In-cylinder direct-injection spark-ignition engine

<b>BACKGROUND OF THE INVENTION</b>

Field of the Invention

The present invention relates to the improvements of an in-cylinder direct-injection spark-ignition internal combustion engine equipped with an open combustion chamber type piston in which fuel is injected directly into engine cylinders and it is ignited by an electric spark, and specifically to techniques for effectively producing strong stable tumble flow for stable combustion (particularly, stable stratified charge combustion), and capable of improving exhaust emission.

Description of the Prior Art

In recent years, there have been proposed and developed various direct-injection spark-ignition engines in which fuel is injected directly into the engine cylinders. Generally, on such direct-injection spark-ignition engines, a combustion mode is changeable between a homogeneous combustion mode (an early injection combustion mode) where fuel-injection early in the intake stroke produces a homogeneous air-fuel mixture, and a stratified charge combustion mode (a late injection combustion mode) where late fuel-injection delays the event until near the end of the compression stroke to produce a stratified air-fuel mixture. Such stratified charging or stratified combustion mode is effective under a low engine-load operating condition where the amount of fuel injected is comparatively less. In contrast to the above, during high engine-load operation where the amount of fuel sprayed out is comparatively great due to demands for more engine power or more engine output torque, there is less requirement for stratified charging, and in lieu thereof it is necessary to form more uniform air-fuel mixture layers, particularly in order to avoid the engine from knocking. One such direct-injection spark-ignition engine has been disclosed in Japanese Patent Provisional Publication No. 9-317479.

<b>SUMMARY OF THE INVENTION</b>

The direct-injection spark-ignition engine disclosed in the Japanese Patent Provisional Publication No. 9-317479, has a spark plug centrally located at a center of the combustion chamber, a fuel injector located at a side wall portion of the combustion chamber and near an intake valve disposed in an intake port, and a piston having a piston bowl cavity combustion chamber in a piston crown. The engine disclosed in the Japanese Patent Provisional Publication No.9-317479 is operable in a stratified charge combustion mode where fuel injection is executed on a compression stroke while introducing a reverse tumble flow to an induced air drawn into the combustion chamber through the intake port. Such a piston bowl cavity acts to promote strong tumble flow. The above reverse tumble flow is a vertical-vortex tumble flow being directed downward from the intake valve, flowing via the vicinity of the fuel injector toward the piston crown, then turned reversely along the piston bowl cavity, and thus directed toward the spark plug. Also, the piston bowl cavity is dimensioned so that the angle between opposite side edge portions of the piston bowl cavity, gradually widening in a direction along a line extending from the intake valve side to the center of the piston crown (or in a direction along streamlines of the tumble flow along the bowl cavity), is substantially equal to a fuel-spray angle. The angle between opposite side edge portions of the piston bowl cavity will be hereinafter referred to as a "bowl-cavity opposite-side-edges tapering angle". Owing to such reverse tumble flow, there is a possibility of spray/wall impingement between fuel spray and the wall surface of the piston ball cavity. Owing to such impingement of fuel spray on the piston bowl cavity, there is an increased tendency for the incoming fuel to adhere to the piston-bowl-cavity combustion chamber wall in the form of a fuel film, and as a rapid carbonization could occur, thus deteriorating an exhaust-emission control performance by the increased amount of exhaust emissions such as smoke and particulate matter (PM) and by formation of unburned hydrocarbons (HCs). The adhered fuel film on the ball cavity results in undesired deposits in the engine, thus resulting in engine power loss and reduced piston life. Additionally, owing to the previously-discussed bowl-cavity opposite-side-edges tapering angle dimensioned to be substantially equal to the spray angle, the engine includes difficulty in concentrating the fuel spray in a desired point while the fuel is guided by the piston bowl cavity having the above-mentioned structural dimensions and geometry and carried on the reverse tumble flow. In other words, there is a possibility of undesired fuel-spray dispersion during the stratified combustion mode. This means that the fuel spray, injected from the injector on the compression stroke during the stratified charge combustion mode, cannot be certainly carried on the reverse tumble flow and delivered to the vicinity of the tip of the spark plug. Additionally, the end of the piston bowl cavity, facing to the exhaust valve side, is slightly raised, and thus there is an increased tendency for the fuel spray injected to collide with or impinge on the slightly-raised portion of the piston crown. This deteriorates fuel economy and lowers engine's output power.

