JP2017194004A - Combustion chamber structure of diesel engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a combustion chamber structure of a Diesel engine capable of improving utilization ratio of air suctioned into a cylinder to further reduce a black smoke discharge amount and improve fuel economy.SOLUTION: A piston 17 has an inlet lip part 35 formed by a first cavity 31 opened to a top surface 29 and a second cavity 32 opened to a first bottom surface 33 of the first cavity 31 and having a smaller opening diameter D2 than an opening diameter D1 of the first cavity 31. The first cavity 31 has the first bottom surface 33, and a first side surface 34 approaching the top surface 29 of the piston 17 toward a portion on a radial outside of the piston 17. A Diesel engine is configured to inject fuel to the inlet lip part 35 at least in an early stage of the fuel injection. A ratio of the opening diameter D2 of the second cavity 32 to a bore diameter Db of a cylinder is within a range of 0.662±0.032.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a combustion chamber structure of a direct injection type diesel engine.

  As a technique for reducing NOx contained in exhaust gas of a diesel engine, exhaust gas recirculation (EGR) in which a part of the exhaust gas is recirculated to an intake passage is known. EGR reduces NOx by supplying a mixed gas of air and exhaust gas as a working gas to a cylinder and lowering the combustion temperature of the mixed gas. However, the reduction in the combustion temperature leads to an increase in the amount of black smoke discharged due to the suppression of the black smoke oxidation reaction. Therefore, in Patent Document 1, by avoiding the formation of a fuel-rich region locally, even when the exhaust gas recirculation amount is increased, the amount of black smoke emission is reduced and the fuel efficiency is improved. A contemplated combustion chamber structure is disclosed. The combustion chamber structure of Patent Document 1 is a step formed by a first cavity recessed on the top surface and a second cavity opening at the bottom surface of the first cavity and having an opening diameter smaller than that of the first cavity. It is constituted by a piston having a certain inlet lip.

Japanese Patent No. 4906005

However, as a result of the inventor's analysis of the combustion chamber structure described in Patent Document 1, there remains room for improvement in reducing black smoke emissions and improving fuel consumption.
An object of the present invention is to provide a combustion chamber structure of a diesel engine that further reduces black smoke emission and improves fuel consumption by improving the utilization rate of air taken into a cylinder.

  A combustion chamber structure of a diesel engine that solves the above problem is a combustion chamber structure of a diesel engine in which fuel is injected radially toward a cavity that is recessed in the top surface of the piston, and the piston includes the top surface An inlet lip portion that is a step formed by a first cavity that opens to a bottom surface of the first cavity and a second cavity that has an opening diameter smaller than the opening diameter of the first cavity; The first cavity has the bottom surface and a side surface that is closer to the top surface of the piston toward a radially outer portion of the piston, and the diesel engine is fueled toward the inlet lip at least at the initial stage of fuel injection. The ratio of the opening diameter of the second cavity to the bore diameter of the cylinder housing the piston 0.662 is ± 0.032.

  According to the combustion chamber structure and piston of the diesel engine, the fuel spray injected toward the inlet lip portion is distributed by the inlet lip portion into a flow flowing into the first cavity and a flow flowing into the second cavity. . The fuel spray flowing into the first cavity is guided by the side surface of the first cavity to form an upward flow toward the squish area which is a space between the top surface of the piston and the upper wall surface of the cylinder. As the piston moves downward, a fuel spray flow is formed toward the inside and outside in the radial direction of the piston. On the other hand, the fuel spray flowing into the second cavity flows toward the bottom surface of the second cavity along the side surface of the second cavity, and then flows toward the inside in the radial direction of the piston along the bottom surface of the second cavity. Along with the downward movement of the piston, a vertically swirling vortex is formed in the vicinity of the side surface of the second cavity. This flow facilitates the spread of fuel spray, avoids the formation of fuel-rich areas locally during both low-speed operation and high-speed operation, significantly reduces black smoke emissions, and improves fuel consumption. Can be achieved.

  As a result of earnest research on the introduction of fuel spray air, the present inventors set the ratio of the opening diameter of the second cavity to the bore diameter of the cylinder to 0.662 ± 0.032 to introduce the air for fuel spray. Has been found to be more promoted. That is, according to the said structure, the air utilization factor which shows the ratio of the air taken in in the fuel spray among the air inhaled in the cylinder can be improved. As a result, it is possible to reduce the black smoke emission amount and the net fuel consumption rate.

