FLAME DISPERSION ARRANGEMENT FOR SWIRL CHAMBER TYPE DIESEL ENGINE PISTON CROWN

Abstract: Wall surfaces (101,102) which define essantially circular recesses are constructed to deflect the flow of burning gases from a swirl chamber (107) in a manner to either tighten the swirl ("c") formed therein or to improve the intial splitting of the flame in a manner which increases the amount of gases which are induced to undergo swirling in the recesses.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to a swirl type diesel engine and more specifically to such a diesel engine wherein the crown of the piston is formed with a two stage flame splitting/swirl inducing arrangement which improves dispersion and mixing of flame which is ejected from the swirl chamber. Description of the Prior Art Fig. 1 shows a so called "clover leaf" type flame dispersing cavity or recess arrangement 1 which is used in combination with a swirl or pre-chamber type diesel engine of the nature disclosed in JP-A-54-59512, JP-U1-50-138403 and JP-U1-57-78724. With this type of engine, in order to lower the peak combustion temperature in the swirl chamber 2 and thus reduce the amount of NOx produced, a fraction of the fuel which is injected thereinto is released into the main combustion chamber prior spontaneous combustion of the main air-fuel charge in the swirl chamber. However, in the absence of any strong turbulence or other mixing influence in the main combustion chamber, most of the fuel which is permitted to escape into the main combustion chamber via a transfer passage 3 tends to remain in or close to the trench portion 4 of the cavity. As the cavity 1 exerts only a relatively weak dispersing influence on the flame which is ejected into the trench 4, it tends to flow up and over the rounded nose portion 5 of the clover leaf type dispersion recess located opposite the trench. This induces the situation wherein the fuel droplets in the trench tend to be entrained in a flame plume P having a shape essentially as illustrated in broken line. Under these conditions, the fuel which is released into the main combustion chamber tends to be carried by the initial flame ejection into a relatively cool section thereof wherein quenching of the flame is apt occur. Accordingly, the relatively rich flame and the entrained fuel (particularly the latter) are not mixed effectively with the air which is available in the main combustion chamber and neither tend to undergo sufficiently rapid nor complete combustion to enable good engine output and low noxious emissions. As a result of this problem, during low load engine operation high HC emissions result while under higher loads smoke formation tends to exceed acceptable limits. US-A-4 785 776 discloses a flame dispersion arrangement which has the features indicated in the pre-characterizing part of claim 1. In this arrrangement, the recess formed in the piston crown comprises a first flame dispersion zone located substantially at the position of the trench 4 shown in figure 1 and relatively large second and third flame dispersion zones comparable to the clover leaf structure shown in figure 1. The first flame dispersion zone is separated from the second and third ones by nose portions formed by the wall of the recess and by an island portion which is located on the same diameter of the piston crown as the transfer passage. Thus. when the piston is close to the TDC position, the flame is ejected into the first flame dispersion zone and passes into the second and third flame dispersion zones through the gaps between the two nose portions and the island portion. When the piston has descended, the flame ejected from the transfer passage rides over the island portion and hits the bottom of the recess on the opposite side of said island, so that a different flow pattern is induced in the second and third flame dispersion zones, thereby promoting an improved mixing of the flame with the air and/or fuel which is remnant in the main combustion chamber. The second and third flame dispersion zones have a circular shape, respectively, so that swirling motions of the flame are induced therein. SUMMARY OF THE INVENTION It is an object of the present invention to provide an arrangement which modifies the spiraling characteristics of the flows in the second and third flame dispersing zones in a manner which further improves combustion characteristics and thus reduces smoke and HC-emissions. This object is achieved with the features indicated in claim 1. In the arrangement according to the invention, the wall portions of the recess defining the second and third flame dispersion zones are arranged to deflect the spiraling flows in the second and third flame dispersing zones in a manner such that they tend to flow into a thightening swirl pattern. More specific optional features of the invention are indicated in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 and 2 show the prior art arrangement discussed in the opening paragraphs of the instant disclosure; Figs. 3 and 4 show a first embodiment of the present invention; Figs. 5 and 6 show a second embodiment of the present invention; Figs. 7 and 8 show a third embodiment of the present invention; and Figs. 9 and 10 show a fourth embodiment of the present invention DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 3 and 4 shows a swirl chamber type diesel engine according to a first embodiment of the the present invention. In this arrangement the piston 100 is formed with a shaped flame dispersing cavity in the crown thereof. The cavity is defined between two opposed shaped walls 101 and 102 and is relatively shallow as compared with the prior art arrangement shown in Figs. 1 and 2. The depth of this dispersion recess is limited to reduce the loss in compression ratio. Portions of the first wall 101 which flank a first flame dispersion zone