CA1071484A - Spark-ignition engine having improved ignition system - Google Patents

Abstract

Abstract of the Disclosure Substantially inert gases such as exhaust gases are added to the air-fuel mixture inducted into the combustion chamber so that the weight ratio of fuel combusted in the combustion chamber and the substan-tially inert gases falls within the range from 1:13.5 to 1:22.5. The fuel mixture mixed with such a high rate of the substantially inert gases is effectively ignited by a spark plug or spark plugs located suitably for shortening the combustion time of the air-fuel mixture in the combustion chamber. Additionally, a reactor is installed downstream of the combustion chamber to treat the exhaust gases discharged from the combustion chamber.

Description

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This invention relates to an improvement in a spark-ignition internal combustion engine of the type where substantially inert gases are added to the charge into the combustion chamber to decrease the formation of nitrogen oxides.
It is well known that the exhaust gases from a spark-ignition internal combustion engine contain, as noxious constituents, nitrogen oxides, carbon monoxide and hydrocarbons. The formation of nitrogen oxides (NOx) is increased as the combustion process in the - combustion chamber is improved. Conversely hydro-carbons (HC) and carbon monoxide (C0) emission levels are increased as the combustion process deteriorates.
; Of the emission levels into the environment of these noxious constituents, those of HC and C0 can be ~ -~
easily decreased by improving the combustion in the combustion chamber, or by means of a catalytic con-verter or a thermal reactor. However, the form&tion of NOx i~ considerably more difficult to decrease, ^ 20 because the formation thereof is increased as the ~ combustion conditions are improved, and NOx once ; generated in the combustion chamber is not easily removed by a catalytical reduction reaction, the catalyst also producing problems with respect to ; 25 performance and durability. Therefore9 the greatest _ 2 -4~4 effort is now directed to the decrease of the NOx generation. Since the NOx emission control downstream of the combustion chamber encounters the above-mentioned problems, it seems better to achieve the emission control of NOx within the combustion chamber. For this purpose, it has been proposed to supply substantially inert gases such as exhaust gases into the combustion chamber in order to lower the maximum temperature and pressure of ;
combustion carried out in the combustion chamber. This is achieved, for example, by a so~called exhaust gas recirculation system (EGR system) D With this supply of substantially inert gases, the emission level of NOx is found to decrease as the amount of the inert gases is increasedO However, by supplying the combustion chamber with a considerable amount of the inert gases, the combustion time of the air-fuel mixture is increased and therefore combustion of the air-fuel mixture in the combustion chamber is not stable and smooth. In view of the above, the amount of the inert gases supplied to the combustion chamber is reqtricted to a relatively low level in due consideration of both stable combustion and NOx emission control. The unstable combustion of : :- .~
the air-fuel mixture causes deterioration of engine - power output and fuel consumption characteristics, and increase of C0 and HC emission levels.

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In this reyard, the inventors' attention is directed to improve the cornbustion in the combusti.on chamber by decreas-i.ng the combustion time of the air-fuel mixture in the combust.ion chamber.
Therefore the present invention provides an improved spark-ignitihn internal combustion engine in which NOx yenera-tion is decreased by adding substantially inert gas to the air-fuel mixture supplied to the combustion chamber.
The present invention also provides an improved 10 spark-ignition internal combustion engine of the type wherein substantially inert gases are added to the air-fuel mixture supplied to the combustion chamber, in which a spark plug or spark plugs are located in the combustion chamber such that the travel di.stance of the flame produced by the spark of the plug : . or plugs in the combustion chamber filled with the air-fuel mixture is shortened to decrease the combustion time-of the air-~ . fuel m.ixture in the combustion chamber, thereby achieving .`. stable and smooth combustion of the air-fuel mixture and decreas-ing the emission levels of CO and HC caused by the unstable 2Q con~ustion of the air-fuel mixture.
~ The present invention further provides an improved spark-ignition internal combustion engine of the type where the air-fuel mixture mixed with a relatively large amount of sub-stantially inert gases is effectively combusted by igniting with a spark of the pl.ug or plugs suitably located to achieve : stable and smooth combustion of the air-fuel mixture, in which the concentrations of CO and HC are fur-ther decreased by oxidiz-ing the unburned constituents contained in the exhaust gases discharged from the combustion chamber in a reactor installed downstream of the combustion chamber.

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According to the present invention there is provided a spark-ignition internal combustion engine 9 comprising: means ~or defining a combustion chamber; air-fuel mixture supply means ~or producing an air-fuel mixture in the combustion chamber by mixing fuel and intake air inducted into the combustion chamber; means for adding substantially inert gas to the air-fuel mixture in the combustion chamber, said substantially inert gas being mixed gases which remain substantiall~ inert in the combustion of the fuel in the combustion chamber, said means for adding inert gas including control means for controlling the ratio of fuel combusted in the combustion chamber and the subtantially inert gas in the range of from 1:13.5 to 1:22.5 by weight during normal engine operation; two spark plugs disposed in the combustion chamber and located to shorten the travel distances of flames produced by sparks thereo~ in the combustion chamber filled with the air-fuel mixture so as to decrease the combus~ion time of the air-fuel mixture; and reactor means for purifying the exhaust gases discharged from the combustion chamber by oxidation of the constituents contained in the exhaust gases.
The present invention will be further illustrated .; ~
; by way of the accompanying drawinys in which like reference numerals designate like parts and elements, and in which:
Fig. 1 is a schematical section view of a preferred ~; embodiment of a spark-ignition internal combustion engine in accordance with the present invention;
Fig. 2 is a vertical section view showing a cylinder head and a piston which defined a combustion chamber of the engine of Fig. l;
Fig. 3 is a plan view of a cylinder head defining ~`

