JPH0621563B2 - Pent roof type piston for direct injection diesel engine - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a pentroof type piston adopted in a direct injection internal combustion engine for directly injecting fuel into a combustion chamber, and more particularly to a cavity shape recessed in a piston crown surface. The present invention relates to a pentroof type piston for an improved direct injection diesel engine.

[Prior Art] Generally, in an internal combustion engine, the diameter of intake / exhaust valves is increased, or the number of intake / exhaust valves per cylinder is increased to increase the cross-sectional area of the intake / exhaust ports. The intake / exhaust efficiency can be improved by increasing.

However, when the flow passage cross-sectional area of the intake / exhaust port is increased in this way, the intake / exhaust valves are forced to incline their valve shafts in a V-shape in order to avoid their interference. Then, due to the arrangement of the valve mechanism, the combustion chamber formed on the lower surface of the cylinder head has a central portion that is recessed upward along the axis of the crankshaft and is gradually inclined on both sides to be substantially perpendicular to the valve axis. It becomes necessary to make a pent roof shape (roof shape) that is formed so that when a pent roof type combustion chamber is adopted for a direct injection internal combustion engine such as a direct injection diesel engine, a high compression ratio of 18 to 20 is obtained. In order to ensure the above, as shown in "Combustion chamber structure of diesel engine" proposed in Japanese Utility Model Application No. 60-143456, the crown of the piston is inevitably upwards corresponding to the shape of the lower surface of the cylinder head. You will have to make it into a pent roof shape that protrudes to the outside.

[Problems to be Solved by the Invention] In the case of a direct-injection internal combustion engine, a cavity for forming a substantial combustion chamber is provided in a substantially central portion of the piston crown surface. As shown in FIG. 6, in the conventional pentroof type piston a, the cavity b had a circular cross section. Therefore, when the inner peripheral wall surface c of the cavity b is developed, it is as shown in FIG. 7, and the upper end edge d of the opening of the cavity b is continuously raised along the circumferential direction with respect to the flat bottom level. Hb was changing. Therefore, when the fuel spray F injected from the injection nozzle e is swirled in the swirl S and swirls in the cavity b, due to the effect of the centrifugal force, from the lower portion g of the cavity inner peripheral wall surface c to the piston crown surface i outside the cavity b. , The reverse squish is stronger than other parts, so that part of the spray and the flame are likely to spill into the gap between the piston crown surface and the cylinder head where the oxygen amount is small. As a result, a part of the spilled spray and the flame cause a shortage of O ₂ , causing a large amount of HC and smoke.

Further, in order to improve this, conventionally, the heights from the bottom of the cavity b are made equal, and the inner peripheral wall c on the opening side of the cavity b has a uniform length inward in the radial direction along the peripheral side. Although the protruding shelf part j was provided, there was still room for improvement.

The present invention has been made in view of the above problems, and an object of the present invention is to reduce the amount of fuel spray spilled from a portion with a low inner peripheral wall height of a cavity to the outside of the cavity as much as possible.
An object of the present invention is to provide a pentroof type piston for a direct injection diesel engine that can reduce the amount of smoke and smoke generated.

[Means for Solving the Problems] In order to achieve the above object, the present invention has a cavity in which injected fuel and a circumferential swirl flow are introduced in a central portion of a pentroof-shaped piston crown surface. , To the inside of the cavity to prevent the injected fuel and swirl flow that circulate inside the cavity from spilling out on both the left and right sides of the pent roof ridge of the cavity opening where the depth of the cavity becomes shallow. The eaves member is extended.

[Operation] When a cavity is recessed in a pentroof piston, the cavity opening becomes a saddle-like shape in which the ridge of the pentroof is high and the two sides of the pentroof orthogonal to it are in the shape of a saddle. Injected fuel and swirl flow easily spill, but since the eaves member was extended toward the inside of the cavity at the part where the depth of the cavity became shallow, the injected fuel and swirl flow inside the cavity spilled out of the cavity. Are prevented. Therefore, the amount of HC and smoke generated is reduced.

[Embodiment] A preferred embodiment of a pent roof type piston for a direct injection diesel engine according to the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a plan view of a pentroof type piston used in a direct injection diesel engine, and FIG. 2 is a sectional view taken along the line II-II of FIG.

As shown in the figure, in the piston 1, the piston crown surface 3 immediately above the piston pin hole 2 is projected upward along the axial direction of the piston pin hole 2 (that is, the crank axis direction), and both side portions are inclined. It is formed into a pent roof shape (roof shape), and this pent roof shape corresponds to the shape of the combustion chamber on the cylinder head side formed on the lower surface of the cylinder head, although not shown.

By the way, a cavity 4 for forming a substantial combustion chamber by the cylinder head at the end of the compression process is provided in a substantially central portion of the piston crown surface 3, and the opening 5 of the cavity 4 has The upper part of the inner peripheral wall 6 of the cavity 4, that is, the opening-side inner peripheral wall part 6a is formed so as to narrow the opening by extending inward of the cavity 4 in a direction orthogonal to the axial direction of the piston pin hole 2. The eaves member 7 is provided, and the eaves member 7 is formed so that the overhanging length (extending length) t ₂ becomes maximum at the portion where the height of the inner peripheral wall 6 of the cavity 4 is the lowest. . That is, the eaves member 7 extends toward the inside of the cavity 4 at both left and right sides of the cavity opening 5 where the depth of the cavity 4 becomes shallower, sandwiching the pentroof ridge.

