. Therefore, although the illustrated thermal efficiency decreases due to an increase in cooling loss, the above-described configuration divides the combustion chamber. The combustion chamber heat insulating layer covers at least a part of the wall surface of the cylinder, the crown surface of the piston, the ceiling surface of the cylinder head in which the intake port and the exhaust port are opened, and the valve head surface of each of the intake valve and the exhaust valve. Constitute. A combustion chamber heat insulation layer can suppress heat dissipation through the surface which partitions a combustion chamber, and can reduce a cooling loss.

  While reducing the cooling loss, this engine can effectively convert the energy of the combustion gas into mechanical work by the high expansion ratio accompanying the high compression ratio, and can also reduce the exhaust loss. That is, while the theoretical thermal efficiency is increased, both the cooling loss and the exhaust loss are suppressed. As a result, the illustrated thermal efficiency can be significantly improved.

  A port heat insulating layer having a lower thermal conductivity than the cylinder head may be provided on a port wall surface defining the intake port.

  The cooling loss is determined by cooling loss = heat transfer rate x heat transfer area x (gas temperature-temperature on the zone screen), so the peak of the gas temperature during combustion is lowered and the difference from the zone screen temperature is reduced. Reducing the temperature is advantageous in reducing the cooling loss. Providing the port heat insulation layer on the port wall surface defining the intake port can suppress the intake air flowing into the cylinder through the intake port from receiving heat from the high-temperature cylinder head and increasing the temperature. This is advantageous in lowering the initial gas temperature and lowering the peak gas temperature during combustion. That is, it is advantageous for improving the thermal efficiency of the engine by suppressing the cooling loss.

  The heat transfer coefficient is a function of the pressure and temperature of the gas in the combustion chamber. The higher the gas pressure and temperature, the higher the heat transfer coefficient and the cooling loss. Lowering the initial gas temperature and lowering the peak gas temperature during combustion is also advantageous in reducing the heat transfer coefficient and suppressing cooling loss.

  The lean burn engine may further include cooling means for cooling the intake air flowing into the cylinder.

  By doing so, as described above, the initial gas temperature can be further reduced, and the peak of the gas temperature during combustion can be further lowered. Therefore, the cooling loss can be suppressed and the engine thermal efficiency can be further improved. Can be.

The combustion chamber heat insulating layer may be made of a heat-resistant ceramic having a thermal conductivity lower than that of the base material and a volume specific heat (specific heat capacity) equal to or smaller than that of the base material. . Specific examples of such ceramics include zirconia (ZrO ₂ ) and zirconia-containing ceramics (for example, partially stabilized zirconia (PSZ)). Since the thermal conductivity is lower than that of the base material, heat dissipation through the surface defining the combustion chamber is suppressed. That is, the heat insulation of the combustion chamber is ensured.

  Also, as the combustion cycle of the engine progresses, the temperature in the combustion chamber (gas temperature) will fluctuate, but the temperature of the section screen that defines the combustion chamber is changed so as to follow the fluctuation in gas temperature. If so, the difference in temperature between the gas temperature and the temperature of the section screen is reduced, so that the cooling loss is reduced. Therefore, it is preferable that the combustion chamber heat insulating layer is composed of a heat-resistant ceramic having a volume specific heat equal to or smaller than that of the base material, and as a result, the temperature followability of the section screen is improved. It can be even more advantageous to further reduce the cooling loss and increase the thermal efficiency of the engine.

  Further, if the heat capacity of the heat insulation layer is large, even if the temperature in the combustion chamber decreases, the temperature in the section screen does not decrease, and the temperature in the combustion chamber is maintained at a high temperature. This results in an increase in exhaust loss and hinders improvement in engine thermal efficiency. On the other hand, when the volume specific heat of the heat insulating layer is reduced, when the temperature in the combustion chamber is lowered, the temperature of the section screen is also lowered. Accordingly, it is possible to avoid maintaining the temperature in the combustion chamber at a high temperature, which can be advantageous in suppressing the exhaust loss in addition to the above-described suppression of the cooling loss accompanying the temperature followability.

  The combustion chamber heat insulating layer may include a porous layer containing air and a coating layer that covers the surface side of the porous layer and forms the surface that defines the combustion chamber.

  Here, the “porous layer” may be a layer containing a large number of bubbles (for example, a layer made of foam metal or a layer in which a large number of voids are formed by anodization), but is not limited thereto. A fiber (including non-woven fabric) layer, a layer in which a large number of hollow particles are aggregated, or the like may be used. Such a porous layer containing air has a relatively low thermal conductivity, and may have a lower thermal conductivity than the base material, and the specific volume heat is also significantly reduced, so that the specific volume heat is higher than that of the base material. Can be smaller.

