U.S. patent application number 10/788324 was filed with the patent office on 2004-10-28 for direct fuel injection engine.
This patent application is currently assigned to Nissan Motor Co., Ltd.. Invention is credited to Hiraya, Koji, Hotta, Isamu, Kakuho, Akihiko, Takahashi, Eiji, Tsuchida, Hirofumi.
Application Number | 20040211388 10/788324 |
Document ID | / |
Family ID | 32964982 |
Filed Date | 2004-10-28 |
United States Patent
Application |
20040211388 |
Kind Code |
A1 |
Hiraya, Koji ; et
al. |
October 28, 2004 |
DIRECT FUEL INJECTION ENGINE
Abstract
A direct fuel injection engine basically comprises a combustion
chamber, a piston with a cavity, a fuel injection valve, a spark
plug and a control unit. The fuel injection valve is configured and
arranged to directly inject a fuel stream into the combustion
chamber in a substantially constant hollow circular cone shape in a
stratified combustion region. The control unit is configured to
ignite a first air-fuel mixture formed directly after the fuel
stream is injected and prior to a majority of the fuel stream
striking the cavity when the direct fuel injection engine is
operating in a low-load stratified combustion region, and to ignite
a second air-fuel mixture formed after a majority of the fuel
stream is guided by the cavity toward an upper portion of the
combustion chamber above the cavity when the direct fuel injection
engine is operating in a high-load stratified combustion
region.
Inventors: |
Hiraya, Koji; (Yokohama-shi,
JP) ; Takahashi, Eiji; (Yokosuka-shi, JP) ;
Tsuchida, Hirofumi; (Yokosuka-shi, JP) ; Hotta,
Isamu; (Yokohama-shi, JP) ; Kakuho, Akihiko;
(Yokohama-shi, JP) |
Correspondence
Address: |
SHINJYU GLOBAL IP COUNSELORS, LLP
1233 20TH STREET, NW, SUITE 700
WASHINGTON
DC
20036-2680
US
|
Assignee: |
Nissan Motor Co., Ltd.
Yokohama
JP
|
Family ID: |
32964982 |
Appl. No.: |
10/788324 |
Filed: |
March 1, 2004 |
Current U.S.
Class: |
123/276 ;
123/295; 123/298 |
Current CPC
Class: |
F02B 23/101 20130101;
Y02T 10/12 20130101; F02D 13/0215 20130101; Y02T 10/44 20130101;
Y02T 10/40 20130101; F02D 37/02 20130101; F02B 17/005 20130101;
F02B 2275/18 20130101; F02B 2075/125 20130101; Y02T 10/125
20130101; F02D 41/3029 20130101; F02D 41/401 20130101; F02D 41/3023
20130101; Y02T 10/123 20130101; Y02T 10/18 20130101; F02D 2041/001
20130101 |
Class at
Publication: |
123/276 ;
123/295; 123/298 |
International
Class: |
F02B 017/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 25, 2003 |
JP |
JP 2003-121610 |
May 30, 2003 |
JP |
JP 2003-154056 |
Claims
What is claimed is:
1. A direct fuel injection engine comprising: a combustion chamber;
a piston including a top surface having a cavity at a substantially
center portion of the top surface, the cavity being defined at
least by a peripheral wall surface and a bottom surface; a fuel
injection valve positioned at an upper portion of the combustion
chamber substantially on a center axis of the piston, the fuel
injection valve being configured and arranged to directly inject a
fuel stream inside the combustion chamber in a substantially
constant hollow circular cone shape during a compression stroke
when the direct fuel injection engine is operating in a stratified
combustion region; a spark plug configured and arranged to ignite
the fuel; and a control unit configured and arranged to control
operations of the fuel injection valve and the spark plug, the
control unit being further configured and arranged to ignite a
first air-fuel mixture formed directly after the fuel stream is
injected from the fuel injection valve and prior to a majority of
the fuel stream striking the cavity when the direct fuel injection
engine is operating in a low-load stratified combustion region, the
control unit being further configured and arranged to ignite a
second air-fuel mixture formed after a majority of the fuel stream
is guided toward an upper portion of the combustion chamber above
the cavity by the bottom surface of the cavity after the fuel
stream first hits the peripheral wall surface of the cavity when
the direct fuel injection engine is operating in a high-load
stratified combustion region.
2. The direct fuel injection engine as recited in claim 1, wherein
the control unit being further configured and arranged to ignite
the first-air fuel mixture prior to a tip of the fuel stream hits
the top surface of the piston.
3. The direct fuel injection engine as recited in claim 1, wherein
the fuel injection valve includes a plurality of nozzles to inject
a plurality of solid-core fuel streams that collectively form the
fuel stream having the substantially constant hollow circular cone
shape.
4. The direct fuel injection engine as recited in claim 1, wherein
the peripheral wall surface of the cavity is slanted radially
inwardly toward the center axis of the piston such that the cavity
forms substantially a partial cone shape.
5. The direct fuel injection engine as recited in claim 1, wherein
the bottom surface of the cavity is a substantially flat
surface.
6. The direct fuel injection engine as recited in claim 1, wherein
the control unit is further configured and arranged to set a start
timing of a fuel injection in the high-load stratified combustion
region more advanced than a start timing of a fuel injection in the
low-load stratified combustion region.
7. The direct fuel injection engine as recited in claim 1, wherein
the control unit is further configured and arranged to set a fuel
injection pressure in the high-load stratified combustion region
that is stronger than a fuel injection pressure in the low-load
stratified combustion region.
8. The direct fuel injection engine as recited in claim 1, wherein
the control unit is further configured and arranged to inject at
least one additional fuel stream during the compression stroke when
the direct fuel injection engine is operating in a relatively
high-load region within the high-load stratified combustion region,
the additional fuel stream being injected such that the additional
fuel stream first hits the bottom surface of the cavity.
9. The direct fuel injection engine as recited in claim 1, wherein
the control unit is further configured and arranged to inject the
fuel stream during an intake stroke when the direct fuel injection
engine is operating in a homogeneous combustion region in which a
load is higher than a load in the high-load stratified combustion
region.
10. The direct fuel injection engine as recited in claim 1 wherein
the control unit is configured and arranged to determine the direct
fuel injection engine is operating in the low-load stratified
combustion region when an engine load is lower than a first
prescribed engine load and an engine rotation speed is higher than
a first prescribed engine rotation speed, and the control unit is
configured and arranged to determine the direct fuel injection
engine is operating in the high-load stratified combustion region
when an engine load is higher than the first prescribed engine load
and an engine rotation speed is lower than the first prescribed
engine rotation speed.
11. The direct fuel injection engine as recited in claim 10,
wherein the control unit is configured and arranged to vary the
first prescribed engine load and the first prescribed engine
rotation speed such that as the first prescribed engine load is
increased the first prescribed engine rotation speed is
increased.
12. The direct fuel injection engine as recited in claim 10,
wherein the control unit is further configured and arranged to
determine the direct fuel injection engine is operating in the
high-load stratified combustion region when the engine load is
higher than a second prescribed engine load which is higher than
the first prescribed engine load regardless of the engine rotation
speed.
13. The direct fuel injection engine as recited in claim 10,
wherein the control unit is further configured and arranged to
determine the direct fuel injection engine is operating in the
low-load stratified combustion region when the engine load is lower
than a third prescribed engine load which is lower than the first
prescribed engine load regardless of the engine rotation speed.
14. The direct fuel injection engine as recited in claim 10,
wherein the control unit is further configured and arranged to
determine the direct fuel injection engine is operating in the
low-load stratified combustion region when the engine rotation
speed is higher than a second prescribed engine rotation speed
which is higher than the first prescribed engine rotation speed
regardless of the engine load.
15. The direct fuel injection engine as recited in claim 10,
wherein the control unit is further configured to change at least
one of fuel injection timing, fuel ignition timing, intake valve
closing timing and fuel injection pressure when the control unit
determines the direct fuel injection engine is transferring between
the low-load and high-load stratified combustion regions.
16. The direct fuel injection engine as recited in claim 15,
wherein the control unit is further configured and arranged to set
the fuel injection timing such that the fuel injection timing in
the low-load stratified combustion region is more retarded than the
fuel injection timing in the high-load stratified combustion
region.
17. The direct fuel injection engine as recited in claim 15,
wherein the control unit is further configured and arranged to set
the intake valve closing timing such that the intake valve closing
timing in the low-load stratified combustion region is more
retarded than the intake valve closing timing in the high-load
stratified combustion region.
18. The direct fuel injection engine as recited in claim 15,
wherein the control unit is further configured and arranged to set
the fuel injection pressure such that fuel injection pressure in
the low-load stratified combustion region is lower than the fuel
injection pressure in the high-load stratified combustion
region.