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the combus*ion chamber of Fig~ 2;
Fig, 4 is a vertical section view showing an exhaust port of the engine of Fig. l;
Fig. 5 is a graph showing the valve overlap of the intake and exhaust valves of the engine of Figo 1;
Fig. 6 i~ a diagram showing the valve overlap similar to that of Fig. 5; ~;
Fig. 7 is a diagram illustrating the amount of adding substantially inert gas to the air-fuel mixture ~ 3 drawn in the combustion chamber of the engine of Fig. l;
Fig. 8 is a schematical section view of another preferred embodiment of the engine according to the - present invention;
~ig. 9 is a schematical section view of a further . .
~ pr~ferred embodiment of the engine according to the - present invention;
Fig. 10 is a graph showing the combustion manner in the combustion chamber of the engine of Fig. 9, in ~; 20 terms of crank angle, combustion rate and combustion volume rate;
Fig. 11 is a vertical section view of the com-bustion chamber of the engine of Fig. 9;
Fig. 12 is a plan view of a cylinder head defining ; 25 the combustion chamber of Fig. 11;

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Fig. 13 is a vertical section view showing the combustion chamber of the engine of Fig. 9;
Fig. 14 is a vertical side section view showing the combustion chamber of the engine of Figo 9; and 5 . Fig~ 15 is a graph showing the exhaust gas re-circulation rate characteristics of the exhaust gas recirculation system of the engines of Figs. 1, 8 and ~ 9, in terms of intake vacuum and engine spe0d.
;~ Referring now to Figs~ 1~ 2~ 3 and 4 of the drawings~ a preferred embodiment of a spark-ignition .~ .
internal combustion engine in accordance with the present invention is shown, in which the engine is generally designated by the reference numeral 10.
The engine in this instance is an in-line, four ~15 cylinder type and therefore the engine proper lOa has four aligned combustion chambers Ci to Cl~ therein.
As clearly shown in Figo 2, each combustion chamber is defined by the cylindrical inner surface of a cylinder 12 formed in a cylinder block 14, the inner surface of a cylinder head 16 closing the one ~upper) end of the cylinder 12 and the crown of a piston 18.
The combustion chamber is communicable through each intake valve 19 with each intake port 20 which is, in turn, communicable through an intake manifold 23 with a carburetor 24 forming part of air-fuel mixture supply 1~7~8~

means 27 or the intake system of the engine. The carburetor 24 is arranged to supply the combustion chambers with an air-fuel mixture having a mean air-fuel ratio slightly higher than stoichiometric, such as from 13.5:1 to 15:1 during normal engine op~ration.
As seen, the outlets (no numerals) of two exhaust ports communicable, respectively, through exhaust valves 21 with two adjacent combustion chambers Cl and C2, and C3 and C4 are combined within the cylinder head 16 of the engine proper lOa to form so-called siamesed ; exhaust port outlets 22a and 22b each having only one exhaust outlet 25 from the engine lOa. The exhaust port outlets 22a and 22b communicate with a reactor 26 or reactor means forming part of the exhaust system of the engine. The reactor 26 functions to reduce the concentration of noxious constituents in the exhaust gases discharged from the combustion chambers by oxidation of the combustibles such as carbon monoxide (C0) and hydrocarbons (HC) therein. It will be under-stood that the reactor 26 may be replaced by an exhaust manifold constructed to serve as the reactorO The r~actor 26, in turn, communicates through an exhaust pipe 28 ~ith the atmosphere~ Disposed adjacent to the inlets (no numeral~) of the reactor 26 is a second-ary air injection manifold 30 having two secondary air ~ 8 , ~07~48~

injection nozzles 32 which open to the exhaust ports 22a and 22b. The secondary air injection manifold 30 forms part of secondary air supply means 34 which is arranged to supply combustion air or secondary air into the reactor 26 to promote the oxidation reaction carried out in same. Accordingl~, the secondary air injection manifold 30 is connected through a secondary air supply pipe 36 and a control valve 38 to an air pump 40. The secondary air supply means mayt as clearly sho~n in Fig. 4, be a device 34~ for inducting --~`~ atmospheric air into the exhaust system of the engine through each secondary air injection nozzlc 32' opened to the exhaust ports 22a and 22b adjacent to each exhaust valve 21. Each injec*ion no~zle 32' communi- -~
cates through a check valve 48 and an air filter 50 with the atmosphere. The induction of the atmospheric air is accomplished by the intermittent YacUUm gen0r~
ated by the pulsation of the exhaust gas pressure due to the open-and-close action of the exh~ust valve 210 20 The check valve 48 is, accordingly, adapted to open to ~ ~

induct the atmospheric air when subjected to a negative ;-pressure generated at the exhaust ports 22a ~nd 22b, and to close to prevent the back-flow of the exhaust gases into the atmosphere when subjected to a positive pressure at the exhaust port.

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Disposed connecting the reactor 26 and the intake manifold 23 is a conduit 42 or conduit means for re-circulating or supplying a portion of the exhaust gases through the intake manifold 23 into the combustion chambers Cl to C4. The conduit 42 forms part of exhaust ; gas recirculating means 44 or an exhaust gas recircu-lation system. A control valve 46 is disposed in the conduit 42 and is arranged to control the amount of .~ the recirculated exhaust. gases with respect to the : 10 amount of the intake air induced thro~lgh the intake ;:
- system in response, for example, to the venturi vacuum which is a function of the amount of the intake air.
The venturi vacuum is generated at the venturi portion (not shown) of the carburetor 24.
15 The exhaust gas recircula*ing means 44 is arranged to add substantially inert gas to the air-fuel mixture in the combustion chamber by only itself or in combi-nation with means for controlling to increase so-called valve overlap of the intake and exhaust valves 19 and 21 or the duration in which both the intake and exhaust valves 19 and 21 are open, as seen in Fig. 5. Accord~
ingly, in this instance, the valve operati.ng mecharlism Inot shown) of the engine 10 may be set to control the valve overlap of the intake and exhaust valves 19 and 21 within the range from 35 to 60 degrees of crank angle ,~

