Patent Application
20160201596
The present invention relates to a combustion chamber structure for an engine, and particularly to a combustion chamber structure for an engine for injecting fuel in a latter half of a compression stroke and igniting the fuel after a top dead center of the compression stroke within a predetermined engine operating range.
Generally, for engines using gasoline or a fuel mainly including gasoline, a spark-ignition method in which ignition is performed by an ignition plug is broadly adopted. Recently, arts for performing compression self-ignition (specifically, premixed compression self-ignition referred to as HCCI (Homogeneous-Charge Compression Ignition)) within a predetermined engine operating range while using gasoline or fuel mainly including gasoline by applying a high compression ratio (e.g., 17:1 or higher) as a geometric compression ratio of the engine are developed in view of improving fuel consumption performance.
One art regarding an engine which performs such compression self-ignition is disclosed in JP2012-172662A, for example. In the art of JP2012-172662A, the engine performs the compression self-ignition within a low engine load range and performs spark ignition within a high engine load range, and within the high engine load range, the fuel is injected into a cavity of a piston of the engine and mixture gas containing the fuel is ignited at a timing at which the mixture gas travels to the vicinity of an ignition plug of the engine.
In such an engine, within the high engine load range (specifically, a range where the engine speed is low and the engine load is high), in view of suppressing pre-ignition (a phenomenon in which the mixture gas self-ignites before a normal combustion start timing triggered by spark ignition), smoke, etc., a target injection start timing is determined to be a timing in a latter half of a compression stroke and a target ignition timing is determined to be a timing after a top dead center of the compression stroke, according to an effective compression ratio, fuel pressure, etc. In this case, to start the fuel injection at the target injection start timing and surely start combusting the mixture gas at the target ignition timing, a distance by which the fuel travels from the target injection start timing to the target ignition timing (fuel spray traveling distance) is preferably at least equal to or longer than a length of a path through which the mixture gas containing the fuel injected by a fuel injector passes to reach the ignition plug (fuel spray traveling path length). In other words, a relationship “fuel spray traveling distance≧fuel spray traveling path length” is preferably established. Therefore, a configuration of the cavity of the piston, etc., may be designed to suitably achieve such a relationship.
The present invention is made in view of solving the issues of the conventional arts described above, and aims to provide a combustion chamber structure for an engine, in which a configuration of a cavity of a piston, etc., are suitably designed to surely start combustion of fuel at a predetermined ignition timing after the fuel is injected at a predetermined fuel injection start timing, and improve combustion stability.
According to one aspect of the present invention, a combustion chamber structure for an engine is provided. The engine injects fuel in a latter half of a compression stroke and ignites the fuel after a top dead center of the compression stroke within a predetermined engine operating range. The combustion chamber structure includes a piston formed with a downward dented cavity at a central part of an upper surface thereof, a fuel injector provided above the piston and in an extension line of a central axis of the piston, and for injecting the fuel toward the cavity of the piston, and at least one ignition plug provided above the cavity of the piston and separated from the fuel injector in radial directions of the piston. A radius of the cavity, a depth of the cavity, and each of positions of the at least one ignition plug are designed to have a fuel spray traveling distance that is equal to or longer than a fuel spray traveling path length, the fuel spray traveling distance being a distance by which mixture gas containing the fuel travels from a fuel injection start timing of the fuel injector to an ignition timing of each ignition plug, the fuel spray traveling path length being a length of a path through which the fuel injected by the fuel injector reaches each ignition plug via the cavity.
With this configuration, the radius of the cavity, the depth of the cavity, and the positions of the ignition plugs are designed to have the fuel spray traveling distance that is equal to or longer than the fuel spray traveling path length. Thus, the fuel injected at the predetermined fuel injection start timing can surely be made to start combusting at the predetermined ignition timing. As a result, the predetermined fuel injection start timing and the predetermined ignition timing can suitably be achieved while securing combustion stability.
The fuel spray traveling path length is preferably a total length of a first distance from a position where the fuel injector is provided, to a position of a surface of the cavity with which the fuel injected by the fuel injector at a predetermined injection angle collides, a second distance from the position of the surface of the cavity with which the fuel collides, to an outer edge portion of the cavity, and a third distance from the outer edge portion of the cavity to the position where each ignition plug is provided.
With this configuration, the fuel spray traveling path length defined suitably is used. Thus, the radius of the cavity, the depth of the cavity, and the positions of the ignition plugs can more accurately be designed to have the fuel spray traveling distance that is equal to or longer than the fuel spray traveling path length.
When the fuel spray traveling path length is “L1,” a cavity radius is “Rc,” a cavity depth is “Dc,” a distance between the fuel injector and each ignition plug is “Rs,” and the predetermined injection angle of the fuel from the fuel injector is “α,” the fuel spray traveling path length L1 is preferably expressed by the following Equation 1.
