JP2005139931A - Combustion chamber structure of direct injection internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form proper stratified air-fuel mixture in a combustion chamber of a direct injection internal combustion engine to achieve satisfactory stratified combustion. <P>SOLUTION: An ignition plug 12 and a fuel injection valve 11 are provided in an upper part of the combustion chamber 1. An outer side cavity 41 having substantially circular column shape and another inner side cavity 42 are formed substantially in the vicinity of center of a crest face of a piston 4 and at a position involved in the outer side cavity 41, respectively. The inner side cavity 42 is formed so as to turn/mix air-fuel mixture two-dimensionally when forming air-fuel mixture via the inner side cavity. The outer side cavity 41 is formed so as to turn/mix air-fuel mixture in the direction in which axis becomes an object for fuel spray axis when forming air-fuel mixture via the outer side cavity 41. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a combustion chamber structure of an internal combustion engine that directly injects fuel into the combustion chamber, and more particularly to a technique for satisfactorily forming a stratified mixture.

  As a conventional in-cylinder direct injection internal combustion engine, a fuel injection valve is mounted on the cylinder head at the center of the upper surface of the combustion chamber, and a spark plug is provided from the exhaust side of the cylinder head toward the center of the combustion chamber. The in-cylinder injection internal combustion engine has a cavity for forming a stratified mixture having a substantially circular cross section with an offset on the exhaust side on the upper surface of the piston and a deep bottom surface on the exhaust side. As a result, it is possible to hold the fuel in the cavity near the spark plug for a relatively long period of time, and to provide an in-cylinder injection internal combustion engine capable of stratified combustion in a wide operating range (Patent Document 1).

On the other hand, it is conceivable to provide a cavity for forming a stratified mixture consisting of a central deep dish portion and a surrounding shallow dish portion on the upper surface of the piston, and to control the size of the mixture formed according to the engine load. In other words, the size of the air-fuel mixture formed via the piston is adjusted according to the size of the cavity. In this case, even when the squish flow generated by the gap between the cylinder head lower surface and the piston crown surface is large, the squish generating gap and the spark plug spark gap position can be moved away from each other and are not affected by the squish flow. In addition, the air-fuel mixture can be formed (Patent Document 2).
JP 2000-34925 A JP 2000-265841 A

  However, in the invention described in Patent Document 1, the piston cavity is formed by one cavity even if the mixture can be homogenized early by improving the fuel injection valve. In particular, there is a concern that the air-fuel mixture formed in the cavity and above the cavity becomes excessive when the load increases. On the other hand, when the size of the cavity is specified so that the concentration of the air-fuel mixture formed in the cavity and over the cavity becomes an appropriate concentration near the stoichiometric air-fuel ratio when the load is high, stable ignition is achieved when the load is low. Is in danger.

More specifically, when the air-fuel mixture is transported to the vicinity of the spark plug via the piston crown and ignited before being diffused into the cavity, the fuel vapor mixing becomes insufficient and inhomogeneous, and the exhaust performance is reduced. It can be exacerbated. That is, in order to suppress the generation of NOx and soot as much as possible, it is desired to form a homogeneous stratified air-fuel mixture and to reduce the excessively concentrated sites.
In this case, even if the air-fuel mixture can be homogenized at an early stage by improving the fuel injection valve, the air-fuel mixture near the spark plug gap is caused by the squish flow generated in the latter half of the compression stroke by the cylinder head lower surface and the piston crown surface. It is considered that stable combustion becomes impossible due to dilution due to excessive diffusion or blow-off of ignition by high-speed gas flow.

In the invention described in Patent Document 2, the shallow dish portion of the cavity has a valve recess shape. In the valve recess shape, the injected fuel spray spills between the recesses and the unburned HC. There is concern about the increase.
The present invention has been made paying attention to such a conventional problem, and a combustion chamber structure of a direct injection internal combustion engine in a cylinder which can generate a stratified air-fuel mixture having good properties by devising a cavity shape. The purpose is to provide.