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of the engine. It will be appreciated that addition of the substantially inert gas to the air-fuel mixture in the combustion chamber results in lowering the maximum temperature of the combustion carried out in the combustion chamber, causing reduction of nitrogen oxides (NOx) farmation level. The substantially inert gas consists of mixed gases which substantially do not participate or remain substantially inert in the combustion in the combustion chamber and therefore the inert gas includes (1) residual gas which is not dis-charged from the combustion chamber during the exhaust stroke and remains in the combustion chamber, (2) ;;
recirculated exhaust gas which is recirculated or ,~ , supplied into the combustion chamber through the ex ~15 haust gas recirculating means 44, (3) nitrogen gas (N2) contained in the intake or inducted air, and (4) oxygen gas (2) contained in excessive air in an air-fuel mixture leaner than stoichiometric in lean operation of the engine. It will be understood that the residual gas and the recirculated exhaust gas contain amongst other things carbon dioxide (C02) 7 water vapour (H20) and nitrogen gas. With rsspect to the residual gas, by controlling the valve overlap of the intake and exhaust valves 19 and 21 within the range from 35 to 60 degrees of the crank angle, about 20 to 30% of the - 11 - ' 7~84 ,,.~, residual gas with respect to the intake air amount remains in the combustion chamber. During idling of the engine, about 40% of the residual gas with respect to the intake air amount remains in the combustion chamber by setting the valve overlap at 50 degrees of the crank angle.
According to the present invention, the weight ratio of the fuel substantially combusted in the com-bustion chamber and the substantially inert gas added to the air-fuel mixture supplied to the combustion chamber, is selected to be within the range from 1:13.5 to 1:22.5 (this weight ratio is referred to as fuel-inert gas ratio" hereinafter).
As hest seen in Figs. 2 and 3, in order to re-~15 liably ignite the air-fuel mixture mixed with such a large amount of the substantially inert gas, two spark plug~ 52a and 52b or spark plug means are disposed in each combustion chamber in such a manner that the two spark plugs 52a and 52b are installed through the cylinder head 16 and their electrodes are projected into the combustion chamber~ The two spark plugs 52a and 5~2b are located spaced apart oppositely from the center axis Xc of the bore of the cylinder 12 and near the periphery of the combustion chamber~ The location~
f the two spark plugs 52a and 52b are preferably such .

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that a midpoint of the spark gap or the distance between the two electrodes (no numeral) of the spark :
plug 52a and the similar midpoint of the spark gap of the spark plug 52b constitute an angle ranging from 110 to 180 degrees with respect to the center axis Xc of the eylindcr bore, and the shortest distance L
between the midpoint of the spark gap of each spark ~.
plug and the center axis Xc is 0~15 to 0.45 times the diameter of the cylinder bore.
10 With this locations of the two spark plugs 52~
and 52b, the combustion volume alotted to each spark : ~:
plug is approximately one half of the combustion ehamber volume causing shortening of the travel length or distance of flame per spark plug and therefore the eombustio~ time is decreased. In general, shortened eombustion ti~e results in an improvement in the efficiency of converting combustion energy or pressure ~:
into meehanical work, thereby achieving the improve-ment in fuel consumption characteristics and engine . power output performanceO Additionally, even when the above-mentioned fuel-inert gas ratio is as high as 1:13.5 to 1:22.5, reliable ignition and stable combustion of the air-fuel mixture in the combustion chambers are aehievedO If the locations of thé two . spark pl~1gs 52a and 52b are excessively close to each ~7~4~34 -:

other, the ignition effect thereof is similar to that of a case wherein the charge in the combustion chamber is ignited by only one spark plug. It will be appre-ciated from the foregoing that the above-mentioned locations of the two spark plugs contribute to reliable ignition and stable combustion of the air-fuel mixture in the combustion chambersO
With respect to the selected fuel-inert gas ratio:
lf it i5 higher than its higher limit 1:13.5, the :10 decreasing effect of NOx is deteriorated; whereas if : it i3 lower than its lo~er limit 1:22~5, stable com-bustion in the combustion chamber is not possible even with the most effective ignition with the two spark plugs 52a and 52b. Additionally, the NOx formation decreasing effect is not improved to any extent by doing sameO The unstable combustion in the combu~tion chamber inevitably causes noticeable deterioration in the fuel economy characteristic~ and the engine output power performanceO
20 In this connection, determination of the fuel-inert gas ratio will be explained hereinafter. Since the weight ratio of the fuel (gasoline or petrol) and the atmospheric air in the stoichiometric air-fuel mixture is 1:14.7 and the volume ratio of oxygen gas (2) and 25 nitrogen gas (N2) is 21:79, the weight ratio of the _ lr~ _ .

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, fuel and the nitrogen gas in the stoichiometric air- :
fuel mixture is 1:11.3. Now, when the substantially inert gas is added to the air-fuel mixture so that the recirculated exhaust gas 7 the residual gas and the excessive air (when the combustion chamber is fed with an air-fuel mixture leaner than stoichiometric) are X%~
Y% and ~% by weight, respectively, with respect to the intake or inducted air of the stoichiometric amount to ~ :
the fuel combusted in the combustion chamber, the fuel-inert gas ratio is obtained by the following equation in consideration of the fact that the weight ratio of the air and the exhaust gases or the re~idual gas is about 1 1 : (11.3 + 1~.7 x (X +lYo+ Z) In connection with the above, the intake air amount stoichiometric to the fuel combusted in the . combustion ch~mber will be determined as follows:
I~ In case ~here the combustion chamber of the engine is supplied with a s*oichiometric air-fuel mixture, the amount of combusted fuel corresponds to the amount of all fuel inducted into the combustion chamber of the engine and accordingly the intake air amount stoichiometric to the total inducted fuel corresponds to the amount of all air inducted into the ~ ' ' , '' ' 7~34 combustion cha~ber. Therefore, the intake air amount stoichiometric to the combusted fuel, in this case, is the amount of all air inducted into the combustion chamber.
II) In case where the combustion chamber is supplied with an air-fuel mixture leaner than stoichio-metric, the combusted fuel amount corresponds to the amount of all fuel inducted into the combustion chamber and accordingly, the air amount stoichiometric to the total inducted fuel corresponds to the remainder of the `~
amount of the excessive air from all of the inducted air.
Therefore, the intake air amount stoichiometric to the combusted fuel, in this case, i8 the remainder of the e~cessive air amount from the whole inducted air.
~15 For instance, when the combustion chamber of the engine is supplied with an air-fuel mixture having an air-fuel ratio of 16:1, 14.7 kg of the induc*ed air amount is stoichiometrically to 1 kg of the combusted fuel and the excessive amounts is 1.3 kg. In this connection, the excessive air is, as preriously described, the inert gas which does not participate in the combustion, and its rate to the inducted air amounts stoichiometrically to the combusted fuel as follows:
~ x 100 8.8%.