L1=Dc(1−sinα)/cosα+2Rc−Rs (1)
The fuel spray traveling distance is preferably determined based on a pressure of the fuel injected by the fuel injector, a predetermined target fuel injection start timing of the fuel injector, and a predetermined target ignition timing of each ignition plug.
With this configuration, the fuel spray traveling distance determined based on the target fuel injection start timing and the target ignition timing which are set to satisfy a predetermined condition is used. Thus, the fuel injected at the target fuel injection start timing can surely be made to start combusting at the target ignition timing while suitably satisfying such a predetermined condition.
When the fuel spray traveling distance is “L2,” the pressure of the fuel injected by the fuel injector is “P,” a time length from the target fuel injection start timing to the target ignition timing is “t,” and a predetermined coefficient is “k,” the fuel spray traveling distance L2 is preferably expressed by the following Equation 2.
L2=k×P0.5×t2 (2)
Hereinafter, a combustion chamber structure for an engine according to one embodiment of the present invention is described with reference to the appended drawings.
Before describing the contents of this embodiment of the present invention, a conditional configuration of an engine in this embodiment is briefly described. The engine of this embodiment is operated at a high compression ratio, for example, a geometric compression ratio is 14:1 or higher (suitably, between 17:1 and 18:1). Within a predetermined operating range of the engine (e.g., a range where an engine speed is low and an engine load is high), the engine injects fuel in a latter half of a compression stroke (retard injection) and ignites the fuel after a top dead center of the compression stroke (CTDC). Further, the engine of this embodiment performs a premixed compression self-ignition referred to as Homogeneous-Charge Compression Ignition (HCCI) within a predetermined low engine load range.
As illustrated in
Further, the single cylinder is provided with two exhaust valves 2A and 2B at the other side (right side in
Moreover, a single fuel injector 3 is disposed in an extension line of the cylinder axis Z. Additionally, a first ignition plug 4A is disposed between the intake valves 1A and 1B, and a second ignition plug 4B is disposed between the exhaust valves 2A and 2B. Hereinafter, when describing the two first and second ignition plugs 4A and 4B without differentiating therebetween, each of the two first and second ignition plugs 4A and 4B may simply be referred to as “the ignition plug 4.”
As illustrated in
Moreover, the upper surface of the piston 10 is formed with four valve recesses 15A, 15B, 16A and 16B concaving downward by about 1 mm, for example. The valve recess 15A is formed at a position corresponding to the intake valve 1A, the valve recess 15B is formed at a position corresponding to the intake valve 1B, the valve recess 16A is formed at a position corresponding to the exhaust valve 2A, and the valve recess 16B is formed at a position corresponding to the exhaust valve 2B. Further, the upper surface of the piston 10, except for the cavity 11 and the valve recesses 15A, 15B, 16A and 16B, is substantially flat in directions perpendicular to the cylinder axis Z. In
In this embodiment, as indicated by the arrows All of
Here, in this embodiment, in view of suppressing pre-ignition, smoke, etc., a predetermined timing in the latter half of the compression stroke is applied as a target injection start timing and a predetermined timing after the CTDC is applied as a target ignition timing, according to an effective compression ratio, fuel pressure, etc. Further, a configuration is adopted so that after the fuel injection is started at the target injection start timing, the fuel can surely be ignited (start to combust) by the ignition plugs 4 at the target ignition timing.
Specifically, in this embodiment, a distance by which the mixture gas containing the fuel travels from the target fuel injection start timing to the target ignition timing (fuel spray traveling distance) is designed to be equal to or longer than a total length of paths indicated by the arrows All, A12 and A13 in
Next, the fuel spray traveling path length of this embodiment is described in detail with reference to
In
Further in
L11=Dc/cosα (3)
Moreover in
L12=Rc−Dc×sinα/cosα (4)
Furthermore in
L13=Rc−Rs (5)
Here, when the fuel spray traveling path length is “L1,” the fuel spray traveling path length L1 is expressed by using L11, L12, and L13 described above, as “L1=L11+L12+L13.” Therefore, by substituting the above Equations 3 to 5 into this equation, the fuel spray traveling path length L1 can be expressed by the following Equation 6.
L1=Dc(1−sinα)/cosα+2Rc−Rs (6)
On the other hand, when the fuel spray traveling distance is “L2,” the pressure of the fuel injected by the fuel injector 3 is “P,” the time length from the target fuel injection start timing to the target ignition timing described above is “t,” and a predetermined coefficient is “k,” the fuel spray traveling distance L2 can be expressed by the following Equation 7.