For this reason, the present invention includes an ignition plug and a fuel injection valve in the upper part of the combustion chamber, and forms a substantially cylindrical outer cavity and an inner cavity near the approximate center of the piston crown, and the inner cavity is defined as the inner cavity. When the air-fuel mixture is formed via the outer cavity, the air-fuel mixture is formed so as to swirl and mix in two dimensions, and when the air-fuel mixture is formed via the outer cavity, the air-fuel mixture is The fuel spray axis is configured to swirl and mix in the axial direction.

In general, in the case of a homogeneous air-fuel mixture, the relationship between the equivalence ratio and the combustion performance is as shown in FIG. That is, fuel efficiency and exhaust performance are best when the equivalence ratio is around 1. The same applies to the air-fuel mixture during stratified combustion. If there is a large distribution or variation in the air-fuel mixture distribution, the combustion performance deteriorates.
On the other hand, in order to form a homogeneous stratified mixture having an equivalent ratio of about 1, the injected fuel needs to be mixed after a sufficient time. When attempting to ignite the mixing process of the injected fuel, it will ignite a heterogeneous air-fuel mixture depending on the characteristics of the fuel injection valve, which may lead to performance degradation such as fuel consumption and exhaust emissions. It is done.

  Therefore, when trying to achieve good stratified combustion, it is desirable to form a homogeneous mixture, and in this case it is best to change the size of the mixture, but the size of the mixture will When the air-fuel mixture is formed via, it depends on the cavity diameter. Therefore, considering the case where the stratified mixture is formed with one cavity diameter as shown in Patent Document 1, the equivalence ratio on the horizontal axis may be considered as a load. That is, with a certain cavity diameter, the engine load is limited to a certain range where the optimum combustion performance, that is, the best fuel consumption is obtained. That is, as shown in FIG. 2, the optimum engine load region differs for each cavity diameter.

Assuming that a homogeneous air-fuel mixture is formed in the cavity and above, the ratio between the cavity diameter and the bore diameter (cavity diameter / bore diameter) and the engine load at which the best fuel efficiency can be obtained are as shown in FIG. In order to improve the stratified combustion performance, when a homogeneous mixture distribution is formed, it is difficult to improve the combustion performance over a wide load range with one cavity diameter.
Therefore, according to the present invention configured as described above, when a small air-fuel mixture is formed via the inner cavity at a low load with a small fuel injection amount, the air-fuel mixture is swirled one-dimensionally in two dimensions. Since the air-fuel mixture is swung in the direction, the air-fuel mixture does not diffuse unnecessarily, so a compact air-fuel mixture distribution can be formed.

On the other hand, when fuel mixture is injected into the outer cavity to form an air-fuel mixture, the air-fuel mixture spreads toward the entire circumference of the cavity, so that an isotropic air-fuel mixture distribution is easily formed.
That is, when the load is low, a compact air-fuel mixture distribution can be formed, and when the load is high, it becomes easy to form a relatively homogeneous air-fuel mixture, and combustion deterioration due to load changes can be reliably prevented.

FIG. 4 shows a combustion chamber structure of a direct injection type internal combustion engine according to the present invention.
Combustion chamber 1 is formed by cylinder head 2, cylinder block 3, and piston 4, and communicates with intake port 5 via intake valve 7 and exhaust port 6 via exhaust valve 8. The intake valve 7 and the exhaust valve 8 are driven to open and close by an intake valve cam 9 and an exhaust valve cam 10, respectively. A fuel injection valve 11 and a spark plug 12 are arranged close to each other near the center of the upper surface (cylinder head) of the combustion chamber 1, and a spark gap 12 a is installed toward the center of the combustion chamber 1. Based on this, fuel injection and ignition are performed.

An outer cavity 41 is formed on the crown surface of the piston 4, and a double cavity comprising another inner cavity 42 is formed at a position enclosed by the outer cavity 41.
In FIG. 4, the fuel injection valve 11 and the spark plug 12 are shown in the front-rear direction of the engine (a direction orthogonal to the direction in which the intake side and the exhaust side are aligned). However, the present invention only requires that the fuel injection valve 11 and the spark plug 12 are provided in the upper part of the combustion chamber 1, and the same effect can be obtained in other arrangements (for example, the intake side and the exhaust side are arranged side by side). .