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III) In case where the combustion chamber is :
supplied with an air-fuel mixture richer than stoichio~
metric, since the amount of combusted fuel corresponds to the fuel of the stoichiom~tric amount to the amount of all air inducted to the combustion chamber, the inducted air stoichiometric amount to the combusted fuel is the amount of all inducted air. For instance, when the combustion chamber is supplied with an air-fuel mixture having an air-fuel ratio of 13:1, the stoichiometric inducted air amount to o.88 ~- ~ ) kg of the combusted fuel is 13 kg. In this case, 0.12 kg of excessi~e fuel (unburned fuel) remains, but is of an extremely small amount and is therefore negligible.
Accordingly, on the assumption that the amounts ~15 f the recirculated exhaust gas, the residual gas, and the excessive air with respect to the inducted air stoichiometric amount to 1:he combusted fuel are X%, Y% and Z%, respectively, when the combustion chamber is supplied with an air-fuel mixture richer than stoichiometric in which Z = 0, *he fuel-inert gas ratio is 1 : (11.3 + 1407 x Xl~OY), While, when the combustion chamber is supplied with an air fuel mixture leaner than stol~hiometric and the excess air factor or actual inducted air weight/calculated air weight required for combustion is represented as A (in the ~.~7~ ~84 stoichiometric air-fuel mixture, ~ = 1), Z = ~ x 100 and therefore the fuel-inert gas ratio is 1 :
(11 3 + 14 7 x X _ -Y + Z 11 3 ~ 14 7 {X Y
1 ) }) As is apparent from the foregoing discussion, when the combustion chamber is supplied with the qtoichiometric air-fuel mixture (its air-fuel ratio, 14.7:1~ 9 and the recirculated exhaust gas and the residual gas are 30% and 15%, respectively, with respect to the amount of the inducted air, the fuel-inert gas ratio (the weight ratio of the combusted fuel and the substalltially inert gas) is 1 : (45% of exhaust gas ~ :
nitrogen gas = 6.6 + 11.3 = 17,9).
With the arrangement described hereinbefore, during ~15 operation of the engine 10, a considerably large amount of the substantially inert gas containing the exhaust gas through the exhaust gas recirculating means 44 and the residual gas remaining in the combustion chamber i8 added to and mixed with the air-fuel mixture drawn into the combustion chamber. The air-fuel mixture mixed with the inert gas is ignited with -the two spark plugs 52a and 52b disposed in each combustion chamber to effectively combust in each combustion chamber.
With these two spark plugs, when the air-fuel mixture is ignited, two ignition sources or flame fronts are - 18 _ ~7~

produced adjacent the inner wall surface of each combustion chamber. These flame fronts mo~e toward the center of the combustion chamber, heating it to a high temperature and accordingly the length of flame travel is shortened. Thus the combustion caused ~ -by the two spark plugs is proceeding fast and completed at the central portion of the combustion chamber there-by accomplishing stable and smooth combustion even though such a large amount of the inert gas is added to the air-fuel mixture combusting in the combustion chamber. Due to the presence of the inert gas, the maximum temperature of the combustion is low and consequently NOx generation in the combustion chamber i5 considerably decreased as compared with a conven-'i5 tional engine without the exhaust gas recirculation system.
Combusted gases or exhaust gases produced in the combustion chamber are thereafter fed into the reactor 26, their temperature kept high~ Within the reactor 26, C0 and HC contained in the exhaust gases are effecti~ely oxidized in the presence of oxygen supplied by the secondary air supply means 34~
Now, a considerably high intake vacuum during idling allows about 40% of the residual gas to tha intake air a~ount to remain in the combustion chamber - 19 - .

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only by the valve overlap of the intake and exhaust valves 19 and 21, and accordingly the desired fuel- ~ `
inert gas ratio is obtained if the exhaust gas recirculation is stopped. As a result of the addition of such a large amount of the inert gas to the air-fuel mixture to be combusted, it becomes impossible to ~ -completely suppress generation of C0 and HC in the combustion of the fuel carried ou$ in the combustion ~ chamber.
: 10 However, in order to overcome the above-mentioned problem~ the engine 10 employs the connected or com-bined exhau~t port outlet passage arrangements 22a and 22b to prevent exhaust gas temperature drop which results from the transfer of exhaust gas heat to the cylinder head 160 Accordingly, the temperat~re within the reactor 26 is maintained high, sufficient for effective oxidation reaction of C0 and HC carried out in the reactor 26. The high reactor temperature main-taining effect is further improved by installing a port liner 54, as clearly shown in Fig. 4~ in each exhaust port 22a (22b) outlet to cover the surface of the exhaust port outlet maintaining a space 55 between the surface cf the exhaust port outlet passage and the outer surface of the port liner 54. In addition to the above measures, the combustion chamber of the ~L~7~4~34 ~ :~
:
engine 10 is supplied with an air-fuel mixture having ~ `
an air-fuel ratio ranging from 13.5 to 15.5 (mean air-fuel ra-tio is slightly higher than stoichiometric) during normal operation range of the engine in order to allow the exhaust gases introduced into the reactor 26 to contain a suitable amount of unburned fuel, and accordingly the unburned constituents such as C0 and :
HC contained in the exhaust gases discharged from the combustion ch~mbers are effectively burned to decrease the emission levels thereof in the presence of the ~econdary air supplied from the secondary air supply ~
means 34. `-Now, the previously described valve overlap for ; controlling the residual ga~ remaining in the combustion ~
~15 chamber will be explained hereinafter with reference particularly to Fig. 6.
In the engine 10 at this instance, the valve overlap of the intake and exhaust valves 19 and 21 is ~et at 35 to 60 degrees of the crank angle of the engine as clearly shown in Fi.g. 6 in which symbols I.0 I.C. 7 EØ, and E.C. represent intake valve opening timing, intake valve closing timing, exhau~t valve opening timing, and exhaust valve closing timing, respectivelyO With this relatively large amount of the valve overlap, the opening time of the exhau~t _ 21 -... ..
' ' . ': : . . , " -, : . ' ' ~

