Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~35534~
This invcntion relatcs to a spark-igl-liti(-n
internal combus-tion engine of a type wllereill a plurality
of spark pJugs are disposed wit}-in the combustion
chamber of the engine to reliably ignite a clarse
mixed with inert gases.
It is well known in tlle art that the emission levol
of nitrogen oxides exhausted from a spark-ignition
internal combustion engine is decreased by introducing
inert gases such as exhaust gases into the combustion
chamber to ]ower tho maximum tomperature and pressure
of the combustion. This is usualLy accomplished by
rocirculatins or supplying a portion of the exhaust
gnses into the combustion chall1ber ~Exhaust Gas ~ecircu-
lation). The thus introduced exhaust gases inevitably
cause deterioration of the combustion of the combustion
of the fuel containins mixture in the combustion chamber.
This deterioration of combustion is cllaracterised by a
prolonged combustion time. As a result of the prolonged
combustion time, combustion energy is not effectively
used and lower engine output power accompanied by
deterioration of fuel economy characterises engine
performance. It will be understood that prevention
of the deterioration of the fuel economy and the engine
output performance lS required in addition to exhaust
gas or exhaust emission control.
.... .. .. . .
1~553a~0`
In this rcgard, an enginc cqllipped with two spark
plugs disposed within tl-c combustion chamber hA.S been
proposod in order to shortcn the combustion time of
the charge and improve the combustion deteriorated by
the e~foct of tho inert gases mixed witll the charge.
Ilowever, a mere two spark plug arrangement in the above-
mentioned typc engine cannot improve the combustion
sufficiently and accordingly further improvements have
been required in both exhaust gas control and performance
characteristic of the engine.
It is, therefore, a gcneral object of thc prcsent
invcntion to provide an improvod spark-ignitiol~ tcr-
nal combustion ongine capable of` achieving tlle required
exhaust gas emission control ~ithout deteriora-tion of
the performance characteristic o-f the engine.
Another object of the present invention is to
provide an improved spark-ignition internal combustion
engine in which the locations of the two spark plugs
disposed in the combustion chamber are selected to
provide stable and reliable combustion of the charge
even though the charge is mixed with a considerably
large amount of inert gases such as exhaust gases;
and accordingly, the emission level of nitrogen oxides
is noticeably decreased without deterioration of the
~5 combustion of the charge in the combustion chamber.
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~ 53~
A furtller object of the prcsent invention is to
provide an im~roved spark-ignition internal combustion
engine~ in w]-ich two spark plugs disposed in one com-
bustion chamber are so located as to respectively
contribute to tile combustion of approximately equal
volumes of the combustible mixture or change in the
combustion chamber.
A still further object of the present invention
is to provide an improved spark-ignition internal.
combustion engine capable of low emission levels o~
carbon monoxide and hydrocarbons even though considerably
large amounts of i.nort gases arc mixed wi.th the charge
in tho combustion challlber.
An even further object of the present invention
is to provide an improved spark ignition internal com-
bustion engine in which the combustion chamber thereof
is shaped to generate squish turbulence of the charge
therein to assist effective combustion of the charge.
An additional object of the present invention is
to provide an improved spark-i.gnition internal combustion
engine which employs a so-called siamesed exhaust port
arrangement to maintain the exhaust gas temperature
high enough to accomplish effective oxidation of the
unburned carbon monoxide and hydrocarbons in the exhaust
gases within a reactor disposed downstream of the exhaust `
1C~553~(~
port.
Othor objects, features ancl advantages of the
spark-ignition internal combustion ensine in aecordance
with the present invention will be more apparellt as the
following doscription of a preferred embodiment thereof
progresses, taken in conjunction with the accomparlying
drawings, in which:
Fig. 1 is a schematical plan view of a preferred
embodiment of a spark-ignition internal combustion
engine in accordanee with the present invention;
Fig. 2 is a vortieal seetional viow showins a
eombustion ehalllber of the engiJIe of Fig. l;
F:ig. 3 is a tr~nsverse soetional view of the
eombustion ehamber of Fig. 2;
Fig. 4 i9 a graphieal representation showing valve
overlap of the intake and exhaust valves of the engine
of ~ig. l;
Fig. 5 is a graphieal representation showing the
eomparison between the ignition systems aecording to
the present invention and prior art in terms of time
and pereent of eombustion of thc eharge in the eombustion
ehamber; and
Fig. 6 is a vertieal sectional view showing the
- 5 -
~lLCI S5340
exhaus-t, I)o.rt of thc crlg~ e or I ig. 1.
