Note: Descriptions are shown in the official language in which they were submitted.
~075988
Background of the Invention
This invention relates to an internal combustion
engine which has two spark plugs for each combustion
chamber and is provided with an exhaust gas recirculation
circuit.
In regard to the suppression of the formation of
nitrogen oxides (NOx) in the operation of internal com-
bustion engines, a recently proposed method is characterized
in that the exhaust gas is recirculated to the combustion
chambers in a large amount and that each combustion chamber
is equipped with two spark plugs which are positioned
at some distance from each other and produce sparks
simultaneously. Accordinq to this method, the maximum
amount of the reci~culated exhaust gas reaches about
25-45% by volume of air admitted into the combustion
chambers as a component of a fresh air-fuel mixture.
The recirculation of exhaust gas in such a large amount
is quite effective in loweri~g the maximum combustion
temperature and hence suppressing the formation of NOx
but is liable to cause an unstable combustion. The simul-
taneously working tow spark plugs for each combustion
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chamber are provided with the purpose of not only ensuring
the ignition of the diluted air-fuel mixture but also
rapidly completing the combustion. Since each of the
two distant spark gaps serves as a starting point of
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flame propagation in the combustion chamber, the pro-
pagation of flame or combustion can be completed with
a greatly shortened propagation distance. In an ideal
case, flames developed at the two starting points can
spread over the entire region of the combustion chamber
in nearly a half of the time needed for the completion
- of the flame propagation in the same combustion chamber
by the use of a single spark plug. - I
. The greatest importance to this method is the
arrangement of the two spark plugs. Shortening the flame
- propagation time, ensuring a stable combustion and avoiding
a significant increase in the fuel consumption can satis-
~- factorily be achieved only when the two spark plugs are
optimumly positioned in the combustion chamber.
; 15 Summary of the Invention
It is an object of the present invention to provide
an engine system which includes an internal combustion
engine of the reciprocating piston type having two spark
plugs for each combustion chamber thereof and an exhaust gas
recirculation circuit, wherein the two spark plugs are
positioned in a manner best suited for accomplishing the
principal object of the engine system, that is, realizing
a stable combustion of an air-fuel mixture diluted with
a large amount of exhaust gas thereby to suppress the
- 25 formation of NOx with only minimized sacrifice of the
107598~
engine performance.
It is another object of the invention to provide
an engine system of the described type, which engine
system features a good fuel economy despite the recir-
culation of a large amount of exhaust gas.
It is still another object of the invention to
provide an engine system of the described type, wherein
intake and exhaust valves for each combustion chamber
are arranged in a manner best suited for practicing an
optimum arrangement of the two spark plugs.
An engine system according to the invention includes
an internal combustion engine of the reciprocating piston
type and an exhaust gas xecirculation circuit which is
arr2nged to recirc~late a portlon of the exhaust gas
from the exhaust line to each combustion chamber at a
controlled flow rate in relation to the flow rate of air
taken into each combustion chamber. Each combustion
- chamber of the engine consistC of a variable volume
cylindrical space defined in a cylinder bore between the
top face of the pistpn and a bottom face of a cylinder
head and a recess formed on this bottom face to have a
larger volume than the cylindrical space at the top dead
center of the piston. Each combustion chamber is provided
with an intake valve, an exhaust valve and two spark
plugs which produce sparks substantially simultaneously.
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As an essential feature of the engine system according
to the lnvention, the spark gaps of the two spark plugs
are positioned in the recess so as to meet the following
conditions: (a) the two spark gaps are located respec-
, 5 tively on different sides of a first plane containing
, the longitudinal axis of the cylinder bore; (b) the two
. spark plugs are gene~ally symmetrically arranged with :
. respect to the bore axis in a plan view taken trans-
'.:~ versal to the bore axis; (c) the ratio of the distance
10 between the two gaps to the diameter of the cylinder bore
is in the range from 0.45 to 0.67; and (d) the ratio of
- the distance from each spark plug to the top face of the
piston to the distance between the two spark gaps is in
the ran~1e from 0.45 to 0.55 when the piston at each
power stroke reaches a position e~pressed by a crank
angle of about 40 degrees after the top dead center.
The recess portion of the combustion chamber is
preferably included entirely in an imaginary and axial
~.` extension of the cylinder bore and has a simple and
: 20 generally symmetrical shape with respect to the bore
axis. More particularly, it is preferable that the
.` periphery of the recess is given generally by a surface of
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revolution on the axis of the cylinder bore and that the
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. cross-sectional area of the recess exhibits fundamentally
:: 25 no increase as the distance from the top face of the piston
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increases.
Preferably, the intake and exhaust valves are located
respectively on different sides of a second plane which
is parallel to the bore axis and contains a line segment
connecting the two spark gaps.