19. The direct fuel injection engine as recited in claims 15,
wherein the control unit is further configured and arranged to set
an interval between the fuel injection timing and the fuel ignition
timing in the low-load stratified combustion region shorter than an
interval between the fuel injection timing and the fuel ignition
timing in the high-load stratified combustion region.
20. The direct fuel injection engine as recited in claim 1, wherein
the bottom surface of the cavity is slanted such that a portion of
the bottom surface that is close to the spark plug has a depth that
is deeper than a depth of a portion of the bottom surface that is
further to the spark plug, and the peripheral wall surface of the
cavity is slanted radially inwardly toward the center axis of the
piston such that a portion of the peripheral wall surface that is
close to the spark plug is less slanted toward the center axis of
the piston than a portion of the peripheral surface that is further
to the spark plug.
21. The direct fuel injection engine as recited in claim 1, wherein
the fuel injection valve has a center axis that is slanted with
respect to the center axis of the piston, and the fuel injection
valve is configured and arranged to inject the substantially
constant hollow circular cone shape substantially symmetrical about
the center axis of the piston.
22. A direct fuel injection engine comprising: means for forming a
combustion chamber; fuel injection means for directly injecting a
fuel stream with a substantially constant hollow circular cone
shape during a compression stroke when the direct fuel injection
engine is operating in a stratified combustion region, fuel stream
guiding means for guiding the fuel stream injected from the fuel
injection means toward an upper portion of the combustion chamber;
ignition means for igniting first and second fuel mixture formed in
the combustion chamber; and control means for controlling
operations of the fuel injection means and the ignition means to
ignite the first air-fuel mixture formed directly after the fuel
stream is injected from the fuel injection means and prior to a
majority of the fuel stream striking the fuel stream guiding means
when the direct fuel injection engine is operating in a low-load
stratified combustion region, and to ignite the second air-fuel
mixture formed after a majority of the fuel stream is guided toward
the upper portion of the combustion chamber by the fuel stream
guiding means when the direct fuel injection engine is operating in
a high-load stratified combustion region.
23. A method of operating a direct fuel injection engine
comprising: injecting a fuel stream into a combustion chamber with
a substantially constant hollow circular cone shape during a
compression stroke when the direct fuel injection engine is
operating in a stratified combustion region; selectively guiding
the fuel stream toward an upper portion of the combustion chamber;
selectively igniting a first air-fuel mixture formed directly after
the fuel stream is injected into the combustion chamber and prior
to a majority of the fuel stream striking a piston when the direct
fuel injection engine is operating in a low-load stratified
combustion region; selectively igniting a second air-fuel mixture
formed after a majority of the fuel stream is guided toward the
upper portion of the combustion chamber when the direct fuel
injection engine is operating in a high-load stratified combustion
region.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention generally relates to a direct fuel
injection engine in which a fuel is directly injected in a
combustion chamber and ignited by a spark plug. More specifically,
the present invention relates to a direct fuel injection engine
that performs stratified combustion and homogeneous combustion by
directly injecting a fuel in the combustion chamber.
[0003] 2. Background Information
[0004] One example of a conventional direct fuel injection engine
is disclosed in Japanese Laid-Open Patent Publication No.
H11-82028. The direct fuel injection engine disclosed in this
reference has a concave cavity or a piston bowl formed on the
piston crown surface. In addition, this conventional direct fuel
combustion engine forms a suitable stratified air-fuel mixture in
the cylinder by arranging a fuel injection valve substantially
directly above the piston bowl. This arrangement allows the fuel
stream to collide against a peripheral side wall of the piston bowl
and form a fuel stream circulation flow towards the center portion
of the piston bowl to reduce fuel consumption.
[0005] In view of the above, it will be apparent to those skilled
in the art from this disclosure that there exists a need for an
improved direct fuel injection engine. This invention addresses
this need in the art as well as other needs, which will become
apparent to those skilled in the art from this disclosure.
SUMMARY OF THE INVENTION
[0006] In the conventional direct fuel injection engine described
above, a volume of the stratified air-fuel mixture that is formed
after colliding with the piston is determined substantially by the
shape of the piston cavity or the capacity of the cavity. In other
words, in the above mentioned conventional direct fuel injection
engine, the volume of the stratified air-fuel mixture formed via
the cavity is always constant regardless of the engine load since
the capacity of the cavity is constant. Therefore, a range of
engine load conditions that allows excellent stratified combustion
operation is limited with the conventional direct fuel injection
engine. More specifically, if the cavity size and other control
parameter is determined to obtain stable combustion and to realize
good fuel efficiency and small exhaust gas emissions during one
stratified operating region with a certain engine load and a
certain engine rotation speed, then good fuel efficiency may not be
realized or the so-called recoil will occur in another stratified
operating region with a different engine rotation speed and a
different engine load.
[0007] For example, when the engine rotation speed is fast, the
advance of the crank angle becomes faster compared to when the
engine rotation speed is slow, and thus, the time allowed to form
an air-fuel mixture becomes shorter. Therefore, if an identical
fuel injection timing and an identical fuel injection duration
(this is basically proportional to the engine load) are used for
both when the rotation speed is fast and slow, then the ignition
timing occurs before the combustible air-fuel mixture reaches the
vicinity of the spark plug when the engine rotation speed is fast.
In order to avoid this problem, it is possible to set the fuel
injection timing to be more advanced as the engine rotation speed
becomes. However, in such case, the fuel stream injected during an
earlier part of the injection might not be received in the cavity
of the piston. In particular, considering executing a homogeneous
combustion in the full load region, in which the fuel is injected
during an intake stroke, the fuel stream must be injected with at
least equal to or more than a certain injection opening angle.
Thus, the fuel injection timing in the stratified combustion state
cannot be arranged to be too early.
[0008] Moreover, when the cavity is made with a larger opening in
order to make it easier to accept the fuel stream, the depth of the
cavity is restricted to be less than a certain depth in view of the
compression ratio. Accordingly, it becomes difficult to receive the
fuel stream due to an insufficient depth.
[0009] Thus, since an air-fuel mixture is always formed via the
cavity and ignited during stratified combustion in a conventional
direct fuel injection engine, it is difficult to obtain stable
combustion and good fuel efficiency as well as small exhaust
emission under various conditions in which the engine rotation
speed fluctuates between fast and slow. Moreover, the engine load
also changes from low-load to high-load or vice versa during the
stratified combustion. When the engine load is low during the
stratified combustion, the stratified air-fuel mixture in the
vicinity of the spark plug tends to be lean in the above mentioned
conventional direct fuel injection engine because the air-fuel
mixture is formed after the fuel stream collides against the
cavity. Thus, the combustion stability is worsened thereby causing
the fuel efficiency to deteriorate. On the other hand, when the
engine load is high during the stratified combustion, the air-fuel
mixture in the vicinity of the spark plug tends to become
excessively dense with the above mentioned conventional direct fuel
injection engine. Thus, smoke and HC is increased.
[0010] Accordingly, one object of the present invention is to
expand a range of engine operation conditions that allows excellent
stratified combustion operation at a low cost by executing two
different stratified combustion operations depending on the engine
load and/or the engine rotation speed. In order to achieve the
above and other objects, a direct fuel injection engine of the
present invention basically comprises a combustion chamber, a
piston, a fuel injection valve, a spark plug and a control unit.
The piston includes a top surface having a cavity at a
substantially center portion of the top surface. The cavity is
defined at least by a peripheral wall surface and a bottom surface.
The fuel injection valve is positioned at an upper portion of the
combustion chamber substantially on a center axis of the piston.
The fuel injection valve is configured and arranged to directly
inject a fuel stream inside the combustion chamber in a
substantially constant hollow circular cone shape during a
compression stroke when the direct fuel injection engine is
operating in a stratified combustion region. The spark plug is
configured and arranged to ignite the fuel. The control unit is
configured and arranged to control operations of the fuel injection
valve and the spark plug. The control unit is further configured
and arranged to ignite a first air-fuel mixture formed directly
after the fuel stream is injected from the fuel injection valve and
prior to a majority of the fuel stream striking the cavity when the
direct fuel injection engine is operating in a low-load stratified
combustion region. The control unit is further configured and
arranged to ignite a second air-fuel mixture formed after a
majority of the fuel stream is guided toward an upper portion of
the combustion chamber above the cavity by the bottom surface of
the cavity after the fuel stream first hits the peripheral wall
surface of the cavity when the direct fuel injection engine is
operating in a high-load stratified combustion region.
