A direct-injection spark-ignition engine having engine cylinders bored in a cylinder block, and a cylinder head mounted on the cylinder block, comprises at least one intake port provided for each of the engine cylinders. The ratio V/A of a stroke volume V (cm.sup.3) per cylinder to a cross-sectional area A (cm.sup.2) is set within a predetermined range defined by 45.ltoreq.V/A.ltoreq.55, where the cross-sectional area A is a minimum cross-sectional area obtained when the at least one intake port is cut by a plane extending in a direction substantially normal to a stream line of intake air flowing through the intake port.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an in-cylinder direct-injection 
spark-ignition internal combustion engine, and specifically to techniques 
for an intake port structure capable of providing the enhanced intake-air 
charging efficiency and ensuring the excellent air/fuel mixture formation, 
and suitable for a cylinder direct-injection spark-ignition engine, 
capable of operating in at least two modes, namely a homogeneous 
combustion mode and a stratified combustion mode. 
2. Description of the Prior Art 
In recent years, there have been proposed and developed various in-cylinder 
direct-injection spark-ignition engines in which fuel is injected directly 
into an engine cylinder. Generally, on such direct-injection 
spark-ignition engines a combustion mode is switchable between a 
homogeneous combustion mode (or an early injection combustion mode) where 
fuel-injection early in the intake stroke produces a homogeneous air-fuel 
mixture, and a stratified combustion mode (or a late injection combustion 
mode) where late fuel-injection delays the event until near the end of the 
compression stroke to produce a stratified air-fuel mixture. The 
previously-noted homogeneous combustion mode is suitable for an engine 
operating condition such as medium or high engine speed and load, whereas 
the previously-noted stratified combustion mode is used generally at an 
engine operating condition such as low engine speed and load. The purpose 
of the stratified combustion is to deliver a readily ignitable mixture 
(richer mixture of a combustible air/fuel mixture ratio at which the 
mixture is ignitable by means of a spark plug provided in the combustion 
chamber) in the vicinity of the spark plug while forming surrounding air 
layer (leaner or ultra-leaner mixture often including part of the exhaust 
gas back through the engine or having the difficulty of direct-ignition by 
the spark plug) that contains little fuel, and to stable lean combustion 
under the condition of the low engine speed and load, and to improve fuel 
economy. In contrast to a conventional engine, the throttle valve is 
opened at the low engine load to increase the intake-air quantity, thus 
reducing a pumping loss. On such cylinder direct-injection spark-ignition 
engine (simply a DI engine) has been disclosed in Japanese Patent 
Provisional Publication Nos. 62-191622 and 2-169834. On the other hand, 
Japanese Patent Provisional Publication No. 7-119472 teaches the provision 
of a so-called swirl control valve to modulate in-cylinder gas motion and 
to create turbulent flow such as swirl flow or tumble flow, thus 
facilitating the mixing of air with fuel spray and improving combustion. 
During the homogeneous combustion mode (or the early injection combustion 
mode) on the intake stroke, it could be expectable to provide the effect 
of cooling the intake air owing to fuel vaporization (or gasification), 
thus ensuring an enhanced intake-air charging efficiency (simply an 
induction efficiency) arising from reduction in the volumetric capacity of 
intake air charged. However, in conventional engines, the effects (the 
enhanced intake-air cooling effect and the enhanced induction efficiency) 
as previously discussed, were inadequate and unsatisfactory. It would be 
possible to highly enhance the intake-air cooling effect by promoting the 
fuel vaporization by virtue of increase in the flow-velocity of intake air 
flowing through the intake port. In addition to the above, the increase of 
the flow-velocity of intake air can contribute to strengthen turbulent 
action (for example tumble flow) in the engine cylinder, thereby assuring 
good mixing of fuel sprayed out with air and thus promoting homogenization 
of the air-fuel mixture within the combustion chamber. Also, it is 
possible to properly carry the fuel spray to the vicinity of the spark 
plug during the stratified combustion mode by swirl motion more greatly 
strengthened by virtue of both the increased intake-air velocity and the 
use of the swirl control valve. This may largely improve combustion 
characteristics (for example, the fuel consumption, the lean misfire 
limit, and the combustion stability) during the stratified combustion 
mode. For the reasons set forth above, it would be desirable to 
effectively increase an intake-air velocity by improving an intake port 
structure of a direct-injection spark-ignition engine. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide an in-cylinder 
direct-injection spark-ignition engine which avoids the aforementioned 
disadvantages of the prior art. 
