JPH02233822A - Engine piston - Google Patents

Abstract

PURPOSE:To reduce waxy white smoke and irritating odor even in low temperature and low load driving by forming a conical projection in the base and a lip in an opening edge, respectively, in a cavity whose horizontal section is a regular pentagon. CONSTITUTION:A horizontal section of a cavity 2 installed in a piston top face 26 is formed into a regular pentagon, making up a corner which feeds a fuel spray from the central side. Then, a side wall 5 of this cavity 2 inclusive of the corner is set to an almost right angle to a fuel spray line. In addition, a conical projection 13 is formed on a bottom surface 11 of the cavity 2 and a lip 6 in an opening edge, respectively. With this formation, a relatively rich content air-fuel mixture exists in each corner, and thereby a diffusion air-fuel mixture of density suitable for flame propagation comes to exist in a swirl turning part 24. Thus, a combustion condition in the cavity 2 at time of low speed and light load driving is improved, thus discharge of hydrocarbon is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] This invention relates to an engine piston having a polygonal cavity formed at the top of the piston to directly supply fuel injection. [Prior Art] Normally, when pursuing high output from a turbocharged diesel engine, the supercharger pressure is set to the maximum at the maximum output point of the engine, that is, the point at which the fuel flow rate is maximum, and the maximum output point is Generally, the compression ratio is lowered so that the combustion pressure generated in the engine is less than the mechanical strength of the engine, and the lower the compression ratio, the lower the fuel consumption can be expected. An engine that has been optimally matched at the maximum output point in this way shows a tendency for the compression pressure, temperature, and combustion chamber (cavity) temperature to become lower in the low speed/light load range, that is, in the range where the supercharger pressure is low.Generally, injection Normally, the fuel injected from the nozzle collides with the side wall of the piston, rebounds, and burns completely in the flame, and the fuel that adheres to the side wall is also completely evaporated or carbonized by the flame.However, as mentioned above, the turbo In a low compression ratio engine matched for a supercharged engine, the fuel that collides with the side wall and is reflected may not be ignited, or the combustion chamber may have a lower temperature due to the delayed evaporation of fuel adhering to IN!!Iu. When it reaches the bottom, it liquefies, is exposed to flame, and floats in the cavity as unburned fuel, which is emitted during the exhaust stroke and produces blue white smoke (HC) and a pungent exhaust odor.For this reason, diesel engines are designed to have low compression. Although it is possible to increase output by reducing friction and increasing the supercharging rate, it has not yet been adopted. Therefore, in order to obtain a low compression ratio supercharged engine, the condition of the fuel supplied to the cavity (evaporation, dispersion, distribution) to a good condition,
Flame propagation must be achieved by igniting accurately. Proposals based on this type of problem include the applicant's proposal “
Combustion chamber of direct injection diesel engine
No. 02720). The proposal is to form a cavity with a rectangular horizontal cross section, and to form a radially inwardly protruding lip at the opening periphery forming the entrance of the cavity, and to align the side wall of the cavity with respect to the fuel spray line of the fuel injection nozzle. It is formed by forming a conical protrusion at the center of the cavity bottom to create a toroidal stirring flow.By colliding the fuel against the side wall at right angles, the injected fuel is While pulverizing some of the fuel particles into smaller particles, the remaining part of the fuel against the side wall adheres to the side wall in the form of a thin film and is diffused. The fuel to be deposited is confined within the cavity. In other words, the proposal promotes atomization of the fuel spray to generate the mixture necessary for ignition and flame propagation, while relying on the strength of the stirring flow to reduce the thin film of fuel and vapor present near the sidewalls. The material is separated from the side wall and transported to the center of the cavity to propagate the flame and improve combustion. [Problems to be Solved by the Invention] However, in order to apply the above-mentioned proposal to the above-mentioned low compression ratio engine, it is necessary to solve the following problems. ■ If you place the fuel injection nozzle on the axis of the combustion chamber and form an oval so that the fuel is injected from the nozzle to the side wall of the cavity, the spray reach will increase! (distance from the injection point to the side wall) is short, so the proportion of fuel that mixes with the air in the cavity during the injection process is small, and the amount of fuel that adheres to the side wall is large. ■ At low speeds and light loads, the excess air ratio is high relative to the amount of injected fuel, and the injected fuel diffuses within the cavity due to swirl, resulting in an excessively lean mixture ratio in the cavity as a whole, resulting in poor ignition and a strong flame-extinguishing effect. The proportion of unburned fuel emitted increases. ■ At the maximum output point of the engine, the engine speed is high, the amount of fuel injected is large, and the angular speed of the injection pump is large, so the dynamic injection timing is considerably retarded. Therefore, if the optimum timing