March 23, 1971
`
`Filed Jan. 6, 1969
`
`SHIGERU ONISHI ET AL
`
`STRATIFIED CHARGE ENGINE
`
`3,572,298
`
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`Page 1 of 17
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`

`
`March 23, 1971
`
`Filed Jan. e, 1969
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`
`SHIGERU ONISHI ET AL
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`STRATIFIED CHARGE ENGINE
`
`3,572,298
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`Page 2 of 17
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`

`
`March 23, 1971
`
`Filed Jan. e, 1969
`
`SHIGERU ONISHI ET AL
`
`STRATIFIED CHARGE ENGINE
`
`3,572,298
`
`4 Sheets-Sheet 3
`
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`Page 3 of 17
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`

`
`Mvch 23, 1971
`
`Filed Jan. e. 1969
`
`SHIGERU ONISHI ET AL
`
`STRATIFIED CHARGE ENGINE
`
`3,572,298
`
`4 Sheets-Sheet 4
`
`17
`
`11 10
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`Page 4 of 17
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`

`
`United' States Patent Office
`
`3,572,298
`Patented Mar. 23, 1971
`
`1
`3,572,298
`STRATIFIED CHARGE ENGINE
`Shigeru Onishi, 31-13 Higashiyama 3-chome, l{auazawa·
`shi, Ishikawa-ken, Japan, and Saburo Yui, 12-7
`Himonya 3-chome, Meguru-ku, Tokyo, Japan
`Filed Jan. 6, 1969, Ser. No. 789,227
`Claims priority, application Japan, Jan. 8, 1968,
`43/599; Jan. 11, 1968, 43/1,083; Feb. 16, 1968,
`43/9,413; June 10, 1968, 43/39,424; July 27, 1968,
`43/52,757
`
`U.S. CI. 123-32
`
`Int. Cl. F02b 72/00
`
`2
`deviated mixtures with each cylinder. Thus, fuels burned
`at excessively lean air-fuel ratios produce only poor
`thermal efficiency and output, and the clean mixtures slow
`down the flame propagation, as physical phenoma of
`inevitable nature. The use of a lean air-fuel ratio thus
`5 causes a drop of burning rate, which in turn results in the
`rise of exhaust temperature, and hence damage to the
`exhaust valve, irregular combustion cycle, engine rough(cid:173)
`ness, insufficient output, poor acceleration and difficulty
`8 Claims 10 of starting. It is for these reasons that the use of lean air(cid:173)
`fuel ratios has not been contemplated in the past.
`More recently, as part of attempts for making use of
`lean air-fuel ratios, a number of proposals have been
`made along the stratified charge concept whereby a spark
`15 plug is provided in a part of main combustion chamber
`or in an auxiliary chamber, and a gaseous mixture of
`combustible range is formed, ignited and burned, and
`then the combustion of a leaner mixture in the rest or
`whole of the main cylinder is effected. It is known that,
`20 according to this stratified charge system whereby the
`engine is operated on a relatively rich fuel mixture main(cid:173)
`tained in the region around the spark plug and on a lean
`overall mixture provided in the entire space inside the
`cylinder (i.e., with excess air), it is made possible to im-
`25 prove the specific heat of active gas for a closer approach
`to an air cycle, prevent heat dissociation, and reduce the
`cooling heat loss to the surroundings, so that improved
`thermal efficiency and reduced fuel consumption can be
`realized and, further, because the combustion is effected
`30 with ample supply of air, the noxious components of
`exhaust emissions can be fairly reduced.
`However, while many versions of the stratified charge
`concept have been studied and developed up to now,
`none of them has attained a satisfactory degree of per-
`35 fection for the control of noxious contents of exhaust
`emissions as well as in the usefulness in actual applica(cid:173)
`tions because of imperfection in design and controllabil(cid:173)
`ity, complicacy and other problems in construction, etc.
`Especially, it is practically much more difficult to main-
`40 tain an air-fuel ratio greater than 15: 1 which corresponds
`(approximately) to the theoretical mixing ratio, or be(cid:173)
`tween about 16: 1 and 17: 1 which is generally believed
`feasible, in every cylinder of the engine, throughout the
`entire load range. Moreover, the higher the air-fuel ratio
`45 the slower the combustion rate and the higher the ex(cid:173)
`haust temperature. This tends to invite troubles in the
`exhaust valve system, increased unsmoothness of com(cid:173)
`bustion process, engine roughness, and frequent occur(cid:173)
`rence of misfire, which in turn results in difficulty of
`50 starting and drop of acceleration. What is worse, the air(cid:173)
`fuel ratio in this range can increase the emission of NOx
`to a maximum. If the compression ratio is increased for
`better performance, the NOx emision will further in(cid:173)
`crease. In order to reduce the latter possibility, attempts
`55 have hitherto been made only at some sacrifice of engine
`performance. Retardation of ignition timing is an example.
`However, this results in a drop of power output and a
`rise of exhaust temperature.
`Recirculating a part of exhaust gas to the intake side
`60 proves effective but it again reduces the available power,
`and use of a very rich mixture to make up for the power
`loss leads to increased CO emission. As a whole, there(cid:173)
`fore, this approach is not contributory to the purification
`of exhaust emissions.
`Charge stratification is beneficial for the improvement
`of thermal efficiency at part-load operation, but for an
`engine to run under full load it fails to ensure good
`gassi:fication and mixing of fuel with air so that the engine
`70 can yield sufficiently high power. In order that the system
`can meet the requirement of engines operating at full
`load, it is necessary to inject the fuel rapidly with an
`
`ABSTRACT OF THE DISCLOSURE
`An internal combustion engine of the stratified charge
`type wherein the purification of the exhaust fan from the
`combustion chambers is provided with elimination of
`noxious components in the emissions thus attaining im(cid:173)
`proved ignitability and combustibility particularly for
`betterment of performance and durability in full load
`operation.
`
`DETAILED EXPLANATION OF THE INVENTION
`This invention relates to a reciprocating, piston-type
`internal combustion engine of stratified charge system
`which achieves charge stratification for complete burning
`of fuel in the cylinder that serves as combustion chamber,
`and more specifically to an internal combustion engine
`of Otto-cycle type for spark ignition of air-fuel mixture,
`wherein stratified charge combustion is accomplished,
`that is, stratified charge of a readily ignitable, combus(cid:173)
`tible mixture is burned in the neighborhood of the region
`including spark plug.
`Efforts for improvements of internal combustion en(cid:173)
`gines, especially gasoline engines, have heretofore been
`directed to higher power output and thermal efficiency
`and better mechanism. As for the prevention of air pollu(cid:173)
`tion due to the exhaust emissions, the manufacturers have
`simply relied upon some attachments for reductions of
`noxious gases. However, those emissions containing car(cid:173)
`bon monoxide, CO, hydrocarbons, HC, and nitrogen
`oxides, NOx, have so rapidly increased in recent years
`that the control and removal of such noxious substances
`posing public hazard, or production of cleaner exhaust
`gases, has come to the fore as a problem of paramount
`importance that must be solved even at some sacrifice
`of the power output, thermal efficiency and other features
`of the engines.
`As is well known, it is important in the combustion
`chambers of internal combustion engines that injected
`fuel, either mist or vapor in form, be thoroughly mixed
`with a necessary amount of air for perfect burning. The
`perfect combustion within the engines incorporating an
`improved system of charge stratification in the combus(cid:173)
`tion chambers would give cleaner emissions. If the engines
`are moreover equipped with means for purifying the
`exhaust with any improvement rather than sacrifice of
`the engine performance, the result would then be more
`than satisfactory.
`It is known that, in conventional gasoline engines, the
`concentrations of such noxious gases can be invariably
`reduced to by far lower levels by effecting good combus(cid:173)
`tion of mixtures at sufficiently lean air-fuel ratios for the
`control of the harmful emissions.
`Generally, a decrease of fuel supply with respect to
`a given air intake improves the fuel consumption although
`the engine develops less power. With leaner air-fuel ratios,
`however, the fuel consumption is. increased and thermal
`efficiency is affected adversely because of slower burning
`rates and higher occasions of misfiring due to rarefied and
`
`65
`
`Page 5 of 17
`
`

