Collision multi-swirl four-leaf clover type combustion system

Abstract

The collision multi-swirl four-leaf clover type combustion system comprises a piston component and an oil injection component, the piston component is provided with a top face, and a combustion chamber is formed in the top face; the combustion chamber comprises a main combustion cavity and a plurality of arc-shaped combustion cavities, the main combustion cavity and the top face are coaxially arranged, the arc-shaped combustion cavities are arranged in the circumferential direction of the main combustion cavity and communicate with the main combustion cavity, and oil distribution protrusions are arranged on the inner walls of the arc-shaped combustion cavities so as to guide fuel oil injected into the arc-shaped combustion cavities to be dispersed in the vertical direction and flow in a convolution mode. A boss is further arranged on the inner wall of the arc-shaped combustion cavity, the boss is located above the oil distribution protrusion and used for guiding fuel oil to flow in a convolution mode, and a plurality of pairs of oil injection holes used for injecting the fuel oil into the combustion chamber are formed in the oil injection component. The space in the combustion chamber is fully utilized, so that fuel oil is fully mixed with air in the combustion chamber, over-concentrated combustion of the fuel oil is avoided, and the condition of high-temperature oxygen-deficient combustion of the fuel oil is improved.

Description

Technical Field

[0001] The invention relates to the technical field of internal combustion engines, and in particular to a collision multi-swirling four-leaf clover type combustion system. Background Art

[0002] With the development of automobile technology, higher requirements are put forward for engine performance and emissions. Combustion system design is the key to improving fuel consumption and bare-metal emissions. The combustion chamber pits of National VI diesel engines are mostly traditional constricted or straight-mouthed gyroscope structures. Ultra-high injection pressure and combustion chamber structure matching are used to promote oil-gas mixing. Cold EGR technology routes are mostly adopted. The exhaust gas is introduced into the intake air into the cylinder according to the required amount through the EGR pipeline, so as to effectively control NOx bare-metal emissions. The intake duct is a straight duct with relatively small duct vortex. The combustion chamber pits are traditional constricted or straight-mouthed gyroscope structures. Ultra-high injection pressure and combustion chamber structure matching are used to promote oil-gas mixing, thereby achieving efficient and clean combustion, improving fuel economy and particulate emissions. The use of straight ducts plus ultra-high pressure injection can effectively promote mixing and improve the diesel engine combustion process. However, ultra-high pressure injection will increase the driving power consumption of the oil pump, thereby affecting the thermal efficiency of the engine.

[0003] In order to improve the combustion process in the cylinder, some researchers have tried to use non-rotating structures to improve the oil-gas mixing, such as a diesel engine combustion chamber and a diesel engine in application No. 202210638635.9, the diesel engine combustion chamber includes a lip, which is opened at the opening of the combustion chamber along the circumference of the combustion chamber; a first boss, which is convexly arranged at the bottom of the combustion chamber; a second boss, which is convexly arranged at the bottom of the combustion chamber and is spaced from the first boss, and the first boss and the second boss are connected by a circular arc transition; a pit, which is opened on the bottom surface of the combustion chamber along the circumference of the first boss and the circumference of the second boss; a ridge, which is arranged at the connection between the second boss and the pit. The combustion chamber is a non-rotating structure, which is obtained by scanning two different surfaces. Surface A is a traditional step structure, the center boss is the first boss, and surface B is provided with a second boss at the bottom of the combustion chamber to achieve rewinding fuel diversion and accelerate the combustion speed.

[0004] However, the problem is that this method can only improve the mixing and distribution of fuel in the axial direction, but cannot improve the mixing of oil and gas in the radial direction. Moreover, the fuel forms a vortex by hitting the throat and then reaches the second boss position through the guiding effect of the bottom pit. The kinetic energy has been greatly reduced. Most of the mixed gas will move obliquely along the second ridge to the upper part of the combustion chamber, and it is difficult to be sucked into the combustion chamber. The fuel is easy to accumulate near the pit, and the particle emission is high. Although the above scheme can improve the mixing and distribution of oil and gas in the axial direction, it cannot improve the mixing and distribution of oil and gas in the radial direction and the particle emission is high. Summary of the invention

[0005] The purpose of the present invention is to overcome the above technical deficiencies and propose a collision multi-volute four-leaf clover type combustion system to solve the technical problem that the prior art cannot improve the radial mixing of oil and gas.