  In the combustion chamber structure of the diesel engine configured as described above, the nozzle cone angle in the injector for injecting the fuel is set in a range of 140 ° to 160 °, and the opening edge of the first cavity in the radial direction of the piston and the first The width, which is the distance from the opening edge of the two cavities, is preferably set in the range of 9% to 19% with respect to the opening diameter of the second cavity.

According to the above configuration, the fuel spray is easily diffused both during the low speed operation and during the high speed operation, and it is possible to avoid the formation of a region where the fuel is dense, thereby obtaining a remarkable black smoke reduction effect.
In the combustion chamber structure of the diesel engine having the above configuration, the depth from the top surface of the piston to the bottom surface of the first cavity is in the range of 4.5% to 9.5% with respect to the opening diameter of the second cavity. It is preferable that it is set to.

  According to the above configuration, the fuel spray is easily diffused both during the low speed operation and during the high speed operation, and it is possible to avoid the formation of a region where the fuel is dense, thereby obtaining a remarkable black smoke reduction effect.

The figure which shows typically schematic structure of the diesel engine to which one Embodiment of the combustion chamber structure of the diesel engine was applied. The fragmentary sectional view which shows an example of the cross-sectional shape in the top part vicinity of a piston. (A) The figure which shows typically an example of the flow of the fuel spray immediately after a fuel spray collides with an inlet lip part, (b) The figure which shows typically an example of the flow of the fuel spray at the time of piston lowering. (A) The figure which shows typically the nozzle cone angle of a porous injector, (b) The figure which shows typically the shape of the cavity of each pattern which performed simulation. It is a figure which shows the area | region where an equivalence ratio is 0.5 or more and an isoline for every equivalence ratio 0.5 in crank angle 50 degree ATDC, Comprising: (a) The figure which shows the case of the pattern 1, (b) Pattern 5 The figure which shows the case of (c) The figure which shows the case of the pattern 3. The graph which shows an example of the relationship between a crank angle and the air utilization factor of in-cylinder space. The graph which shows an example of the relationship between the diameter ratio in each crank angle, and an air utilization factor. (A) The graph which shows an example of the relationship between a crank angle and the air utilization factor of a squish area, (b) The graph which shows an example of the relationship between a crank angle and the air utilization factor of an internal area. The graph which shows an example of the relationship between a net average effective pressure and a net fuel consumption rate. The graph which shows an example of the relationship between a net average effective pressure and black smoke discharge concentration. The graph which shows an example of the relationship between a crank angle and a heat release rate.

  With reference to FIGS. 1-11, one Embodiment of the combustion chamber structure of a diesel engine is described. First, a schematic configuration of a diesel engine to which the combustion chamber structure of the diesel engine is applied will be described with reference to FIG.

  As shown in FIG. 1, the engine 10 is a direct injection diesel engine having a plurality of cylinders 11. The engine 10 includes an intake passage 12 through which intake air flows and an exhaust passage 13 through which exhaust gas flows. The engine 10 includes an EGR passage 15 that connects the exhaust passage 13 and the intake passage 12, and an EGR valve 16 that can change the cross-sectional area of the EGR passage 15. When the EGR valve 16 is in the open state, the engine 10 is supplied with a mixed gas of intake air and exhaust gas as a working gas.

  A piston 17 is accommodated in the cylinder 11 so as to be movable up and down, and an in-cylinder space 20 that is a space surrounded by the piston 17, the side wall surface 18 of the cylinder 11, and the upper wall surface 19 of the cylinder 11 is formed. Yes. Fuel is injected into the in-cylinder space 20 from an injector 21 located on the central axis A of the piston 17. The injector 21 injects fuel radially outward of the piston 17. In the engine 10, the air-fuel mixture of the working gas and the fuel spray injected into the in-cylinder space 20 is sequentially burned by self-ignition, whereby the piston 17 is moved up and down to rotate the crankshaft 23.