is set at the maximum output point, the dynamic injection timing will be delayed at low speeds. The injection timing is advanced. This combined with ■ causes fuel to be injected quite early before top dead center and adhere to the top surface of the piston (upper surface of the lip), so an auto-timer that automatically adjusts the injection timing is required as an additional device. As is clear from the above ■, ■, and ■, an excessive amount of HC (blue-white smoke) is generated in the low boost pressure range (low speed, light load). [Means for Solving the Problems] The present invention aims to solve the above problems, and the horizontal cross section of the cavity recessed in the top surface of the piston is made into a regular pentagon to form a corner for supplying fuel spray from the central surface. The side wall of the cavity including the corner is approximately perpendicular to the fuel spray line, a conical protrusion is formed on the bottom surface of the cavity, and a lip is formed on the opening edge to form an engine piston. ] The cavity has a regular pentagonal horizontal cross section and a lip formed on the opening edge, and the lip attenuates the airflow introduced into the cavity. In other words, as the strength of the air flow decreases, the penetration force of the injected fuel increases relatively, increasing the force reaching the side wall. Therefore, even at low speeds and light loads when the penetration force of the fuel spray is small, the fuel spray reaches each corner. When the fuel spray collides with the side wall forming the corner, part of it is reflected and diffused, and part of it adheres to the side wall as a fuel film and diffuses. The degree of reflection and diffusion is uniquely determined by the angle of inclination of the corners and side walls with respect to the fuel spray line, but if this is made approximately at right angles, reflection and diffusion will occur symmetrically to the central axis of the fuel spray, and the degree of reflection will be approximately uniform. Become. As the fuel penetration force increases with speed and load, the degree of reflection and diffusion also increases, but the flow out of the combustion chamber is blocked by the squish flow generated by the lip. In other words, the strength of the squish flow is also the same as the speed. This is because it increases according to the load. Therefore, the fuel supplied at all speeds and load ranges is distributed within the corner, and within the corner, fuel is generated that is further broken down into fine particles by collision and evaporated, and fuel that evaporates at the rM wall 6 These fuels are It is taken into the above squish flow and swirls in the vertical direction, producing a mixture (premixture) with excellent ignitability and flame propagation performance in the corner. In other words, even at low speeds and light loads, it is possible to partially supply fuel with a penetration force that does not excessively cool the sidewalls, making it possible to generate a partially rich mixture. In addition, since the flight distance of the fuel spray to be supplied increases relative to the flight distance to the side walls before and after the corner, the time for mixing with the air on the way of flight is increased in accordance with the increase in the flight distance, and the fuel spray around the protrusion is The air-fuel mixture that primarily causes flame propagation is distributed by diffusion. Therefore, while an increase in the flight distance of the 01 fog functions to relatively reduce the amount of fuel reaching the corner and, as a result, reduce the thickness of the fuel film adhering to the sidewall, this time lag It functions to match the actual compression ratio of the piston when it increases. In other words, at low speeds and light loads, the advance of the injection timing is relatively absorbed by the time delay that accompanies the increased flight distance of the fuel, making it possible to synchronize the injection timing with the engine rotational speed. As a result, while preventing fuel from being injected and deposited on the top surface of the piston, the thickness of the fuel film deposited on the side wall can be appropriately reduced, and the amount of unburned fuel can be greatly reduced while improving fuel evaporation performance. Improvements can be made. By the way, if a pentagonal cavity is formed compared to a square cavity, the ratio of distributing the diffused mixture within the fuel spray interval in the circumferential direction increases due to the increase in the number of corners, so the existence ratio of the lean part between injections increases. Therefore, the flame generated by ignition at the corner will propagate to the adjacent diffused mixture, resulting in rapid flame propagation combustion. [Embodiment] A preferred embodiment of the present invention will be described below with reference to the accompanying drawings. As shown in FIGS. 1 and 2, a general piston 1 used in a small direct injection diesel engine has a small piston diameter, and a cavity 2 formed in the piston 1 is located at a location where a fuel injection nozzle 3 is located. It is uniquely determined by the position, that is, the position of the injection port 4 of the fuel injection nozzle 3 is relative to the cylinder center axis 0. When the central axis 02 of the cavity is offset radially outward from
Formed at a position offset by χ2 to the side. Offset χ1. The relationship between χ2 is usually set as χ2 = χ1/2. In an engine that satisfies such a relationship, it is difficult to maintain equal distances from each injection port (each injection point) of the injection port section 4 to the side wall 5 of the cavity 2.