[0011] These and other objects, features, aspects and advantages of
the present invention will become apparent to those skilled in the
art from the following detailed description, which, taken in
conjunction with the annexed drawings, discloses preferred
embodiments of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Referring now to the attached drawings which form a part of
this original disclosure:
[0013] FIG. 1 is a partial cross sectional view of an injection
portion of a combustion chamber of a direct fuel injection engine
in accordance with a first embodiment of the present invention;
[0014] FIG. 2 is a diagrammatic chart illustrating the relationship
between engine operation regions and an engine load and an engine
rotation speed in accordance with the first embodiment of the
present invention;
[0015] FIG. 3(a) is a diagrammatic cross sectional view of the
combustion chamber shown in FIG. 1 illustrating distribution of the
air-fuel mixture in the combustion chamber in a low-load stratified
combustion region in accordance with the first embodiment of the
present invention;
[0016] FIG. 3(b) is an enlarged, partial diagrammatic side view of
a spark plug and a fuel stream illustrating distribution of the
fuel stream in a time-series manner in the low-load stratified
combustion region in accordance with the first embodiment of the
present invention;
[0017] FIG. 4 is a diagrammatic cross sectional view of the
combustion chamber shown in FIG. 1 illustrating distribution of the
air-fuel mixture in the combustion chamber in a high-load
stratified combustion region in accordance with the first
embodiment of the present invention;
[0018] FIG. 5 is a diagrammatic chart illustrating the relationship
between engine operation regions and an engine load and an engine
rotation speed in accordance with a second embodiment of the
present invention;
[0019] FIG. 6 is a diagrammatic chart illustrating the relationship
between engine operation regions and an engine load and an engine
rotation speed in accordance with the second embodiment of the
present invention;
[0020] FIG. 7(a) is a diagrammatic cross sectional view of the
combustion chamber shown in FIG. 1 illustrating distribution of the
air-fuel mixture in the combustion chamber in a high-load
stratified combustion region when an engine load is relatively high
in accordance with the second embodiment of the present
invention;
[0021] FIG. 7(b) is a diagrammatic cross sectional view of the
combustion chamber shown in FIG. 1 illustrating distribution of the
air-fuel mixture in the combustion chamber in a high-load
stratified combustion region when an engine load is relatively low
in accordance with the second embodiment of the present
invention;
[0022] FIG. 8 is a diagrammatic chart illustrating a change in
control parameters including fuel pressure, intake valve closing
timing, fuel injection timing, and fuel ignition timing between the
high-load and low-load stratified combustion regions with respect
to the engine load in accordance with the second embodiment of the
present invention;
[0023] FIG. 9 is a diagrammatic chart illustrating a change in
control parameters including fuel pressure, intake valve closing
timing, fuel injection timing, and fuel ignition timing between the
high-load and low-load stratified combustion regions with respect
to the engine rotation speed in accordance with the second
embodiment of the present invention;
[0024] FIG. 10 is a diagrammatic cross sectional view of a
combustion chamber illustrating distribution of the air-fuel
mixture in the combustion chamber in accordance with a third
embodiment of the present invention;
[0025] FIG. 11 is a diagrammatic chart illustrating the
relationship between engine operation regions and an engine load
and an engine rotation speed in accordance with the third
embodiment of the present invention;
[0026] FIG. 12 is a partial cross sectional view of an injection
portion of a combustion chamber of a direct fuel injection engine
in accordance with a fourth embodiment of the present invention;
and
[0027] FIG. 13 is a partial cross sectional view of an injection
portion of a combustion chamber of a direct fuel injection engine
in accordance with a fifth embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] Selected embodiments of the present invention will now be
explained with reference to the drawings. It will be apparent to
those skilled in the art from this disclosure that the following
descriptions of the embodiments of the present invention are
provided for illustration only and not for the purpose of limiting
the invention as defined by the appended claims and their
equivalents.
[0029] Referring initially to FIGS. 1-4, a direct fuel injection
engine is illustrated in accordance with a first embodiment of the
present invention. FIG. 1 is a partial cross sectional view of an
injection portion of a combustion chamber 1 of a direct fuel
injection engine of the first embodiment. The combustion chamber 1
is basically formed by a cylinder head 2a, a cylinder block 2b and
a piston 3. A gasket 14 is placed between the cylinder head 2a and
the cylinder block 3. A substantially cylindrical cavity 4 is
provided at the center of a crown surface or a top surface of the
piston 3. The cavity 4 is formed with a peripheral wall surface or
an inner peripheral surface 4a and a flat bottom surface 4c that
are smoothly joined by a curved surface 4b. As seen in FIG. 1, the
inner peripheral surface 4a is preferably inclined or slanted
towards a center axis of the piston 3 to form a reentrant shape of
the cavity 4. The cavity flat bottom surface 4c is preferably a
smooth surface without any unevenness and disposed substantially
perpendicular to the center axis of the piston 3. Therefore, the
cavity 4 forms a substantially cone shape with a portion including
an apex of the cone being cut off. An intake port 5 is arranged to
send air required for combustion to the combustion chamber 1
through an intake valve 7 that is operatively controlled by an
intake valve cam 9. The intake valve 7 is preferably coupled to a
variable valve timing mechanism that allows at least the intake
valve closing timing to be varied (i.e., delayed and advanced). For
example, a variable valve timing mechanism that changes the
relative phase between the camshaft and the crankshaft can be
coupled to the intake valve 7. Such variable valve timing mechanism
is well known in the art, and thus, not discussed in detail herein.
An exhaust port 6 discharges exhaust gases combusted in the
combustion chamber 1 through an exhaust valve 8 that is operatively
controlled by an exhaust valve cam 10.
[0030] A spark plug 12 is positioned substantially adjacent to the
fuel injection valve 11 so that a spark gap 12a of the spark plug
12 is positioned in the vicinity of the center of the combustion
chamber 1. The spark plug 12 is configured and arranged to ignite
the fuel stream injected by the fuel injection valve 11 to cause
combustion.
[0031] A fuel injection valve 11 is provided on the cylinder head
2a and preferably positioned substantially on the center axis of
the piston 3, which is substantially coincident with a center axis
of a cylinder, at the upper portion of the combustion chamber 1.
The fuel injection valve 11 is preferably configured and arranged
to have a plurality of through-holes or nozzles with identical
shapes through which the fuel is injected into the combustion
chamber 1. More specifically, a plurality of solid-core fuel
streams is injected from these nozzles towards the piston 3. Thus,
the plurality of solid-core fuel streams injected from the fuel
injection valve 11 collectively forms a fuel stream with a
substantially constant hollow cone shape. The "substantially
constant hollow cone shape" as used herein basically refers to that
an injection opening angle or an apex angle (umbrella angle) of the
cone is substantially constant. In other words, the fuel injection
valve 11 does not have a special variable mechanism for changing
the injection opening angle. Rather, the present invention utilizes
the fuel injection valve 11 whose injection opening angle is not
greatly affected by the factors such as an amount of fuel injected
or fuel injection timing (i.e., pressure inside the cylinder during
fuel injection). Therefore, the fuel stream can be reliably
injected toward a desired direction. More specifically, in the
present invention, the fuel injection valve 11 is configured and
arranged to inject the fuel stream toward the discharge gap 12a of
the spark plug 12 or in the vicinity of the electrodes of the spark
plug 12 when the engine is operating in the low-load stratified
combustion state or region, and toward the cavity inner peripheral
surface 4a when the engine is operating in the high-load stratified
state or region.
[0032] An engine control unit 13 is operatively coupled to the
spark plug 12 and the fuel injection valve 11 and configured and
arranged to control various operations of the direct fuel injection
engine, such as the fuel injection timing and duration of the fuel
injection valve 11 and the fuel injection timing of the spark plug
12, based on the engine operational conditions. More specifically,
the control unit 13 preferably includes a microcomputer with a
control program that controls the direct fuel injection engine as
discussed below. The control unit 13 can also include other
conventional components such as an input interface circuit, an
output interface circuit, and storage devices such as a ROM (Read
Only Memory) device and a RAM (Random Access Memory) device. The
microcomputer of the control unit 13 is programmed to control the
direct fuel control engine. The memory circuit stores processing
results and control programs that are run by the processor circuit.
The control unit 13 is operatively coupled to the various
components of the direct fuel injection engine including the fuel
injection valve 11 and the spark plug 12 in a conventional manner.
The internal RAM of the control unit 13 stores statuses of
operational flags and various control data. The control unit 13 is
capable of selectively controlling any of the components of the
control system in accordance with the control program. It will be
apparent to those skilled in the art from this disclosure that the
precise structure and algorithms for control unit 13 can be any
combination of hardware and software that will carry out the
functions of the present invention. In other words, "means plus
function" clauses as utilized in the specification and claims
should include any structure or hardware and/or algorithm or
software that can be utilized to carry out the function of the
"means plus function" clause.
[0033] The direct fuel injection engine of the present invention is
configured and arranged to perform combustion of the air-fuel
mixture in a homogeneous combustion operating region or a
stratified combustion operating region depending on an operating
condition of the direct fuel injection engine. In the homogeneous
combustion operating region, the fuel is injected during an intake
stroke (preferably in the first half of the intake stroke) to form
a homogeneous fuel air mixture throughout the combustion chamber 1
to perform combustion in a stoichiometric air-fuel ratio operation.