Accordingly, it is an object of the invention to provide an in-cylinder direct-injection spark-ignition engine which avoids the aforementioned disadvantages of the prior art.

It is another object of the invention to provide an in-cylinder direct-injection spark-ignition engine having specified piston-bowl-cavity dimensions and geometry, capable of avoiding undesired spray/wall impingement by way of normal tumble flow opposite in rotation from the previously-discussed reverse tumble flow, and of certainly reliably carrying the fuel spray on the normal tumble flow and delivered to the vicinity of a tip of a spark plug to create a richer air/fuel mixture layer around the spark plug, by efficiently concentrating air flow (the normal tumble flow) in a desired point as much as possible.

It is a further object of the invention to provide an in-cylinder direct-injection spark-ignition engine having specified piston-bowl-cavity dimensions and geometry, capable of ensuring a good normal tumble flow control performance and a good delivery of a small very rich layer of air-fuel mixture toward around a spark plug, and enhancing a stratified charge combustion stability by way of an optimally-controlled fling-up action, during a stratified combustion mode. The optimally-controlled fling-up action is obtained by full cooperation of a strong in-cylinder normal tumble flow concentrating to a desired point and a relatively wide angle between opposite side edge portions of the piston bowl cavity, gradually widening in a direction along a line extending from the intake valve side to the center of the piston crown or gradually narrowing in a direction along the normal tumble-flow streamline on the bowl cavity, as compared to a fuel-spray angle.

In order to accomplish the aforementioned and other objects of the present invention, a direct-injection spark-ignition engine operable in at least a stratified charge combustion mode where fuel injection is executed on a compression stroke while introducing a vertical-vortex tumble flow to an induced air drawn into a combustion chamber through an intake port, comprises a cylinder block having a cylinder, a piston movable through a stroke in the cylinder and having a piston bowl cavity combustion chamber in a piston crown, a cylinder head mounted on the cylinder block, a spark plug centrally located at a center of the combustion chamber, and a fuel injector valve provided at a side wall portion of the combustion chamber and near an intake valve disposed in the intake port for injecting fuel directly into the combustion chamber. The piston bowl cavity combustion chamber is formed in the piston crown for producing a normal tumble flow being directed toward a vicinity of said fuel injector valve and then directed toward a vicinity of the spark plug while being guided by the piston bowl cavity combustion chamber. The width of the piston bowl cavity combustion chamber is relatively wide at an exhaust valve side and relatively narrow at an intake valve side. A depth of the piston bowl cavity combustion chamber is relatively shallow at the exhaust valve side and relatively deep at the intake valve side.

<b>BRIEF DESCRIPTION OF THE DRAWINGS</b>

<ul id="ul0001" list-style="none" compact="compact"><li>Fig. 1 is a cross section illustrating an embodiment of an in-cylinder direct-injection spark-ignition internal combustion engine having a piston formed with a bowl-shaped piston cavity.</li><li>Fig. 2 is a top view of the piston of the engine of the embodiment, having the improved bowl-in-piston combustion chamber structure.</li><li>Fig. 3 is a perspective view showing the piston crown portion of the engine of the embodiment.</li><li>Fig. 4 is a graph illustrating the relationship between an air/fuel mixture ratio (A/F) and a rate of change in indicated mean effective pressure, and showing the difference of lean misfire limit between the prior art engine and the improved engine.</li></ul>