It is another object of the invention to provide an in-cylinder 
direct-injection spark-ignition engine which is capable of achieving 
enhancement of the engine output power and improvement of the combustion 
characteristics such as fuel economy by enhancing the induction efficiency 
and by increasing the intake-air velocity during the homogeneous 
combustion mode, and of improving the combustion characteristics such as 
the fuel economy, the lean misfire limit, and the stable lean combustion 
during the stratified combustion mode. 
In order to accomplish the aforementioned and other objects of the present 
invention, a direct-injection spark-ignition engine having engine 
cylinders bored in a cylinder block, and a cylinder head mounted on the 
cylinder block, comprises at least one intake port provided for each of 
the engine cylinders, wherein a ratio V/A of a stroke volume V (cm.sup.3) 
per cylinder to a cross-sectional area A (cm.sup.2) is set within a 
predetermined range defined by 45.ltoreq.V/A.ltoreq.55, where the 
cross-sectional area A is a minimum cross-sectional area obtained when the 
at least one intake port is cut by a plane extending in a direction 
substantially normal to a stream line of intake air flowing through the at 
least one intake port. It is preferable that the direct-injection 
spark-ignition engine may further comprise a swirl control valve disposed 
in the at least one intake port, for strengthening in-cylinder swirl flow 
created by the intake air. More preferably, the at least one intake port 
has a narrowed-down portion for locally narrowing down a cross section 
thereof, while providing the ratio V/A defined by 45.ltoreq.V/A.ltoreq.55. 
A minimum cross-sectional area of the narrowed-down portion is set at the 
minimum cross-sectional area A. It is more preferable that a cross section 
of the narrowed-down portion is dimensioned so that the minimum 
cross-sectional area of the narrowed-down portion is obtained 
substantially in a middle position of the at least one intake port. 
Preferably, the cross section of the narrowed-down portion may be 
dimensioned so that the cross section gradually increases from the middle 
position to an upstream position of the at least one intake port and 
gradually increases from the middle position to a downstream position of 
the at least one intake port. More preferably, the at least one intake 
port has a substantially straight port portion. It is preferable that the 
angle between the central axis of the substantially straight port portion 
and the bottom face of the cylinder head is set within a predetermined 
angle range defined by 30.degree..ltoreq..theta..ltoreq.50.degree..