  Further, forming the surface that covers the surface side of the porous layer and is covered with the coating layer to define the combustion chamber can protect the porous layer and be advantageous in improving durability and stability. .

  The control means controls the excess air ratio λ during combustion so that the maximum combustion temperature becomes a predetermined temperature or less when the load increases in the operation range of the partial load. Is preferred.

  The “predetermined temperature” referred to here may be set, for example, as a temperature at which RawNOx can be generated in the combustion chamber, and by doing so, discharge of RawNOx from the combustion chamber can be suppressed or avoided. In particular, as described above, since this engine reduces exhaust loss by increasing the expansion ratio and is disadvantageous for the activation of the catalyst, suppressing or avoiding the release of RawNOx from the combustion chamber is an It is advantageous for improving the emission performance.

  The lean burn engine may further include ignition means including an ignition plug disposed facing the combustion chamber, and the ignition means may be plasma type ignition means.

  The plasma-type ignition means has a relatively high ignition energy, and therefore is advantageous in improving the ignitability and stabilizing the combustion in a lean burn engine in which the excess air ratio λ can be set to 2.5 to 6. .

  The lean burn engine further includes fuel supply means including a fuel injection valve that directly injects fuel into the combustion chamber, and the fuel supply means has an air-fuel mixture on the center side in the combustion chamber at the start of combustion. At the same time, fuel may be supplied into the combustion chamber so that fresh air is interposed between the air-fuel mixture and the section screen.

  Since fresh air intervening between the air-fuel mixture present on the center side in the combustion chamber and the section screen at the start of combustion can function as a heat insulating layer after the start of combustion, heat release through the section screen can be suppressed. That is, the fuel supply form is advantageous in reducing the cooling loss.

  Here, as one of the measures for interposing new air between the air-fuel mixture and the section screen, for example, the combustion injection timing may be set at the latter stage of the compression stroke. Further, for example, the air-fuel mixture may exist only on the center side in the combustion chamber by devising the arrangement and shape of the nozzle holes. Moreover, you may combine those measures.

  As described above, the lean burn engine improves the theoretical thermal efficiency by a combination of a high compression ratio and lean air-fuel mixture, and at least a part of the section screen partitioning the combustion chamber is formed by the combustion chamber heat insulating layer. As a result of reducing the cooling loss and increasing the expansion ratio that accompanies the higher compression ratio, the combustion gas energy is effectively converted into mechanical work and the exhaust loss is reduced, resulting in improved thermal efficiency and improved fuel efficiency. Significant improvements can be realized.

It is a figure showing roughly the composition of a lean burn engine. It is a figure which shows an example of the relationship of the illustration thermal efficiency with respect to geometric compression ratio. It is a figure which shows an example of the relationship of the illustration thermal efficiency with respect to an excess air ratio. It is a figure which shows an example of the division screen temperature change of the combustion chamber with respect to a crank angle. It is sectional drawing which shows the structural example of the heat insulation layer provided in the ward screen.

  Hereinafter, an embodiment of a lean burn 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 (engine body) 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 type internal combustion engine, and has a plurality of cylinders 11 although not shown. 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 not shown, a water jacket through which cooling water flows is formed inside the cylinder block 12 and the cylinder head 13.

  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, a recess is formed in the crown surface of the piston 15. 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 opens to the lower surface of the cylinder head 13 (the ceiling surface that defines the upper surface of the combustion chamber 17). And communicates with the combustion chamber 17. 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 in the ceiling 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 for adjusting the intake flow rate is interposed in the intake passage, and the opening degree of the throttle valve 20 is adjusted in response to a control signal from the engine controller 100. 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. Although not shown, an exhaust gas purification system having one or more catalytic converters is disposed in the exhaust passage.

  As shown in the figure, 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 in a state inclined to the exhaust side with respect to the central axis of the cylinder 11, and the 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. As will be described later, since the engine 1 makes the air-fuel mixture lean, the use of a plasma ignition type plug with high ignition energy is advantageous in improving ignition stability.