The fuel injection valve 11 and the spark plug 12 are attached with a slight angle with respect to the cylinder central axis (this is because both are placed on the upper part of the combustion chamber 1 without reducing the diameter of the intake valve 7 and the exhaust valve 8. (It is a means for attachment and is not limited.)
The fuel spray is injected substantially parallel to the cylinder central axis. That is, the fuel is injected in a direction inclined with respect to the fuel injection valve shaft.

Here, the outer cavity 41 is installed on an axis object with respect to the fuel spray central axis.
FIG. 5 shows the piston shape according to the present invention.
The outer cavity 41 is installed on an axis object with respect to the fuel spray central axis.
The inner cavity 42 is set so that the upper surface opening shape from the direction of the cylinder head 2 forms a rectangle, and the fuel spray is biased toward the bottom surface of the inner cavity 42 in the fuel injection valve-ignition plug side direction. Are provided to collide with each other.

Here, an outline of the fuel injection method for each operating condition will be described.
When the engine load is lower than the predetermined load, injection is performed only once, and the fuel injection start timing is advanced as the load increases. At that time, the fuel injection timing is set so that the fuel spray is received by the inner cavity 42. As the engine load increases, the injection period extends, so that two split injections are started before the injection period during which the fuel spray is not received by the inner cavity 42.

When performing split injection twice, the fuel injection amount for the second injection is retarded as compared to the case of only one injection, and the fuel is injected only once by reducing the fuel injection amount. The injection end timing of the second injection in the case of split injection is set so as to be substantially the same timing.
At the time of split injection, the increase in the fuel injection amount corresponding to the increase in engine load is dealt with by increasing the first injection amount. At that time, the fuel injection start timing is advanced, and the fuel injection end timing is substantially the same.

FIG. 6 shows the fuel behavior when the fuel is injected only once under the stratified low load operation condition.
The fuel injection timing is set so as to be received by the inner cavity 42, and the fuel spray collides with the bottom surface of the inner cavity 42. Thereafter, the spray proceeds along the bottom surface of the inner cavity 42 by the penetrating force of the spray, and goes toward the combustion chamber 1. At this time, since the bottom surface of the inner cavity 42 where the spray collides is formed so that the fuel is directed to the spark plug 12, the fuel spray flows two-dimensionally along the piston 4 on the crown surface of the piston 4, It advances in the direction of the spark plug 12, and the direction of travel is changed upward by the piston 4, whilst swirling over the combustion chamber 1, whilst surrounding air is drawn in, the inner cavity in the vicinity of the spark plug 12 A stratified mixture is generated above 42. By performing only one fuel injection toward the inner cavity 42, the air-fuel mixture formed can be formed relatively compactly near the ignition plug 12 on the exhaust side slightly from the center of the combustion chamber 1.

Here, by arranging the spark gap portion of the spark plug 12 substantially at the upper portion of the outer peripheral portion of the inner cavity 42, the air-fuel mixture is reliably transported to the vicinity of the spark gap portion even when the fuel injection amount is small. Certain ignition is possible.
Moreover, by making the opening shape of the inner cavity 42 rectangular, diffusion of the air-fuel mixture other than the direction from the fuel injection valve 11 toward the spark plug 12 is reliably suppressed.

Further, by setting the injection direction of the fuel injection valve 11 to be deviated from the center of the inner cavity 42 toward the fuel injection valve 11 side, the fuel movement direction is changed from the bottom surface of the inner cavity 42 on the fuel injection valve 11 side to the spark plug 12 side. Therefore, the combustible air-fuel mixture can be reliably transported even in the case of a minimum injection amount such as idling.
Further, by setting the depth of the bottom surface of the inner cavity 42 low on the side where the fuel spray collides, the liquid film does not adhere to the piston 4 even in the case of fuel injection near the compression top dead center. .

On the other hand, the spray behavior under the stratified high load operation condition is as shown in FIG.
At stratified high load, two injections are performed. First, near the middle of the compression stroke, the first fuel injection is performed toward the outer cavity 41. As in the case of one-time injection at the time of stratified low load, the spray collides with the bottom surface of the outer cavity 41, travels to the sky above the combustion chamber 1 via the bottom surface of the outer cavity 41 by the penetration force of the spray, A homogeneous mixture is formed. Here, the size of the air-fuel mixture formed by the first fuel injection depends on the size of the outer cavity 41, becomes a relatively large mass, and the central portion of the combustion chamber 1 becomes lean.