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valve 21 during intake stroke becomes prolonged to suck the exhaust gases from the exhaust system in addition to the intake of the air-fuel mixture from the intake system, and accordingly the amount of the exhaust gas or the substantially inert gas added to the air-fuel mixture in the combustion chamber is increased~ This allows a decrease in the amount of the exhaust gas supplied through the exhaust gas re-circulated system 44. With this valve overlap~ the exhaust gas amount dependent on the valve overlap occupies about 20 to 40% of all the substantially inert gas added to the air-uel mixture in the com-bustion charnber. In this connection, ~ith prior art engine is in which the valve overlap is set at about ,15 10 to 30 degrees of the crank angle, the exha~s* gas amount dependent on its valve overlap occupies about 10 to 30% of the total substantially inert gas added to the air-fuel mixture. Accordingly, by setting -the valve overlap at 35 to 60 degrees of the crank angle 9 the amount of the exhaust gases recirculated through the exhaust gas recirculation system 44 can be decreased and therefore the contamination of the inner surface of the intake manifold and the surface of the intake valve 19 due to adhesion of contaminants such as carbon particles is considerably decreased.

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For instance, as shown in Fig. 7, when the com-bustion chamber is fed with a rich air-fuel mixture, the fuel-inert gas ratio is set at 1:17.2, and the valve overlap is set at 40 degrees of the crank angle, about 21% of the residual gas to the amount of the intake air (residual gas rate) remains in the combus~
tion chamber, and accordingly about 19% of the recirculated exhaust gas to the amount of the intake air (Exhaust Gas Recirculation, EGR, rate) will be sufficient to obtain the fuel~inert gas ratio range in accordance with the present invention.
In case of employing the secondary air supply means 34' as shown in Fig. 4, in which the above-mentioned valve overlap is at 35 to 60 degrees of the crank angle9 the intake vacuum generated at the intake ~ystem is also applied to the exhaust ports 2Za and 22b during low speed cruising and accordingly the vacuum generated at the exhaust port by the exhaust ~ gas pulsation is increasedO This increases the amount of secondary air inducted through the secondary air injection nozzle 32' and the check valve 48, c~using effec*i~e burning of C0 and HC in the exhaust gases and fuel which leaks into the exhaust port in the reactor 26 connected downstream of the exhaust portn Additionally, the previously described increased ~7~.4~

valve overlap can increase the inertia effect of intake air column and therefore the maximum engine output power is increased during high engine speed operation in which the throttle valve of the engine is fully open. Moreover, with this increased valve overlap, the air-fuel mixture at the intake port 20 leaks to the e~haust ports 22a and 22b during the valve overlap which occurs before the piston lo reaches its top dead center, but the leaked air-fuel mixture is sucked back to the combustion chamber during the valve overlap which occurs during the intake stroke in which the piston 18 is below the top dead center. Subse~uently, the air-fuel mixture sucked back to the combustion chamber is ignited by spark plug means according to the present invention and effectively burned in the combustion chamber, and consequently G0 and HC emission levels are decreased particularly during low and medium J
speed cruising.
As a result of the experiments in which the valve overlap of a four-cylinder type engine, designed ac-cording to the present invention~ having a displacement of 2000 cc was changed, it was revealed that the required volume of the residual gas with respect to the volume of the total substantially inert gas could not remain in the combustion chamber when the valve _ 21~ --- 1~7~4~4 - overlap is less than 35 degrees of crank angle;
whereas the amount of the residual gas remaining in the combustion chamber was too much causing failure of stable and smooth engine running when the valve overlap was more than 60 degrees of crank angle.
Fig~ 8 illustrates another prefexred embodiment of the spark-ignition internal combustion engine according to the present invention, which is similar to the embodiment of Fig. 1 with the exception that `~ 10 this engine 10' is not equipped with the secondary air supply means which supplies the reactor 26 with the secondary air for promoting the oxidation reaction ;~
of the unburned constituents contained in the exhaust gases discharged from the combustion chambers Cl to C4. The reason why the secondary air supply means is removed is that the carburetor 24, in this instance, is arranged to supply the combustion chambers with a lean air-fuel mixture having an air-fuel ratio ranging from 16.5:1 to 21:1 and therefore excessive oxygen gas is 20 supplied to the reactor 26 (called "a lean reactor~
with the exhaust gases discharged from the combustion chambers.
Fig~ 9 illustrates a -further preferred embodiment of the spark-ignition internal combustion engine ac-cording *o the present invention, which is similar to ,: , . . :
', . '', ' :