Referring now to l`~`ls. ]., " and 3 of t:~e clrawi.ngs,
ther~ i,s showll a pre:rerred embodimont of a spark-i.gllition
internal combustion engirlo in accordallce with tl~e present
invention, in whicl~ tlle ellgirle :is general.ly designated
by thc reference numeral 10. The ensine c>f this installce
is an in-line, four cylinder typo and therefore the
engine proper 11 has four alisnod combustion chambers
C1 to CIL therein. As clearly shown in Fig. 2, each
combustion chambor is dofined by the cylindrical inner
wall of a cylinder 1,2 formed in a cyl-i.nder bloclc 1.3,
the inner wal] of a cylindor hoa(l 1~l closing the ono .,:
(upE)er) end of the cylinder 12 and the crown of a piston '';"
16. Tlle spark-plugs 17a and 17b are disposed within
each combustion chamber. The intake port 18 of each
:
combustion chambe,r is communicablo through an intake ,`
manifol,d 20 with a carburetor 22 forming part of air- ~ :~
fuel mixturo supply means 2ll. The carburetor Z2 is
arranged to supply the combustion chambers with an air-
fuol mixture ha~ing a mean air-fuol ratio sli.~htly
richer than stoichiometric, such as from 13.5:1 to 15:1 !,
during normal engine operation. Ilowever, the air-fuel
ratio may be alternately be slightly leaner than the
stoichiometric. As seen, the outlets (no numerals)
from the two exhaust ports of the adjacent two combustion
io5534(3
chaml)cl-c C and C2, and C3 and Cll are combined within
tlle cyl:incler head 1ll of the engirlc proper 11 to form
~o-called siamesed e~haust ports 26a or 26b eac}l having
only one exhaust outlet 28. The cxhaust ports 26a and
26b commullicate with a reactor 30 or an exhaust gas pu ~ ~:
purifying device forming part of the exhaust system of
the engine. The reactor 30 functions to reduce the
concentration of noxious constituents in the exhaust
gases discharged from the combustion chambers by oxi.dizing
the combustibles such as carbon monoxide (C0) and hydro-
carbons (lIC). It will be understood that the reactor
30 may be replaced by an exhaust man:ifold cc>nstructed
to serve as the reactor, or a catalytic convorter. The
reactor 30, in turn, communicates through an exhaust
pipe 32 with the atmosphere. Disposed adjacent to the
inlets (no numerals) of the reactor 30 is a secondary
air injection manifold 34 having two secondary air
injection nozzles 36 which open adjacent the exhaust
ports 26a or 26b. The secondary air injection manifold
3l~ forms part of secondary air supply means 38 which is
arranged to supply combustion air or secondaxy air into
the reactor 30 to promote the oxidation react:ion carried
out in same. Accordingly, the secondary air injection - . .
manifold 34 is connected through a secondary air supply :
. 25 pipe 40 and a control valve 42 to an air pump 1111. The
105534~ ~
secon~ary air Sllpp]y air supp]y moans 3~3 rnay be a
device for inducting air into the oxhaust system of the
engine throu$2l a cl~eck valvo using the intermittent
vacuun1 generatod by the pul.sation of the exhaust gas
5 pressure. Disposed connocting the roactor 30 and the
intako manifolcl 20 is a conduit ~16 or conduit means
for recirculating or supp].ying a portion of the exhaust
gases passing the roactor 30 through the exhaust mani~
fold 20 into the combustion chambers Cl to Cll. The
eonduit ~16 forms part of exhaust gas recir-culating
means 1l8 or an exhaust gas recircu.l.at.ion system. A
eontrol valve 50 is disposed in the eondui-t /18 and is
arranged to eontrol tho amour~t of the rceirculated exhaust
gases with respect to the amount of the intake ai.r induced
through the intalce system in response, for example, to
tlle venturi vacuum which is a function of the amount of
.*he int,ake air. The venturi vacuum is generated at
the venturi por-tion of the earburetor 22.