  After the first fuel injection, the second fuel injection is performed toward the inner cavity 42 in the second half of the compression stroke, near the top dead center. That is, the second fuel injection timing is set to such a fuel injection timing that the injection angle tends to be slightly narrowed by the in-cylinder pressure even if the injection angle is the same as that of the first fuel injection, and is reliably received by the inner cavity 42. ing. Through a mixture formation process similar to the case of only one injection at the time of stratified low load, a homogeneous mixture of small lumps is formed by the second fuel injection, and it has a substantially donut shape formed by the first fuel injection. The air-fuel mixture is transported to the vicinity of the spark plug 12 by the close air-fuel mixture and the compact air-fuel mixture formed by the second fuel injection, and reliable ignition is performed. Further, stratified combustion can be achieved without impairing exhaust and fuel consumption performance by a homogeneous air-fuel mixture.

  Here, by setting the outer cavity 41 to be concentric with the fuel spray axis, the relative angle between the fuel spray and the piston 4 is similarly set with respect to the vertical movement of the piston 4, and the fuel injection timing is different. Even when the fuel injection period is long, spillage of fuel outside the outer cavity 41 can be reduced, so that an increase in the discharge of unburned HC can be suppressed. Further, the spray that has collided with the outer cavity 41 is rolled up from the outer cavity 41 approximately isotropically, and it is easy to form a homogeneous mixture distribution over the inner cavity 42.

  In addition, when the load is low, only one fuel injection is performed, and the fuel injection timing is set so that the injected fuel enters the inner cavity. Therefore, the engine load is low and the total fuel injection amount is small. In this case, a homogeneous air-fuel mixture can be formed in the vicinity of the spark plug without diluting more than necessary, and an air-fuel mixture that can be ignited reliably is formed, resulting in fuel instability and exhaust performance deterioration at low loads. Can be prevented.

  On the other hand, when the load is high, the fuel injection period is extended by only one injection, so that the injected fuel protrudes not only to the inner cavity but also to the outer cavity. When only a small amount of fuel goes to the outer cavity, the air-fuel mixture formed via the outer cavity forms a lean air-fuel mixture that cannot be ignited and may be discharged as unburned HC. On the other hand, the concentration of the air-fuel mixture formed in the vicinity of the spark plug via the inner cavity may become excessive when the fuel injection amount is a certain amount or more.

Therefore, when the load is high, by performing the split injection twice in the compression stroke, it is possible to distribute an appropriate amount of fuel injection to the outer cavity 41 and the inner cavity 42 in each divided injection period. A good stratified mixture can be formed without being diluted or excessively concentrated.
In particular, when fuel is injected in the compression stroke, the injection timing is set so that the injected fuel enters the outer cavity 41 in the first fuel injection, and the fuel enters the inner cavity 42 in the second fuel injection. Since the injection timing is set so as to enter the fuel, the fuel that has passed through the inner cavity 42 forms a small air-fuel mixture in the vicinity of the spark plug, enables reliable ignition, and is injected into the outer cavity 42 to be injected into the outer cavity. The fuel passing through 41 forms a large air-fuel mixture in a relatively wide range. By setting the first fuel injection at a relatively early injection timing, the mixture can be homogenized.

FIG. 8 shows the direction of air-fuel mixture movement in the inner cavity 42 in the above embodiment. Within the inner cavity 42, there are a pattern that turns symmetrically as two small vortexes and a pattern that turns as one vortex.
In addition, as shown in FIG. 9, the upper surface opening shape of the inner side cavity 42 may be elliptical. In this case, the winding of the fuel on one side of the inner cavity 42 where the spark gap of the spark plug 12 is provided is easily concentrated by the elliptical inner cavity 42. More specifically, by making the opening shape of the inner cavity 42 an ellipse, while suppressing the movement of the air-fuel mixture in the direction orthogonal to the direction in which the fuel injection valve 11 and the spark plug 12 are arranged, the inner side of the lower side of the spark plug 12 The action of collecting the air-fuel mixture works in the cavity 42, so that the rich air-fuel mixture can be reliably collected near the spark plug 12. For this reason, even in the case of the minimum amount injection such as idling, the ignition can be surely performed by preventing the diffusion of the air-fuel mixture.