where the flame from a transfer passage 106 which leads from a swirl chamber 107 to the main combustion chamber, are arranged to define two essentially inwardly directed nose-like guide portions or fingers 101A and 101B. In this embodiment the fingers are relatively blunt. A wall portion 101C which extends between the two guide portions is arranged to have a curvature which approximately parallels the periphery of the piston 100. As will be noted the first flame dispersing zone has an essentially bay-like configuration. Curved wall portions 101D, 101E of the first wall and 102A and 102B of the second wall are arranged to define therebetween second and third essentially circular and mirror image dispersion zones. As shown in Fig. 3 in the instant embodiment the wall portions 102A and 102B are arranged such that the extrapolations thereof pass inboard of the extrapolation of wall portions 101E and 101D. Viz., as illustrated in the lower half of this figure, the extrapolation 102B′ of wall portion 102B passes inboard of the extrapolation 101E′ of the wall portion 101E. Further, the inwardly extending extrapolations 101E′ and 101D′ of the wall portions 101E and 101D are such as to intersect essentially mid portions of a single island-like deflection portion 108 which is located amid the the first, second and third flame dispersion zones. This island 108, as it will be referred to hereinafter, is arranged to project up from the floor of the cavity at a site which located in the mouth of the bay-like first flame dispersing zone and located on a line A which extends diametrically across the piston crown and with respect to which the second and third flame dispersing zones are symmetrically arranged. The transfer passage 106 is oriented so that this deflection portion 108 is arranged in the path of the flame which is ejected therefrom during the initial stage of the expansion phase while the piston 100 is still relatively close to its TDC position. The upstream face of the deflection portion or island 108, is arranged to have angled sides which face into the flame and which are designed to induce a first splitting or division of the same. These two sides define a first angle. A second flame splitting or deflecting portion is defined by a V-shaped nose portion 102C which forms part of the second wall 102 and which projects into the cavity in a manner which tends to separate the second and third circular dispersion zones. The sides of the nose are arranged to define a second angle. In this instance the first angle is larger than the second one. It will be noted that the second and third zones of the cavity are open and extend to the very perimeter of the piston crown in the illustrated manner. For disclosure relating generally to this type of cavity, reference may be had to United States Patent No. 4,662,330 issued on May 5, 1987 in the same of Giichi Shiyoyama et al. The operation of the above described arrangement is such that while the piston is close to TDC, the major portion of the flame which passes through the transfer portion 106 during the expansion phase of the engine, flows toward and impinges on the island 108 as shown by the flow lines "a". This induces a first splitting of the flame and causes the divided parts to encounter portions of the flame which have spread almost laterally outward from the transfer port and which are flowing along flow lines "b". Portions of the flame which tend to flow back away from the island 108 as indicated by flow lines "e" are deflected by the wall portion 101C in a manner wherein they tend to curl back and blend with the flows "a" and "b". The blended flows are subject to deflection by the guide fingers 101A and 101B in a manner they pass through the openings defined between the island 108 and the fingers and assume a flow indicated by flow lines "c". This blending process tends to engulf most of the air which is present in the first flame dispersion zone and thus promote desirable combustion characteristics during the initial stage of the expansion phase. As the piston 100 descends, the location where the flame from the transfer port 106 impinges on the piston crown travels from a site upstream of the island 108, over the top of the same into the region defined between the island 108 and the nose 102C. Thus, upon a given amount of decent of the piston 100, the flame passes over the top of the island 108 and is subject to a second division or splitting by the nose 102C. This splitting produces swirling flows which blend into the flows established by the first splitting and result in the formation of contra-rotating swirling flows indicated by the flow lines "d". In this embodiment the curvature of the swirl which is induced by the deflection of wall portions 102A and 102B tend to be tighter than that induced by the deflection by wall portions 101E and 101D. This dual stage splitting of the flame tends to produce an extensive and well developed plume of the nature generally indicated by the broken line and which results in a considerable amount of the air which is resident in the main combustion chamber at this time, being engulfed by the flame. This results in the fraction of the fuel which is permitted to escape into the main combustion chamber prior the beginning of the expansion phase being entrained in the burning gases which constitute the flame which is ejected from the transfer port 106 in a manner which promotes efficient combustion. This reduces HC emissions during low load operation and reduces smoke formation under high loads. SECOND EMBODIMENT Figs. 5 and 6 show the construction which characterizes a second embodiment of the present invention. In this arrangement the nose portion 102C' is arranged to be higher than the remainder of the piston crown. The lower face of the cylinder head is recessed in a manner to receive the elevated