Br_ef Descriytion of the Drawings
Fig. 1 is a schematic plan view of an engine system
according to the invention;
Fig. 2 is a cross-sectional view of a combustion
chamber as an embodiment of the invention;
Fig. 3 is a sectional view taken along the line
3-3 of Fig. 2 and partly rotated through an angle
indicated in Fig. 2;
Fig. 4 is a s_ctional view taken along the line
4-4 of Fig. 2;
Fig. 5 is a graph showing the dependency of the
fuel economy of the engine system of Fig. 1 on the
distance between the two spark gaps in each combustion
chamber;
Fig. 6 is a cross-sectional view of two adjacent
combustion chambers as another embodiment of the invention;
Fig. 7 is a sectional view taken along the line
7-7 of Fig. 6;
Figs. 8 and 9 are cross-sectional views of two
different combustion chambers, respectively, as still
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different embodiments of the invention.
Description of Preferred Embo iments
In Fig. 1, a four-cylinder internal combustion
engine 10 is equipped with a carburetor 12 to supply
S an air-fuel mixture to the respective engine cylinders
through an induction passage 14, an intake manifold 16
and intake ports 18. An intake valve 20, an exhaust
valve 22 and two spark plugs 24A and 24B are provided
for each combustion chamber defined in each engine
cylinder. The exhaust line of this engine 10 includes
exhaust ports 26, an exhaust manifold 28 and an exhaust
pipe 30. An exhaust gas recirculation passage 32
interconnects the exhaust manifold 28 to either the
induction passage 14 at a secticn downstream from the
carburetor 12 or the intake manifold 16. A flow control
valve 34 is arranged to control the volumetrlc flow rate
of the exhaust gas through the recirculation passage 32
in dependence on the operational condition of the engine
10. A considerably high exhaust recirculation rate is
employed in this engine system. Numerically, the re-
circulated exhaust gas amounts to about 25-45% by volume
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of air taken into the combustion chambers when the flow
rate of the exhaust gas through the control valve is
maximized during a steady state operation of the engine
10 in a medium speed range. Accordingly, no extra means
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~075988
is needed for preventing the emission of NOx into the
atmosphere. The exhaust line may optionally include
; either a thermal reactor or a catalytic converter (not
shown) for the removal of carbon monoxide and hydrocarbons.
Referring to Figs. 2-4, each engine cylinder of this
engine 10 receives therein a reciprocating piston 36 to
leave a thin cylindrical space as part of a combustion
chamber 40 between the piston top 36a and the bottom face
of the cylinder head 38 at the top dead center of the
piston 36. A recess 42 is formed on the bottom face of
the cylinder head 38 to serve as a major portion of the
combustion chamber 40: the recess 42 has a larger volume
than the cylindrical space at the top dead center. The
recess 42 has a roughly hemispherical shape in this example
but may alternatively have a ~ifferent but yet simple shape
such as a pentroof shape or a pancake shape. Preferably,
the bottom face of the cylinder head 38 is recessed to
generally give a surface of revolution on the longitudinal
axis 44 of the cylinder bore. Accordingly the recess 42 is
circular in any cross section. It is preferable that the
, cross-sectional area of the recess 42 exhibits no increase
as the distance from the piston top 36a increases. (Of
course the recess 42 may exhibit some deformation needed
. to the installation of the spark plugs 24A, 24B and/or the
valves 20, 22.) Furthermore, the recess 42 is preferably
included entirely in an imaginary and axial extension of
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107S988
the cylinder bore.
The two spark plugs 24A and 24B are screwed into
the cylinder head 38 in such an arrangement that their
spark gaps 25A and 25B are positioned in the recess 42
with a distance L therebetween. This distance L is
determined in correlation to the diameter D of the
cylinder bore such that the ratio L/D is in the range
from 0.45 to 0.67. In addition, the position of the
spark gaps 25A, 25B has a specific relationship to the
top face 36a of the piston 36. When the piston 36 at
each power stroke is in a position expressed by a crank
angle of about 40 after the top dead center, each of
the spark gaps 25A, 25B lS at a dis~ance H from the top
face .~a of the piston 36 (the distance H is measured
normal to the top face of the piston 36). The spark
gaps 25A, 25B are positioned such that the ratio H/L is
in the range from 0. 45 to 0.55. As another requisite to
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the arrangement of the two spa~k gaps 2 5A and 25B, a
- spark gap should be located on either side of a plane
containing the longitudinal axis 44 of the cylinder bore
to give a symmetrical arrangement to the axis 44.
The intake valve 20 and the exhaust valve 22 need
to be positioned so as not to interfere with the two
spark plugs 24A, 24B. To provide sufficient space for
the installation of the valves 20, 22 and the distantly
arranged spark plugs 24A, 24B, it is preferable that the
107S988
intake valve 20 is located on one side of a plane
(indicated at 46 in Fig. 2) which is parallel to the
axis 44 and contains the line segment connecting the
two spark gaps 25A, 25B while the exhaust valve 22 on
5 the other side of this plane 46. When the thus arranged
intake and exhaust valves 20 and 22 are operated by a
single cam shaft 50 as shown in Fig. 4, the intake and
; exhaust valves 20 and 22 are positioned such that a
plane which is parallel to the axis 44 and contains the
centers of the valves 20 and 22 (on the periphery of the
- recess 42) is normal to none of the cam shaft 50, the
plane 46 and the firstly mentioned plane between the spark
gaps 25A, 25B. By the employment of this valve arrange-
ment; no interferer.-e occurs b~tween two rocker arms 52
and 54 for the intake and exh~u~t valves 20 and 22 or
. between two cams 56 and 58 for the two valves 20 and 22.