  In this embodiment, the fuel injection valve 33 is disposed along the central axis of the cylinder 11 and is attached to the cylinder head 13 with a known structure such as using a bracket. The tip of the fuel injection valve 33 faces the center of the ceiling of the combustion chamber 17. The arrangement of the fuel injection valve 33 is not limited to this. The fuel injection valve 33 is also a multi-injection type fuel injection valve, for example, but is not limited thereto. The fuel supply system 34 includes a fuel supply system that supplies fuel to the fuel injection valve 33, and an electric circuit that drives the fuel injection valve 33. The electric circuit receives a control signal from the engine controller 100 and operates the fuel injection valve 33 to inject a desired amount of fuel into the combustion chamber 17 at a predetermined timing. Here, although the fuel of the lean burn engine 1 is gasoline in this embodiment, the fuel is not limited to this, and various liquefied fuels including light oil and bioethanol, and various gases including natural gas, etc. These gaseous fuels can be appropriately employed.

  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.

  The engine controller 100 includes 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, an accelerator opening signal from the accelerator opening sensor 73 that detects the amount of depression of the accelerator pedal, A vehicle speed signal from the vehicle speed sensor 74 is 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.

  Thus, the most characteristic feature of the lean burn engine 1 is that the geometric compression ratio ε of the engine 1 is set to 20 from the viewpoint of increasing the indicated thermal efficiency of the engine and greatly improving the fuel efficiency. While the ultrahigh compression ratio is set to 50 or more and the excess air ratio λ is set to 2.5 or more and 6 or less at least in the partial load operation region, the combustion chamber 17 The heat insulation structure is to be further combined.

  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.

  Further, 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, 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. Further, a heat insulating layer may be provided on the port wall surface near the opening on the ceiling surface side of the combustion chamber 17 in the intake port 18 and the exhaust port 19, although it is not the wall surface that directly partitions the combustion chamber 17. 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.

Hereinafter, the configuration related to the improvement of fuel consumption in the lean burn engine 1 will be described with reference to the drawings. As described above, in the lean burn engine 1, the geometric compression ratio ε is set to 20 ≦ ε ≦ 50. FIG. 2 is a graph showing the thermal efficiency of the engine 1 with respect to the change in the geometric compression ratio ε. An example of the result of calculating the change by model calculation is shown. First, the theoretical thermal efficiency η _th in the Otto cycle, which is a theoretical cycle, is η _th = 1-1 / (ε ^κ-1 ) as shown in FIG. 2, and the higher the compression ratio ε, the theoretical thermal efficiency. η _th 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 .

An engine with an excess air ratio λ = 1 (no adiabatic) shown by a black diamond in FIG. 2 increases in the illustrated thermal efficiency as the geometric compression ratio ε increases. However, the illustrated thermal efficiency is significantly lower than the theoretical thermal efficiency η _th . This decrease is mainly due to cooling loss and exhaust loss. On the other hand, a lean burn engine with an excess air ratio λ = 6 (no heat insulation) shown by a white triangle in FIG. 2 has a higher theoretical thermal efficiency due to lean air-fuel mixture. The illustrated thermal efficiency is higher than that of an engine with λ = 1. However, the lean burn engine without insulation exhibits a thermal efficiency peak when the compression ratio ε is about 15, and even if the geometric compression ratio ε is further increased, the thermal efficiency shown does not increase. Will be reduced. This is because FIG. 2 shows the change in the indicated thermal efficiency when the geometric compression ratio is changed with the engine load kept constant. This is because the higher the compression ratio becomes, the higher the compression ratio becomes, and 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 disclosed herein, the illustrated thermal efficiency is increased in the range of the geometric compression ratio ε, for example, the geometric compression ratio ε, such as the geometric compression ratio ε, in which the illustrated thermal efficiency is reduced. Thus, the heat insulation structure of the combustion chamber 17 is combined. That is, the heat loss of the combustion chamber 17 is reduced to reduce the cooling loss, thereby increasing the indicated thermal efficiency. The details of the heat insulation structure will be described later. As shown by the black circles in FIG. 2, the lean burn engine with an excess air ratio λ = 6 (with heat insulation) has a heat insulation rate of 50% higher than that of the engine without “heat insulation”. Unlike the lean-burn engine without insulation, which is indicated by white triangles, this engine does not decrease the thermal efficiency even when the geometric compression ratio ε is increased. For example, an engine with an excess air ratio λ = 1 Compared with the above, the illustrated thermal efficiency is significantly increased.

  If the combustion chamber 17 is insulated to reduce the cooling loss, the reduced cooling loss is converted into the exhaust loss and does not contribute much to the improvement in the illustrated thermal efficiency. In the lean burn engine 1, as described above, As a result of the higher expansion ratio accompanying the higher compression ratio, the energy of the combustion gas corresponding to the reduced cooling loss is efficiently converted to 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.