This engine 10 with the above-mentioned arrangement
. ._ .
reduces the en1ission level of the nitrogen oxides (NOx)
by adding a relatively largo amount of substantially
inert gas to the air-fuol mixture to lower the maximum
temperature of the combustion carried out in the com-
bustion chamber. The substantially inert gas consists ~'
of mixed gases whieh do not substantially participate
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1~5S3~0
in the coml)ll.ctioll -in tllc combllst:i.ol-l charllL~er ancl there-
fore inc:lucles res-iclucll sa~ or coml)llstiorl gas ~ ich is
not discllargcd frolll thc! combust:ion chamber during the
exhaust stroke and rcmai.ns :in the combusti.on chamber,
recirculated exllaust gas ~llich is recirculated or
supplied into the combustion chamber by the exhaust
gas recirculation systent/lo, nitrogell gas (N2) contained
in the intake air, and oxygen gas (2) contained in
excessive air in an air-fuel m:ixture leaner than stoichio-
metric in lean operation of the engi.ne. It will beunderstoocl that the resiclual gas ancl the rec:irclllated
c.~xhaust gas contairl amongst other th:ings carbc)n monox:ide
(C02), water vapour ~ 0) and n:itrogen gas.
According to the present invention, the fuel-inert
gas ratio R or the weight ratio of the fuel combusted
in the combustion chamber and the substantially inert
gas is selected to be 1:13.5 to 1:22.5, and more preferably
1:18.5 during normal engine operation. ~ith respect to
the fuel-inert gas ratio R: if the rate of the inert
sas is decreased below its lower limit 13.5, the NOx
decreasing effect is reducecl, whereas if it is i:ncreased
above its upper limit 22.5, stable combustion in the
combustion chamber is not possible even with the ~ost
. effective ignition with the two spark plugs 17a and 17b..
Additionally, tle NOx decreasing effect is noi improved
~OS5340
to any extetlt by doins same. Tlle unstable combustion
in the combus-tioll chcllllber inovi-tab]y causes noticeable
deter:ioration of the fuo] econollly characteristics and ;
the eng:ine output po~Yer porI`ormanco.
~lany exper:illlents rovea] thclt, by setting the fuel-
inert gas ratio R at the above-mentioned range, more
than 80% of the total combustion carried out in -the
combustion chamber is completcd during the period when
the piston doscends by about /lO degrees of crank angle ~;
from top dead ccnter (i.e. during t}-e expansion stroke).
This shortens the combustion timo o r the charge in the
combustion chamber and therefore the combustion enersy
generated -is offectively talcell out as engine output
power.
In order to actually select the fuel-inert gas
ratio R, the amounts of the recirculated exhaust gas,
the residual gas and the excess oxygen (which does not
participate in the combustion) are varied. The vari-
ation of the recirculated exhaust gas is accomplislled
by the control valve 50 shown in Fig. l. The amollnt
of the residual gas is increascd by increasing a so-
called valve overlap of the intalce and exhaust valves.
For example, when the valve overlap of the intake and -~
exhaust valves is about 40 to 55 degrees of the crank
angle and the diameters Di and De of the intake and
,
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~OS534~
exhaust valves 53a and 53b are respectively 0./15 to
0.55 times and 0./10 to 0.50 times of the cylinc1er bore
diameter D as indicated in Fig. 3, about 20 to 30(,' of
the residual gas remaininS in the cylinde]r during ~ow
engine speed operating range. The amount of the residual
gas can also be controlled by an exhaust pressure re-
gulating valve (not shown) whic11 may be installed in
tho exhaust passage downstream of the combustion chamber.
The amount of the excess oxygen w}lich does not participate
in the combustion is increased by supplying the combustïo
chan)ber with the air-fuel mixture leaner than stoicl-io-
metric. It will be understoo(l that if the amount of
the residual gas increased by the selected valve over-lap
falls within the range of the fuel-inert gas ratio
according to the present invention, it may be unnecessary
to supply the exhaust gases into the combustion chamber
via the exhaust gas recirculation system llo.