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the embodiment of Figo ~ except for the spark plug means in which only one spark plug 52' is disposed in each combustion chamber.
The location of the spark plug 52 ~ is selected to travel distance of flame produced by the spark plug in the combustion chamber filled with the air-fuel mixture in order to achieve stable and smooth combustlon of the air~fuel mixture in the combustion chamber by shortening the travel distance of the flame produced .10 by the spark of the plug to decrease the combustion time of the air-fuel mixture in the combustion chamber, even though a large amount of the substantiall.y inert gas is added to the air-fuel mixture so that the fuel-inert gas ratio is maintained within the range from ~15 1:13.5 to 1:22.5.
; As clearly shown in Figs. 11 and 12, in order to ~ttain the above-mentioned purpose, the spark plug 52 is located in a hemispherical combustion chamber such that the volume defined in the combustion chamber by the spheri.cal surface of an imaginary sphere S (only its cross-section shown) occupies more than 35% of the combustion chambe.r which is defined between the cylinder head and the crown of the piston at top dead center; the imaginary sphere being formed with its center at the midpoint (not iden-tified) of the spark _ 26 -~ 7~4~34 plug gap ~ or the distance between the central and side electrodes 52'a and 52'b with a radius R~m), the radius R being calculated by the following equation:

R = N x ~ x Vc Rc ~ n ~ o o ~ o ~ o ~ Eq~

where, ~ - spark advance (the minimum advance, repre-sented at crank angle~ before top dead center for attaining suitable exhaust gas temperature and stable engine operation) N = engine speed (1200 - 2400 r.p.m.) Rc_ combustion rate The above-mentioned radius R corresponds to the radius of the combustion volume (the volume produced ~15 by the combustion of the air-fuel mixture in the com-bustion chamber) which is spherically spread with the center at the midpoint of the spark gap of the spark plug 52' while the inducted air-fuel mixture is ignited before top dead cen-ter of the piston 18 and the piston reaches the top dead center. Accordingly, th~s radius R i~ represented as the product of -the combustion rate Rc and the time t required for the piston movement from the position at which the ignition takes place to the top dead center, and conse~uently , - ~7~4~34 the time t is calculated as follows, using the engine qpeed N and the spark advance ~:

t = N x ~

= N x ~ ....... ,.. 0................ ~.. Eq. (2) It will be understood from the foregoing that the radius R of the imaginary sphere S i~ obtained from the above-described equation Eq~ (l). This radius R is approximately constant regardless of engine speeds during the normal engine speed range or an engine speed ranging from 1200 to 2400 r.p.m., because the spark advance is arranged to increase with the increase of engine speed within such an engine speed range.
~15 In case in which the spark advance 0 is set at 30 degrees of crank angle for attaining the best com-bustion of the air-fuel mixture during city cruising at the engine speed N of 2000 r.p.m., the radius of the imaginary sphere is calculated as follows, based on the fact that~ in general, the combustion rate Rc is approximately 15 m/s:

R = 2600 x ~ x 15 - 3.75 x 10 2 (m) Z 37.5 mm : ' ' , ~ 714~34 Now, the experiments by the inventors have revealed that stable and smooth combustion of the air-fuel mixture was achieved if more than 80% lby weight) of air-fuel mixture of the total inducted air~fuel mixture had been combusted ~hen the piston 18 descended from the top dead center by 15% of the total travel distance of the piston 18 or when the piston is at about 40 degree3 of crank angle after top dead center.
With reference to Fig. 10 *he graph of which was experimentally plotted, it will be understood that the above-mentioned more than 80% combustion will be attained by completing the combustion of about 20% (by weight) of the air-fuel mixture of the total inducted ~15 air-fuel mixture, when the piston is at th0 top dead center. As shown, when the above-mentioned about 20%
of the air-fuel mixture is combusted, the volume of the air-fuel mixture combusted occupieQ about 35% of the volume of the combustion chamber defined between ` 20 the cylinder head 16 and the head of the piston 18 at the top dead center (this refers to "combustion volume rate~ in Figo 10). In this connection, "combustion rate~ in Fig. 10 represents the weight % of the com-busted air-fuel mixture with respect to the total inducted air-fuel mixture.

~ 29 , .

3 6~7~ 4 It will be apparent from the foregoing discussion that, by locating the spark plug 52' so that the volume defined in the combustion chamber by the imaginary sphere S ~ith the above-mentioned radius ~ occupies 35% of the combustion chamber defined between the cylinder head 16 and the crown of the piston at the top dead cent~r, the distance of travel of the flame , produced by the spark of the plug in the combustion chamber is shortened to decrease the combustion time of the air-fuel mixture in the combustion chamber and therefore stable and smooth combustion of the air-fuel mi~ture are achieved even though a large amount of the substantially inert gas is added to the air-fuel mixture inducted into the combustion chamber so that .lS the fuel-inert gas ratio is maintained within the range from 1:13.5 to 1:22.5.
~ eferring again to Fig. 11, the reference numeral 18a indicates a circular cavity formed on the crown of the piston 16. It is to be noted that the cavity 18a contributes to increase the volume in a portion of the combustion chamber defined by the imaginary sphers S
in order to promote the effect obtained by the location o~ the spark plug 52'. Additionally, the cylinder head 16 has a concavity 56 of a hemispherical shape, formed concentrically with respect to the bore of the cylinder : ' 12. As shown, the concavity 56 forms a major part of the combustion chamber. As best seen in Fig. 12, the diameter d of the hemispherical concavity 56 is smaller than D of the bore of the cylinder 12 and consequently an annular space 58, called a squish area, is formed between the annular flat portion 16a of the cylinder head 16 defining the combustion chamber and the peri-pheral portion of the crown of the piston 18 when it is at the top dead center thereof. With this squish area 58, during the last stage of th~ compression stroke of the piston 18, most of the air-fuel mixture or the charge supplied into the combustion chamber is squeezed out of the squish area 58 and moved toward the central portion of the combustion chamber to pro-duce squish turbulence within the combustion chamber.The squish turbulence promotes smooth and rapid burning of the air-fuel mixture in the combustion chamber and therefore contributes to further shortening of the combustion time.