In the stratified combustion operating region, a fuel is injected
during a compression stroke (preferably in the second half of the
compression stroke) to form a stratified fuel-air mixture inside
and/or above the cavity 4 to achieve a lean operation to improve
fuel economy. Moreover, in the first embodiment of the present
invention, two different operations are performed in the stratified
combustion operating region depending on the engine load. In a
low-load stratified combustion region, the fuel injected from the
fuel injection valve 11 is ignited directly after the fuel is
injected before a majority of the fuel stream collides against the
cavity 4. In a high-load stratified combustion region, the fuel
stream is ignited after the fuel stream collides against the inner
peripheral surface 4a of the cavity 4 and rises upwardly in the
center portion of the cavity 4 as guided by the curved surface 4b
and the cavity bottom surface 4c. Moreover, by utilizing the fuel
injection valve 11 that injects a fuel stream with a substantially
constant hollow cone shape, a comparatively small air-fuel mixture
mass in a low-load stratified combustion region and a comparatively
large air-fuel mixture mass in a high-load stratified combustion
region are obtained at a low cost. Accordingly, with the
arrangement of the direct fuel injection engine of the present
invention, the stratified combustion operating region can be
expanded at a low cost.
[0034] FIG. 2 is a diagrammatic chart illustrating the relationship
between the homogeneous combustion operating region, the high-load
stratified combustion region and the low-load stratified combustion
region with respect to an engine load and an engine rotation speed
in accordance with the first embodiment. Among all of the operating
regions, the stratified combustion operating region (including the
high-load and low-load stratified combustion regions) is set to a
comparatively low load and slow rotational speed region. In the
stratified combustion operating region, a stratified combustion is
performed in which an air-fuel mixture is formed within a portion
above the cavity 4 and/or inside the cavity 4. As seen in FIG. 2,
the stratified combustion operating region is divided into the
high-load stratified combustion region where the engine load is
relatively high and the low-load stratified combustion region where
the engine load is relatively low.
[0035] When the direct fuel injection engine is operating in the
low-load stratified combustion region, the control unit 13 is
configured and arranged to operate the spark plug 12 to ignite an
air-fuel mixture that is formed directly after the fuel stream is
injected from the fuel injection valve 11 and before the majority
of the fuel stream collides against the cavity 4. When the direct
fuel injection engine is operating in the high-load stratified
combustion region, the control unit 13 is configured and arranged
to operate the spark plug 12 to ignite an air-fuel mixture formed
after a majority of the fuel stream is guided toward an upper
portion of the combustion chamber 1 above the cavity 4 by the
bottom surface 4c of the cavity 4 after the fuel stream first
collides against the inner peripheral wall surface 4a of the cavity
4. In other words, an interval between when the fuel is injected
and when the fuel is ignited is set relatively shorter in the
low-load stratified combustion region and relatively longer in the
high-load stratified combustion region.
[0036] As seen in FIG. 2, the homogeneous combustion operating
region is set such that the engine load is higher and the engine
rotation speed is faster in the homogeneous combustion operating
region than in the stratified combustion operating region. In the
homogeneous combustion operating region, a fuel is injected from
the fuel injection valve 11 during an intake stroke. Air is
introduced from the intake port 5 to the combustion chamber 1 to
form a homogeneous air-fuel mixture throughout the entire
combustion chamber 1 so that homogeneous combustion is
performed.
[0037] Referring now to FIGS. 3(a) and 3(b), distribution of the
air-fuel mixture (first air-fuel mixture) in the combustion chamber
1 in the low-load stratified combustion region is described. In the
low-load stratified combustion region, the fuel injection pressure
is set to a comparatively low pressure. Thus, the penetration force
of the fuel stream is reduced and a size of the air-fuel mixture
mass is also reduced. Moreover, in the low-load stratified
combustion region, the fuel injection timing is set to inject the
fuel stream during the second half of the compression stroke close
to the compression top dead center. Thus, a relatively small
air-fuel mixture is ignited when the crank angle is close to the
compression top dead center. Consequently, the fuel consumption
efficiency is improved.
[0038] Since the amount of fuel injected in the low-load stratified
combustion region is set to a small amount, the fuel injection is
preferably completed before the tip of the hollow cone shape fuel
stream reaches the piston 3. Thus, the air-fuel mixture is ignited
by the spark plug 12 during the fuel injection or directly after
the fuel injection is completed before the tip of the fuel stream
reaches the cavity 4. More specifically, the fuel ignition timing
is preferably set such that the air-fuel mixture is ignited when
the fuel stream is still floating in the air before the tip of the
hollow cone shape fuel stream reaches the piston 3. Accordingly, as
seen in FIG. 3(a), a size of the air-fuel mixture mass at the time
of ignition is relatively small in the low-load stratified
combustion region.
[0039] The injection opening angle .theta. of the fuel injection
valve 11 is preferably set to a relatively wide angle, for example,
from approximately 60.degree. to approximately 80.degree.. Thus,
the fuel stream injected from the fuel injection valve 11
preferably passes through the discharge gap 12a of the spark plug
12 or a through an area that is close to the electrodes of the
spark plug 12. This arrangement of the relatively wide injection
opening angle .theta. of the fuel injection valve 11 is
advantageous because the fuel stream can be ignited by the spark
plug 12 directly after the fuel injection ends as explained above.
If the injection opening angle .theta. of the fuel injection valve
11 is relatively narrow, the discharge gap 12a of the spark plug 12
must protrude deeper into the combustion chamber 1 in order to
ignite the fuel stream directly after injected, which makes it
difficult to ensure the durability of the spark plug 12.
[0040] As seen in FIG. 3(b), the injected fuel stream is mixed with
the surrounding air from the tip or periphery of the fuel stream.
By setting the fuel injection timing close to the compression top
dead center, the temperature inside the cylinder during fuel
injection is high. Thus, the fuel is quickly vaporized and mixed
with the air after injected. Accordingly, in the low-load
stratified combustion region, the air-fuel mixture formed in the
periphery of the fuel stream is ignited during the fuel injection
or directly after the fuel injection is completed as the fuel
stream is quickly vaporized and mixed with the air. Consequently,
stable combustion is performed in the low-load stratified
combustion region even when the engine rotation speed is fast and
sufficient time cannot be provided to form an air-fuel mixture
after the fuel stream collides against the cavity 4. Moreover, when
the engine load is low and the amount of fuel injected is small,
the density of the air-fuel mixture formed by colliding the fuel
stream against the piston 3 and defusing the fuel stream inside the
cavity 4 generally tends to be excessively thin. Thus, since the
fuel is directed ignited after injected with the direct fuel
injection engine of the present invention, stable combustion is
performed in the low-load stratified combustion region even when
the engine load is low and the amount of fuel injected is
relatively small. In addition, because the fuel is ignited before
the tip of the fuel stream makes contact with the crown surface of
the piston 3, the fuel is combusted without adhering to the piston
3. Thus, the amount of unburned HC produced is reduced. Moreover,
the air-fuel mixture is combusted in a state in which the thermal
insulation layer (air layer) is positioned between the air-fuel
mixture and the piston 3, and thus, cooling loss is reduced.
[0041] Next, referring to FIG. 4, distribution of the air-fuel
mixture (second air-fuel mixture) in the combustion chamber 1 in
the high-load stratified combustion region is described. In the
high-load stratified combustion region, the fuel injection pressure
is set to a comparatively high pressure. Thus, the penetration
force of the fuel stream is increased thereby making it possible to
produce a strong circulation flow of the fuel stream. Moreover, it
is further preferable to set the fuel injection pressure higher as
the engine load grows larger while the engine is operating in the
high-load stratified combustion region. Furthermore, the fuel
injection timing in the high-load stratified combustion region is
set to the second half of the compression stroke that is more
advanced than the fuel injection timing in the low-load stratified
combustion region. Consequently, a sufficient time is ensured to
mix the fuel and air by utilizing a circulation flow.
[0042] Since the fuel injection valve 11 is configured to inject a
fuel stream to form a substantially constant hollow cone shape, the
shape of the fuel stream in the high-load stratified combustion
region is substantially identical to the shape of the fuel stream
in the low-load stratified combustion region (i.e., the injection
opening angle .theta. is from approximately 60.degree. to
approximately 80.degree.). Thus, the fuel stream injected from the
fuel injection valve 11 reaches the piston 3 and collides against
the inner peripheral surface 4a of the cavity 4 as seen in diagram
(A) of FIG. 4. Since the cavity inner peripheral surface 4a is
preferably inclined inwardly such that the cavity 4 forms an
substantially cone shape, the collision angle between the cavity
inner peripheral surface 4a and the fuel stream is relatively small
(e.g., an acute angle). Thus, majority of the fuel stream is guided
downwardly toward the cavity 4 and remained within or above the
cavity 4. Of course, it will be apparent to those skilled in the
art from this disclosure to modify the cavity inner peripheral
surface 4a to extend substantially parallel to the center axis of
the piston 3 or slightly inclined radially outwardly in order to
make the fabrication of the cavity 4 easier, although the amount of
fuel overflowing from the cavity 4 will increase slightly in such
cases.
