Combustion chamber for a Diesel engine

Abstract: A projection (3) positioned in the center of a concave section of a toroidal combustion chamber (2) is divided into a section with a steep slope (3c) extending from the bottom (2b) to the head surface and a section with a gentle slope (3b). The head surface is in the form of a flattened conical trapezoid. Because of its shape, the projection makes it possible for the air which is close to the center section to flow close to the surface of a side wall of the concave section. The air which had been at the center section is added to the amount of air supplied for combustion and is mixed with the fuel in the vicinity of the surface of the side wall of the concave section. By setting the angle at which the air flows along the side wall surface (re-entrant angle) in the combustion chamber, the flow of the air close to the side wall surface is made turbulent, which improves the agitation of the fuel.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a combustion chamber with an injection nozzle for a diesel engine, and more particularly to the configuration of a combustion chamber for reducing the creation of noxious components in the exhaust gas comprising the features of the preamble of claim 1. Discussion of Background Conventionally, the structure is known for a combustion chamber of a diesel engine wherein the combustion chamber is provided as a concave section in the head surface of a piston and fuel is injected directly into the air which is compressed in this combustion chamber. This type of combustion chamber is referred to as a toroidal combustion chamber. FR-A-2 388 997 describes an combustion chamber for a direct injection type of diesel engine and a injection nozzle, the combustion chamber comprising a built-up projection formed in a conical shape at the center of a bottom surface of a circular concave section formed in the head section of a piston, the circular concave section comprising a peripheral edge section , formed as the arc of a circle, which is joined to this projection, and is connected to a side wall surface. BE-A-496 710 discloses a combustion chamber wherein the projection comprises two regions with different slope angles. EP-A-271 478 discloses a combustion chamber wherein the projection has a convex shape. When the piston rises, the air inside the combustion chamber is caused to rotate to produce an eddying flow. Then, in order for the flow of fuel to intersect this eddying flow and be accelerated, the eddying flow must have the strength to rotate over the entire periphery so that the fuel is uniformly mixed in the circumferential direction, in the interval between consecutive injections. In addition, the mixture of fuel and air is made uniform, and, in particular, there is no temporary concentration of fuel. Therefore, because the fuel is injected continuously, there is an effective reduction of black smoke and NO x which are noxious substances in the exhaust gases. For these reasons, conventionally, it has been proposed that the shape of the concave section in the combustion chamber be made heart-shaped, or hemispherical or globular. However, with a combustion chamber of any of the abovementioned shapes, the combustion at the side wall surface, where the best mixing of the fuel and air is considered to occur, is not complete, and there is a tendency for incomplete combustion to be obtained. Specifically, the fuel which is injected into the center of the combustion chamber is not sufficiently dispersed and is not broken down into fine particles. It is therefore not in a state in which ignition can occur from contact with the air. As a result, the air at the center of the eddying flow does not particularly contribute to the effect of smooth ignition of the fuel. The mixture of the fuel and air close to the surface of the side wall is vigorously mixed, but there is a tendency toward a lack of air which is necessary for combustion. Therefore, when sufficient air to burn the fuel is lacking, incomplete combustion occurs and there is a large amount of noxious components produced in the exhaust gases. In addition, because the mixing and agitation of the air and fuel at the concave section of the side wall surface depends on the strength of the eddying flow, it is difficult to guarantee a uniform mixing performance. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide, with due consideration to the drawbacks of such conventional combustion chambers, a combustion chamber for a diesel engine which can reduce the occurrence of noxious components, and, in particular, reduce the occurrence of black smoke and NO x in the exhaust gases. Another object of the present invention is to provide a combustion chamber for a diesel engine into which a maximum amount of air can be fed at a position where the fuel can be vigorously mixed with the air. A further object of the present invention is to provide a combustion chamber for a diesel engine in which the fuel and air are mixed with improved blending and agitation. These objects are achieved in the present invention by the provision of a combustion chamber for a direct injection type of diesel engine, wherein the center of the bottom surface of a circular concave section formed