Therefore, in this embodiment, a cavity 2 having a shape corresponding to the number of nozzles of the fuel injection nozzle 3 and the spread angle θ of the fuel spray injected from the nozzle is formed on the top surface of the piston 26. The fuel injection nozzle 3 is formed with five nozzles arranged at equal intervals in the circumferential direction in a nozzle part 4, and when a valve body (not shown) such as a 21 dollar valve housed therein is opened. It is configured to simultaneously inject fuel spray in five directions (diagonally downward) in the circumferential direction. On the other hand, the horizontal section of the cavity 2 has a regular pentagonal shape, and a lip 6 is formed on the periphery of the opening forming the entrance 10 of the cavity 2, projecting radially inward along the circumferential direction. Tip of lip 6 (protruding end)
7 is rounded into an arc shape with a curvature whose center is radially outward of the lip 6 from the lip tip 7, and the [11
! It is continuous with the upper part of the middle part of lu5 and the lower lip surface 9 which is tangential to the arc 8. The side wall 5 below the connection part with the lower lip surface 9 is It is formed as an inclined surface that slopes radially inward from the connecting part and faces diagonally upward. The angle θ of this slope is in the range of 90° to 110° with respect to the fuel spray line from the fuel injection nozzle 3, and the interval in the depth direction of the slope is the same as that of the fuel spray F1. ~
Diffusion interval J of F, The distance D1 in the radial direction formed between the bottom surface 11 and the side wall 5 is approximately equal to the distance D2 between the lip tips @7 in the same cross section, while being equal to or greater than (2).