That is, in this embodiment, the cavity 4 has a circular cross section below the eaves member 7, and the shape of the opening 5 formed by the eaves member 7 has a long axis in the axial direction of the piston pin hole 2. The eave member 7 is formed to have an elliptical shape, and the eave member 7 is formed to further extend radially inward along the circumferential side of the cavity 4. Therefore, the overhanging length t ₁ of the eaves member 7 at the highest height portion of the inner peripheral wall 6 of the cavity 4 becomes the minimum, and the elongating length t of the eaves member 7 increases as the height of the inner peripheral wall 6 becomes lower. Become the inner wall 6
The overhang length t ₂ of the eaves member 7 at the portion where the height is lowest is the maximum. The overhanging length t of the eaves member 7 is the minimum value t ₁ = d / 50 to d / 51 and the maximum value t ₂ = d / 10 to d / 3 with respect to the inner diameter d of the eaves lower portion of the cavity 4. It is desirable to form it.

On the other hand, as shown in FIG. 1, an injection nozzle 8 which is attached to the cylinder head and injects fuel into the cavity 4 is disposed above the substantial center of the cavity 4. The injection nozzle 8 has four injection ports 9 that face the inner peripheral wall 6 of the cavity 4 and inject fuel at the same angle θ in the horizontal direction with the axial center of the piston pin hole 2 as a reference. 9 is directed obliquely downward so that the fuel spray F injected from them collides with the lower side of the eaves member 7 of the inner peripheral wall 6, and the colliding fuel spray F and the air-fuel mixture G produced thereby are swirled by the swirl S. It is designed to flow downstream.

Therefore, as shown in FIGS. 1 and 3, the fuel spray F or its mixture G injected toward the points A and C is flown through the swirl S and along the inner peripheral wall 6 of the cavity 4. The height h ₆ flows to the lowest part. However, at this time, in the cavity 4 of the present invention, the overhanging length of the eaves member 7 increases as the height h ₆ of the inner peripheral wall 6 decreases as described above, and the overhanging length t ₂ of the eaves member 7 at the lowest portion thereof. Is the largest. For this reason, even if strong reverse squish occurs on the upper side of the piston crown surface 3 on both sides of the portion of the cavity 4 where the inner peripheral wall height h ₆ is low, the spray / mixture, flame, etc. in the cavity 4 have its upper side covered by the eaves member 7. It is in a state in which it is difficult to flow out because it is covered with, and the spray / mixture and flame spilling from the lower portion of the inner peripheral wall height h ₆ near the lowest portion to the upper side of the piston crown surface 3 are suppressed as much as possible, As a result, it becomes possible to reduce the amount of smoke and HC emitted and increase the output.

The swirl S in the cavity 4 is generated by intake air from the intake port. In addition, the eaves member 7 of the cavity 4
As shown in FIG. 4, the extension ends 7a extending from both sides of the lower portion of the inner peripheral wall 6 in the direction orthogonal to the axial direction of the piston pin hole 2 are parallel to the piston pin hole 2. Alternatively, the opening 5 may be formed into a substantially oval shape.

Further, in the embodiment shown in FIG. 1, the combustion chamber opening 5 is formed into an elliptical shape with respect to the combustion chamber (cavity 4) having a circular cross section, and the eaves member 7 is formed due to the difference in diameter. The eaves member 7 may be formed by making the (cavity 4) oval and making the combustion chamber opening 5 circular.

[Effects of the Invention] In summary, according to the present invention, the following excellent effects can be exhibited.

． A cavity was provided in the pentroof type piston, and the eaves member was extended toward the inside of the cavity at the part where the depth of the cavity became shallow, so the injected fuel and swirl flow circulating inside the cavity spilled out of the cavity. Can be prevented.

． Therefore, the amount of fuel spray and air-fuel mixture / flame in the cavity spilling from the portion with the lower inner peripheral wall height to the upper side of the piston crown surface is suppressed as much as possible, and the smoke / HC emissions are reduced and the output is reduced. Can be improved.

[Brief description of drawings]

FIG. 1 is a plan view showing a preferred embodiment of a pentroof type piston according to the present invention, FIG. 2 is a sectional view taken along the line II-II of FIG. 1, and FIG. 3 is an inside view of the cavity shown in FIG. FIG. 4 is a plan view showing a modified example of the opening shape of the eaves member of the cavity, FIG. 5 is a plan view of a conventional pentroof type piston, FIG. 6 is a side sectional view of FIG. FIG. 7 is a development view of a conventional cavity. In the figure, 1 is a piston, 2 is a piston pin hole, 3 is a piston crown surface, 4 is a cavity, 5 is an opening, 6 is an inner peripheral wall, 6
Reference numeral a is an inner peripheral wall portion on the opening side, and reference numeral 7 is an eave member.

Claims (1)

[Claims] 1. A pent roof ridge of a cavity opening in which a cavity into which injected fuel and a swirl flow in the circumferential direction are introduced is provided in the center of a crown surface of a piston formed in a pent roof shape so that the depth of the cavity becomes shallow. Direct injection type characterized by extending the eaves members toward the inside of the cavity to prevent the injected fuel and swirl flow circulating inside the cavity from spilling out of the cavity on both left and right sides sandwiching the Pent roof type piston for diesel engine.