In this connection, the fue]-nert gas ratio R is
calcu]ated as follows: Since air (atmospllere) is appro-
ximately composed of 21 Wt% of oxygen and 79 Wt~o of
nitrogen, l4.7 kg of air is required to combust l kg
of the fuel (gasoline). Then the weight of nitrogen
(in air) which does not participate in the combustion
is obtained by the following equation, given that the
molecular weights of N2 and 2 are 28 and 32, respectively:
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~L05534(1 . ~:
7 3 1 ~ x O ~ 1 + r) ~ ~ () / () = 1 1 ~ 3 ( lc ~ q
~dclitionally, i-t will be unclerstoocl -t}-at the specific
gravity of tlle cxl-aust gases is approximately same as
that of air, an~ all the excessive air in the air-fuel
mixture (if leaner than stoichiometric) does not parti-
cipate in the combustion of the charge carried out in
tl~e combustion chamber. Accordingly, when the rate of
mixture gas containing the recirculated exhaust gas7
the re~sidual gas and the excess air is 10% witl~ respect
to the air required for the combustion, the Weig}lt of
thc mixture gas is l/1.7kg x 10/100 = 1.5 Icg; wheTI the
rate is 20%~ the weight is lilcewise l/1.7 kg x 20/100 = 2. 9 kg.
In view of tho above, the ratio of the weigh-t of the ~?
inert gas to the fuel in the mixture is the sum of the
ratio of nitrogen in the abo~e-mentioned Eq. (l) alld
the above-mentioned mixture gas weight (or the weight -~
ratio to the fuel), and -therefore is obtained as shown
in the following table-
~ 12
';~ ~''
ii340
_ -~
inert gas incroased inert total incrt
ratc gas to ni-tlosen gas to fuel
ratio ratio
.
10/'. 1.1l7 :L.1~7+11.3 = 12.~3
15% 2.2~ 1 2.21+11.3 = 13.S
20% 2. 9l~ 2.91~+11.3 = 1/l.2
30% Il.llO 4.ll0~11.3 = 15.7
llo% 5.88 5.~3~11.3 = 17.3
50~0 7.35 ' 7.35~11.3 = lo.7
60% 8.~2 ~.o2+11.3 = 20.1
_ . : .
Consequently, for example, when the amOUrltS of tlle
rcsidllal gas and the recirculated exhaust gas are
respectively 15% and 30% with respect to the amount of
air required for combustion of the air fuel mixture,
the ratio of the inert gas to the fuel is the sum of
45% f the mixture gas containing tlle residual gas and
the recirculated exhaust gas, and the nitrogen from the
inducted air, i.e. 6.62 ~ 11.3 = 17.9. Accordingly,
the fuel-inort gas ra-tio R or the weight ratio be-tween
fuel and the inert gas is 1:17.9. Similarly, when the
residual gas and the recirculated gas are respectively
10% and the air-fuel ratio is 16.2:1 ( the excess air
factor ~ = actual intake air weight/calculated air
weight required for combustion = 1.1), the ratio of
the inert gas to the fuel is the sum of 20% of the
exhaust gas, 10% of the excess air and the nitrogen in
.
'.
.
,
: i: . . . . . .
. . . , ~ . . , :
~05~340
tl~e intake air, i.e. 3.9l~ 7 + 11.3 = 15.7, and
thererore the f`uel-inert gas ratio 1~ is 1:15.7.