  Further, during high output operation such as full load, so-called homogeneous combustion is performed, in which fuel injection is performed during the intake stroke and sufficient mixture time is taken to homogenize the in-cylinder mixture distribution.

The figure which shows the relationship between an equivalence ratio and combustion performance. The figure which shows the relationship between the engine load for every cavity diameter, and optimal fuel consumption. The figure which shows the relationship between the ratio with respect to the cylinder bore diameter of a cavity diameter, and the optimal engine load in a homogeneous mixture formation state. 1 is a cross-sectional view showing a combustion chamber structure of a direct injection type internal combustion engine according to an embodiment of the present invention. The figure which shows the shape of the outer side cavity and inner side cavity in embodiment same as the above. The figure which shows the behavior of the fuel spray at the time of fuel injection only once in the stratified low load operation condition in the embodiment same as the above. The figure which shows the behavior of the fuel spray in a stratified high load operation condition. The figure which shows the air-fuel | gaseous mixture motion direction in the inner side cavity in the said embodiment. The top view which shows the combustion chamber structure in the 2nd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Combustion chamber 2 Cylinder head 3 Cylinder block 4 Piston 11 Fuel injection valve 12 Spark plug 41 Outer cavity 42 Inner cavity

Claims (11)

  An ignition plug and a fuel injection valve are provided at the upper part of the combustion chamber, a substantially cylindrical outer cavity is formed near the center of the piston crown surface, and another inner cavity is formed at a position enclosed by the outer cavity. When the mixture is formed through the inner cavity, the mixture is swirled and mixed two-dimensionally, and the outer cavity is formed through the outer cavity. A combustion chamber structure of a direct injection type internal combustion engine, characterized in that the air-fuel mixture swirls and mixes in the axial direction with respect to the fuel spray axis when performing.   The length of the opening part of the inner cavity in the direction in which the fuel injection valve and the spark plug are arranged is longer than the length of the opening part in the direction perpendicular to the arrangement direction. A combustion chamber structure of a direct injection type internal combustion engine.   3. The combustion chamber structure of a direct injection type internal combustion engine according to claim 1 or 2, wherein a spark gap portion of a spark plug is disposed substantially above the outer peripheral portion of the inner cavity.   The combustion chamber structure of a direct injection type internal combustion engine according to any one of claims 1 to 3, wherein the opening shape of the inner cavity is substantially rectangular.   The combustion chamber structure of a direct injection type internal combustion engine according to any one of claims 1 to 3, wherein the opening shape of the inner cavity is elliptical.   The direct injection type internal combustion engine according to any one of claims 1 to 5, wherein an injection direction of the fuel injection valve is biased toward the fuel injection valve side from a center of the inner cavity. Combustion chamber structure.   The combustion chamber structure of a direct injection type internal combustion engine according to any one of claims 1 to 6, wherein the depth of the bottom surface of the inner cavity is set low on the side where the fuel spray collides. .   The combustion chamber structure of a direct injection type internal combustion engine according to any one of claims 1 to 7, wherein the outer cavity is set to be concentric with the fuel spray axis.   9. The fuel injection according to claim 1, wherein the injection is performed only once in the compression stroke when the load is lower than the predetermined engine load, and the divided injection is performed twice in the compression stroke when the load is higher than the predetermined load. The combustion chamber structure of the direct injection type internal combustion engine described in 1.   When injecting fuel divided into compression strokes, the first fuel injection sets the injection timing so that the injected fuel enters the outer cavity, and the second fuel injection injects the fuel into the inner cavity. 6. The direct injection type internal combustion engine according to claim 1, wherein the timing is set.   7. The fuel injection timing is set so that the injected fuel enters the inner cavity when the engine load is equal to or less than a predetermined value and fuel is injected only once in a compression stroke. The combustion chamber structure of an in-cylinder direct injection internal combustion engine.

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