The bottom surface 11 of the cavity 2 is a horizontal surface, and a conical protrusion 13 is formed in the center of the bottom surface 11, with the center axis 02 of the cavity 2 as the center and the bottom surface 11 of the cavity 2 as the bottom (FIG. 2). However, the apex of the protrusion 13 should be at a height below the lip lower surface 9, and the protrusion 13
A constant distance is formed in the radial direction between the bottom outer periphery and the side wall 5. By the way, each side wall 5 and the lower lip surface 9 are smoothly connected by an arc of constant curvature. This curvature causes the squish flow V generated by the rig 6 at the end of the compression stroke to be separated from the lip lower surface 9 side and to form the bottom surface 11 of the cavity 2.
Let it be a curvature that can be inverted to 1FI. Other connections are connected smoothly with arcs of curvature that improve stress concentration against fluid turbulence, explosions, pressure increases, etc.
Also, the corner corners 14, 15, 16 of the cavity 2,
17.18 are formed by smoothly connecting the adjacent sides u5 with curved surfaces 19, 20, 21, 22, and 23 in the circumferential direction, respectively. These curved surfaces 19 to 23 are also formed as inclined surfaces in the range of 90° to 110° with respect to the fuel spray line. The fuel injection nozzle 3 is arranged so that the fuel sprays Fl to F5 reach within the curved surfaces 19 to 23 connecting the corners 19 to 23 in the circumferential direction of the cavity 2 formed in this way. Determine the formation position of the spout. In addition, as shown in FIG. 2, the starting point and ending point of fuel spray for each curved surface 19 to 23 are designated as A and B, respectively, and the angle formed between each of these points A and B and the center 02 of the fuel injection nozzle 3. When is θr,
The nozzle diameter is determined so that the spread angle θ of the fuel injections MF+ to F is θr>θ. With the above configuration, the lip lower surface 9. At the lower part of the side wall 5 and under the lip lower surface 9 by the protrusion 13, the swirling flow SI of the combustion air supplied into the cavity 2 is turned into a stirring flow S2 in the toroidal direction, and in the vertical direction Squish style■
A space 24 (hereinafter referred to as the swirl section) is formed into which the flow is introduced. Next, the effect will be explained. When the horizontal section of the cavity is formed from a conventional quadrangle to a regular pentagon, the air flow introduced as the swirling flow S1 is temporarily attenuated at the piston top surface 26 due to the increase in the number of corners. That is, as the strength of the swirling flow S decreases, the penetration force of the fuel injections i1+ to F is relatively increased to increase the rate of fuel reaching each corner 14 to 18. On the other hand, when fuel sprays F+ to Fs are supplied from the nozzle between each corner 14 to 18, the spray flight distance of the fuel sprays F1 to Fs is [4! ．． j. =0.323 From D', =0.353
Increases to D. That is, as the spray flight distance jl increases, the time for mixing with the combustion air in the cavity 2 increases, and during the flight, a diffused mixture necessary for flame propagation is generated around the protrusion 13. In addition, the spray flight distance illit
When 1+ increases, the amount of fuel reaching the side wall 5 decreases in inverse proportion to the increase in the amount of diffused mixture generated. As a result,
The injection timing is retarded at low speeds and light loads, and the result is consistent with increasing the actual compression ratio of piston 1. As a result, the thickness of the fuel film adhering to the side wall 325 may become thick enough to cool the side wall 5 excessively.The evaporation capacity of the wall surface is maintained above a certain level even at low speeds and light loads. Become. In this way, a relatively rich mixture exists in the corners 14 to 18, and a diffused mixture with a concentration suitable for flame propagation exists in the swirl portions 24 before and after the corners 14 to 18 in the circumferential direction. It comes to exist. In other words, the combustion conditions (air-fuel mixture concentration, ambient temperature inside the cavity, etc.) inside the cavity 2 during low-speed, light-load operation are improved, and the amount of HC discharged is reduced. When ignition begins, the generated flame propagates in the circumferential direction and burns relatively quickly. In other words, the combustion temperature improves, a constant engine output is ensured, and HC emissions decrease accordingly. Next, the comparative performance of the square cavity and the pentagonal cavity 2 in terms of HC emissions when the piston 1 with the cavity 2 described above is adopted for a low compression ratio supercharged engine is shown in Figures 3 to 3. The explanation will be based on the test data shown in Figure 6. Figure 3 shows that the performance of the pentagonal cavity 2 is superior to that of the rectangular cavity even when the rotational speed changes, and that the performance of the pentagonal cavity 2 is superior to that of the rectangular cavity regardless of the rotational speed. It shows that the performance is excellent, and Figure 4 shows that when the rotation speed and compression ratio are fixed and the injection timing is varied, when the injection timing is set to about 18 degrees before top dead center, the amount of HC emissions decreases. It shows that On the other hand, in Fig. 5, the rotation speed is 1000RPM, 220O
This study investigated the relationship between He emission amount and compression ratio ε during RPM. As a result, it was confirmed that the amount of HC discharged was reduced by lowering the compression ratio ε for each rotation speed. As is clear from the above results, Suginari's pentagonal cavity 2 as described above is suitable for low compression ratio engines, with a compression ratio ε of approximately 15.4 and a fuel injection timing of approximately 18'.
(BTDC) ensures steady ignition and flame propagation combustion. Therefore, there is no need to adjust the injection timing using an automatic timer in low compression ratio engines, and it is suitable for low speed and low load operation. It can also reduce blue-white smoke (HC) and irritating odors, improving fuel efficiency. In addition, although the embodiment describes an example in which the fuel injection nozzle 3 is offset with respect to the central axis of the cavity 2,
It is naturally possible to make the injection point of the fuel injection nozzle 3 coincide with the center axis of the cavity 2, and if the piston is a large one, the center axes of the cylinder bore, the cavity 2, and the fuel injection nozzle 3 should be made to coincide. Of course, this is also possible. Furthermore, if an offset is necessary, it is of course possible to adjust the aperture of the nozzle to adjust the penetration force and atomization degree of the fuel sprays F1 to F. [Effects of the Invention] As is clear from the above explanation, the horizontal cross section of the cavity recessed in the top surface of the piston is a regular pentagon to form a corner to which fuel spray is supplied from the center surface, and the side wall of the cavity including the corner is approximately perpendicular to the fuel spray line,
A conical protrusion is provided on the bottom surface of the cavity, and a cavity with a regular pentagonal horizontal cross section is recessed on the top surface of the piston with a lip formed on the opening edge to which fuel spray is supplied directly into each corner,
Since a lip is formed at the top of this cavity, low-speed
It is possible to provide an engine piston with a cavity that can reduce blue-white smoke (HC) and irritating odors without using an automatic timer even during low-load operation.

[Brief explanation of the drawing]

Fig. 1 is a sectional view showing a preferred embodiment of the present invention, Fig. 2 is a view taken along the line ■-■ in Fig. 1, and Figs. 3 to 6 are performance lines showing the HC performance of square and pentagonal shapes. This is a diagram. In the figure, 1 is a piston, 2 is a cavity, 5 is a side wall, 6 is a rig, and 14 to I8 are corners. Patent applicant Isuzo Jidosha Co., Ltd.

Claims (1)

[Claims] 1. The horizontal section of the cavity recessed in the top surface of the piston is a regular pentagon to form a corner that supplies fuel spray from the center surface, and the side wall of the cavity including the corner is approximately perpendicular to the fuel spray line. An engine piston characterized by having a conical protrusion on the bottom and a lip on the opening edge.