Swirl chamber type diesel engine

Abstract: A piston crown is formed with a cavity including a first flame deflecting member (108) which is located amid three flame dispersion zones and which splits the burning gases which are ejected from a transfer port (106) into a first dispersion zone while the piston (100) is proximate its TDC position, into two flows. The arrangement additionally includes an arrange­ ment which induces the two flows to assume two essentially symmetrical swirling patterns in second and third dispersion zones. After the piston has descended by a predetermined amount the gases are supplied directly into the second and third zones Important design considerations which influence the efficiency with which the air in the main combustion chamber is used come in: the angle at which the gases are directed toward the piston crown; the location of the first flame splitting member with respect to the site (P) where the flame initially impinges in the first dispersion zone; the angle (α) at which the flame is split and directed toward the second and third zones; the area of the first zone into which the flame is ejected and the zones in which the swirling air patterns are produced; and the velocity at which gases flow into the second and third zones.

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 prechamber type diesel engine of the nature disclosed in JP-A-54-59512, JM-A-50-138403 and JM-A-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, most of the fuel which is permitted to escape into the main combustion chamber via a transfer passage 3 tends to either remain in, or close to the trench portion 4 of the cavity. As the cavity 1 exerts only a relatively weak dispersive 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 FP 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. A diesel engine of the type specified in the pre-characterizing part of claim 1 has been disclosed in US-A-4 785 776. In this arrangement, the piston crown is formed with a cavity and with an island portion which is located between three flame dispersion zones defined within said cavity. The island portion splits the burning gases which are ejected from a transfer port into the first dispersion zone while the piston is close to its TDC position into two flows. The arrangement is such that if induces the two flows to assume two essentially symmetrical swirling patterns in the second and third dispersion zones. After the pistion has decended by a predetermined amount the gases are supplied directly into the second and third zones. In this prior art arrangement, the first flame dispersion zone is provided with a trench portion similar to the one described above in conjunction with figure 2. The island portion is so arranged that the inboard extremum of the extrapolation of the transfer port meets the apex of the island portion between the bottom and the top of the apex when the pistion is in the TDC position. SUMMARY OF THE INVENTION It is an object of the present invention to provide an arrangement which further promotes improved spreading of the flame which ejects via the transfer port from the swirl chamber and mixing with the air and/or fuel which is present in the main combustion chamber in a manner which promotes more efficient combustion and thus reduced smoke and HC emissions. This object is achieved with the features indicated in the claims. In brief, the island portion is so positioned that is splits the flame ejected through the transfer port only after the piston has decended from the TDC position by a certain amount. It has been found that important design considerations which influence the effeciency with which the air in the main combustion chamber is used come in: the angle at which the gases are directed toward the piston crown; the location of the island portion with respect to the side where the flame initially impinges in the first dispersion zone; the angle at which the flame is split and directed toward the second and third flame dispersion zones; the area of the first zone into which the flame is ejected and the zones in which the swirling air patterns are produced; and the velocity at which gases flow into the second and third zones. The invention will be more fully understood from the detailed description of preferred embodiments given hereinbelow. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 and 2 show the prior art arrangement discussed in the opening paragraphs of the instant disclosure; Fig. 3 is a sectional elevation showing a first embodiment of the present invention; Fig. 4 is a plan view showing the crown of the piston of Fig. 3; Fig. 5 is a graph which shows in terms of piston displacement and crank angle, the timing with which the leading and trailing (inboard, outboard) extremum of the transfer port extrapolation pass over an apex a V-shaped portion formed on an island-like projection which is arranged in the path of the flame as it ejects from the swirl chamber; Fig. 6 is a graph which shows the effect of transfer port angle on the output of the engine at fixed engine speed and smoke levels; Fig. 7 is a graph which shows the effect of injection volume on the amount of smoke which is produced; Fig. 8 is a graph which shows the effect of the magnitude of the angle at which the first flame splitting occurs, on the output of the engine; Fig. 9 is a graph which shows the relationship between the output of the engine and the area ratio of two essentially circular flame dispersing recesses; and Fig. 10 is a graph which shows the relationship between the engine output and the ratio W w /W s (the width of the first flame dispersing island-like projection and the width of the cove-like bay in the mouth of which the island-like projection is disposed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 3 shows a swirl chamber type diesel engine which embodies the present invention. In this arrangement the piston 100 is formed with a flame dispersing cavity in the crown thereof. The cavity is defined between two opposite shaped walls 101 and 102 and is relatively shallow in order to prevent loss of compression. To facilitate description of the configuration and arrangement of the various portions of the cavity a symmetry line A is shown which extends along a diameter of the piston and which passes through the center of the crown. Portions of the first wall 101 are formed in a manner to define two essentially inwardly directed nose-like guide portions 101A and 101B. These portions 101A and 101B define a first flame dispersion area or zone within the cavity. A wall portion 101C which extends between the two guide portions is arranged to have a curvature which tends to parallel the outer periphery of the piston. In this instance the first flame dispersion zone has what shall be referred to as a bay-like configuration. Curved wall portions 101D and 101E of the first wall 101 and portions 102A and 102B of the second wall 102 are arranged to define therebetween second and third dispersion areas or zones which have essentially circular shapes and which both partially overlap one another and the first dispersion zone. These second and third zones are arranged symmetrically with respect to line A. A single island-like deflection portion 108 is located amid the first, second and third flame dispersion zones. This element is arranged to project up from the floor of the cavity in the mouth of the bay-like first flame dispersion zone, on the line A at a location between the center of the piston the wall portion 101C. The island 108 (as it will be referred to hereinafter) is arranged with respect to the transfer passage 106 which fluidly communicates the swirl chamber 107 with the main combustion chamber in a manner to be located in the path of the flame which ejects from the transfer passage while the piston is close to its TDC position. Further comment concerning this feature will be given hereinlater. The side of the island 108 which faces the wall portion 101C is formed with angled walls which define a first V-shaped flame splitting portion. The splitting angle defined by these walls will be discussed in more detail hereinlater. A second flame splitting portion is defined by a V-shaped nose-like portion 102C which forms part of the second wall 102. This portion 102C projects into the cavity in a manner which tends to separate the second and third circular dispersion zones. The sides of this nose are arranged to define a second angle. In this instance the first angle is preferably larger than the second one. It will be noted that the second and third dispersion zones are open and such as to extend to the very perimeter of the piston 100, in the manner illustrated. For disclosure relating to this type, of dispersion arrangement wherein the petals of the clover leaf have portions which are open in the above mentioned manner, reference may be had to United States Patent No. 4,662,330 issued on May 5, 1987 in the name of Shioyama et al. In general, the operation of the above described arrangement is such that when the piston is close to its TDC position, the flame (burning gases) which initially ejects from the transfer passage following spontaneous combustion of the air-fuel charge in the swirl chamber 107, impinges on the first flame dispersion zone. As a result, the gases firstly tend to flow outwardly in the manner indicated by the initial portions of the arrows "b". After encountering the nose-like guide portions 101A and 101B the gases tend to curl back toward the island and proceed in the direction of the second wall. Due to the shape of the second wall 102, two contra rotating swirl patterns are established in the second and third dispersion zones in the manner depicted. Subsequently, as the piston descends the location where the gases tend to impinge on the piston crown shifts toward the center thereof. Thus, following a given amount of descent, the gases pass over the top of the island 108 and impinge on the overlapping portions of the second and third dispersion zones. Under these conditions