L2=k×P0.5×t2 (7)
Note that for the target fuel injection start timing, a timing in the latter half of the compression stroke, for example, a timing corresponding to “−9°,” is applied as a fuel injection start timing capable of suitably suppressing pre-ignition when the high compression ratio is applied. Further, for the target ignition timing, a timing immediately after the compression stroke (i.e., an early half of expansion stroke), for example, a timing corresponding to “3°,” is applied as an ignition timing that is close to an ignition timing with which a highest engine torque is obtained (Minimum advance for the Best Torque (MBT)), and capable of suitably suppressing smoke (knocking may be included). With these example timings, when the engine speed is 2,000 rpm, the time length t from the target fuel injection start timing to the target ignition timing becomes “t(sec)=){(3°+9°)/360°}/(2000/60).”
Moreover, as the fuel pressure P, a comparatively high fuel pressure may be applied so that the time length from the fuel injection start timing to the ignition timing can be shortened (i.e., the fuel injection start timing can be retarded and a response period of time from the retarded fuel injection start timing to the ignition can be shortened) so as to suppress abnormal combustion (e.g., pre-ignition). For example, a highest fuel pressure may be applied. In one example, “120 MPa” is applied as the fuel pressure P.
Further, the predetermined coefficient k is applied as a value obtained in advance based on experiment(s), predetermined equation(s), etc.
To summarize, in this embodiment, the cavity radius Rc, the distance Rs between the fuel injector 3 and each ignition plug 4, and the cavity depth Dc are designed based on the following Equation 8 applying the above Equations 6 and 7, so that the fuel spray traveling distance L2 becomes equal to or longer than the fuel spray traveling path length L1, in other words, the condition “L2>L1” is satisfied.
k×P
0.5
×t
2
≧Dc(1−sinα)/cosα+2Rc−Rs (8)
Next, a specific example of the cavity radius (and therefore cavity diameter) applied in this embodiment is described with reference to
In
Based on the graphs G1 to G3, it can be understood that the ignitable timing is retarded as the fuel spray traveling path length becomes longer. In other words, it can be understood that the fuel spray traveling path length needs to be shortened to advance the ignitable timing. Moreover, based on the graphs G1 to G3, it can be understood that the ignitable timing is advanced as the fuel pressure becomes higher.
Here, a case where an ignition timing within a range indicated by the reference character “R1” (e.g., approximately between 2° to 4°) is applied as the target ignition timing is considered. When the fuel pressure indicated by the graph G1 (e.g., 60 MPa) is used, to suitably start combusting the mixture gas by the ignition plugs 4 within the target ignition timing range R1, a fuel spray traveling path length D1 (e.g., about 37 mm) may be applied. In this case, a cavity diameter CD1 (e.g., about 50 mm) corresponding to the fuel spray traveling path length D1 may be applied. When the fuel pressure indicated by the graph G2 (e.g., 80 MPa) is used, to suitably start combusting the mixture gas by the ignition plugs 4 within the target ignition timing range R1, a fuel spray traveling path length D2 (e.g., about 40 mm) may be applied. In this case, a cavity diameter CD2 (e.g., about 54 mm) corresponding to the fuel spray traveling path length D2 may be applied. When the fuel pressure indicated by the graph G3 (e.g., 120 MPa) is used, to suitably start combusting the mixture gas by the ignition plugs 4 within the target ignition timing range R1, a fuel spray traveling path length D3 (e.g., about 42 mm) may be applied. In this case, a cavity diameter CD3 (e.g., about 58 mm) corresponding to the fuel spray traveling path length D3 may be applied.
Note that within the engine operating range where the engine speed is low and the engine load is high, a comparatively high fuel pressure is preferably applied so that the time length from the fuel injection start timing to the ignition timing can be shortened (i.e., the fuel injection start timing can be retarded and the response time period from the retarded fuel injection start timing to the ignition can be shortened), so as to suppress the abnormal combustion (e.g., pre-ignition). Therefore, in the example of
Next, the operations and effects of the combustion chamber structure for the engine according to this embodiment of the present invention are described. According to this embodiment, the cavity diameter, the cavity depth, and the positions of the ignition plugs 4 are designed so that the fuel spray traveling distance (the distance by which the mixture gas containing the fuel travels from the target fuel injection start timing to the target ignition timing) becomes equal to or longer than the fuel spray traveling path length (the length of the path through which the mixture gas containing the fuel injected by the fuel injector 3 reaches each ignition plug 4 via the cavity 11). Thus, the fuel injected at the target fuel injection start timing can surely be made to start combusting at the target ignition timing. As a result, the target fuel injection start timing and the target ignition timing can suitably be achieved while securing combustion stability.
It should be understood that the embodiments herein are illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof, are therefore intended to be embraced by the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-003373 | Jan 2015 | JP | national |