in the head section of a piston has a projection which is built up in a conical shape. This projection is made up of a bottom section with a steep slope, positioned at the bottom surface of the concave section, and a conical trapezoid with a gentle slope on the top section positioned at the head surface. Accordingly, because the head is flat, the projection is prevented from interfering with the injection nozzle and the air at the center of the combustion chamber is guided toward the side wall surface. In addition, the head angle of the conical trapezoid is made 10° to 30° smaller than the cone angle of the injection nozzle, and the angle between the sloped surface of the bottom section which faces the side wall surface of the concave section is set at 0° to 30°. Accordingly, the fuel which is sprayed from the injection nozzle is prevented from impacting the bottom section of the projection, and this fuel flows toward the side wall surface. Further, in the fuel chamber for a diesel engine of the present invention, the re-entrant angle of the side wall surface is set at 20° to 30°. Accordingly, when the piston is elevated the flow of the air which enters the combustion chamber is turbulent so that laminar flow at the sidewall surface is prevented and the agitation of the fuel is improved. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects, features, and advantages of the present invention will become more apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings, in which: FIG. 1 is a cross-sectional view of one embodiment of a combustion chamber for a diesel engine according to the present invention. FIG. 2 is a cross-sectional view of another embodiment of a combustion chamber for a diesel engine according to the present invention. FIG. 3 is a graph which illustrates the relationship between the diameter and the surface area of the combustion chamber shown in FIG. 2. DESCRIPTION OF PREFERRED EMBODIMENTS Now referring to FIG. 1, in a combustion chamber 2 formed at the center of the head-section of a piston 1, the inner diameter D₂ of an open end 2a of a circular depression is smaller than the maximum inner diameter D₃ of the inside of the combustion chamber (D₂ < D₃). A projection 3 is formed at the center of the bottom surface. The projection 3 is larger than a conventional projection, and approximately one-half of the projection bottom (hereinafter referred to as the bottom section) 3a is a cone which rises steeply upright. The top half (hereinafter referred to as the top section) 3b, which is joined to the bottom section 3a, is an almost conical trapezoid with a surface which has a more gentle slope than the lower section 3a. The depth of a head section level surface, specifically, the section at the center of the combustion chamber 2 is slightly less than that of a conventional projection. A head angle β of the top section of the projection 3, specifically, of the conical trapezoid 3b is a slightly smaller angle than a cone angle ϑ of an injection nozzle 4 which is mounted above the projection 3 on the centerline of the combustion chamber 2. Specifically, the head angle β is set at an angle 10° to 30° smaller than the cone angle ϑ [β = ϑ-(10° to 30°)] . Therefore, by forming the top section 3b of the projection 3 as a conical trapezoid and the head section as a flat surface, obstruction of the injection nozzle 4 is avoided. In addition, by forming the bottom section 3a of the projection 3 as a steeply upright sloping surface and the top section 3b as a gently sloping surface, the fuel sprayed from the injection nozzle 4 is prevented from reaching a sloped surface 3c of the bottom section 3a of the projection 3. A peripheral edge section and a side wall surface of the bottom section, specifically, a bottom arc section 2b which is joined to the bottom section 3a of the projection 3 of the combustion chamber 2, is formed so that a spherical body can follow along the circumference of the bottom section 3a of the projection 3 when moved with a rolling action, and the outer side section of the bottom surface of the combustion chamber 2 has a cross section which includes a part of a circle, specifically, in the shape of an arc. The radius R of this arc is set in the range of R = 0.12 D₂ to 0.18 D₂. Here the value D₂ is the inside diameter of the open end 2a of the combustion chamber 2, as previously stated. A side wall surface 2c joined to the bottom arc section 2b of the combustion chamber 2 is a sloped surface tapering with a decreasing diameter in the upward direction. The side wall surface 2c and the sloped surface 3c of the bottom section 3a of the projection 3 are opposedly faced, either in parallel or at a slight angle α. The angle α is set in a range of 0° to 30°. The maximum diameter D₃ of the combustion chamber 2 is close to the joint of the side wall surface 2c of the bottom arc section 2b of the combustion chamber 2. The re-entrant angle γ° on the side wall 2c is set in a range of 20° to 25°. In this configuration, the inner diameter D₂ of the combustion chamber 2 is small, the maximum diameter D₃ is large, and the deepest section has a large depth h, so that the projection 3 is large. In the combustion chamber 2 of the shape shown in FIG. 1, the fuel sprayed from the