[0006] In order to achieve the above technical objectives, the present invention adopts the following technical solutions: The present invention provides a collision multi-puff four-leaf clover type combustion system, comprising: A piston component having a top surface, on which a combustion chamber is formed, wherein the combustion chamber comprises a main combustion chamber coaxially arranged with the top surface and a plurality of arc-shaped combustion chambers circumferentially arranged around the main combustion chamber and connected with the main combustion chamber, wherein an oil separation protrusion is formed on the inner wall of the arc-shaped combustion chamber to guide the fuel injected into the arc-shaped combustion chamber to disperse and flow in a convolution in a vertical direction, and a boss is further formed on the inner wall of the arc-shaped combustion chamber, wherein the boss is located above the oil separation protrusion to guide the convolution flow of the fuel; and A fuel injection component is provided with a plurality of pairs of fuel injection holes for injecting fuel into the combustion chamber.

[0007] In some embodiments, the bottom inner wall of the main combustion chamber is provided with a central protrusion arranged around the central axis, and a central annular portion circumferentially recessed around the central protrusion, and the central annular portion smoothly transitions with the oil separation protrusion so that the cross-section of the main combustion chamber along the central axis is ω-shaped.

[0008] In some embodiments, the inner wall of the arc-shaped combustion chamber is arc-shaped, and the inner diameter of the arc-shaped combustion chamber gradually decreases in a direction away from the main combustion chamber, so that the arc-shaped combustion chamber forms a leaf shape.

[0009] In some embodiments, the diameter of the arc-shaped combustion chamber gradually decreases in the vertical direction to form a necking structure.

[0010] In some embodiments, four arc-shaped combustion chambers are opened on the top surface, and the four arc-shaped combustion chambers are connected to the main combustion chamber to form a four-leaf clover shape.

[0011] In some embodiments, the oil separation protrusion is arranged on the inner wall of the arc-shaped combustion chamber and protrudes toward the main combustion chamber, transitioning in an arc shape with the inner wall of the arc-shaped combustion chamber, so that a first recess located below the oil separation protrusion and a second recess located above the oil separation protrusion are formed on the inner wall of the arc-shaped combustion chamber.

[0012] In some embodiments, the oil separation protrusion is bent into an arc shape along the inner wall of the arc-shaped combustion chamber.

[0013] In some embodiments, the boss is bent into an arc shape along the inner wall of the arc-shaped combustion chamber.

[0014] In some embodiments, the oil spray component includes a main body and a spray head, the spray head is fixed on the bottom of the main body, and a plurality of pairs of oil spray holes are formed on the spray head.

[0015] In some embodiments, each pair of the oil injection holes is symmetrical about the radial axis of the main body, and is used to inject two oil beams that intersect each other and have an acute angle.

[0016] Compared with the prior art, the collision multi-volute four-leaf clover type combustion system provided by the present invention comprises a piston component and an injection component, wherein the piston component has a top surface, and a plurality of combustion chambers are provided on the top surface, wherein the combustion chamber comprises a main combustion chamber coaxially arranged with the top surface and a plurality of arc-shaped combustion chambers circumferentially arranged around the main combustion chamber and connected with the main combustion chamber, and an oil separation protrusion is provided on the inner wall of the arc-shaped combustion chamber to guide the fuel injected into the arc-shaped combustion chamber to disperse and convolute in the vertical direction, and a boss is also provided on the inner wall of the arc-shaped combustion chamber, and the boss is located above the oil separation protrusion to guide the convolute flow of the fuel, and a plurality of pairs of injection holes for injecting fuel into the combustion chamber are provided on the injection component, and each pair of the injection holes injects two oil beams that intersect each other and have an acute angle. When the piston moves to near the top dead center, the fuel is injected into the combustion chamber in the form of an oil beam through the injection component, and gasifies to form fuel vapor. After that, the fuel vapor will first contact the oil separation protrusion, and will be split by the oil separation protrusion to form two parts, one of which will be reflected downward by the oil separation protrusion 1 and flow into the main combustion chamber, and the other part will be reflected upward by the oil separation protrusion, and then contact and collide with the boss to form a further upward convolution oil beam, guiding the fuel to further disperse and convolute. In this way, the fuel will not be concentrated in the main combustion chamber or the arc-shaped combustion chamber, and the space in the combustion chamber will be fully utilized to fully mix the fuel with the air in the combustion chamber, avoiding excessively concentrated combustion of the fuel and improving the high-temperature and oxygen-poor combustion of the fuel; in addition, the oil beams sprayed by each pair of the fuel injection holes will intersect, so that the oil beams collide with each other and disperse to form oil mist, so that the fuel can be further dispersed and mixed with the air, improving the high-temperature and oxygen-poor combustion of the fuel.