<b>DESCRIPTION OF THE PREFERRED EMBODIMENTS</b>

Referring now to the drawings, particularly to Fig. 1, the in-cylinder direct-injection spark-ignition engine of the invention is exemplified in a four-valve, gasoline-fuel, spark-ignition internal combustion engine having a pent-roof combustion chamber. As shown in Fig. 1, in the direct-injection spark-ignition engine of the embodiment, the ignition plug (the spark plug) 9 is located essentially at the center of the combustion chamber 4. The cylinder head 2 is mounted on a cylinder block 1 having an engine cylinder. A piston 3 is provided in the cylinder to be movable through a stroke in the cylinder. The combustion chamber 4 is defined by the cylinder wall of the cylinder block 1, the bottom face of the cylinder head 2, and the top surface (or the piston crown or the piston head) of the piston 3. The engine is equipped with two intake ports (7, 7), offsetting from the center axial line of the cylinder bored in the cylinder block 1. As clearly seen in Fig. 2, the cylinder head 2 is equipped with two intake valve ports (7, 7), offsetting from the center axial line of the cylinder formed in a cylinder block 1, and two exhaust valve ports (8, 8), offsetting from the center axial line of the cylinder in the opposite direction to positions of the intake ports. As appreciated from Fig. 1, the engine has a so-called cross-flow port structure. Two intake valves (5, 5) are disposed in the respective intake ports (7, 7) to open and close them, whereas two exhaust valves (6, 6) are disposed in the respective exhaust ports (8, 8) to open and close them. A fuel-injector valve 10 is provided in a side wall portion of the combustion chamber 4 and in the vicinity of the substantially middle portion of two downstream opening ends of the intake ports (7, 7), so as to inject or spray out fuel directly into the combustion chamber 4. Each of the intake ports (7, 7) is contoured to easily affect sufficient turbulent action, that is, a strong normal tumble flow (denoted by a in Fig. 1) to the air-fuel mixture, in the form of a vertical vortex (in-cylinder normal tumble flow) tumbling within the combustion chamber 4. The strong normal tumble flow indicated by the arrow <b>a</b> in Fig. 1, is a vertical-vortex tumble flow being directed upward from the intake valve, flowing via the vicinity of the fuel injector 10 toward the piston crown, and then turned reversely along a bowl-shaped piston bowl cavity combustion chamber 11 (fully described later), and thus directed toward the vicinity of the tip of the spark plug 9. As shown in Figs. 1, 2, and 3, the bowl-shaped piston cavity combustion chamber (simply, a piston bow cavity) 11 is formed in the piston crown of the piston 3. The piston bowl cavity 11 functions as a tumble-flow guide groove. Therefore, the normal tumble flow <b>a</b> is guided by virtue of the piston bowl cavity 11 of the piston 3, and thus directed to the vicinity of the tip of the spark plug 9. Structural details of the piston bowl cavity 11 are hereunder described.