: In operation, the two spark plugs 24A, 24B- produce
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; sparks substantially simultaneously a short time before
the end of each compression stroke. Ideally, combustion
- 20 initiated at the two spark gaps 25A, 25B should be
- completed in the entire volume of the combustion chamber
40 upon meeting of two flame fronts propagated respec-
tively from the two spark gaps 25A, 25B. Actually, such
a manner of flame propagation cannot be realized whatever
25 arrangement of the sparl- gaps 25A, 25B may ke employed
since the flame fronts are generally spherical surfaces.
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1075988
However, it was experimentally confirmed that the
engine 10 runs with good stability, even though a large
amount of exhaust gas is recirculated, if combustion is
completed in about 80% or more of the total volume of
; 5 the combustion chamber 40 when the piston 36 at each
power stroke reaches a position expressed by a crank
angle of about 40 after the top dead center. The above
described arrangement of the two spark gaps 25A, 25B has
been devised on the basis of this experimentally confirmed
fact. When the distance ratios L/D and H/L are respec-
tively within the above specified ranges, two flame
fronts propagated from the two spark gaps 25A, 25B meet
each other upon arrival of the flam~ fronts on the top
` facé ~6a of the piston 36 at the latest even if the
~ombustion proceeds so slowly that the piston 36 is
below the top dead center by a crank angle ~f about 40
when the flame fronts arrive on its top-face 36_. (Note
that the distance H is specified so as to be approximately
half of the distance L.) Then the propagation of flames
is accomplished in about 80% of the entire volume of the
combustion chamber 40 with a piston 36 in this position.
Accordingly the combustion is stable to a practical
satisfaction and the engine 10 exhibits a satisfactorily
high output efficiency.
We have experimentally confirmed also that the above
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1075988
, specified range of the distance L between the two
spark gaps 25A, 25B relative to the diameter D of the
cylinder bore is quite favorable to the fuel economy of
the engine 10. As shown in a qualitative graph of Fig.
S, the fuel economy becomes best when each of the spark
gaps 25A, 25B is at a distance of 0.225D - 0.335D from
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the axis 44 of the cylinder bore.
Thus, the spark plug arrangement according to the
invention allows the combustion chambers of the two-
point ignition type to exhibit their maximum advantage
~' and fully accomplishes the primary object, suppression
-~; of the formation of NOx without substantially sacrificing
', the engine'performance.
Figs. 6 and 7 show a different arrangement of the
intake valve 20 and the exhaust valve 22 in the above
described combustion chamber 40. The two spark gaps
25A, 25B are arranged as specified with reference to
~' Figs. 2-4. The intake valve 20 and the exhaust valve
22 are respectively located on different sides of the plane 46
also in this case as,shown in Fig. 5. However, a plane
, ' which is parallel to the axis 44 and contains the centers
of the two valves 20 and 22 is normal to the plane 46.
When the engine 10 is a multi-cylinder engine as is
usual, the plane 46 coincides with a plane (indicated
at 47) containing the axes 44 of the engine cylinders
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75988
arranged in a rank, and the intake valve 20 and the
exhaust valve 22 are arranged generally symmetrical with
respect to this plane 46 (47) in each engine cylinder.
In this case, the engine 10 has two separate and parallel
S cam shafts 50A and SOB respectively in association with
; the intake valve 20 and the exhaust valve 22 via usual
valve lifter mechanisms 60A and 60B. This type of cam
shaft arrangement , which is called a double overhead
cam shaft type, is quite convenient to realize the
specified arrangement of the two spark plugs 24A, 24B
, without any interference with the intake and exhaust
valves 20 and 22. The spark plugs 24A, 24B can be
screwed into the cylinder head 38 generally vertically
to t~e top face 36a of the piston 36 without interfering
with the valves 20, 22 so that the positioning of the
spark gaps 25A, 25B can be settled with great ,reedom
(of course within the above specified L/D and H/L values).
If desired, each combusti~n chamber 40 may be
equipped with the intake valve 20 and/or the exhaust
valve 22 in plural number. Fig. 8 shows a case when
a combustion chamber 40A is equipped with two intake
valves 20A, 20B and a single exhaust valve 22. The two
intake valves 20A, 20B are located on the same side
of the plane 46 and the exhaust valve 22 on the other
side. In Fig. 9, a combustion chamber 40B is equipped
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with the two intake valves 20A, 20B both located on the
same side of the plane 46 and two exhaust valves 22A,
22B both located on the other side. The two spark gaps
25A, 25B are arranged as specified hereinbefore also
in Figs. 8 and 9.
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