  As shown in FIG. 2, the cylinder 11 has a bore diameter Db. The piston 17 has a cavity 30 recessed in the top surface 29 thereof. The cavity 30 has a first cavity 31 that opens to the top surface 29 of the piston 17 and a second cavity D2 that opens to the first bottom surface 33 of the first cavity 31 and has an opening diameter D2 that is smaller than the opening diameter D1 of the first cavity 31. And a cavity 32. The piston 17 has a step formed by a difference in diameter between the opening diameter D 1 of the first cavity 31 and the opening diameter D 2 of the second cavity 32, and an inlet lip portion 35 that protrudes radially inward of the piston 17. Is formed. The front end portion of the inlet lip portion 35 is formed in a gently curved shape.

  The first cavity 31 is a surface that connects the first bottom surface 33, the outer peripheral edge of the first bottom surface 33, and the top surface 29 of the piston 17, and is a curved surface that is closer to the top surface 29 toward the radially outer side of the piston 17. A first side surface 34. The first cavity 31 has an opening diameter D1 centered on the central axis A of the piston 17, a depth Dp that is a distance from the top surface 29 of the piston 17 to the first bottom surface 33 of the first cavity 31, and the piston 17 It has a width L that is the distance between the opening edge of the first cavity 31 and the opening edge of the second cavity 32 in the radial direction.

  The second cavity 32 includes a second side surface 36 that is continuous with the inner periphery of the first bottom surface 33 of the first cavity 31, and a second bottom surface 37 that extends inward in the radial direction of the piston 17 from the lower end periphery of the second side surface 36. Yes. The second side surface 36 is a surface connecting the upper side surface 36 a that protrudes radially outward of the piston 17 from the inner peripheral edge of the first bottom surface 33, and the upper side surface 36 a and the second bottom surface 37, and is radially inward of the piston 17. The lower side surface 36b is gently curved toward the bottom. The second bottom surface 37 is an inclined surface that approaches the top surface 29 of the piston 17 as the portion is closer to the central axis A of the piston 17. By the second bottom surface 37, a center cone 38 having a flat conical shape having a tip portion on the central axis A is formed on the piston 17. The cavity 30 may be a toroidal cavity whose upper side surface 36a extends in the vertical direction while maintaining the opening diameter D2 of the second cavity 32.

  The flow of fuel spray in the in-cylinder space 20 during low speed operation and high speed operation of the engine 10 will be described with reference to FIGS. 3 and 4. In the engine 10, fuel is injected from the injector 21 toward the inlet lip 35 at the initial stage of the fuel injection period, and the injected fuel spray collides with the inlet lip 35.

  As shown in FIG. 3A, the fuel spray injected from the injector 21 is distributed by the inlet lip portion 35 into a flow flowing into the first cavity 31 and a flow flowing into the second cavity 32. The fuel spray flowing into the first cavity 31 is guided by the first side surface 34 and flows upward toward the squish area 40 which is a space between the top surface 29 of the piston 17 and the upper wall surface 19 of the cylinder 11. Form. On the other hand, the fuel spray flowing into the second cavity 32 flows along the second side surface 36 toward the second bottom surface 37, and then flows along the second bottom surface 37 toward the inside in the radial direction of the piston 17.

  As shown in FIG. 3 (b), the upward flow toward the squish area 40 accompanying the downward movement of the piston 17 is caused by the swirl vortex combined with the downward movement of the piston 17 in the radial inner side and the outer side. The fuel spray is moved to Further, the fuel spray moves outward in the radial direction by the momentum of the injection remaining in the upward flow toward the squish area 40. The flow of the fuel spray toward the radially inner side of the piston 17 in the second cavity 32 is promoted to spread by the swirling vortex in the vertical direction coupled with the downward movement of the piston 17. By forming such a flow of fuel spray, it becomes easy for the fuel spray to diffuse during both low speed operation and high speed operation, thereby avoiding the formation of a fuel-rich region locally.