[0017] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the preferred embodiments of the present invention are described in detail as follows with reference to the accompanying drawings. The specific implementation of the present invention is given in detail by the following embodiments and their accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic structural diagram of the collision multi-puff four-leaf clover type combustion system provided by the present invention; Figure 2 yes Figure 1Schematic diagram of top view of the piston component: Figure 3 yes Figure 1 Schematic diagram of the cross section of the piston component: Figure 4 yes Figure 1 Schematic diagram of the structure of the fuel injection components.

[0019] Description of reference numerals: 1-piston component, 11-top surface, 12-combustion chamber, 121-main combustion chamber, 121a-center convex portion, 121b-center annular portion, 122-arc-shaped combustion chamber, 122a-first concave portion, 122b-second concave portion, 123-oil separation protrusion, 124-boss, 2-injection component, 21-main body, 22-nozzle, 23-injection hole. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0021] Most of the current internal combustion engine systems use a pit formed on the top surface of the piston base as the main space for fuel combustion. The pit formed on the top surface is the combustion chamber. The fuel beam sprayed from the hole-type fuel injector of the internal combustion engine enters the pit of the combustion chamber and performs a compression stroke as the piston moves upward. The fuel is gradually vaporized and mixed with the air in the pit to form a mixture. Finally, the combustion and expansion of the fuel push the piston downward to do work.

[0022] From the working process of the above-mentioned internal combustion engine, it can be known that after the fuel enters the combustion chamber pit, it is only fully mixed with the air in the combustion chamber pit, but is not thoroughly mixed with the air outside the combustion chamber pit, especially with the air outside the combustion chamber pit and above the top surface, which aggravates the situation of concentrated local combustion of the fuel, causing high temperature and oxygen deficiency during diesel combustion, and then producing a large amount of carbon soot particles and NOx.

[0023] Based on this, see Figure 1-4The present invention provides a collision multi-volute four-leaf clover type combustion system, comprising: a piston component 1 and an injection component 2, wherein the piston component 1 has a top surface 11, and a combustion chamber 12 is opened on the top surface 11, and the combustion chamber 12 comprises a main combustion chamber 121 coaxially arranged with the top surface 11 and a plurality of arc-shaped combustion chambers 122 circumferentially arranged around the main combustion chamber 121 and connected with the main combustion chamber 121, an oil separation protrusion 123 is provided on the inner wall of the arc-shaped combustion chamber 122 to guide the fuel injected into the arc-shaped combustion chamber 122 to disperse and convolute in the vertical direction, a boss 124 is also provided on the inner wall of the arc-shaped combustion chamber 122, and the boss 124 is located above the oil separation protrusion 123 to guide the convolute flow of the fuel, and a plurality of pairs of injection holes 23 for injecting fuel into the combustion chamber 12 are opened on the injection component 2, and each pair of the injection holes 23 injects two oil beams that intersect with each other and have an acute angle.