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, particularly to FIGS. 1A and 1B, the 
direct-injection engine of the invention is exemplified in case of an 
in-cylinder direct-injection spark-ignition DOHC four-valve engine 
employing two intake-port valves and two exhaust-port valves for each 
individual engine cylinder. As seen in FIG. 1B, the cylinder head 1 is 
mounted on a cylinder block (not numbered), and is formed with two 
intake-valve ports (intake ports) 3 and two exhaust-valve ports (exhaust 
ports) 4, all communicating the combustion chamber 2. The combustion 
chamber 2 is defined by the recessed portion formed at the bottom of the 
cylinder head 1 and the piston crown (not numbered) of the piston which is 
fitted to an engine cylinder formed in the cylinder block, and is movable 
through a stroke in the cylinder. The two intake ports (3, 3) have the 
substantially same shape and size. Two intake valves (not numbered) are 
installed at an intake-valve mounting portion 5 of the cylinder head 1, to 
permit intake air (or induced air) to be drawn into the combustion chamber 
through the intake ports 3 with the intake valves opened. Two exhaust 
valves (Not numbered) are installed at an exhaust-valve mounting portion 6 
of the cylinder head 1, to permit burned gases to exhaust from the engine 
cylinder with the exhaust valves opened during the exhaust stroke. The 
cylinder head 1 is also formed with a substantially centrally-located 
spark-plug installation hole 7 into which the spark plug is screwed so 
that the tip of the spark plug is exposed to the center of the combustion 
chamber 2. A fuel injector valve installation hole 8 is formed in the 
cylinder head in the vicinity of the two intake ports (3, 3), more 
precisely, between the two intake ports (3, 3), to install the fuel 
injector valve therein, so that the injection nozzle of the fuel injector 
is projected toward within the outer peripheral portion of the combustion 
chamber 2, and so that fuel is injected or sprayed out downwards obliquely 
with respect to the horizontal plane of the bottom face of the cylinder 
head (or the upper face of the cylinder block) (see the conically-injected 
fuel spray of FIG. 1B). The fuel-injection timing is set at the intake 
stroke in the homogeneous combustion mode, whereas the fuel-injection 
timing is set at the compression stroke in the stratified combustion mode. 
In the shown embodiment, a swirl control valve 10 is located in the middle 
of the intake pipe 9 connected to or communicating with one of the intake 
ports (3, 3). As described later, the swirl control valve serves as a 
turbulence control means or a swirl-flow plus tumble-flow control means. 
Generally, the opening and closing of the swirl control valve 10 is 
electronically controlled by means of an electronic engine control unit 
(ECU) or an electronic engine control module (ECM). As seen in FIG. 2A, 
the swirl control valve 10 is closed during the stratified combustion 
mode, to create swirling gas flow, often called "in-cylinder swirl flow", 
simply "swirl flow". As seen in FIG. 2B, the swirl control valve 10 is 
opened during the homogeneous combustion mode, to create in-cylinder 
vertical vortex, often called "in-cylinder tumble flow", simply "tumble 
flow". 
In the engine of the embodiment, the intake port structure is designed and 
dimensioned so that the velocity v of intake-air flow entering the engine 
cylinder, which will be hereinafter referred to as an "intake-air flow 
velocity v", is effectively increased, for the purpose of fully enhancing 
the induction efficiency (the intake-air charging efficiency) arising from 
the enhanced intake-air cooling effect obtainable as a result of rapid 
fuel vaporization during the homogeneous combustion mode. The intake-air 
flow velocity v is represented as a ratio V/A of the stroke volumetric 
capacity (simply the stroke volume) V (cm.sup.3) to the minimum 
cross-sectional area A (cm.sup.2) of the intake port 3. Hereupon, the 
cross-sectional area of the intake port means a total cross-sectional area 
obtained when the two intake ports (3, 3) is cut by a plane extending in a 
direction substantially normal to a stream line of intake air flowing 
through each of the intake ports 3 associated with one of the engine 
cylinders, because, in the embodiment, the two intake ports (3, 3) are 
provided for each of the engine cylinders. In the embodiment, the two 
intake ports, substantially parallel to each other, are formed for every 
engine cylinder, and therefore the intake-port cross-sectional area per 
cylinder is the sum of the cross-sectional area of one of the two adjacent 
intake ports, taken in a direction substantially normal to a stream line 
of intake air flowing through the one intake port, and the crosssectional 
area of the other intake port of the same cylinder, taken in a direction 
substantially normal to a stream line of intake air flowing through the 
other. As can be appreciated, assuming that the stroke volume V (cm.sup.3) 
is fixed to a constant value, the minimum intake-port cross-sectional area 
A (cm.sup.2) must be decreased in order to increase the intake-air flow 
velocity v. However, if the minimum intake-port cross-sectional area A is 
decreased for the increase in the intake-air flow velocity v, there is a 
tendency the intake-air flow resistance to increase. The increased 
intake-air flow velocity v contributes to enhancement of the induction 
efficiency, whereas the increased intake-air flow resistance contributes 
to reduction of the induction efficiency. To balance these two 
contradictory factors, that is, the increased intake-air flow velocity v 
and the increased intake-air flow resistance, it is necessary to optimally 
set the minimum cross-sectional area A or the ratio V/A. By way of 
repetition of various sorts of experiments, the inventors of the invention 
have discovered optimal values of the previously-described ratio V/A, as 
discussed in detail hereunder by reference to FIGS. 3 and 4. 