With reference to FIG. 4, various conditions in the simulation performed by the present inventor for the air utilization rate U in the in-cylinder space 20 will be described.
As shown in FIG. 4A, in this simulation, the nozzle cone angle θn, which is the angle formed by the fuel injection direction of the injector 21 around the central axis A of the piston 17, is set to 140 ° or more and 160 ° or less. In the piston 17, the width L is set in a range of 9% or more and 19% or less (= L / D2) with respect to the opening diameter D 2 of the second cavity 32, and the depth Dp of the first cavity 31 is set to the second It was set in the range of 4.5% to 9.5% (= Dp / D2) with respect to the opening diameter D2 of the cavity 32. The simulation was performed by changing the diameter ratio R (= D2 / Db) of the opening diameter D2 of the second cavity 32 to the bore diameter Db of the cylinder 11 while keeping the volume of the cavity 30 constant. In this embodiment, as shown in FIG. 4B, a pattern 1 having a diameter ratio R = 0.58, a pattern 2 having a diameter ratio R = 0.63, a pattern 3 having a diameter ratio R = 0.661, and a diameter ratio. A simulation was performed on five patterns: a pattern 4 with R = 0.694 and a pattern 5 with a diameter ratio R = 0.73. As shown in FIG. 4B, the cavity 30 becomes shallow as the diameter of the second cavity 32 is increased.

An example of the result of the above-described simulation and the like will be described with reference to FIGS.
FIGS. 5A to 5C are diagrams showing regions where the equivalence ratio is 0.5 or more with dots at a crank angle of 50 ° ATDC, and also showing isolines for each equivalence ratio of 0.5.

  As shown in FIG. 5A, in the pattern 1, fuel diffusion over a wide range is recognized in the squish area 40, but the fuel in the inner area 41 including the cavity 30 is an area inside the squish area 40. The diffusion of was found to be partial.

  As shown in FIG. 5B, in pattern 5, it was recognized that the fuel diffusion in the squish area 40 was partial, although the fuel diffusion over a wide range was recognized in the inner area 41. In the entire cylinder space 20, fuel diffusion over a wider range than that of the pattern 1 was recognized.

  As shown in FIG. 5 (c), in pattern 3, a wide range of fuel diffusion is recognized in the squish area 40, and in the inner area 41, fuel diffusion is recognized in a wider range than pattern 1 although not as much as pattern 5. It was. In the entire cylinder space 20, fuel diffusion over a wider range than both patterns 1 and 5 was observed.

  FIG. 6 is a graph showing an example of the relationship between the crank angle and the air utilization rate of the in-cylinder space. In FIG. 6, the portion of the air-fuel mixture with an equivalence ratio of 0.1 or more is set as the air introduction region for fuel spray. Since 99% or more of the fuel is distributed in a region having an equivalence ratio of 0.1 or more, it was considered that air in this region can be regarded as air introduced by fuel spray. And the ratio of the air quantity of the air introduction area | region of the fuel spray with respect to the air quantity of the cylinder space 20 was set to the air utilization factor U. FIG. 7 is a graph showing an example of the relationship between the diameter ratio R and the air utilization rate U at each crank angle.

  As shown in FIGS. 6 and 7, it was recognized that the air utilization rate U improved as the crank angle advanced in each of the patterns 1 to 5. After the crank angle of 15 ° ATDC, it was recognized that the air utilization rate U of pattern 1 was the lowest. In the range of a crank angle of 15 ° ATDC to a crank angle of 40 ° ATDC, the air usage rate U of patterns 1 and 2 is lower than the air usage rate U of pattern 3, and the pattern 4 is lower than the air usage rate U of pattern 3. , 5 was found to have a high air utilization rate U. After the crank angle of 50 ° ATDC, it is recognized that the air utilization rate U of the pattern 3 is the highest, and the air utilization rate U tends to decrease as the deviation from the pattern 3 diameter ratio R = 0.661 increases. It was.

  In response to the simulation results described above, the inventor has analyzed the relationship between the crank angle and the air utilization rate of the squish area 40 and the relationship between the crank angle and the air utilization rate of the internal area 41.

  As shown in FIGS. 8A and 8B, the air utilization rate U in the squish area 40 is lower at each crank angle as the diameter ratio R is larger, and the diameter ratio R is larger as the diameter ratio R is larger. It was observed that the rate of decrease of the air utilization rate U with respect to the decrease of the air rate increases. On the other hand, although the air utilization rate U in the internal area 41 increases as the diameter ratio R increases, it has been recognized that the increase in the air utilization rate relative to the increase in the diameter ratio R decreases as the crank angle advances. That is, the air utilization rate U in the squish area 40 increases as the diameter ratio R decreases, and decreases as the diameter ratio R increases. The air utilization rate U in the inner area 41 increases as the diameter ratio R increases. Was recognized.