[0024] Specifically, when the piston moves to near the top dead center, the fuel is injected into the combustion chamber 12 through the fuel injection component 2 in the form of an oil beam, and gasifies to form fuel vapor. After that, the fuel vapor will first contact the oil separation protrusion 123, and be diverted by the oil separation protrusion 123 to form two parts, one of which is reflected downward by the oil separation protrusion 123 and flows into the main combustion chamber 121, and the other part is reflected upward by the oil separation protrusion 123, and then contacts and collides with the boss 124, forming a convoluted oil beam toward the main combustion chamber 121 to prevent the fuel from rushing out of the combustion chamber 12. In this way, the fuel will not be concentrated in the main combustion chamber 121 or the arc-shaped combustion chamber 122, nor will it rush out of the combustion chamber into the narrow gap area, and the space in the combustion chamber 12 will be fully utilized, so that the fuel is fully mixed with the air in the combustion chamber 12, avoiding excessive concentrated combustion of the fuel, and improving the high-temperature and oxygen-poor combustion of the fuel. In addition, the oil beams sprayed by each pair of the oil injection holes 233 will intersect, causing the oil beams to collide with each other and disperse to form oil mist, so that the fuel can be further dispersed and further mixed with the air, thereby improving the high-temperature and oxygen-poor combustion of the fuel.

[0025] Furthermore, the bottom inner wall of the main combustion chamber 121 is provided with a central protrusion 121a arranged around the central axis, and a central annular portion 121b circumferentially recessed around the central protrusion 121a, and the central annular portion 121b and the oil separation protrusion 123 are smoothly transitioned, so that the cross section of the main combustion chamber 121 along the central axis is ω-shaped. In other words, the bottom of the main combustion chamber 121 is not flat, and the uneven bottom can appropriately interfere with the vortex movement of the fuel vapor in the main combustion chamber 121 to reduce the mixture suitable for NOx production. The purpose of this design is, on the one hand, to prevent dead angles at the joints of the components to avoid residual oil mist, and on the other hand, to adopt an undulating structure to allow the oil mist to collide with each other and fully contact and mix with the air.

[0026] Furthermore, the inner wall of the arc-shaped combustion chamber 122 is arc-shaped, and the inner diameter of the arc-shaped combustion chamber 122 gradually decreases in the direction away from the main combustion chamber 121, so that the arc-shaped combustion chamber 122 forms a leaf shape. It can guide the oil beam through the radial flow guidance of the arc-shaped combustion chamber 122 and divide it into two beams, which flow convolutively along the left and right side walls of the arc-shaped combustion chamber 122 respectively. In the absence of vortex, it effectively promotes the radial oil and gas mixing in the combustion chamber and improves the radial mixing quality of oil and gas. The purpose of this design is, on the one hand, to prevent dead corners in the arc-shaped combustion chamber 122 and avoid residual oil mist. On the other hand, the gradually shrinking arc-shaped combustion chamber 122 can allow the oil mist to collide with each other and fully contact and mix with the air.

[0027] Furthermore, the diameter of the arc-shaped combustion chamber 122 gradually decreases in the vertical direction to form a constriction structure, which can prevent the fuel from rushing out of the combustion chamber and entering the narrow gap area. On the one hand, it can reduce particle emissions, and on the other hand, it can prevent the fuel from entering the engine oil and causing the engine oil dilution problem. At the same time, the constriction structure of the arc-shaped combustion chamber 122 can further improve the oil-gas mixing inside the combustion chamber. The purpose of this design is to reduce the splashing of oil mist from the arc-shaped combustion chamber 122.

[0028] In this embodiment, four arc-shaped combustion chambers 122 are opened on the top surface 11, and the four arc-shaped combustion chambers 122 are connected to the main combustion chamber 121 to form a four-leaf clover shape.

[0029] Furthermore, the oil separation protrusion 123 is arranged on the inner wall of the arc-shaped combustion chamber 122 and protrudes toward the direction of the main combustion chamber 121, and transitions in an arc shape with the inner wall of the arc-shaped combustion chamber 122, so that a first recess 122a located below the oil separation protrusion 123 and a second recess 122b located above the oil separation protrusion 123 are formed on the inner wall of the arc-shaped combustion chamber 122, and the oil beam performs vortex motion in the first recess 122a and the second recess 122b to be fully mixed with the air.

[0030] Furthermore, the oil separation protrusion 123 is bent into an arc shape along the inner wall of the arc-shaped combustion chamber 122. The purpose of this design is to converge the oil beams reflected by the oil separation protrusion 123, make them collide and disperse with each other, and further fully mix with the internal air.

[0031] Furthermore, the oil separation protrusion 123 and the opening of the combustion chamber 12 are transitioned by an inclined surface. The inclined surface can guide the oil mist so that the oil mist can quickly move along the inclined surface toward the opening of the combustion chamber 12.