the effect of the second flame splitting portion 102C induces flows of the nature indicted by arrows "c". This dual stage flame splitting action produces an extensive and well developed flame plume FP of the nature depicted by the chain line. The swirling and various changes in gas flow direction induce and promote mixing with the air and/or fuel droplets which are present in the main combustion chamber. In accordance with the present invention it is preferred that the upper corner of the apex defined by the first V-shaped flame splitting portion defined on the island 108 is such as to be intersected by the inboard extremum I/E of an extrapolation of the transfer port 106 when the piston assumes a position within a crank angle range of ATDC 17 - 21° and to intersect the outboard extremum O/E when the piston has descended to a position which falls in the crank angle range of 33 - 35°. The angle at which the extrapolation intersects the crown of the piston is arranged to be ϑ. When the piston assumes its TDC position, the extremum I/E is arranged to intersect the first flame dispersion zone at point P. This point is located on line A and with respect to the centers Q, Q of the circular second and third dispersion zones in a manner which defines an angle α. In this instance the angle (α/2)° which is defined on each side of the line A, can have a tolerance of +/- 2°. The timing with which the splitting occurs and the angle α are critical to achieving the desired air usage efficiency and combustion characteristics. In the event that the value of ϑ is small the splitting action by the island 108 is short lived and vice versa. In the event that the flame flow into the first flame dispersion zone terminates quickly, the efficiency with which the flame is mixed with the air in the main combustion chamber is reduced. Conversely, if the flame is allowed to impinge on the first flame dispersion zone for an excessively long period, the flow into the second and third dispersion zones is delayed to the point wherein an adequate supply of gas thereinto is not achieved. This results in the air which is available therein not being efficiently used. Fig. 5 is a graph which is recorded in terms of piston displacement and crank angle and which shows the effect of ϑ on the timing which the first splitting begins to attenuate to the time it finishes. In this figure data for five different values of  are given. In accordance with the present invention it is preferred that the angle at which the extrapolation intersects the crown of the piston ϑ = 35° which is accompanied by range of (15.0 - 31.6° ATDC) over which the flame splitting begins to attenuate to the time it terminates. It should be noted the angle at which the flame is split by the island 108 is important. That is to say, in the event that the splitting angle is too small, the amount of flow into circular second and third dispersion zones becomes excessive for the amount of air which is available therein and vice versa. By way of example, if the angle QPQ (viz., α) is set at 77° with ϑ = 33°, if the leading edge island 108 is moved from position I to position II in the manner shown in Fig. 3, then the valve of α changes to oe' = 92°. As will be appreciated from Fig. 8 the amount of power which is produced, reduces notably. The reason for this is deemed to be the reduction in the swirl intensity in the second and third dispersion zones which reduces the efficiency with which the air therein is used. From Figs. 6 and 7 the effect on engine power output and smoke formation by the value of ϑ and the amount of fuel which is injected can be observed. From this data it has been determined that the (α/2)+/- 2° is required to produce the desired combustion characteristics. In this connection, it is required to set the shape and position of the island 108 carefully in the main combustion chamber with a view of achieving the required gas flow and air usage characteristics. A further consideration comes in the setting of the ratio of the area of the first dispersion zone to the area of the second and third zones. As shown in Fig. 9, it has been discovered in connection with the present invention that if the area of the first dispersion zone is set in the range of 19.5 - 22.5% desirable combustion characteristics and engine output are achieved. It should be noted that, in this instance the area of the second and third dispersion zones are determined by extrapolating the curved wall portions 101D and 101E inwardly toward the island 108 as indicated by the broken lines "ex" When the flaming gases are ejected from the swirl chamber by way of the transfer port 106 into the main combustion chamber, the piston is approximately at its TDC position. As a result, the gases are delivered into the first flame dispersion zone. In the event that the area of this zone is set too small, while the piston is at or close to its TDC position, the pressure and temperature in this zone elevate to the point which prevents the cooler air from entering the