injection nozzle 4 proceeds toward the bottom section of the side wall surface 2c of the combustion chamber 2 across a rotating flow of air, specifically, an eddying flow, which is created in the combustion chamber 2, and the spray is gradually distributed throughout a flow of new air. In addition, because the center projection 3 is large, the excess air in the vicinity of the side wall surface 2c of the combustion chamber 2 close to the center of the combustion chamber, which does not participate in the combustion, can be effectively utilized at the center of the flow of air to the combustion chamber 2. Furthermore, because the combustion chamber 2 has a small diameter D₂ at an open end 2a, a large maximum diameter D₃, and a large maximum depth h, even when the piston 1 is descending, a good eddying flow, specifically, a good airflow, can be maintained. As a result, the atomization of the fuel is accelerated, ignition is improved, and the combustion of the fuel in the combustion chamber 2 is good so that the creation of black smoke is restrained. This results in an improvement in engine performance. FIG. 2 shows another embodiment of the combustion chamber of the present invention. Now referring to FIG. 2, a combustion chamber 2', in addition to the configuration of the combustion chamber 2 shown in FIG. 1 has an outer curved surface 2'd with a radius R₁ connected to a side wall surface 2'c of a bottom arc section 2'b, an inner curved surface 2'e with a radius R₂ connected to a bottom section 3'a of a projection 3' (where R₁ ≠ R₂), and has an area (combustion area) S i = π x d₁ x ℓ₁ at a position where the diameter of the combustion chamber 2' is d₁. In addition, so that the area S i at the approximate center of the combustion chamber 2' is expanded as shown in FIG. 3, the centers O₁, O₂ of the radii R₁, R₂ of the curved surfaces 2'd, 2'e respectively, are separated only by a distance a (where a = approximately 2 to 6 mm), and a flat section 2'f is provided at the bottom surface of the combustion chamber 2', specifically, the bottom arc section 2'b. The combustion chamber 2' with this shape causes the flame to expand toward the center of the combustion chamber 2', further improving combustion after ignition and thus providing reduced fuel costs.

Claims: A combustion chamber (2) with an injection nozzle (4) for a direct injection type of diesel engine, the combustion chamber comprising a built-up projection (3) formed in a conical shape at the center of a bottom surface of a circular concave section, formed in the head section of a piston, and a bottom arc section (2b) comprising an arc of a circle, which is joined to this projection (3), and is connected to a side wall surface (2c) characterized in that the projection (3) combines a bottom section (3a) positioned on a bottom surface side formed as a surface (3c) with a steep slope, and a top section (3b) positioned on a head section side formed as a conical trapezoid which has a gently sloping surface; the head angle (β) of the conical trapezoid is formed 10° to 30° smaller than the cone angle (Θ) of the injection nozzle (4); and the angle (α) between the side wall surface of the combustion chamber (2) and the sloped surface of the bottom section which faces the side wall surface is set at 0° to 30°. The combustion chamber as claimed in claim 1, wherein the bottom arc section (2b) of the concave section, which connects the bottom section (3a) of the projection (3) and the side wall surface (2c), is formed in a shape which allows a spherical body to be rolled in the circumferential direction; and the radius R of the bottom arc section (2b) is equal to 0.12 D to 0.18 D (R = 0.12 D to 0.18 D), where D is the inner diameter of the open end (2a) of the combustion chamber (2). The combustion chamber as claimed in claim 1, wherein the side wall surface (2c) has a re-entrant angle of 20° to 25°. The combustion chamber as claimed in claims 1 to 3, wherein the height of the projection (3) is such that the projection (3) extends to a position at which it does not obstruct the spray of fuel from the injection nozzle (4). The combustion chamber as claimed in claim 1, wherein the bottom arc section (2b) of the concave section which connects the bottom section (3'a) of the projection (3') and the side wall surface (2c) has a curved surface (2'd) with a radius of R₁ on the outer side which is connected to the side wall, and has a curved surface (2'e) with a radius of R₂ on the inner side which is connected to the bottom section; and a flat section (2'f) is provided between the curved surface (2'd) on the outer side and the curved surface (2'e) on the inner side, with the centers of the radii R₁, R₂ being separated only by a distance a.

Inventors: KAWATANI TORU MITSUBISHI-JIKO, KOHKETSU SUSUMU MITSUBISHI-JIK, MIZOTE EIJI, NAKAGAWA TOMOMI, SUZUKI TETSUO MITSUBISHI APART, YAMAKI YOSHIHISA MITSUBISHI SA, KAWATANI, TORU ,/MITSUBISHI-JIKO MIYAZAKIDAI-RYO, KOHKETSU, SUSUMU,/MITSUBISHI-JIKO, MIZOTE, EIJI, NAKAGAWA, TOMOMI, SUZUKI, TETSUO, /MITSUBISHI APARTMENT 625, YAMAKI, YOSHIHISA, /MITSUBISHI SAKURADAI-
Applicants: MITSUBISHI MOTORS CORP, MITSUBISHI JIDOSHA KOGYO KABUSHIKI KAISHA
Application year: 1990
Main classification: F02F3
IPC codes: F02B23, F02B3