[0032] Furthermore, the boss 124 is arranged on the inner wall of the arc-shaped combustion chamber 122 and is located above the oil separation protrusion 123, and has an arc-shaped transition with the second recess 121b. The boss 124 protrudes relative to the inner wall of the arc-shaped combustion chamber 122 to guide the oil beam to move in a vortex flow for full mixing with the air.

[0033] Furthermore, the boss 124 is bent into an arc shape along the inner wall of the arc-shaped combustion chamber 122. The purpose of this design is to converge the oil mist reflected by the boss 124, make it collide and disperse with each other, and further fully mix with the internal air.

[0034] Specifically, the fuel injection component 2 includes a main body 21 and a nozzle 22. The nozzle 22 is fixed to the bottom of the main body 21. The nozzle 22 is provided with a plurality of pairs of fuel injection holes 23. The fuel is injected into the combustion chamber through the fuel injection holes 23.

[0035] Furthermore, the bottom of the main body 21 forms a conical surface that matches the inclined surface.

[0036] Furthermore, each pair of the oil injection holes 23 is symmetrical about the radial axis of the main body 21 .

[0037] In this embodiment, four pairs of the oil injection holes 23 are formed on the nozzle 22 , and each pair of the oil injection holes 23 corresponds to the arc-shaped combustion chamber 122 one by one.

[0038] During use, when the piston moves to near the top dead center, the fuel is sprayed from the nozzle 22 into the arc-shaped combustion chamber 122 and vaporized to form fuel vapor. The fuel vapor contacts the oil separation protrusion 123, disperses and flows in a convolution in the vertical direction, a part of it flows to the bottom of the main combustion chamber 121 and flows back into the main combustion chamber 121, and the other part flows to the boss 124. The inner wall of the arc-shaped combustion chamber 122 converges the oil beams reflected by the oil separation protrusion 123, so that they collide and disperse with each other, and further mix fully with the air inside. After the fuel vapor flowing to the boss 124 collides with the boss 124, a part of it is further dispersed and reflected, guiding the fuel vapor to further disperse and flow in a convolution. The fuel vapor is dispersed in the entire space within the combustion chamber 12, so that the fuel is fully mixed with the air in the combustion chamber, so as to avoid excessively concentrated combustion of the fuel and improve the high-temperature and oxygen-poor combustion of the fuel. In addition, in the horizontal direction, the inner wall of the arc-shaped combustion chamber 122 is arc-shaped, and the fuel vapor injected into the arc-shaped combustion chamber 122 can be dispersed in the horizontal direction when it hits the inner wall of the arc-shaped combustion chamber 122, and under the guidance of the inner wall of the arc-shaped combustion chamber 122, it flows back to the main combustion chamber 121, and the fuel vapor is dispersed in the horizontal direction, so that it is fully mixed with the air in the horizontal direction, so as to avoid excessively concentrated combustion of the fuel and further improve the high-temperature and oxygen-poor combustion of the fuel.

[0039] Beneficial effects of the present invention: The collision multi-volute four-leaf clover type combustion system provided by the present invention comprises a piston component and an injection component, wherein the piston component has a top surface, and a plurality of combustion chambers are provided on the top surface, wherein the combustion chamber comprises a main combustion chamber coaxially arranged with the top surface and a plurality of arc-shaped combustion chambers circumferentially arranged around the main combustion chamber and connected with the main combustion chamber, and an oil separation protrusion is provided on the inner wall of the arc-shaped combustion chamber to guide the fuel injected into the arc-shaped combustion chamber to disperse and convolute in the vertical direction, and a boss is also provided on the inner wall of the arc-shaped combustion chamber, and the boss is located above the oil separation protrusion to guide the convolute flow of the fuel, and a plurality of pairs of injection holes for injecting fuel into the combustion chamber are provided on the injection component, and each pair of the injection holes injects two oil beams that intersect each other and have an acute angle. When the piston moves to near the top dead center, the fuel is injected into the combustion chamber through the injection component in the form of an oil beam, and is gasified to form fuel vapor. After that, the fuel vapor will first contact the oil separation protrusion, and will be split by the oil separation protrusion to form two parts, one of which will be reflected downward by the oil separation protrusion and flow into the main combustion chamber, and the other part will be reflected upward by the oil separation protrusion, and then contact and collide with the boss to form a further upward convolution oil beam, guiding the fuel to further disperse and convolute. In this way, the fuel will not be concentrated in the main combustion chamber or the arc-shaped combustion chamber, and the space in the combustion chamber will be fully utilized to fully mix the fuel with the air in the combustion chamber, avoiding excessively concentrated combustion of the fuel and improving the high-temperature and oxygen-poor combustion of the fuel; in addition, the oil beams sprayed by each pair of the fuel injection holes will intersect, causing the oil beams to collide with each other and split to form oil mist, so that the fuel can be further dispersed and mixed with the air, improving the high-temperature and oxygen-poor combustion of the fuel.