same. Accordingly, combustion in the main combustion chamber tends to be delayed. On the other hand, if the area of the first dispersion zone is too large, the velocity of the gases which flow into the second and third dispersion zones is insufficient to induce the required swirl intensity. Accordingly, combustion in these latter zones is slowed and the efficiency with which air is used, is reduced. Further parameters which are deemed important to the operation of the instant invention are the width W T of the transfer port 106, the width W M of the island, and the width W S defined between the nose-like guide portions 101A and 101B. As changing the dimensions of the island 108 is at to induce a number of other changes, the ratio of W M /W S is varied mainly by changing W S . By holding the above mentioned area ratio constant and increasing WS by reducing the lengths of the portions 101A and 101B, it is possible to reduce the velocity of the gases which flow out of the first dispersion zone and thus reduce the swirling in the second and third ones. On the other hand, by increasing the lengths of 101A, 101B, W S can be reduced. However, in excess this can lead to throttling the communication between the dispersion zones and induce the situation wherein the gas flow up over wall 101 toward the perimeter of the piston. Under these conditions the gases tend to encounter the relatively cool bore wall and undergo quenching. In connection above mentioned facet of the invention it is preferred that ratio of the area ratio of the first dispersion zone be held at 20% and the ratio of W M /W S be set between 27.5 - 31.5%. It is possible to set the value of W S at about the distance the centers Q, Q of the second and third dispersion zones are separated.

Claims: A swirl chamber type diesel engine comprising: a piston (100) reciprocatively disposed in a bore to define a variable volume combustion chamber; and a swirl chamber (107), said swirl chamber being fluidly communicated with the combustion chamber by a transfer passage (106) which is inclined relative to the crown of the piston in an imaginary median plane of the piston, the crown of said piston comprising: first and second peripheral raised portions and an island portion (108) which is formed between said first and second raised portion and is surrounded by a lower crown surface which is, in turn, bounded between the first and second raised portions, said island portion (108) having a first approximately V-shaped splitting portion having two side walls forming a first splitting angle (α) to split a gas stream ejected from said transfer passage (106), the second raised portion having a second V-shaped splitting portion (102C) for splitting the gas stream ejected from the transfer passage, said first and second splitting portions being arranged so that the tips of both splitting portions are arranged in an imaginary median line (A) contained in said median plane and both point in the same direction toward the first raised portion, the frist peripheral raised portion having first and second guide portions (101A,101B) which are located on both sides of the median line (A) and project from the periphery of the piston crown toward the island portion (108), and a middle portion defining a first flame dispersion zone which receives the gas stream ejected from the transfer passage when the piston is its TDC position and whose mouth is defined between the tips of the first and second guide portions (101A, 101B); said second raised portion having first and second arched side walls (102A,102B) defining second and third substantially circular flame dispersion zones on both sides of the median line (A), characterized in that the middle portion of said first peripheral raised portion has a continous smooth concave side wall (101C) facing toward the island portion (108) and defining a bay-like shape of said first flame dispersion zone, and in that the first splitting portion of the island portion (108) is so arranged that the top end of its apex is located on the inboard extremum (I/E) of the extrapolation of said transfer passage (106) when the piston assumes a first predetermined postion wherein the crank angle is in the range of 17-21° ATDC and that it is located on the outboard extremum (O/E) of the extrapolation when the piston has decended from the first predetermined position to a second predetermined postion wherein the crank angle is in a crank angle range of 33-35° ATDC. A swirl chamber type diesel engine according to claim 1, wherein the ratio (W M ,W S ) of the width (W M ) of the island portion (108) measured along a direction perpendicular to the median line (A) to the distance (W S ) between the tips of the first and second guide portions (101A,101B) which are symmetrical with respect to the median line (A) is in the range of 27,5 - 31,5%.

Inventors: SHINZAWA MOTOHIRO, SHINZAWA, MOTOHIRO
Applicants: NISSAN MOTOR, NISSAN MOTOR CO., LTD.
Application year: 1990
Main classification: F02B19
IPC codes: F02B19, F02B3, F02B23