  In addition, the present inventor, from the results of the experiments conducted using the pattern 1 and pattern 2 pistons, the relationship between the net average effective pressure and the net fuel consumption rate, the relationship between the net average effective pressure and the black smoke emission concentration, The relationship between the crank angle and the heat generation rate was analyzed.

  Here, in a high load state of the engine 10 where black smoke is likely to be generated, the combustion of the fuel starts from the vicinity of the top dead center of the piston 17, and the crank angle of 20 ° to 30 ° ATDC is relatively the main combustion of the fuel. Premixed combustion and diffusion combustion with a high heat release rate occur. Thereafter, afterburn combustion with a relatively low heat generation rate occurs up to a crank angle of 70 ° ATDC. Therefore, the higher the air utilization rate U before the crank angle 20-30 ° ATDC, the more the combustion in the main combustion is promoted, and the net fuel consumption rate is reduced by improving the isovolume. Further, as the air utilization rate U at a crank angle of 70 ° ATDC at which combustion ends is higher, the oxidation of black smoke in the afterburning combustion is promoted, and the black smoke emission concentration is reduced.

  As shown in FIG. 9, the net fuel consumption rate changes with the same tendency with respect to the net average effective pressure in both pattern 1 and pattern 2, but in each net average effective pressure, pattern 2 is more likely than pattern 1. Was found to be small. That is, when the net average effective pressure is the same value, the net fuel consumption rate is recognized to be smaller in the pattern 2 having a high air utilization rate U at a crank angle of 25 ° ATDC.

  As shown in FIG. 10, although the black smoke discharge concentration of pattern 2 is maintained at a value close to 0% for each net average effective pressure, the black smoke discharge concentration of pattern 1 is within a predetermined net average effective pressure range. It was observed that the black smoke emission concentration of Pattern 2 was larger. That is, it was recognized that the black smoke discharge density was lower in the pattern 2 having a high air utilization rate U at a crank angle of 70 ° ATDC. In addition, in FIG. 10, the black smoke discharge density | concentration is shown by the ratio with respect to the maximum black smoke discharge density | concentration obtained by this experiment.

  As shown in FIG. 11, the heat generation rate with respect to the crank angle was observed to change in the same manner in pattern 1 and pattern 2 until slightly before crank angle 10 ° ATDC. Then, it was recognized that the heat generation rate thereafter was larger in the pattern 2 than in the pattern 1 until slightly before the crank angle of 20 ° ATDC. In Pattern 2, since the continuous decrease in the heat generation rate occurs earlier than Pattern 1, it was recognized that the main combustion ended earlier than Pattern 1. That is, pattern 2 was found to have a lower net fuel consumption rate due to a higher isovolume than pattern 1. In addition, in FIG. 11, the heat release rate is shown by the ratio with respect to the maximum heat release rate [J / deg] obtained in this experiment.

  As shown in FIG. 7, the pattern 5 has a higher air utilization rate U than the pattern 1 at a crank angle of 25 ° ATDC. Therefore, it is easily estimated that the net fuel consumption rate and the heat generation rate in pattern 5 are superior to the values in pattern 1. Pattern 3 has a higher air utilization rate U than pattern 2 at a crank angle of 25 ° ATDC. Therefore, it is easily estimated that the net fuel consumption rate and the heat generation rate in Pattern 3 are superior to the values in Pattern 2.

  Further, patterns 2 and 4 have a higher air utilization rate U than patterns 1 and 5 at a crank angle of 70 ° ATDC. Therefore, it is easily estimated that the black smoke discharge density in patterns 2 and 4 is superior to patterns 1 and 5. Moreover, since the pattern 3 has the highest air utilization rate U at a crank angle of 70 ° ATDC, it is easily estimated that the black smoke discharge density is the best among the patterns 1 to 5.

  From the above, the diameter ratio R has a higher air utilization rate U than the pattern 1 at each crank angle, and the combustion reaction is caused by a decrease in the in-cylinder temperature accompanying the downward movement of the piston 17 even under a high load condition. It is preferable that the diameter ratio R = 0.62 ± 0.032 (0.63 ≦ R ≦ 0.694) at which the air utilization rate U is 95% or more at a crank angle of 70 ° ATDC at which the air temperature is frozen.