Referring now to FIG. 3, there are shown the air-flow resistance 
characteristic curve and the engine-torque characteristic curve at various 
ratios (V/A) during the homogeneous combustion mode. As seen in FIG. 3, 
during the homogeneous combustion (i.e., during the early injection 
combustion mode on the intake stroke), when the ratio V/A is greater than 
55, that is, in case of V/A&gt;55, the intake-air flow velocity v tends to 
increase remarkably, while the flow resistance of intake air flowing 
through the intake port 3 tends to increase to excess. The above-mentioned 
intake-air flow resistance has been measured by the inventors under a 
specified condition where the intake pipe is removed and also the intake 
valve is mounted on the cylinder head. As a result of the excessively 
great intake-air flow resistance, when V/A&gt;55, the induction efficiency 
(the intake-air charging efficiency) is reduced remarkably, thus lowering 
the engine torque. On the other hand, when V/A&lt;45, the intake-air flow 
resistance tends to reduce, however the intake-air flow velocity v cannot 
be increased adequately. This lowers the effect of cooling the intake air, 
and consequently results in reduction in the induction efficiency. 
Additionally, owing to the undesiredly decreased intake-air flow velocity 
v, the turbulent action of air (e.g., the tumble flow) within the cylinder 
(or the combustion chamber) cannot be strengthened adequately. As a 
result, the mixing between fuel spray and air, that is, the homogenization 
of the air-fuel mixture cannot be promoted satisfactorily. This lowers the 
engine torque. As seen in the intermediate portions 
(45.ltoreq.V/A.ltoreq.55) of the two characteristic curves shown in FIG. 
3, the inventors of the invention have discovered that it is preferable 
that the ratio V/A is set within the range defined by 
45.ltoreq.V/A.ltoreq.55 when the engine is operating in the homogeneous 
combustion mode, in order to promote fuel vaporization by virtue of the 
increased intake-air flow velocity, and to enhance the induction 
efficiency utilizing the excellent intake-air cooling effect obtained by 
the promoted fuel vaporization, and consequently to enhance the engine 
torque, and also to strengthen the in-cylinder gas motion (for example, 
the tumble flow) with the increased intake-air flow velocity v, and 
consequently to ensure good mixing and adequate homogenization of the 
air-fuel mixture and to improve combustion. 
Referring now to FIG. 4, there are shown the swirl-strength characteristic 
curve and the fuel-consumption characteristic curve at various ratios 
(V/A) during the stratified combustion mode. The test is made by the 
inventors of the invention under a specified condition where the swirl 
control valve 10 is closed so as to strengthen the swirl flow within the 
cylinder. As seen in FIG. 4, during the stratified combustion mode (i.e., 
during the late injection combustion mode on the compression stroke), the 
airflow velocity v can be effectively increased, when the ratio V/A is set 
within the previously-noted predetermined range defined by 
45.ltoreq.V/A.ltoreq.55. Therefore, in the engine of the embodiment, the 
swirl flow can be more highly strengthened by way of full cooperation of 
the increased intake-air flow velocity v and the swirl control valve 10 
closed. Hitherto, only the use of the swirl control valve is considered 
and the ratio V/A is below 45. In comparison with the conventional engine 
having the ratio defined by V/A&lt;45, the swirl flow can be sufficiently 
strengthened or the swirl ratio can be satisfactorily enhanced by the 
improved engine of the invention having the ratio defined by 
45.ltoreq.V/A.ltoreq.55. The more strengthened in-cylinder swirl flow 
functions to more effectively carry the fuel spray injected to the 
vicinity of the spark plug during the stratified combustion mode. This 
improves the combustion characteristics (e.g., the fuel consumption, the 
lean misfire limit at which the air/fuel mixture is no longer ignitable, 
and the combustion stability). Due to the optimal setting 
(45V/A.ltoreq.55) of the ratio V/A, a required engine performance or an 
overall engine performance in the homogeneous combustion mode, such as 
engine output power and fuel consumption, can be largely improved. 