[0043] After the collision, the fuel stream is guided downwardly in
the cavity 4 along the cavity inner peripheral surface 4a. The
travel direction of the fuel stream is then curved inwardly by the
cavity curved surface 4b. Then, the fuel travels towards radial
inner direction of the cavity 4 along the cavity bottom surface 4c.
Since the cavity bottom surface 4c is preferably formed without any
unevenness on its surface, the fuel stream traveling from the
radial peripheral direction to the center of the cavity bottom
surface 4c collide against each other in the vicinity of the center
of the cavity bottom surface 4c. Thus, a flow that rises upwardly
is effectively formed in the cavity 4 as seen in diagram (B) of
FIG. 4. This flow of the fuel stream results in the fuel stream
encompassing the circumference air as the fuel streams travels.
Thus, a circulation flow of air-fuel mixture within the space
between the cylinder head 2a and the piston 3 is formed as seen in
diagram (C) of FIG. 4. This circulation flow promotes mixing of
fuel and air and creates a substantially homogeneous air-fuel
mixture mass within the cavity 4 and thereabove. In other words,
when the fuel streams collide in the vicinity of the center of the
bottom surface 4c, a moderate disturbance occurs resulting in
favorable mixing of the fuel and air and form a substantially
homogeneous air-fuel mixture within the cavity 4 and thereabove.
Then, the spark plug 12 is configured and arranged to ignite this
substantially homogeneous air-fuel mixture mass. Accordingly, in
the high-load stratified combustion region, the size of the
air-fuel mixture mass at the time of fuel ignition is relatively
large.
[0044] The arrangement of the relatively wide injection opening
angle .theta. of the fuel injection valve 11 is advantageous in the
high-load stratified combustion region as well as in the low-load
stratified combustion region. If a fuel injection valve 11 with a
narrow injection opening angle is used, the fuel stream collides
with the cavity bottom surface 4c resulting in a circulation flow
that rotates in a direction opposite to the circulation flow
described above. In such case, an air-fuel mixture mass created
within the cavity 4 and thereabove tends to have a less dense
air-fuel ratio at the center of this air-fuel mixture mass that is
close to the spark plug 12. On the other hand, since the injection
opening angle .theta. of the fuel injection valve 11 is relatively
wide in the present invention, the air-fuel mixture mass can be
obtained in which the air-fuel ratio is substantially uniform
throughout the mass or the air-fuel ratio is slightly more dense at
the center of the mass and becomes less dense as moving towards the
periphery of the mass. Accordingly, with the direct fuel injection
engine of the present invention, an effective ignition and stable
combustion are obtained. Consequently, EGR can be introduced in
large quantities making it possible to operate the engine with a
small amount of NOx occurring. In the high-load stratified
combustion region, the fuel may adhere to the cavity 4 of the
piston 3 when the fuel stream collides against the cavity 4.
However, the circulation flow of air-fuel mixture within the cavity
4 promotes the vaporization of the adhering fuel. Thus, any sudden
increase in the amount of unburned HC produced is prevented in the
high-load stratified combustion region.
[0045] Next, the fuel injection and the distribution of the
air-fuel mixture during the homogeneous combustion region will be
described. During the homogeneous combustion region, the fuel is
injected from the fuel injection valve 11 in the second half of the
intake stroke. As in the stratified combustion region, the fuel
injection valve 11 is configured and arranged to inject a fuel
stream with the relatively wide injection opening angle .theta..
Thus, the fuel stream is diffused throughout the combustion chamber
1 including areas outside of the cavity 4 and thoroughly mixed with
the air to create an air-fuel mixture having a substantially
stoichiometric air-fuel ratio in the combustion chamber 1.
Accordingly, the combustion with good fuel efficiency and a small
amount of exhaust gas emissions is achieved.
[0046] Accordingly, by changing the operations in the stratified
combustion region depending on the engine load and by utilizing the
fuel injection valve 11 that injects a fuel stream with a
substantially constant hollow cone shape, the direct fuel injection
engine of the present invention enables to obtain an excellent
combustion in a wide range of engine operating conditions.
Second Embodiment
[0047] Referring now to FIGS. 5-8, a direct fuel injection engine
in accordance with a second embodiment will now be explained. In
view of the similarity between the first and second embodiments,
the parts of the second embodiment that are identical to the parts
of the first embodiment will be given the same reference numerals
as the parts of the first embodiment. Moreover, the descriptions of
the parts of the second embodiment that are identical to the parts
of the first embodiment may be omitted for the sake of brevity.
[0048] Basically, the direct fuel injection engine of the second
embodiment is identical to the direct fuel injection engine of the
first embodiment, except that operation of the direct fuel
injection engine is in either the low-load or high-load stratified
combustion regions is determined based on the engine rotation speed
as well as the engine load. Moreover, in the second embodiment of
the present invention, the operations of the intake valve opening
timing and the fuel injection timing as well as the operations of
the fuel injection pressure and the fuel injection timing are
changed depending on whether the direct fuel injection engine is
operating in the low-load or high-load stratified combustion
regions.
[0049] When the engine rotation speed increases in the high-load
stratified combustion region, there is a concern that there is no
sufficient time to form a homogeneous air-fuel mixture. Moreover,
there is also a danger of the flow inside the cylinder becoming so
strong that the air-fuel mixture is excessively diffused and the
air-fuel ratio in the vicinity of the spark plug 12 becomes
excessively thin. On the other hand, if an excessive amount of fuel
is injected during the low-load stratified combustion and the fuel
is ignited while the fuel is being vaporized and mixed with the
air, then an excessively dense air-fuel mixture may exist during
flame propagation. In such case, the fuel combustion efficiency is
reduced and exhaust gas emissions is increased.
[0050] Thus, in the second embodiment of the present invention, the
operation in the low-load stratified combustion is executed when
the engine rotation speed is relatively fast so that an air-fuel
mixture with an air-fuel ratio suitable for ignition is always
formed near the spark plug 12 regardless of the engine rotation
speed. Generally, the gas flow inside the cylinder becomes larger
as the engine rotation speed becomes faster. When the engine
rotation speed is fast, the fuel stream is diffused and mixed with
the air relatively fast due to the relatively large gas flow inside
the cylinder. Thus, mixing of fuel is promoted in the flame
propagation process in the low-load stratified combustion region
after the fuel stream is injected due to the flow inside the
cylinder when the engine rotation speed is relatively fast. Thus,
the formation of an excessively dense air-fuel mixture is prevented
in the low-load stratified combustion region. When, however, the
engine rotation speed is relatively slow and the engine load is in
relatively high, it is preferable to provide sufficient time for
the fuel stream to be vaporized and mixed with the air.
[0051] Accordingly, in the second embodiment of the present
invention, the control unit 13 is configured to consider an engine
rotation speed as well as an engine load in determining the
operational regions of the direct fuel injection engine as seen in
FIG. 5. More specifically, the control unit 13 is configured and
arranged to determine the direct fuel injection engine is operating
in the high-load stratified combustion region when the engine
rotation speed is slower than a first prescribed engine rotation
speed and the engine load is higher than a first prescribed engine
load. The control unit 13 is configured to determine the direct
fuel injection engine is operating in the low-load stratified
combustion region when the engine rotation speed is faster than the
first prescribed engine rotation speed and the engine load is lower
than the first prescribed engine load. As seen in FIG. 5, the first
prescribed engine load and the first prescribed engine rotation
speed are preferably set such that the higher the first prescribed
engine load becomes, the faster the first prescribed engine
rotation speed becomes. In the second embodiment of the present
invention, the homogeneous combustion is also performed in which
the fuel is injected during a intake stroke in the homogeneous
combustion region in which the engine load is equal to or more than
a predetermined engine load and the engine rotation speed is equal
to or more than a predetermined engine rotation speed as shown in
FIG. 5.
[0052] Also, as seen in FIG. 6, under certain engine operation
conditions, the control unit 13 is configured to determine the
direct fuel injection engine is operating in the high-load or
low-load stratified combustion engine regardless of the engine
rotation speed. Specifically, when the engine load increases more
than a certain load (a second prescribed engine load), sufficient
mixing of the fuel and air becomes difficult even though the flow
is intensified by increasing the engine rotation speed if the fuel
is ignited during fuel injection or directly after completion of
the fuel injection but before the fuel stream collides against the
piston 3 (i.e., the operation in the low-load stratified combustion
region). In other words, if the control unit 13 determines the
direct fuel injection engine is operating in the low-load
stratified combustion engine when the engine load is equal to or
higher than the second prescribed engine load, a risk increases
that an excessively dense air-fuel mixture will be partially
formed. Therefore, as shown in FIG. 6, the control unit 13 is
configured to determine the direct fuel injection engine is
operating in the high-load stratified operating region when the
engine load is equal to or more than the second prescribed engine
load regardless of the engine rotation speed. Thus, when the engine
load is equal to or more than the second prescribed engine load,
the fuel stream is ignited after colliding against the cavity 4 and
forming an air-fuel mixture inside and above the cavity 4.