As seen from the side view in Fig. 1, the piston bowl cavity 11 is formed into a circular-arc shape in a direction along streamlines of the normal tumble flow <b>a</b> of intake air. As appreciated from the cross section of the piston 3 shown in Fig. 1, a substantially right-hand half of the piston bowl cavity closer to the exhaust valve 6 than the intake valve is formed as a moderately sloped or curved, comparatively shallow cavity portion, while a substantially left-hand half of the piston bowl cavity 11 closer to the intake valve 5 than the exhaust valve is formed as a steeply sloped or curved, comparatively deep cavity portion. The deepest portion α of the piston bowl cavity 11 is slightly offset from the center of the piston crown (the axis of the piston 3) toward the intake valve side. The offset between the deepest portion α and the center of the piston crown, that is, the distance of the deepest portion α from the axis of the piston 3, is dimensioned so that the ratio of the offset to the cylinder bore is within a specified range of 5 to 20%. The depth of the deepest portion α, measured from the uppermost face of the piston crown of the piston 3, is dimensioned so that the ratio of the depth of the deepest portion α to the cylinder bore is within a specified range of 5 to 20%. This is because an excessively shallow piston bowl structure produces a relatively weak tumbling action, and on the contrary an excessive deep piston bowl structure results in an increase in the thickness of the piston crown, that is, increased piston-head weight. As shown in Fig. 1, the moderate curved line R1, included in the moderately sloped or curved, comparatively shallow cavity portion (see the substantially right-hand half of the piston bowl cavity 11 shown in Fig. 1), continues with the steep curved line R2, included in the steeply sloped or curved, comparatively deep cavity portion (see the substantially left-hand half of the piston bowl cavity 11 shown in Fig. 1), on the same tangential line at the deepest portion α. The radial distance β of each of diametrically-opposing cavity ends of the piston bowl cavity 11, in the direction along a substantially central streamline of tumble flow on the piston bowl cavity 11, to the edged portion of the circumference of the piston crown of the piston 3, is dimensioned so that the ratio of the radial distance β to the cylinder bore is within a specified range of 5 to 15%. The lower limit (such as 5%) of the ratio of the radial distance β to the cylinder bore is determined depending on a mechanical strength of the piston 3. On the other hand, the upper limit (such as 15%) of the ratio of the radial distance β to the cylinder bore is determined, accounting for an optimally-controlled fling-up action of air-fuel mixture caused by the normal tumble flow <b>a</b>. An excessively large radial distance β (at the exhaust valve side) is a difficulty to entry of the tumble flow (turned reversely along the pent-roof side combustion chamber) into the piston bowl cavity 11. Conversely, if the radial distance β (at the intake valve side) is excessively large, the fuel spray, injected from the injector 10 on the compression stroke during the stratified charge combustion mode, cannot be effectively carried on the normal tumble flow <b>a</b> and delivered to the vicinity of the tip of the spark plug 9. For the reasons set out above, in the improved piston structure of the embodiment, the radial distance β is dimensioned so that the ratio of the radial distance β to the cylinder bore is within the specified range of 5 to 15%. Additionally, as appreciated from the top view of Fig. 2, as per a cavity width of the piston bowl cavity 11, measured in a direction perpendicular to the direction along a substantially central streamline of normal tumble flow whose central streamline passes through the axis of the piston and extends from the exhaust-valve side to the intake-valve side, the moderately sloped or curved, comparatively shallow cavity portion of the piston bowl cavity 11 (the exhaust-valve side shallow cavity portion) is relatively wide, whereas the steeply sloped or curved, comparatively deep cavity portion of the piston bowl cavity 11 (the intake-valve side deep cavity portion) is relatively narrow. In the shown embodiment, the central cavity width γ (defined as a line segment between and including two diametrically-opposing points located on the circumferential edge of the piston bowl cavity 11 and passing through the axis of the piston 3 in the direction perpendicular to the direction along the substantially central streamline of tumble flow on the piston bowl cavity) is dimensioned so that the ratio of the central cavity width γ to the cylinder bore is within a specified range of 50 to 70%. As per a cross-sectional area of a cross section of the piston bowl cavity 11, cut in an arbitrary plane perpendicular to the direction along the substantially central stream line of tumble flow on the bowl cavity, a curvature of the piston bowl cavity 11 at each cross section, cut in the arbitrary plane, is dimensioned so that the cross-sectional area is substantially same over a specified range offsetting from the above line segment (passing the axis of the piston 3) by a predetermined distance <b>s</b> (or a predetermined offset <b>s</b>) in two opposite directions along the substantially central streamline of tumble flow on the bowl cavity 11. In the piston bowl structure of the embodiment, as seen from the top view of Fig. 2, the rightward offset <b>s</b> (or the leftward offset <b>s</b>) is dimensioned so that the ratio of the offset <b>s</b> to the cylinder bore is within a specified range of 5 to 20%. As previously discussed, the cavity width of the exhaust-valve side shallow cavity portion of the piston bowl cavity 11 is relatively wide, while the cavity width of the intake-valve side deep cavity portion of the piston bowl cavity 11 is relatively narrow. The edged portion of the circumference of the piston bowl cavity combustion chamber 11 formed in the piston crown of the piston 3 is dimensioned so that the angle δ between opposite side edged portions of the piston bowl cavity, gradually widening from the intake valve side to the exhaust valve side or gradually narrowing in the direction along the substantially central streamline of tumble flow on the bowl cavity 11, is greater than a fuel-spray angle θ of the fuel spray, injected from the injector on the compression stroke during the stratified charge combustion mode. For instance, suppose that the spray angle θ is 50 degrees, it is preferable to set the "bowl-cavity opposite-side-edges tapering angle" δ to 60 degrees.