[0040] The specific implementation of the present invention described above does not constitute a limitation on the protection scope of the present invention. Any other corresponding changes and modifications made based on the technical concept of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A collision multi-puff four-leaf clover type combustion system, characterized in that: It includes: A piston component having a top surface, on which a combustion chamber is formed, wherein the combustion chamber comprises a main combustion chamber coaxially arranged with the top surface and a plurality of arc-shaped combustion chambers circumferentially arranged around the main combustion chamber and connected with the main combustion chamber, wherein an oil separation protrusion is formed on the inner wall of the arc-shaped combustion chamber to guide the fuel injected into the arc-shaped combustion chamber to disperse and flow in a convolution in a vertical direction, and a boss is further formed on the inner wall of the arc-shaped combustion chamber, wherein the boss is located above the oil separation protrusion to guide the convolution flow of the fuel; and A fuel injection component is provided with a plurality of pairs of fuel injection holes for injecting fuel into the combustion chamber.

2. The collision multi-plummet four-leaf clover type combustion system according to claim 1, characterized in that: The bottom inner wall of the main combustion chamber is provided with a central protrusion arranged around the central axis, and a central annular portion circumferentially concave around the central protrusion, and the central annular portion smoothly transitions with the oil separation protrusion so that the cross-section of the main combustion chamber along the central axis is ω-shaped.

3. The collision multi-plummet four-leaf clover type combustion system according to claim 1, characterized in that: The inner wall of the arc-shaped combustion chamber is arc-shaped, and the inner diameter of the arc-shaped combustion chamber gradually decreases in a direction away from the main combustion chamber, so that the arc-shaped combustion chamber forms a leaf shape.

4. The collision multi-plummet four-leaf clover type combustion system according to claim 3, characterized in that: The diameter of the arc-shaped combustion chamber gradually decreases along the vertical direction to form a shrinkage structure.

5. The collision multi-plummet four-leaf clover type combustion system according to claim 1, characterized in that: Four arc-shaped combustion chambers are provided on the top surface, and the four arc-shaped combustion chambers are connected with the main combustion chamber to form a four-leaf clover shape.

6. The collision multi-plummet four-leaf clover type combustion system according to claim 5, characterized in that: The oil separation protrusion is arranged on the inner wall of the arc-shaped combustion chamber and protrudes toward the main combustion chamber, and transitions in an arc shape with the inner wall of the arc-shaped combustion chamber, so that a first recess located below the oil separation protrusion and a second recess located above the oil separation protrusion are formed on the inner wall of the arc-shaped combustion chamber.

7. The collision multi-plummet four-leaf clover type combustion system according to claim 6, characterized in that: The oil separation protrusion is bent into an arc shape along the inner wall of the arc-shaped combustion chamber.

8. The collision multi-plummet four-leaf clover type combustion system according to claim 5, characterized in that: The boss is bent into an arc shape along the inner wall of the arc-shaped combustion chamber.

9. The collision multi-plummet four-leaf clover type combustion system according to claim 8, characterized in that: The oil spray component comprises a main body and a spray head, wherein the spray head is fixed on the bottom of the main body, and a plurality of pairs of oil spray holes are formed on the spray head.

10. The collision multi-plume four-leaf clover type combustion system according to claim 9, characterized in that: Each pair of the oil injection holes is symmetrical about the radial axis of the main body, and is used for injecting two oil beams that intersect each other and have an acute angle.

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