nose portion when the piston reaches TDC. With this construction, the height of the wall against which the flame impinges and is split into two flows is increased. This results in the efficiency with which the flame is split into two flows is increased and tendency for the flame to ride up over the same, reduced. This feature both improves the swirl formation and improves high load operation wherein the velocity of the flame and the tendency for the flame to flow over the nose, are higher than in the case of low - medium load operation. By recessing the cylinder head in a manner to admit the upwardly projecting nose portion, any loss in compression ratio is offset. THIRD EMBODIMENT Figs. 7 and 8 show a third embodiment of the present invention. In this embodiment the depth of the flame dispersing cavity increases from the first wall 101 toward the second wall 102. In this arrangement in order to prevent loss of compression ratio, the depth of the cavity at the first wall end is less than in the case of the first embodiment which exhibits a uniform depth, and increase to a maximum depth at the second wall. This increases the height of the wall defined at the nose 102C and thus provides advantages similar to those derived with the arrangement shown in Figs. 5 and 6. FOURTH EMBODIMENT Figs. 9 and 10 show a fourth embodiment of the present invention. In this arrangement concave dished recesses 501 are formed in the second and third flame dispersing zones. These recesses 501 are such as to increase the strength with which the swirling of the gases in the second and third dispersion zones takes place and thus improves the efficiency with which the air remnant in the combustion chamber can be used. Viz., the gradually increasing depth of the recesses 501 is such as to tend to tighten the curvature of the swirling patterns which are established following the second flame splitting on the nose 102C and thus improve the mechanism with which the flame mixes with the remaining air. The depth of the flame dispersing cavity outside of the sites of the recesses 501 is uniform.

Claims: An internal combustion engine having a piston (100) reciprocatively disposed in a bore to define a variable volume combustion chamber; a swirl chamber (107), said swirl chamber being fluidly communicated with the combustion chamber by a transfer port (106), said piston comprising: an essentially flat crown; a shaped cavity formed in said crown, said cavity being in part bounded by first and second opposed walls; (101,102) ; an island-like flame deflecting projection (108), said island-like flame deflecting projection comprising a member which projects up from the floor of said cavity and which is arranged in the path of the flame which ejects from said transfer passage during the expansion phase of the engine and when the piston is close to its TDC position; said first wall being arranged to have first and second guide portions (101A,101B), said first and second guide portions extending inwardly toward one of the center of the piston crown or the island-like flame deflecting projection, said first wall further comprising a first curved section (101C) which merges with said first and second guide portions, said first and second guide portions and said curved portion defining in part a first flame dispersion zone in said cavity, said first curved wall portion being so constructed and arranged as to deflect a portion of the flame which flows theretoward and which promotes mixing of the flame with the air contained in said first flame dispersing zone; second and third concavely curved wall portions (101D,101E) formed in said first wall, said second and third wall portions being formed in said first wall at locations outboard of said first and second guide portions, respectively; fourth and fifth concavely curved wall portions (102A,102B) formed in said second wall, said fourth and fifth wall portions cooperating to define a nose portion (102C) which projects into said cavity, said nose portion being arranged in the path of the flame which ejects from said transfer passage after said piston descends from its TDC position by a predetermined amount, said fourth and fifth wall portions further cooperating with said second and third curved wall portions of said first wall in a manner to define second and third essentially circular flame dispersing zones in which spiralling flows are established; characterized in that said fourth and fifth curved wall portions (102A,102B) are curved concavely in a manner so that the curved extrapolations (102B') thereof pass inboard of the curved extrapolations (101E') of said second and third curved wall portions. An internal combustion engine as claimed in claim 1. wherein said nose portion (102C') projects above the level of said essentially flat crown and thus increases the area of the nose against which the flame impinges and is split into flows which tend to spiral in the second and third dispersion zones. An internal combustion engine as claimed in claim 1, wherein the depth of said shaped cavity increases from the first wall (101) in the direction of the second wall (102) and thus defines a tapering arrangement which tends to increase the area of the nose portion (102C'') against which the flame impinges and is split into flows which tend to spiral in the second and third dispersion zones. An internal combustion engine as claimed in any of the preceding claims, comprising first and second discrete dish-shaped concavities (501) formed within the second and third flame dispersing zone, respectively, the perimeter of said first and second dish-shaped concavities being arranged to proximate said first and second walls (101,102).

Inventors: SEKIYA, YOSHIKI, SHINZAWA, MOTOHIRO
Applicants: NISSAN MOTOR CO., LTD.
Application year: 1990
Main classification: F02B23
IPC codes: F02B3, F02B23, F02F3