It will be understood that as the area of the annular flat portion 16a of the cylinder head adjacent the hemispherical concavity 56 increases, the squish area is increased and accordingly the effect of the squish turbulence is increased. However, since the squish area contacts the inner surface of the cylinder - ~7~4~

12 which is cooled by a cooland (not shown) flowing outside the cylinder 12, the air-fuel mixture in the squish area 58 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 (HC) is increased with the increase of the area of the annular flat portion of the cylinder head adjacent the hemispherical concavity 56. In view of the foregoing, the area o~ the annular flat portion 16a of the cylin-der head 16 should be determined such that the emission level of-hydrocarbons is not, to any extent increased, but that sufficient squish turbulence is produced.
Experiments reveal that it i5 preferable for obtaining the above-described intended purpose that the area of the annular flat portion 16a is in the range from 0.1 to 0.45 times the cross-sectional area of the bore of the cylinder 12. Furthermore 9 the thickness St of the squish area 58 or the distance between the annular flat portion 16a of the cylinder head 16 and the crown of the piston 18 at the top dead center is preferably in the range from 1.05 to 2.5 mm in order to obtain good effect of the squish area 58.
It will be understood that imprvved mixing of fuel and air and uniform dispersion of the recirculated -,32 ~

7~ 4 exhaust gas in the combustion chamber may be attained by producins swirl turbulence due to the momentum of the high-speed incoming gas in addition to the squish turbulence due to the squish area 58.
It is desirable to project each spark plug to the central portion of the combustion chamber for decreas-ing combustion time of the charge, since the flame produced by the spark of the plug spreads spherically with the center at.a point where the spark is produced. ~.
lG However, it was thought that the spark plugs located at or near the central portion of the combustion chamber and subjected to an extremely high temperature would undergo thermal damage. However, experiments reveal that combustion efficiency is improved and the ~15 durability of the spark plugs is not reduced by locat-ing them such that the midpoints of the spark gaps thereof are projected and spaced apart from the inner surfaces of the cylinder head 16 by a distance ranging from 2 to 7 mm.
In order to improve the combustion 9 in addition to the effect due to the projected spark plugs the spark gap of spark plug 52l is set at 1.1 to 2.0 mm to produce a rela*ively long spark between the elec-trodes of the spark plug. ln this connection, the spark plug is arranged to generate a spark energy of - ~7~L484 about 100 mj (milli-joule) to provide stable and reliable sparking even though the spark gap is con-siderably long. Furthermore, it was observed that the electrodes of the spark plug 52' was no-t subjected to a thermal damage but improved the ignition of the air-fuel mixture in the combustion chamber by forming the diameter of the top surface of the central elec-trode 52'a of the spark plug 52' within the range from 0.50 to o.63 mm, the top surface being opposed to the side electrodes 52's of the spark plug 52~.
The experiments by the inventors have revealed that, by setting the diameter de of the intake valve 19 within the range from 0.40 to 0.50 times of the diameter D of the cylinder bore as shown in Fig. 12, the preventing effect of exhaust gas *emperature drop is further promoted in addition to the combined exhaust port outlet arrangements 22a and 22b with a liner 54, disposed within the surfaces of the combined port outlets. In this connection, it has been observed that the pwnping loss of the engine was decreased and accordingly the fuel consumption characteristics was improved by setting the diarneter di of the exhaust Yalve 21 within the range from 0.45 to 0.55 times of the diameter D of the cylinder bore as shown in Fig.
12.

- 3l~ ~

..

:' :. . .. .

7~

With respect to the exhaust gas recirculation system 44, its characteristics is desirable to be designated as shown in Fig. 15 in which the center Ct of the maximum value of the exhaust gas recirculation rate (the rate of the recirculated exhaust gas to the intake air) resides over the line Lr representing the engine load during even road cruising, at ths normal engine operating range or the city cruising range at the engine speed of 1400 to 2000 r.p.m., and at the range in which the frequency of the acceleration is the highest or the range in which the intake manifold vacuum is from 150 to 200 mmHg. As seen in Fig. 15, the exhaust gas recirculation rate is decreased from the center Ct toward circumferential contour lines.
~15 With the exhaust gas recirculation system of this characteristics, stable and smooth combustion of the air-fuel mixture is accomplished with the ignition of the spark of the plug 52' of this instance and NOx emission level is remarkedly decreased without dete-riora-tion of the fuel consumption characteristics and the drivability of the vehicleO
Figs. 13 and 14 illustrate examples in which the spark plugs 52' are installed in the cylinder heads forming thereinside a wedge type combustion chamber and a bath-tub type combustion chamber, respectively.

- 35 ~

.

~' - ~7~484 In these cases, the spark plugs 52' are, of course, located in the respective combustion chambers to shorten the travel distance of the flame produced by the spark of the plugs 52' in the combustion chambers to decrease the combustion time of the air-fuel mixture in th0 combustion chambers. Consequently, the spark plugs 52' disposed in the combustion chambers are, as shown, installed at the cylinder head 16 to project into portions of the combustion chambers which portions are approximately farthest from the crown of the piston 18. Additionally, in these cases9 the piston~s 18 are formed with cavities 18a, at their crowns, respectively, in order to promote the combustion time decreasing effeet by increasing the combustion chamber volume contained in the imaginary spheres S.
~While not described in the explanation of the embodiment of ~ig. 9 nor shown, the secondary air sup-ply means for supplying secondary air into the reactor 26 may be the device 34' arranged to supply secondary air to the reactor by using the pulsation effect of the exhaust gas pressure generated at the exhaust ports 22a and 22b, as shown in Fig~ 4. Additionally, it will be appreciated that the valve overlap of the ; intake and exhaust valves of the engine 10'l shown in ~ 25 Fig. 9 is controlled within the range from 35 to 60 , :, . , .

.; ,..... . .
.. . .