ReI`errins again to l~igs. 2 ancl 3, two spark pluss
17a and 17b are disposed w-ithin eacll cornbllstion chamber
so that the electrodes projocte froo1 the inner wall of
the cylinder head 14 into the combustion chamber. Each
spark plug 17a or 17b has two elec-trodes (no numerals)
defining therebetweon a spark gap or a distance between
the two electrodes. The two spark plugs 17a and 17b
are located, as shown in Fig. 3, inside the bore of` the
cylindcr 12 such that the intermediate points (not
identified) of the two spalk gaps constitute an ansle
ransing from 110 to 1~0 degrees with resyect to the
center axis 0 of the cylinder 12. Furthermore, as
shown in Fig. 2 the two spark plugs 17a and 17b are
separa-ted from each other such that the area defined
between the wall of the combustion chamber and the arcs
of the curves of two imaginary circles 52a and 52b is
at least 20% of the virtical sectional area of the
combustion chamber which is defined by the crown of the
piston 16 which is at the position as shown in Fig. 2,
i.e. descended by 15% of the distance of one stroke of
the piston 16 from the top dead center toward the bottom
dead center. The imaginary circ-Les 52a and 52b are
respectively drawn with their center at the intermediate
.:
:, ~
,`: :~
.'' . ' ' ` .
,'.': ` ~ ' ' " . : . ` ` .~
~(~S5340
points of the spark gap of the spark plugs 17a ancl 17b
with a radius wllich is one llalf of an imaginary line L.
The line L connects the s}~ortest di~tance the i.ntermediate
po:ints of the spark gap of tl~O spark plugs and passes `.
througll the center axis 0 of the cylinder, on an imaginary
surface (not identified) which extends along the cen-ler
axis of the cylinder 12 and contains the imaginary line
L. It will be understood that if the imaginary line L
passing through the center axis 0 of the cylinder 12 is
not straight, the imaginary surface may include two
imaginary plane surfaces one of wh:i.ch contains a segmellt
of the lille L connectinS the intermediate po:int of thc
spark gap of the spark plug 17a and the center axis 0
and the other contains a segment of the line L connecting
the intermediate of the spark gap of the spark plug 17b
and the center axis 0. In Fig. 3, the reference numerals ~ :
53a and 53b indicate an intake valve and an exhaust valve,
respectively.
With the above-described spark plug arrangement,
tho combustion volume allotted to each spark plug 17a
or 17b is approximately one half of the combustion
chamber volume causing shortening of flame propagation
distance per spark plug, and therefore the combustion
time. In general, shortened combustion time resuIts in.
2$ an improvement in the efficiency of converting combustion
_
. . . .
. . ~ ~ : . .
' ...... :
~5534~ -:
::
enorsy Ol` l)ressure illtO mecllallical work thereby
acheiving improvcmonts in fuel consulllptiorl characteristics
and engille output powcr pcrformarlce. Additiorlall.y7 even
w}cn tlle fucl-inert gas ratio R mentioncd before is
considerably increased stable combustion of the charse
in the combustion chamber is still possible. Ho~1ever
if the locations of the two spark plugs 17a and 17b are
excessive~.y closo to each other the ignition effect
thereof is similar to that of the prior art wherein the
charge in the combustion chamber is i$nited by only one
spark plUS
~ ig. 5 shows the combust:ion rate of the charge in
the combustion chambcr in whicll a curve a represents
the rate acheived by the spark plug arrangement according
to the present invention and a curve b represents the
rate acheived by the prior art wherein the charge in
the combustion chamber is ignited by only one spark
plug. This figure indicates that the combusti.on rate
according to the present invention :is considerably
higher than that of the prior art at a certain time
after ignition.
It will be appreciated from the foregoing that the ~
above-described spark plug arrangement according to the ~ :
present invention makes possible the stable combustion .
carried out in the combustion chamber even urlder
- 16 -
~534(~
conclitiolls wherein relatively large amounts o~` the
substantially inert gas is added to thc charse in the
combustion chaml)er.
In orcler to fur-ther iml)rove the effect of the
above-described two spark arrangement, various other
features of this engine according to the invention will
be discussed hereinafter.
As illustrated in Fig. 2, the cylinder head 1l~
has a hemispherical concavity 5/l formed concentrically
with respect to the bore of the cylinder 12. As shown,
tl-o concavity 5~l l`orms major part of the combust:ion
chamber. It is to be noted that the diameter d of the
llel-lisphcrical concavity 5~ is smal:Ler than that D of
the bore of the cylinder 12 and consequently an annular
space 56, called a squish area, is formed between the
annular flat portion 58 of the cylinder head 1~1 defining
the combustion chamber and the peripheral portion of
the crown of the piston 16 whicll is at the top dead ~`~
center thereof. With this squish area 56, during the
last stage of the compression stroke of the piston 16,
most the air-fuel mixture or the charge supplied into
the combustion chamber is squeezed out of the sqllish
area 56 and moved toward the central portion of the ~ ~;
combustion chamber to produce squish turbulence within
the combustion chamber.