Additionally, the required engine performance in the stratified combustion 
mode, such as minimum fuel consumption, enhanced lean misfire limit, and 
enhanced combustion stability, can be highly attained. To ensure the 
previously-noted optimal setting (45.ltoreq.V/A.ltoreq.55), the 
cross-sectional area of the intake port can be dimensioned so that the 
minimum cross-section area A is formed over the entire length of the 
intake port 3. Alternatively, only a part of the intake port 3 may be 
formed as a narrowed-down portion (or a throttling portion) 3A having the 
minimum cross-sectional area A, in order to locally narrow down the cross 
section of the intake port 3, while providing the ratio defined by 
45.ltoreq.V/A.ltoreq.55. The latter intake-port structure is superior to 
the former intake-port structure in that the flow resistance of intake air 
flowing through the intake port 3 is kept as small a value as possible, 
while providing the ratio defined by 45.ltoreq.V/A.ltoreq.55. Therefore, 
the intake-port structure (having the partly narrowed-down portion 3A) 
shown in FIG. 1A enables an easy increase in the intake-air flow velocity 
v, remarkably enhancing the induction efficiency. Thus, the required 
engine performance in the homogeneous combustion mode and the required 
engine performance in the stratified combustion mode can be attained at a 
higher level. 
Referring now to FIG. 5, there is shown the relation between the induction 
efficiency (or the engine torque) and the position of the locally-formed, 
narrowed-down portion 3A of the intake port 3. As seen from the enlarged 
cross section shown in FIG. 1A, the position a corresponds to the upstream 
end of the substantially straight portion of the intake port 3, the 
position b corresponds to the middle portion of the substantially straight 
portion of the intake port 3, and the position c corresponds to the 
downstream end of the substantially straight portion of the intake port 3. 
The case where the narrowed-down portion 3A is formed such that the 
minimum cross section of the narrowed-down portion 3A matches the position 
a, is superior to the other case where the narrowed-down portion 3A is 
formed such that the minimum cross section of the narrowed-down portion 3A 
matches the position b or the position c, from the viewpoint of avoidance 
of undesired separation of airflow from the inner wall of the intake port 
3 and from the viewpoint of avoidance of degraded flow-straightening 
performance. As discussed hereunder, the specified shape of narrowed-down 
portion 3A located at the middle position b of the intake port 3 is 
effective to suppress or avoid both undesired separation of airflow from 
the inner wall of the intake port 3 and degradation of the airflow 
straightening performance, while keeping the airflow resistance as small 
as possible and satisfying the specified condition of 
45.ltoreq.V/A.ltoreq.55. As shown in FIGS. 1A and 1B, as regards the 
specified shape of the locally-narrowed-down portion 3A, the cross section 
of the intake port 3 is narrowed down at the middle position b between the 
substantially upstream end a and the substantially downstream end c, and 
additionally the cross section is designed to gradually moderately 
increase from the middle position b to the upstream position a, and to 
gradually moderately increase from the middle position b to the downstream 
position c. The locally, moderately narrowed-down portion 3A greatly 
contributes to avoidance of undesired separation of airflow from the inner 
wall of the intake port 3 and avoidance of degraded flow-straightening 
performance. With the airflow velocity v increased, the narrowed-down 
portion 3A located at the middle position b can effectively produce the 
previously-described various effects, such as the promotion of fuel 
vaporization, the enhanced intake-air cooling effect, the enhanced 
induction efficiency, the enhanced engine output power, the strengthened 
in-cylinder gas motion involving the swirl motion and tumble motion, the 
promotion of mixing and homogenizing the air/fuel mixture, the improvement 
of combustion. 