  Note that, as shown by white circles and black triangles in FIG. 2, even when the excess air ratio λ = 2.5, by setting “with heat insulation”, a decrease in the illustrated thermal efficiency is avoided, and the excess air ratio λ = The illustrated thermal efficiency can be significantly higher than that of one engine. The white circle and the black triangle have different engine loads, and the white circle has a higher engine load than the black triangle (however, it corresponds to a partial load rather than a full load).

  In the model calculation shown in FIG. 2, “with heat insulation” increases the heat insulation rate of the combustion chamber by 50% as compared with the engine without “heat insulation”, but the heat insulation structure of the combustion chamber 17 has a higher heat insulation performance. The cooling loss can be further reduced, which is advantageous in improving the illustrated thermal efficiency.

  Here, the upper limit value “50” of the geometric compression ratio ε does not substantially increase the illustrated thermal efficiency even when the geometric compression ratio exceeds 50, as is remarkable in the example indicated by the black circle in FIG. It is a value set based on. In addition, setting the geometric compression ratio ε to be larger than 50 requires significantly higher accuracy as machining accuracy of various engine parts including the cylinder 11 and the piston 15, while This is disadvantageous in that the solid difference in the chemical compression ratio ε can be very large. On the other hand, the lower limit value “20” of the geometric compression ratio ε is a value set as the lower limit value of the geometric compression ratio at which the illustrated thermal efficiency decreases in a conventional lean burn engine that does not insulate the combustion chamber 17. In other words, it is the lower limit value of the geometric compression ratio in which the illustrated thermal efficiency is expected to be improved by the heat insulation of the combustion chamber 17.

  FIG. 3 shows an example of a result obtained by calculating the change in the indicated thermal efficiency with respect to the change in the excess air ratio λ by model calculation. In an “insulated” engine in which the combustion chamber 17 is insulated (geometric compression ratio ε = 40, low load), the thermal efficiency shown in the drawing is basically increased as the excess air ratio λ is increased to make the air-fuel mixture leaner. And the illustrated thermal efficiency peaks at an excess air ratio λ = 6. It should be noted that the “non-insulated” engine in which the combustion chamber 17 is not insulated (geometric compression ratio ε = 20, low load) is more illustrated than the “insulated” engine regardless of the excess air ratio λ. Thermal efficiency is low.

  Here, when the excess air ratio λ is lower than 2.5, 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 this lean burn engine 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 2.5 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, an increase in the excess air ratio λ 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 with the excess air ratio λ lowered.

  On the other hand, as shown in FIG. 3, since the illustrated thermal efficiency peaks when the excess air ratio λ = 6, the range of excess air ratio λ is preferably 2.5 ≦ λ ≦ 6. Note that, in the operation range of the full load of the engine 1, the excess air ratio λ may be further reduced to λ = 1, for example, with priority on torque. The numerical range of the excess air ratio λ is a preferable range of the engine 1 in at least a partial 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, for example, as shown by a broken line in FIG. 4, 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 as shown by a solid line in FIG. .

  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.

The heat insulating layer 6 is formed on 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 section screen that defines the combustion chamber 17. 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, so that the thermal conductivity is lower than that of the base material and the heat capacity is the same as or lower than that of the base material. Layer 6 is constructed.

  In addition, as illustrated in FIG. 5, the heat insulating layer 6 includes a porous layer 601 containing air, and a coating layer 602 that forms a section screen that covers the surface side of the porous layer 601 and partitions the combustion chamber 17. You may comprise. Since the porous layer 601 containing air has a relatively low thermal conductivity, the thermal conductivity is lower than that of the various metal base materials described above, and the volume specific heat is significantly smaller. The volume specific heat is smaller than that of the base material 7.

  Here, the porous layer 601 may specifically be constituted by various metal foams (foam metal), or by a fiber molded body such as a metal nonwoven fabric such as stainless steel, for example. May be configured. When the porous layer 601 is constructed using such foam metal or metal nonwoven fabric, the porous layer 601 is porous by reducing the thermal conductivity by forming an oxide film on the surface of the foam metal or metal nonwoven fabric by performing an oxidation treatment. The heat insulating performance of the layer 601 can be improved. The porous layer 601 using such a foam metal or a metal nonwoven fabric forms the heat insulating layer 6 on the crown surface of the piston 15, the ceiling surface of the cylinder head 13, the valve head surfaces of the intake valve 21 and the exhaust valve 22, and the like. Can be used for

  For example, when a heat insulating layer is provided on the surface of an aluminum alloy part such as the piston 15 or the cylinder head 13, that is, when the heat insulating layer 6 is provided on the aluminum alloy base material 7, a partition wall is formed. A porous layer 601 having a large number of pores each extending in the thickness direction may be provided by subjecting the surface of the base material 7 to anodic oxidation (alumite treatment). As described above, the porous layer 601 by the anodizing treatment can be used also when a heat insulating layer is provided on the port wall surface near the opening of the intake port 18 or the exhaust port 19.