.
.
' . :
:, . . . .
~553~0 `
The squ~ turbul.erIcc promotes sIlloo-t;Il and rapid burn.ing
of the air-fuel mixture in tI~e combusti.on chaIllber and
therefore contributes to further sIIorteni.ng of the
combustion ti.me.
It will b~ understood tIIat as the area of thc
annular flat portion 58 of the cylinder head adjacent
the hemispherical concavity 5II increases, the squisI
area i.s increased and accordingly the effect of the
squish turbulence is increased. IIowovor, sincc the
squish area contacts the inner surfaco of tho cylinder
12 which is coolod by a coo:I.ant (not showrI) flo~:ins
outside the cylinder 12, tho air-fuel mixture residing
in the squish area 56 is cooled and does not react
readily, which may cause the flame in the combustion ~.
chamber to go out and possibly cause misfire of the ~ :
engine. Therefore, the emission level of unburned :~
hydrocarbons (IIC) is increased with the increase of the
area of tho annular flat portion 58 of the cyli.nder
head adjacent the hemispherical concavity 51I. As will
be appreciated from the foregoing discussion, although
the squish turbulence produced by the squish area 56 :i
promotes the smooth and rapid burning of the air-fuel
mixture in the combustion chamber, it invites increase
of the emission level of unburned hydrocarbons. In thi.s
connection, the squish area is also called a quench area
- 18 -
~a~s~3~ `
(an arca ~here tl-c flame will go out). In view of the
foregoing, tlle area of tl~e annular flat portion 58 of
the cylincltr head 1~l should bc detcrnlined such tllat
the elllission level of`llydrocarbons is not, to any extent~
increased but sufficiellt squish turbulence is produced.
Experimcnts revcal that it is preferable for obtaining
the above-described intended purpose that thc area of
the antlular flat portion 58 is in the range from 0.1
to 0.35 times of the cross-sectional area of the bore
of the cylinder 12. Furthermore, the thickness of the
squish area 56 or the distance between the annular flat
por-tion 5~ of the cylirlder llcad 14 and tlle crown O:r the
piston 16 at the top dcad center :is preferably less than
about 2.5 mm in addition to the thickness of a gasket
(no numeral) disposed between the upper end of the
cylinder block 13 and the cylinder llead 14 in order to
obtain better effect due to the squish area 56.
It will be understood that improved mixing of fuel
and air and uniform dispersion of the recirculated exhaust
gas in the combustion chamber may be attained by producing
swirl turbulence due to the momentum of the higl~-speed
incoming gas iII addition to the squish turbulence due
to the squish area 56. The swirl turbulence may be
produced with a suitable inlet-tract design sucll as
tangential location of the intake port 18 to tlle
- 19 -
.
,
~ . . . .
~5534(~
i
combllstion cham~or, or by providing mearls for chang.ing
the flow d:irect:ion of` the incoming ~as, such as an .
obstructi.on plate, a guide plate or a guide groovo,
on tho surface of the i.ntako valve 53a and its valve
seat ~not idcntified).
In order -to proverlt undesirable close positi.oning
of the two spark plugs 17a and 17b, they are positioned : .
such that the shortest distanco between the interlllediate
points of the spark gaps of the two spark plugs 17a and .
17b is in th( range from 0./l to o.8 times of tlle di.ameter
D of tho bore of the cy:l.i.ncler 12, and adcliti.orlally tlle
shortost distanco l' betweon tho intermediate point
of the sparlc gap of eac}l spark plug and the center axis
O of the cylinder is in the ran~e from 0.15 to 0.1~5
,
times of thc diameter D of the bore of the cylinder 12. ~:
It is more preferable that the distances between the
interme~iate points of the spark gaps of the two spark
plugs and the center axis 0 of the cyli.nder are equal.