[0053] Moreover, when the engine load is equal to or less than a
certain load (a third prescribed engine load), the amount of the
fuel injected from the fuel injection valve 11 becomes small.
Therefore, there is a risk of the density of the air-fuel mixture
becoming excessively thin if the fuel is ignited after colliding
against the cavity 4 and forming the air-fuel mixture as the
operation in the high-load stratified combustion region.
Accordingly, the control unit 13 is configured to determine the
direct fuel injection engine is operating in the low-load
stratified combustion region when the engine load is equal to or
lower than the third prescribed engine load regardless of the
engine rotation speed, as seen in FIG. 6. Thus, when the engine
load is equal to or lower than the third prescribed engine load,
the fuel stream is ignited during fuel injection or directly after
completion of the fuel injection but before the fuel stream
collides against the piston 3.
[0054] Furthermore, the control unit 13 is configured to determine
the direct fuel injection engine is operating in the low-load
stratified combustion region regardless of the engine load under
certain engine operation conditions. When the engine rotation speed
becomes faster than a certain rotation speed (second engine
rotation speed), there is a risk that sufficient time is not
provided to form an air-fuel mixture by hitting the fuel stream
against the cavity 4. Moreover, in such case, if the fuel injection
timing is set to reliability receive the fuel stream in the cavity
4, there is a risk that the air-fuel mixture has not reached near
the spark plug 12 when the fuel ignition timing occurs. Therefore,
the control unit 13 is configured to determine the direct fuel
injection engine is operating in the low-load stratified combustion
region when the engine rotation speed is faster than the second
prescribed engine rotation speed regardless of the engine load, as
shown in FIG. 6. Thus, when the engine rotation speed is faster
than the second prescribed engine rotation speed, the fuel stream
is ignited during fuel injection or directly after the fuel
injection completes before the fuel stream collides against the
piston 3.
[0055] Moreover, the control unit 13 is configured and arranged to
change operation parameters including the intake valve closing
timing, the fuel injection pressure, the fuel injection timing and
the fuel ignition timing depending on whether the direct fuel
injection engine is operating in the high-load or low-load
stratified combustion regions. As seen in FIG. 7(a), when the
direct fuel injection engine is operating in the high-load
stratified combustion region and when the engine load is relatively
high within the high-load stratified region, a relatively dense
homogeneous air-fuel mixture is formed in the space above the
cavity 4. However, as the engine load becomes smaller in the
high-load stratified combustion region, the density of the air-fuel
mixture formed in the space above the cavity 4 becomes thinner as
shown in FIG. 7(b). Thus, the second embodiment of the present
invention is configured to control the fuel injection timing and
the fuel ignition timing of the direct fuel injection engine to
keep the density of the air-fuel mixture formed inside the
combustion chamber within an air-fuel ratio which is combustible
and which does not worsen the exhaust gas emissions. In other
words, under the conditions where the engine load is so high that
the density of the homogeneous air-fuel mixture formed in the space
above the cavity 4 becomes very dense which results in worsening
the exhaust emission, the fuel injection timing and fuel ignition
timing is controlled so that a part of the air-fuel mixture is
drawn to the outside of the cavity 4. Under the conditions where
the density of the homogeneous air-fuel mixture formed in the space
above the cavity 4 becomes so lean that there is a danger that an
accidental combustion may occur, the fuel injection timing and
ignition timing is controlled such that an air-fuel mixture with a
mild air-fuel ratio distribution is ignited before the air-fuel
mixture spreads throughout the entire space above the cavity 4.
[0056] More specifically, FIG. 8 illustrates variations in each
parameter with respect to the engine load assuming the engine
rotation speed is constant. FIG. 9 illustrates variations in each
parameter with respect to the engine rotation speed assuming the
engine load is constant. As shown in FIGS. 8 and 9, the fuel
injection timing in the low-load stratified combustion region is
set to be more retarded with respect to the fuel injection timing
in the high-load stratified combustion region. Moreover, in both
the low-load and high-load stratified combustion regions, the fuel
injection timing is controlled such that the fuel injection timings
is more advanced as the engine load becomes higher or the engine
rotation speed becomes faster.
[0057] Moreover, the fuel ignition timing in the low-load
stratified combustion region is also set more retarded with respect
to the fuel injection timing in the high-load stratified region.
However, an amount of change in fuel ignition timing between the
low-load and high-load stratified combustion regions is kept
smaller than an amount of change in fuel injection timing between
the low-load and high-load stratified combustion regions. In other
words, as seen in FIGS. 8 and 9, intervals T1 and T2 between the
fuel injection timing and the fuel ignition timing in low-load and
high-load stratified combustion regions, respectively, are
preferably set such that the interval T1 of the low-load stratified
combustion region is shorter than the interval T2 of the high-load
stratified combustion region. This arrangement of the control
parameters provides reliable ignition of the fuel stream that is
floating in the combustion chamber 1 before the fuel stream
collides against the piston 3 in the low-load stratified combustion
region.
[0058] In FIGS. 8 and 9, the fuel ignition timing is simplified to
be substantially constant within the high-load or low-load
stratified combustion region. However, it will be obvious to one of
ordinary skill in the art from this disclosure that the fuel
ignition timing can be varied in each stratified combustion region
as the fuel ignition timing is preset to realize optimum fuel
efficiency and exhaust emission as explained above.
[0059] Moreover, the intake valve closing timing in the low-load
stratified combustion region is more retarded with respect to the
intake valve closing timing in the high-load stratified combustion
region. Accordingly, the pressure inside the cylinder during the
fuel injection timing is lowered in the low-load stratified
combustion region. Thus, the actual fuel stream angle (injection
opening angle) of the fuel stream in the low-load stratified
combustion region expands slightly due to the pressure differential
between the high-load and low-load combustion regions. Thus, the
fuel stream is further reliably directed in the vicinity of the
spark plug 12 to achieve stable combustion.
[0060] Furthermore, the fuel injection pressure (fuel pressure) in
the low-load stratified combustion region is set lower than the
fuel injection pressure in the high-load stratified combustion
region. Thus, the travel speed of the fuel stream in the low-load
stratified combustion region is reduced such that the air-fuel
mixture formed around the fuel stream main axis remains in the
vicinity of the spark plug 12. Thus, even more reliable and stable
combustion can be achieved in the low-load stratified combustion
region.
[0061] According to the second embodiment of the present invention,
the operation of the direct fuel injection engine is switched
between the low-load stratified combustion region and the high-load
stratified combustion region depending on the engine load and
engine rotation speed to provide sufficient time required to form
the air-fuel mixture, and thus, stable stratified combustion is
obtained regardless of the engine rotation speed.
[0062] Since the air-fuel mixture is formed and ignited after the
fuel stream hits against the cavity 4 and guided upwardly above the
cavity 4 in the high-load stratified combustion region, the fuel is
sufficiently vaporized and mixed with the air to form a relatively
homogeneous air-fuel mixture in the space above of the cavity 4.
Thus, smoke or CO discharge from an excessively dense air-fuel
mixture can be reduced and the engine operation is prevented from
being affected by the fluctuations in the cycle of the air-fuel
mixture distribution.
[0063] However, when the engine rotation speed increases in the
high-load stratified combustion region, there is a concern that the
time may be insufficient to form a homogeneous air-fuel mixture.
Moreover, there is also a danger of the flow inside the cylinder
becoming too strong that the air-fuel mixture is excessively
diffused and the area near the spark plug 12 becomes excessively
thin.
[0064] Thus, the second embodiment of the present invention is
configured to execute the operation in the low-load stratified
combustion region when the engine rotation speed is relatively
fast. In the low-load stratified combustion region, the fuel stream
floating in the air forms the air-fuel mixture from the peripheral
areas of the fuel stream by mixing with the air and vaporizing the
fuel directly after the fuel is injected. Thus, the fuel stream
without going through the cavity 4 is quickly formed into a compact
air-fuel mixture directly below the fuel injection valve 11 near
the spark plug 12. Therefore, in the second embodiment of the
present invention, stable stratified combustion can be obtained
regardless of the engine rotation speed.