  Further, the porous layer 601 may be configured by aggregating many hollow particles such as alumina and ceramics. If the hollow particles are bonded to each other, the strength of the porous layer 601 can be improved. The joining of the hollow particles may be performed by, for example, spark plasma sintering, or may be joined by, for example, impregnating alumina sol or silica sol so as to fill the gap between the hollow particles. . Since the porous layer 601 constituted by the hollow particles does not cause air convection, it can be advantageous in improving the heat insulation performance. The formation of the porous layer 601 by hollow particles is, for example, porous on the crown surface of the piston 15, the cylinder wall surface of the cylinder block 12, the ceiling surface of the cylinder head 13, and the valve head surfaces of the intake valve 21 and the exhaust valve 22. It can be used to form the quality layer 601.

  The coating layer 602 covering the surface side of the porous layer 601 protects the porous layer 601 and can be advantageous in improving durability and stability. The coating layer 602 is, as an example, but not limited thereto, and may be formed of a sprayed coating of zirconia or partially stabilized zirconia. The coating layer 602 containing zirconia can be advantageous in improving heat insulation performance and temperature followability by making its thermal conductivity and volume specific heat lower than that of the base material 7.

  In order to further suppress the cooling loss of the engine 1, the supply of fuel into the combustion chamber 17 through the fuel injection valve 33 may be devised. Specifically, the air-fuel mixture locally exists at the center side in the combustion chamber 17 at the start of combustion, while the fresh air is interposed between the air-fuel mixture and the section screen. The fuel may be supplied (injected). Thereby, after the start of combustion, the fresh air interposed between the air-fuel mixture and the section screen functions as a kind of heat insulating layer, and the release of heat through the section screen can be further suppressed. The configuration in which the air-fuel mixture is locally present at the center side in the combustion chamber 17 and the surroundings is surrounded by fresh air is that the air-fuel mixture does not exist in the entire combustion chamber 17 and the air-fuel mixture is present throughout the combustion chamber. This is different from the conventional stratified combustion with the concentration distribution. Here, as a method of causing the air-fuel mixture to exist locally, for example, the fuel injection timing by the fuel injection valve 33 may be set at the latter stage of the compression stroke. Further, for example, by devising the arrangement of the nozzle holes of the fuel injection valve 33, the air-fuel mixture may exist only on the center side in the combustion chamber 17, or these techniques may be combined. In addition, from the viewpoint of promoting the mixing of the fuel injected from the fuel injection valve 33 and air while allowing the air-fuel mixture to exist locally, a configuration is adopted in which the fuel is injected into the combustion chamber 17 in a supercritical state. May be.

  In order to reduce the cooling loss, it is also effective to reduce the difference between the gas temperature and the section screen. In the lean burn engine 1, a heat insulating layer is provided in the intake port 18 to reduce the gas temperature. Yes. Specifically, as shown in FIG. 1, a port liner 181 made of aluminum titanate having a very low thermal conductivity, excellent heat insulation, and excellent heat resistance is integrally cast on the cylinder head 13. As a result, a heat insulating layer is provided in the intake port 18. With this configuration, when fresh air passes through the intake port 18, it is possible to suppress or avoid an increase in temperature due to heat received from the cylinder head 13. 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 can lower the heat transfer rate, which is advantageous for reducing the 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, and the structure of the heat insulation layer of the partition wall described above may be applied as appropriate.

  Here, a means for cooling the intake air may be provided in the engine 1 in order to lower the temperature of the fresh air introduced into the cylinder 11. As the cooling means, for example, a cooling system that is different from the cooling system of the engine 1 including the water jacket formed in the cylinder head 13 and the cylinder block 12 and has a lower temperature zone than that of the cooling system and a high cooling effect of the intake air. A system may be provided around the intake port 18 to actively cool the intake air before cylinder introduction. Further, a cooler for cooling the intake air may be provided in the middle of the intake passage. This cooler may be constituted by, for example, an intercooler for cooling the intake air after being supercharged by a supercharger on the intake passage. The turbocharger may be a turbocharger including a turbine disposed on the exhaust passage side. However, the lean burn engine 1 takes into account that it is difficult to secure a sufficient boost pressure due to reduction of exhaust loss. An electric supercharger may be provided. Further, a combination of a turbocharger and an electric supercharger can be used.