Furthermore, exporiments reveal that stable combustion
is accomplished by disposing one spark plug per comt)ustion
chamber volume (defincd by the cylinder head 14 and the ~.
crown of the piston 16 at the top dead center) in the :
range from about 15 to about 45cc, preferably in the .:
range from 15 to 40cc. If one spark plug is disposed
per volume greater than ll0cc of the combustion chamber,
~ 20 ~
~1~5534~
stable combustion in tlle combustion chamber is not
obtainecl. Fur-ther it is difficult to disl)oso one spark
plug per vo]ume less thall 15cc of the combllstion cllamber
from v:iewpoillts of structure and space recluired for the
spark plugs and, additionally, the eombustion is not
improved to any extent by doing same. To obtain the
above-described combustion chamber volume for one spark
plug, the displacement of tho cylinder 12 (or the volu!ne
of a space defilled by the cylindrical wall of the cylinder
12, the inner waLl of the cyl:inder head lfl and the crown
of the piston 16 at the bottom dead eentor~ should be
at lt-~ast 290 ee. With this eonnoetion, the eompressio
ratio i~Y preferab~y o to 10.5.
It is desirable to projeet eaeh spark plug to the
eentral portion of the eombustion ehamber for effective
eombustion of the eharge, sinee the flame produeed by
the spark plug spreads spherieally with the eenter at
a point where the spark is produeed. However, it was
thought that the spark plugs loeated at or near the
eentral por-tion of the eombustion ehamber and subjeeted
to an extremely higb temperature would undergo thermal
damage. However, experiments reveal that eombustion
effieieney is improved and the durability of the spark
plugs is not redueed by~loeating them sueh that the
intermediate points of the spark gaps thereof are
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,
.
~55340
~roj~cte(l an(l sl~ace(l apclrt .I`rom the inner ~all o:f the
cyl.inder hcad 1ll by a di.~tance ranS:irlg rrom 2 to 7 mm,
prcfc I` al~ly ll.5 mm.
In ordcr to improve tl-e coml)ustion~ in addi-tion
to the effect due to t]-le Irojccted spark p:lugs the spark ~;
gap of each spark plug 17a ancl 17b is set a-t 1.1 to
2.2 mm to produce a relativ(ly long sparlc bet~eerl the ~ ~
electrodes of the spark plug. With this connection, . :.
the engine 10 according to the present invention is
equi.pped with ineans for generating spark ~nergy rallging
from 50 to 1''0 mj (miri-~jou.Le), preferably 50 mj to .
provide stable and rcL.iable spark:ing even -tl-ough -the
sl)ark g~p is considerably long.
The combustion chamber is preferably formed into
a hemispherical type as shown in Fig. 2, a bath-tub type,
or a heron type to decrease the combustion chamber
surface area-to-volume ratio and accordingl.y decrease : ...
the quench area. This prevents generation of large
amounts of.llC and therefore contributes to decrease of
HC omission ].evel.
In general, the ignition timing of an engine is
...... set slightly before top dead center to effectively use
the combustion pressure produced. Normally, in order
to improve exhaust gas control, the ignition timing
must inevitably be retarded with respect to that required :
:.
- 22 -
~05S3~0
for beqt torque. On the contrary, in tho engjne
according to tho present invention, the i~gnition tirning
is set at the minim~lm advance required for hest torque
or near where fuel consuml>tion is most improved ancl
theref`ore contribu-te to fllel economy. To tllis end, the
engine 10 of the present invention is eqllipped with a
device (not shown) for advancing the ignition timing
t~ meet the operation condition of the ensine.