[0065] Moreover, the first prescribed engine load and the first
prescribed engine rotation speed are set such that the first
prescribed engine load becomes larger as the first prescribed
engine rotation speed becomes faster. When the engine load is
higher, the amount of fuel injected is larger, which sometimes
results in forming an excessively dense air-fuel mixture. Thus, by
setting the first prescribed rotation speed such that the first
prescribed rotation speed gets faster as the first prescribed
engine load becomes larger, when the engine load is high, the
operation of the low-load stratified combustion region is executed
only when the engine rotation speed is relatively fast. Thus, a
danger of smoke discharge is reduced. Moreover, when the engine
load is relatively high and the engine rotational speed is
relatively slow in the stratified combustion operating region, the
operation in the high-load stratified combustion region is
executed. Since the fuel injection amount is relatively large when
the engine load is relatively high, the mixing of the air and fuel
is promoted by the gas flow inside the cylinder to prevent forming
a less dense air-fuel mixture in the vicinity of the spark plug 12
in the high-load stratified combustion region in which the air-fuel
mixture is formed after the fuel stream collides against the cavity
4.
[0066] However, because the total fuel injection amount increases
when the engine load is larger than the second prescribed engine
load, sufficient mixing of the fuel and air is difficult even
though the flow is intensified by faster engine rotation speed in
the low-load stratified combustion region, which increases the
danger that an excessively dense air-fuel mixture will be partially
formed. Thus, in the second embodiment of the present invention,
the control unit 13 is configured to determine the direct fuel
injection engine is operating in the high-load stratified
combustion region when the engine load is higher than the second
prescribed engine load. Accordingly, when the fuel injection amount
exceeds certain amount due to increase in the engine load, smoke
discharge can be controlled and good combustion obtained by
executing the operation in the high-load stratified combustion
engine in which the fuel stream is sufficiently vaporized and mixed
with the air as the air-fuel mixture is formed after the fuel
stream collides against the cavity 4 and guided upwardly above the
cavity 4.
[0067] Furthermore, when the engine load is equal to or less than
the third prescribed engine load, the fuel injection amount is
small. Therefore, there is the danger of the density of the
air-fuel mixture becoming excessively thin when attempting to
ignite the fuel after the air-fuel mixture is formed via the cavity
4 in the high-load stratified combustion region. Accordingly, the
control unit 13 is configured to determine the direct fuel
injection engine is operating in the low-load stratified combustion
region when the engine load is lower than the third prescribed
engine load. Thus, the air-fuel mixture formed directly after the
fuel is injected is ignited thereby making it possible to prevent
the combustion stability from worsening when the engine load is
low.
[0068] As described above, when the engine rotation speed becomes
faster, there is the danger that there is no sufficient time to
form an air-fuel mixture formed via the cavity 4. Even when the
engine rotation speed is relatively high and engine load is
relatively high, the stratified combustion can be obtained by
igniting an air-fuel mixture formed via the cavity 4 although the
air-fuel mixture may contain relatively dense air-fuel ratio due to
the large amount of the fuel injection. However, when the engine
rotation speed is fast with respect to time required to form an
air-fuel mixture using the cavity 4 (which is usually determined
based on the fuel injection pressure, the shape of the cavity, and
the like), there may not be sufficient time to form an air-fuel
mixture in the vicinity of the spark plug 12 via cavity 4. Thus,
the control unit 13 is configured to determine the direct fuel
injection engine is operating in the low-load stratified combustion
region when the engine rotation speed is higher than the second
prescribed engine rotation speed. Accordingly, the combustible
air-fuel mixture is positioned near the spark plug 12 even when the
engine rotation speed is fast.
[0069] Moreover, according to the second embodiment of the present
invention, the control unit 13 is configured to change at least one
of the fuel injection timing, fuel ignition timing, intake valve
closing timing and fuel injection pressure in conjunction with
switching operations among the low-load stratified combustion
region and the high-load stratified combustion region. Since these
parameters can be instantly controlled without using special
equipments, suitable operation of the direct fuel injection engine
is obtained without increasing the cost. Thus, even when a driver
of the vehicle suddenly changes the operating state of the vehicle,
the operations among the high-load and low-load stratified
combustion regions are switched without degrading the driving
performance of the vehicle.
[0070] In order to achieve good fuel efficiency and less exhaust
emission regardless of the operating state of the direct fuel
injection engine, a heat generation time after fuel ignition must
be optimized. In other words, the fuel ignition timing must be set
such that an optimum heat generation time is obtained for each
operating state. However, the fuel ignition timing is required to
be in a certain range of time span in a cycle and cannot exceed
this range. Thus, in order to accommodate this situation, the fuel
injection timing is varied to optimize the heat generation time in
the second embodiment of the present invention. Since the fuel
injection timing in the low-load stratified combustion region is
set more retarded than the fuel injection timing in the high-load
stratified combustion region, the fuel stream is allowed to pass
through the cavity 4 and sufficiently mixed after the fuel is
injected in the high-load stratified combustion region, and the
fuel stream forms an air-fuel mixture that is ignited directly
after the fuel is injected in the low-load stratified combustion
region.
[0071] In addition, in the low-load stratified combustion region,
the travel direction of the fuel stream must approach the spark
plug 12 in order to reliably position the air-fuel mixture in the
vicinity of the spark plug 12. If the travel direction of the fuel
stream is preset to direct at the spark plug 12, there is a concern
that smoldering may occur on the spark plug 12 especially when the
engine load is high. Accordingly, in the present invention, the
intake valve closing timing in the low-load stratified combustion
region is set to more retarded relative to the intake valve closing
timing in the high-load stratified combustion region so that the
pressure inside the cylinder during the fuel injection is reduced
in the low-load stratified combustion region. Thus, the actual fuel
stream injection angle (injection opening angle) is slightly
increased to reliably position the combustible air-fuel mixture
close to the spark plug 12 in the low-load stratified combustion
region. Moreover, in the high-load stratified combustion region, by
closing the intake valve early, the pressure inside the cylinder is
increased and the actual fuel stream injection angle is slightly
reduced. Thus, the fuel stream is further reliably directed toward
the cavity 4 to form the air-fuel mixture inside and above the
cavity 4.
[0072] According to the second embodiment of the present invention,
the fuel injection pressure in the low-load stratified combustion
region is set lower than the fuel injection pressure in the
high-load stratified combustion region. Thus, the penetrative force
of the fuel stream is reduced in the low-load stratified combustion
region and the fuel undergoes even more vaporization because the
fuel is floating. Moreover, the air-fuel mixture distribution is
further made compact and further reliable ignition is obtained.
[0073] Furthermore, in the second embodiment of the present
invention, the interval T1 between the fuel injection timing and
ignition timing in the low-load stratified combustion region is set
shorter than the interval T2 between the fuel injection timing and
ignition timing in the high-load stratified combustion region.
Accordingly, the fuel stream is further reliably ignited while the
fuel is floating and a compact air-fuel stream formation is
achieved to prevent the exhaust emissions from worsening.
Third Embodiment
[0074] Referring now to FIGS. 10 and 11, a direct fuel injection
engine in accordance with a third embodiment will now be explained.
In view of the similarity between the first and third embodiments,
the parts of the third embodiment that are identical to the parts
of the first embodiment will be given the same reference numerals
as the parts of the first embodiment. Moreover, the descriptions of
the parts of the third embodiment that are identical to the parts
of the first embodiment may be omitted for the sake of brevity.
[0075] Basically, the third embodiment is identical to the first
embodiment, except that an additional fuel stream is injected
during the compression stroke when the direct fuel injection engine
is operating in a relatively high-load region within the high-load
stratified combustion region. The additional fuel stream is
injected such that the additional fuel stream first hits the bottom
surface 4c of the cavity 4 and guided upwardly by the curved
surface 4b and the peripheral surface 4a.
[0076] When the fuel injection pressure is not controlled to be set
higher as the engine load becomes higher when the direct fuel
injection engine is operating in the high-load stratified
combustion region, it becomes difficult to have majority of the
fuel stream collide against the cavity inner peripheral surface 4a
because the duration of the fuel injection becomes longer as the
engine load becomes higher. In such case, the fuel stream injected
during the second half of the duration of the fuel injection would
collide against the curved surface 4b of the cavity 4. The
collision angle formed between the fuel stream and the curved
surface 4b is substantially a right angle and the fuel stream
collided against the curved surface 4b would not travel in a
specific direction. Thus, the movement of the fuel stream is not
converted to a circulation flow.
[0077] Accordingly, in the third embodiment of the present
invention, when it is determined that the majority of the fuel
stream would not collide against the cavity inner peripheral
surface 4a based on the engine load, the injection of the fuel
stream is divided into two injections as seen in FIG. 10. The first
injection causes the fuel stream to collide against the cavity
inner peripheral surface 4a creating a circulation flow in the same
manner as the first embodiment as seen in diagram (B) of FIG. 10.