With the previously-discussed arrangement, during a stratified charge combustion mode (a late injection combustion mode) where late fuel-injection delays the event until near the end of the compression stroke to produce a stratified air-fuel mixture, and the flame begins in a small very rich air-fuel mixture layer around the tip of the spark plug 9 and after ignition, spreads to the leaner mixture filling the rest of the combustion chamber 4, the engine of the embodiment having the improved piston bowl cavity structure produces a vertical-vortex in-cylinder normal tumble flow <b>a</b> within the combustion chamber 4, so that the fuel spray injected by the injector 10 is directed or delivered to the vicinity of the tip of the spark plug 9. The normal tumble flow <b>a</b> is useful to avoid spray/wall impingement between fuel spray and the wall surface of the piston ball cavity 11. Furthermore, in the engine of the embodiment, the width of the piston bowl cavity 11, functioning as a guide groove for the normal tumble flow <b>a</b>, is relatively wide at the exhaust-valve side and relatively narrow at the intake-valve side. Also, the depth of the piston bowl cavity 11 is relatively shallow at the exhaust valve side and relatively deep at the intake valve side. Therefore, as seen in Fig. 3, the piston bow cavity structure of the embodiment can smoothly and reliably carry or guide the normal tumble flow <b>a</b>, entering from the exhaust valve side into the piston bowl cavity 11, toward the intake valve side (the underside of the fuel injector 10) without overflowing the tumble flow out of the bowl cavity 11. Moreover, the piston bowl cavity structure of the embodiment allows streamlines of tumble flow on the piston bowl cavity 11 to be gradually concentrated toward within the substantially central streamline of tumble flow whose central streamline passes through the center of the piston crown of the piston 3. This produces a strong normal tumble flow directed to a desired point. Owing to the concentrated streamlines of tumble flow properly guided and concentrated by the reasonably curved recessed inner peripheral wall surface of the piston bowl cavity 11 having unique shape, dimensions and geometry, the engine of the embodiment can provide an effective fling-up action according to which fuel spray, injected from the injector 10 on the compression stroke during the stratified charge combustion mode, are flung upward and reliably directed toward the tip of the spark plug 9. Thus, by virtue of the controlled strong normal tumble flow, the fuel spray can be effectively carried to the vicinity of the tip of the spark plug 9 to certainly create the richer air/fuel mixture layer around the tip of the spark plug 9 during the stratified combustion mode. This ensures a clean, stable combustion during the stratified charge combustion mode.

Referring now to Fig. 4, there is shown the result of comparison between a rate of change in indicated mean effective pressure obtained by the improved piston bowl cavity structure of the present invention and a rate of change in indicated mean effective pressure obtained by the conventional piston bowl cavity structure. The improved piston bowl cavity structure of the invention, can reduce the rate of change in indicated mean effective pressure at a low level even at an ultra-lean air/fuel mixture ratio AFR. That is to say, the combustion stability, particularly the stratified combustion stability can be remarkably enhanced, and thus effectively enlarging the lean misfire limit. Also, the controlled strong normal tumble flow a contributes to a good delivery of the fuel spray on the tumble flow to the vicinity of the tip of the spark plug 9. Also, with the aid of the controlled strong normal tumble flow <b>a</b>, there is less possibility of fuel film adhered to the inner wall of the bowl cavity 11. This avoids undesired carbonization from occurring due to the fuel film, and also reduces exhaust emissions (unburned hydrocarbons, and particulate matter) and deposits in the engine, thereby improving fuel economy and thus enhancing the exhaust-emission control performance. Moreover, as previously discussed, a curvature of the piston bowl cavity 11 at each cross section is dimensioned, so that the cross-sectional area of a cross section of the bowl cavity 11, arbitrarily cut in a plane perpendicular to the direction along the substantially central streamline of tumble flow on the bowl cavity, is substantially same over a specified range offsetting from the above line segment by a predetermined offset <b>s</b> in two opposite directions along the substantially central streamline of tumble flow on the bowl cavity 11. Thus, it is possible to more certainly reliably prevent the tumble flow from overflowing out of the bowl cavity 11. That is, the unique piston bowl cavity structure of the invention serves as an excellent tumble-flow guidance. Additionally, the edged portion of the circumference of the piston crown of the piston 3 is dimensioned, so that the angle δ between opposite side edged portions of the piston bowl cavity, gradually widening in a direction along a line extending from the intake valve side to the center of the piston crown or gradually narrowing in the direction along the substantially central streamline of tumble flow on the bowl cavity 11, is greater than a fuel-spray angle θ. Such a relatively wide angle δ between opposite side edged portions of the piston bowl cavity 11 is effective to certainly carry the fuel spray on the tumble flow, without spray/wall impingement between fuel spray and the wall surface of the piston crown (the piston top surface).

The entire contents of Japanese Patent Application No. P11-159214 (filed June 7, 1999) is incorporated herein by reference.

While the foregoing is a description of the preferred embodiments carried out the invention, it will be understood that the invention is not limited to the particular embodiments shown and described herein, but that various changes and modifications may be made without departing from the scope or spirit of this invention as defined by the following claims.