With the arrangement hereinbefore discussed, the
emission level of NOx is noticeably decreased without
loss of s-table and re1iclble comb1lstiorl of the charge
in the combustion chamber. The elllission levols of CO
and IIC are decreasecl by means for oxidizins the l~nburned
constituents in the exhaust gases discharged from the
combustion chamber, such as a thermal reactor 30, a
catalytic converter, or an exhaust rnanifold serving as
a thermal reactor. It is necessary to maintain the
temperature in the thermal reactor as high as practical
for the purpose of promotion of the oxidation reaction
of the unburnod constituents. ~rith this connection,
the engine 10 according to the present invention employs
so-called siamesed exhaust port arrangement as indicated ~ `
by the reference numerals 26a and 26b to provide the
-- reactor 30 ~ith high temperature exhaust gases. This
is achieved by preventing the temperature drop of the
- 23 ~
10553~0 ` " " ~ ,
e~llaust ga.~e.s pass:irlg nl).5 tr-ealll of` tl10 reactor 30 with
the rolat.:iv(?ly slllall ~sur:~`ace arca of tho s:i.amesed
exhaust por-t. Th:is tomper.Ituro drop preverltion effect
is further irnproved by d:i.Ypos:i.r1s a so-cal:k?d port linor
5 Go made o~` a heat-resi.stant matorial on the inner surface
of the siamosed ex1laust port ~6a or 26b as sl-own in
I~ig. 6. As seen, the port linor 60 linos and covers
the inner surface of tho exhaust port maintaining a
dosired space therehet~een. This port liner arrangement
can prevent exhaust h(?at transmissi.on through the inner
wall of the exhaust por1. to tho cylindor hoacl l/l.
'~xper:i.nlents rovea:l. that the oxl1a1l.st gas temperaturo in
the onsine with th1.s port :I.inor a.rrang(?ment is mair1-
tained lO0 to 150 C h:i.gher than that in prior art engine
15 which does not employ the port liner arrangement. .
Whilc? only a thermal reactor 30 i.s shown and
described hereinbefore to treat the exhaust gases dis-
charged from the combustion chambers, it will be under-
stood that, to further improve the emission control of
NOx, the reactor 30 may be roplacad wi-th a three-way
catalyt:i.c converter which reduces NOx and oxidi~.es CQ
and ~IC in the exhaust gases. This three-way cata1ytic ~ `
conver-ter effectively f1mctions only when suppli.ed with
the exhaust gases having approximately stoichi.ometr:ic
oxygen-to-combustibles ratio and therefore requires
~ .
'
.
.,
' , .
....... . . .
.. . ~ , . . .
~55340
ovorall colItrol of the air-fIlel ratio. The overall
air-fuel ratio i~s a ratio of` tIIe total a.iI' (the
sum of the intake air and the secondary air supplied
upstream of the exIlaust gas pIlr-ifyin$ device 30) and
the fuel supplied to the in-taIce systen- of tI-e engine.
~or this purpose, the air-fuel mixture supply means
24 may include an electronical:Ly controllecl fuel injection
system or an electronically controlled carburetor wIIich ~-
may be operated by a feedback control device which is
well known in the art. The feecIback control device
is, for example, arranged to regulate the air-fuel ratio
of an air-fuel mixture produced by the carburetor and
the fuel in~jectioIl sy~tem to a required value in responsa
to the composition of the exha-Ist gases, the composition
being sensed by a sensor disposed in the exhaust system
of the engine to generate and transmit the information ~ -
signal corresponding to the composition to the feedback
control device.
As is apparen-t from the foregoing discussion,
according to the present invantion, a relatively large
amount of the substantially inert gas is added to the
charge in the combustion chamber to lower the combustion
temperature causing a noticeable decrease of the NOx
emission level. The charge containing the substantially
inert gas is ignited by two spark plugs disposed per
1~55i3~ ~
combus-t:ion cllaml~cr, proventitl$ dctcr:i.ora-ti.on Or com- -
busti.oll o:f thc charge carricd oui. in tlle combl.lstior~
chambor. ~`hol~eforo, t11e NOx etll:ission ]evol. is remarkeclly
rcduccd ~ithout doteriorat:ion of` the pcrfc)rmance of tl-e
engine. ~urthc~rnlore, s:illce tho tomperature of the
exhaust gases dischargcd from the colnbllstion chamber
is maintained higll eno~gh -to promote oxidation roactior
in the exllaust gas purifyi.ng device, the emission levels ~ ~:
of CO and ~IC are effecti.vely decreased.
- 26 -
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