The second injection causes the fuel stream to collide against the
cavity bottom surface 4c as seen in diagram (C) of FIG. 10. This
second injection creates a circulation flow in the opposite
direction to the first circulation flow as seen in diagram (D) of
FIG. 10. Therefore, the first half of the fuel injection creates an
air-fuel mixture in the vicinity of the center of the cavity 4 and
thereabove, while the second half of the fuel injection creates an
air-fuel mixture in the vicinity of the inner peripheral surface 4a
of the cavity 4 and thereabove. As an overall result, one large
air-fuel mixture mass is created within the cavity 4 and
thereabove. Since the two gas flows in directions opposite from
each other are created, mixing of fuel and air is promoted
resulting in excellent stratified combustible air-fuel mixture
layer. Although two fuel streams are injected in the third
embodiment of the present invention, it will be apparent to those
skilled in the art from this disclosure to inject more than two
fuel streams in a compression stroke in order to create an air-fuel
mixture optimum for combustion.
[0078] FIG. 11 illustrates the relationship between the operating
regions with respect to the engine load and engine rotation speed
in accordance with the third embodiment. As seen in FIG. 11, the
relatively high-load region within the high-load stratified
combustion is a region in which it is determined the it is
difficult to allow the majority of the fuel stream to collide
against the cavity inner peripheral surface 4a. Thus, the
relatively high-load region within the high-load stratified
combustion is regarded as a multiple fuel injection region.
[0079] According to the third embodiment of the present invention,
even when the fuel injection pressure is not controlled to increase
in response to the engine load increases in the high-load
stratified combustion region, a large air-fuel mixture mass can be
created within the cavity 4 and thereabove by injecting the fuel
multiple times during the compression stroke. Moreover, since the
circulation flows can be created in different directions by
injecting the fuel multiple times, a disturbance occurs within the
cavity 4 making it possible to promote mixing between the injected
fuel and air. Accordingly, stable combustion can be obtained while
introducing large quantities of EGR as well as combustion with good
fuel efficiency and a small amount of NOx can be obtained.
[0080] It will be apparent to those skilled in the art from this
disclosure that the multiple injections of the third embodiment can
be adapted to the direct fuel injection engine of the second
embodiment explained above. For example, the direct fuel injection
engine of the second embodiment can be configured and arranged to
execute multiple fuel injections in the region where the engine
load is higher than the second prescribed engine load so that a
large air-fuel mixture mass can be created within the cavity 4 to
obtain an excellent combustion in the relatively high-load region
within the high-load stratified combustion region.
Fourth Embodiment
[0081] Referring now to FIG. 12, a direct fuel injection engine in
accordance with a fourth embodiment will now be explained. In view
of the similarity between the first and fourth embodiments, the
parts of the fourth embodiment that are identical to the parts of
the first embodiment will be given the same reference numerals as
the parts of the first embodiment. Moreover, the descriptions of
the parts of the fourth embodiment that are identical to the parts
of the first embodiment may be omitted for the sake of brevity. The
parts of the fourth embodiment that differ from the parts of the
first embodiment will be indicated with a prime (').
[0082] Basically, the fourth embodiment of the present invention is
identical to the first embodiment, except that the spark plug 12 is
positioned further away from the fuel injection valve 11 and a
piston 3' is substituted for the piston 3 of the first embodiment.
The piston 3' of the third embodiment is basically identical to the
piton 3 of the first embodiment, except that the shape of the
cavity 4' has been modified from the cavity 4 of the first
embodiment.
[0083] When the spark plug 12 cannot be arranged close to the fuel
injection valve 11 due to limitations on the construction of the
cylinder head 2, the spark plug 12 is installed at a position away
from the fuel injection valve 11 as seen in FIG. 12. In the fourth
embodiment of the present invention, a bottom surface 4c' of the
cavity 4' is inclined such that a part of the bottom surface 4c'
including a bottom surface 4c.sub.2 that is farther from the spark
plug 12 is shallower than a part of the bottom surface 4c'
including a bottom surface 4c.sub.1 that is closer to the spark
plug 12. Moreover, the cavity 4' includes an inner peripheral
surface 4a' in which a surface 4a.sub.2 that is farther from the
spark plug 12 is less inclined toward the center axis of the piston
3' than a surface 4a, that is closer to the spark plug 12. The
surfaces 4a.sub.1 and 4a.sub.2 are smoothly joined in
circumferential direction of the cavity 4' to form a smooth surface
of the inner peripheral surface 4a'. Thus, a stratified air-fuel
mixture layer suitable for combustion is formed in the vicinity of
the spark plug 12 by injecting the fuel stream onto the cavity
inner peripheral surface 4a' when the direct fuel injection engine
is operating in a high-load stratified state.
[0084] Accordingly, in the second embodiment, even when the spark
plug 12 cannot be arranged close to the fuel injection valve 11 as
in the first embodiment, stable combustion can be obtained while
introducing large quantities of EGR when the direct fuel injection
engine is operating in the high-load stratified combustion region
as in the first embodiment. Moreover, combustion with good fuel
efficiency and a small amount of NOx can be also obtained.
[0085] It will be apparent to those skilled in the art from this
disclosure that the structure of the spark plug 12 and the cavity
4' of the fourth embodiment can be adapted to the direct fuel
injection engine of the second or third embodiment as explained
above in case the spark plug 12 cannot be arranged close to the
fuel injection valve 11.
Fifth Embodiment
[0086] Referring now to FIG. 13, a direct fuel injection engine in
accordance with a fifth embodiment will now be explained. In view
of the similarity between the first and fifth embodiments, the
parts of the fifth embodiment that are identical to the parts of
the first embodiment will be given the same reference numerals as
the parts of the first embodiment. Moreover, the descriptions of
the parts of the fifth embodiment that are identical to the parts
of the first embodiment may be omitted for the sake of brevity. The
parts of the fifth embodiment that differ from the parts of the
first embodiment will be indicated with a prime (').
[0087] Basically, the direct fuel injection engine of the fifth
embodiment is identical to the first embodiment, except that a fuel
injection valve 11' is substituted for the fuel injection valve 11
of the first embodiment. More specifically, the direct fuel
injection engine of the fifth embodiment utilizes the fuel
injection valve 11' in cases when the fuel injection valve 11 of
the first embodiment cannot be set substantially parallel to the
center axis of the piston 3. The injection valve 11' is a
multi-hole injection valve that allows non-symmetrical fuel
injection in the axial direction of the fuel injection valve 11'.
Thus, in the fifth embodiment, the fuel injection valve 11' is
installed such that the center axis of the fuel injection valve 11'
is inclined with respect to the center axis of the piston 3 and the
fuel stream injected from the fuel injection valve 11' forms a
hollow cone shape that is substantially symmetrical with respect to
the center axis of the piston 3, as seen in FIG. 13.
[0088] Accordingly, in the fifth embodiment, even when the fuel
injection valve 11 cannot be set substantially parallel to the
center axis of the piston 3, the shape of fuel stream is formed
into a hollow cone that is substantially symmetrical with respect
to the piston 3. Consequently, unburned HC as well as cooling loss
can be reduced when the engine is operating in the low-load
stratified state as in the first embodiment. Also, combustion with
good fuel efficiency and a small amount of exhaust gas emissions
can be obtained. When the engine is operating in the high-load
stratified state, stable combustion is obtained while introducing
large quantities of EGR as well as combustion with good fuel
efficiency and a small amount of NOx.
[0089] It will be apparent to those skilled in the art from this
disclosure that the structure of the fuel injection valve 11' of
the fifth embodiment can be adapted to the direct fuel injection
engine of the second or third embodiment as explained above in case
the fuel injection valve 11 cannot be arranged substantially
parallel to the center axis of the piston 3.
[0090] As used herein, the following directional terms "forward,
rearward, above, downward, vertical, horizontal, below and
transverse" as well as any other similar directional terms refer to
those directions of a vehicle equipped with the present invention.
Accordingly, these terms, as utilized to describe the present
invention should be interpreted relative to a vehicle equipped with
the present invention.
[0091] The term "configured" as used herein to describe a
component, section or part of a device includes hardware and/or
software that is constructed and/or programmed to carry out the
desired function.
[0092] Moreover, terms that are expressed as "means-plus function"
in the claims should include any structure that can be utilized to
carry out the function of that part of the present invention.
[0093] The terms of degree such as "substantially", "about" and
"approximately" as used herein mean a reasonable amount of
deviation of the modified term such that the end result is not
significantly changed. For example, these terms can be construed as
including a deviation of at least .+-.5% of the modified term if
this deviation would not negate the meaning of the word it
modifies.
[0094] This application claims priority to Japanese Patent
Application Nos. 2003-121610 and 2003-154056. The entire
disclosures of Japanese Patent Application Nos. 2003-121610 and
2003-154056 are hereby incorporated herein by reference.
[0095] While only selected embodiments have been chosen to
illustrate the present invention, it will be apparent to those
skilled in the art from this disclosure that various changes and
modifications can be made herein without departing from the scope
of the invention as defined in the appended claims. Furthermore,
the foregoing descriptions of the embodiments according to the
present invention are provided for illustration only, and not for
the purpose of limiting the invention as defined by the appended
claims and their equivalents. Thus, the scope of the invention is
not limited to the disclosed embodiments.
* * * * *