~C17~

degrees of the crank angle of the engine.
As is apparent from the foregoing discussion, according to the present invention, the emission level of NOx can be decreased by adding substantially inert gas to the air-fuel mixture inducted into the combustion chamber so that the weight ratio of the substantially combusted in the combustion chamber is within the range from 13:1 to 22.5:1. Moreover, the deteriorated com-bustion due to addition of such a high rate of the ~ubstantially inert gas is improved by locating the spark plug or spark plugs such that the travel distance of the flame produced in the combustion chamber is ~hortened to decrease the combustion time of the air-fuel mixture in the combustion chamber. Accordingly ~15 ~table and s~ooth combustion is carried out in the combustion chamber, causing improvements in engine power output characteristics and fuel consumption characteristicsO Addi-tionally, C0 and HC contained in the exhaust gases from the combustion chamber are introduced into the reactor, maintaining the exhaust gas temperature high, to be effectively oxidized in the reactor, and therefore the emission levels of C0 a~d HC are remarkedly decreased to desirable values.

:

- 37 ~

Claims (17)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A spark-ignition internal combustion engine, comprising:
means for defining a combustion chamber;
air-fuel mixture supply means for producing an air-fuel mixture in the combustion chamber by mixing fuel and intake air inducted into the combustion chamber;
means for adding substantially inert gas to the air-fuel mixture in the combustion chamber, said substantially inert gas being mixed gases which remain substantially inert in the combustion of the fuel in the combustion chamber, said means for adding inert gas including control means for controlling the ratio of fuel combusted in the combustion chamber and the substantially inert gas in the range of from 1:13.5 to 1:22.5 by weight during normal engine operation;
two spark plugs disposed in the combustion chamber and located to shorten the travel distances of flames produced by sparks thereof in the combustion chamber filled with the air-fuel mixture so as to decrease the combustion time of the air-fuel mixture; and reactor means for purifying the exhaust gases discharged from the combustion chamber by oxidation of the constituents contained in the exhaust gases.

2. A spark-ignition internal combustion engine as claimed in Claim 1, in which said substantially inert gas contains residual gas which remains in the combustion chamber during the exhaust stroke, exhaust gas which is supplied from the exhaust system of the engine to the combustion chamber, and nitrogen gas contained in the intake air.

3. A spark-ignition internal combustion engine as claimed in Claim 2, in which said substantially inert gas further includes oxygen gas contained in excess air contained in an air-fuel mixture leaner than stoichiometric when the engine is operated on a lean air-fuel mixture.

4. A spark-ignition internal combustion engine as claimed in Claim 2, in which said combustion chamber defining means defines a combustion chamber by the cylindrical inner surface wall of a cylinder of the engine, the inner surface wall of the cylinder head of the engine closing one end of the cylinder, and the crown of a piston reciprocally movably disposed within the cylinder.

5. A spark-ignition internal combustion engine as claimed in Claim 4, in which said two spark plugs are disposed through the cylinder head and project into the combustion chamber, each spark plug having two electrodes defining therebetween a spark gap.

6. A spark-ignition internal conbustin engine as claimed in Claim 5, in which said spark plugs are located such that the midpoints of the spark gaps of said two spark plugs constitute an angle ranging from 110 to 180 degrees with respect to the center axis of the cylinder bore.

7. A spark-ignition internal combustion engine as claimed in Claim 6, in which said two spark plugs are located such that the shortest distance between the midpoint of the spark gap of each spark plug and the center axis of the cylinder bore is 0.15 to 0.45 times of the diameter of the cylinder bore.

8. A spark-ignition internal combustion engine as claimed in Claim 1, in which the substantially inert gas adding means includes exhaust gas recirculating means for supplying exhaust gas of the engine into the combustion chamber through the intake system of the engine.

9. A spark-ignition internal combustion engine as claimed in Claim 8, in which said exhaust gas recirculating means includes conduit means connecting a portion of the exhaust system of the engine and a portion of the intake system of the engine for feeding exhaust gases into the intake system, and a control valve disposed in said conduit means for controlling the amount of the exhaust gases passing through the conduit means.

10. A spark-ignition internal combustion engine as claimed in Claim 8, in which said inert gas adding means includes means to control the valve overlap of the intake and exhaust valves of the engine within the range from 35 to 60 degrees of the crank angle of the engine.

11. A spark-ignition internal combustion engine as claimed in Claim 4, in which said combustion chamber defining means defines another combustion chamber by the inner surface wall of another cylinder of the engine, the inner surface wall of the cylinder head of the engine closing one end of the cylinder, and the crown of another piston reciprocally movably disposed within the cylinder.

12. A spark-ignition internal combustion engine as claimed in Claim 11, further comprising means for forming in the cylinder head a combined exhaust port outlet in which two exhaust outlets of the two exhaust ports of said combustion chamber and said another combustion chamber are combined to form one exhaust outlet.

13. A spark-ignition internal combustion engine as claimed in Claim 12, in which said reactor means includes a reactor having an inlet communicating with the one exhaust outlet of said combined exhaust port outlets to receive the exhaust gases discharged from the two combustion chambers.

14. A spark-ignition internal combustion engine as claimed in Claim 12, further comprising a liner disposed covering the inner surfaces of said combined exhaust port outlets and maintaining a space between the inner surfaces of said combined exhaust port outlets and the outer surface of said liner.

15. A spark-ignition internal combustion engine as claimed in Claim 13, in which said air-fuel mixture supply means is arranged to supply the combustion chambers with an air-fuel mixture having a mean air-fuel ratio higher than stoichiometric, in which the engine further comprises secondary air supply means for supplying the reactor means with air.

16. A spark-ignition internal combustion engine as claimed in Claim 15, in which said secondary air supply means includes a secondary air injection nozzle open to said combined exhaust port, and an air pump connected to said secondary air injection nozzle for admitting pressurized air to said secondary air injection nozzle.

17. A spark-ignition internal combustion engine as claimed in Claim 15, in which said secondary air supply means includes a secondary air injection nozzle open to the combined exhaust port outlet and disposed adjacent the exhaust valve of the combustion chamber, and a check valve connected to said secondary air in-jection nozzle, said check valve being arranged to open or close in dependence on the pulsations of the pressure of the exhaust gases passing through said combined exhaust port outlet.