Referring now to FIG. 6, there is shown the relation between the induction 
efficiency (or the engine torque) and the angle (.theta.) between the 
central axis of the substantially straight port portion of the intake port 
and the bottom face of the cylinder head (or the upper face of the 
cylinder block). As seen in FIG.6, the angle range of 
30.degree..ltoreq..theta..ltoreq.50.degree. (or the angle range defined by 
40.degree..+-.10.degree.) is more effective to strengthen the in-cylinder 
gas motion (such as swirl flow and tumble flow). When the previously-noted 
angle .theta. is set within the predetermined angle range defined by 
30.degree..ltoreq..theta..ltoreq.50.degree., the fuel spray injected from 
the injector valve can be effectively impinged against or collided on the 
air flow entering the cylinder. This ensures the plural effects as 
previously discussed, in both the homogeneous combustion mode and the 
stratified combustion mode. 
In the shown embodiment, the direct-injection spark-ignition engine 
employing two intake ports per one engine cylinder, and an injector valve 
located between the two intake ports, is exemplified. The concept of the 
invention is applicable to the other arrangement of the engine, for 
example a direct-injection spark-ignition engine employing a sole intake 
port per one cylinder, and an injector valve located near the sole intake 
port. In this case, the minimum cross-sectional area A of the intake port 
corresponds to an area of the minimum cross section of the narrowed-down 
portion of the sole intake port. The use of the swirl control valve 10 is 
effective to improve both homogeneous-combustion characteristics and 
stratified-combustion characteristics. For the purpose of improving only 
the homogeneous-combustion characteristics, the swirl control valve 10 may 
be eliminated. In the shown embodiment, the swirl control valve 10 is 
provided in the middle of the intake pipe 9 connected to one of the two 
intake ports to create the swirl flow with the one intake port closed by 
the swirl control valve and the other intake port constantly opened during 
the stratified combustion mode, and to create the tumble flow with the one 
intake port opened by the swirl control valve and the other intake port 
constantly opened. Alternatively, the swirl control valve may be replaced 
a swirl control valve as disclosed in the previously-described Japanese 
Patent Provisional Publication No. 7-119472. The swirl control valve 
serves to close only the lower-half cross section of the intake pipe (or 
the intake-air passage) to strengthen the in-cylinder gas motion. 
Furthermore, the narrowed-down portion 3A having the minimum cross section 
of the intake port can be formed in each of two intake ports of each 
cylinder. Alternatively, the narrowed-down portion 3A may be formed in 
either one of the two intake ports of each cylinder. In the shown 
embodiment, the cross sections of the two intake ports (3, 3), included in 
the associated cylinder, are substantially identical in shape and size. 
That is, the two intake ports (3, 3) are substantially congruent. Thus, 
the cross-sectional area Ak of one of the intake ports is designed to be 
almost equal to the cross-sectional area A.sub.m of the other. In lieu 
thereof, in a multi-intake-port engine having a plurality of intake ports 
for every engine cylinders, the minimum cross-sectional area A.sub.m of a 
certain intake port included in the associated cylinder may be different 
from the minimum cross-sectional area A.sub.k of the other intake port 
included in the same cylinder, if the sum (A.sub.1 +A.sub.2 +A.sub.k +. . 
. +A.sub.m +A.sub.n) of the minimum cross-sectional areas A.sub.1, 
A.sub.2, . . . , A.sub.n corresponds to the total minimum cross-sectional 
area A, and the condition of 45.ltoreq.V/A.ltoreq.55 is satisfied. 
While the foregoing is a description of the preferred embodiments carried 
out the invention, it will be understood that the invention is not limited 
to the particular embodiments shown and described herein, but that various 
changes and modifications may be made without departing from the scope or 
spirit of this invention as defined by the following claims.