  The lean burn engine 1 can be incorporated into a hybrid system as an engine mounted on a hybrid vehicle, for example. By doing so, the engine 1 can be operated only in the operation region where the illustrated thermal efficiency is high. As a result, it can be further advantageous in improving the efficiency of the engine and hence the fuel efficiency.

  The numerical range (20 ≦ ε ≦ 50) related to the geometric compression ratio ε described above can be partially reduced by arbitrarily narrowing the range within the range in which the same operation effect is obtained. . Further, the numerical range (2.5 ≦ λ ≦ 6) associated with the excess air ratio λ can also be partially eliminated by arbitrarily narrowing the range in which the same effect as the above-described effect can be obtained. .

1 Lean burn engine (engine body)
11 Cylinder 12 Cylinder block 13 Cylinder head 15 Piston 17 Combustion chamber 18 Intake port 181 Port liner (port heat insulation layer)
19 Exhaust port 100 Engine controller (control means)
21 Intake valve 22 Exhaust valve 31 Spark plug (ignition means)
32 Ignition system (ignition means)
33 Fuel injection valve 34 Fuel supply system (fuel supply means)
6 Combustion chamber heat insulation layer 61 Combustion chamber heat insulation layer 62 Combustion chamber heat insulation layer 63 Combustion chamber heat insulation layer 64 Combustion chamber heat insulation layer 65 Combustion chamber heat insulation layer

Claims (8)

Cylinder block in which a cylinder is formed, a piston fitted into the cylinder, a cylinder head placed on the cylinder block, and an intake valve and an exhaust valve that open and close an intake port and an exhaust port formed in the cylinder head An engine body having a combustion chamber surrounded by
Control means for controlling the operation of the engine body through control of at least the excess air ratio λ,
The combustion chamber is
A wall surface of the cylinder;
A crown surface of the piston;
A ceiling surface that is formed in the cylinder head and has the intake port and the exhaust port each open, and closing the upper end opening of the cylinder;
A valve head surface of each of the intake valve and the exhaust valve;
At least a part of a section screen defining the combustion chamber is provided on a surface side of a base material that constitutes the cylinder block, the piston, the cylinder head, the intake valve, and the exhaust valve, which surrounds the combustion chamber; , Constituted by a combustion chamber heat insulating layer that suppresses the heat of the gas in the combustion chamber from being released to the outside through the partition wall;
The engine body has a geometric compression ratio ε set to 20 ≦ ε ≦ 50,
The lean burn engine in which the control means sets the excess air ratio λ during combustion to 2.5 ≦ λ ≦ 6 when the engine body is at least in a partial load operation region. The lean burn engine according to claim 1,
A lean burn engine in which a port heat insulating layer having a lower thermal conductivity than the cylinder head is provided on a port wall surface defining the intake port. The lean burn engine according to claim 2,
A lean burn engine further comprising a cooling means for cooling the intake air flowing into the cylinder. The lean burn engine according to any one of claims 1 to 3,
The lean combustion engine in which the combustion chamber heat insulating layer is made of a heat-resistant ceramic having a thermal conductivity lower than that of the base material and a volume specific heat equal to or lower than that of the base material. The lean burn engine according to any one of claims 1 to 4,
The combustion chamber insulation layer is
A porous layer containing air;
A lean burn engine comprising: a coating layer covering the surface side of the porous layer and constituting the section screen. In the lean burn engine according to any one of claims 1 to 5,
The control means is a lean burn engine that controls the excess air ratio λ during combustion so that the maximum combustion temperature is equal to or lower than a predetermined temperature when the load increases in the operating range of the partial load. The lean burn engine according to any one of claims 1 to 6,
Further comprising ignition means including an ignition plug disposed facing the combustion chamber;
The ignition means is a lean burn engine which is a plasma type ignition means. The lean burn engine according to any one of claims 1 to 7,
Fuel supply means including a fuel injection valve for directly injecting fuel into the combustion chamber;
The fuel supply means supplies fuel into the combustion chamber so that an air-fuel mixture exists in the center of the combustion chamber at the start of combustion, and fresh air is interposed between the air-fuel mixture and the section screen. A lean burn engine to supply.

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