According to the combustion chamber structure of the diesel engine of the above embodiment, the following effects can be obtained.
(1) In the piston 17 having the inlet lip portion 35, the air utilization rate U can be improved by setting the diameter ratio R to 0.662 ± 0.032.

  (2) The nozzle cone angle θn in the injector 21 is set in the range of 140 ° to 160 °, and the width L of the first cavity 31 is in the range of 9% to 19% with respect to the opening diameter D2 of the second cavity 32. Is set to According to such a configuration, it becomes easy for the fuel spray to diffuse both during low speed operation and during high speed operation, and it is possible to avoid the formation of a fuel rich region locally. Thereby, the remarkable black smoke reduction effect is acquired.

  (3) The depth Dp of the first cavity 31 is set in the range of 4.5% or more and 9.5% or less with respect to the opening diameter D2 of the second cavity 32. According to such a configuration, it becomes easy for the fuel spray to diffuse both during low speed operation and during high speed operation, and it is possible to avoid the formation of a fuel rich region locally. Thereby, the remarkable black smoke reduction effect is acquired.

In addition, the said embodiment can also be suitably changed and implemented as follows.
The depth Dp of the first cavity 31 is preferably set in the range of 4.5% to 9.5% with respect to the opening diameter D2 of the second cavity 32. The effect described in 1) can be obtained.

  The nozzle cone angle θn in the injector 21 is preferably set in the range of 140 ° or more and 160 ° or less, but the effect described in (1) above can be obtained even if it is not in this range.

  The width L of the first cavity 31 is preferably set in a range of 9% or more and 19% or less with respect to the opening diameter D2 of the second cavity 32. However, the width L is not limited to this range but is described in (1) above. An effect can be obtained.

  In short, the piston 17 has the inlet lip portion 35, the cavity 30 is constituted by the first cavity 31 and the second cavity 32, and the fuel is injected into the inlet lip portion 35 at least at the initial stage of the fuel injection period. In the configuration, the diameter ratio R may be set to 0.662 ± 0.032.

  DESCRIPTION OF SYMBOLS 10 ... Engine, 11 ... Cylinder, 12 ... Intake passage, 13 ... Exhaust passage, 15 ... EGR passage, 16 ... EGR valve, 17 ... Piston, 18 ... Side wall surface, 19 ... Upper wall surface, 20 ... In-cylinder space, 21 ... Injector, 23 ... crankshaft, 29 ... top surface, 30 ... cavity, 31 ... first cavity, 32 ... second cavity, 33 ... first bottom surface, 34 ... first side surface, 35 ... inlet lip, 36 ... second Side surface, 36a ... Upper side surface, 36b ... Lower side surface, 37 ... Second bottom surface, 38 ... Center cone, 40 ... Squish area, 41 ... Inner area.

Claims (3)

A combustion chamber structure of a diesel engine in which fuel is injected radially toward a cavity recessed in a top surface of a piston,
The piston is an entrance that is a step formed by a first cavity that opens to the top surface and a second cavity that opens to the bottom surface of the first cavity and has an opening diameter smaller than the opening diameter of the first cavity. Has a lip,
The first cavity has the bottom surface and a side surface that approaches the top surface of the piston toward a radially outer side of the piston,
The diesel engine is configured to inject fuel toward the inlet lip at least at an initial stage of fuel injection,
A combustion chamber structure of a diesel engine, wherein a ratio of an opening diameter of the second cavity to a bore diameter of a cylinder accommodating the piston is 0.662 ± 0.032. The nozzle cone angle in the injector for injecting the fuel is set in a range of 140 ° to 160 °,
The width, which is the distance between the opening edge of the first cavity and the opening edge of the second cavity in the radial direction of the piston, is set in a range of 9% to 19% with respect to the opening diameter of the second cavity. The combustion chamber structure of the diesel engine according to claim 1. The depth from the top surface of the piston to the bottom surface of the first cavity is set in a range of 4.5% to 9.5% with respect to the opening diameter of the second cavity. Diesel engine combustion chamber structure.