`
`3,572,298
`
`4
`3
`and further effect matching of the charge of air with the
`injection pump into the cylinder while it is fully open and
`combustion process or, in other words, to materialize
`provide means for adjusting the injection timing that is
`stable stratified charge by obtaining a rich mixture on the
`capable of varying the vaporizing and mixing time over
`circumferential side and a lean mixture in the concentric
`a broad range. To prevent emission of the noxious con-
`side.
`tents of exhaust (i.e., the three constituents above men-
`tioned) from the combustion chamber notwithstanding 5
`Another object is to provide an internal combustion
`engine capable of maintaining sound burning conditions
`the.se additions is almost impossible. Among other dis-
`over the range from idling to full-load operation by the
`advantages is an inevitable increase in equipment cost.
`injection of fuel from a suitable position at a suitable
`Stratified charge engines thus far developed not only
`require expensive attachments of complicated designs but 10 angle against the scavenging air stream or a swirling air
`involve difficulties in achieving accurate and effective
`stream based thereon in the cylinder of an ordinary valve-
`less 2-cycle engine, thereby realizing positive and stable
`charge stratification at all loads, with lack of stability
`in the charge obtained, and thus they have failed to dis-
`stratified charge which can be spread over the inner sur-
`play performance well comparable to those of conven-
`face of cylinder head by the resulting action of the com-
`15 posite stream with the swirling air and the motion of
`tiona! internal combustion engines.
`According to one version of the stratified charge system,
`atomized particles, and extending the mutual tolerance
`a recess is formed in the piston crown and fuel is directly
`of injection and firing.
`injected therein for vaporization and stratification. This
`Another object is to provide a stratified charge engine
`naturally increases the thermal load upon the piston,
`which dispenses with the valve mechanism of internal
`renders the temperature control and hence cooling dif- 20 combustion engine and can easily pick up speed without
`valve hunting that is otherwise inevitable, has the effect
`ficult, and may shorten the piston life. In addition, the
`extra weight that is inevitably given to the reciprocating
`of evaporation and cooling due to direct fuel injection
`pmt because of the special construction hampers the
`during the compression stroke of piston, and shortens
`high-speed operation of the engine and makes it impos-
`the residence time of mixture in the hot cylinder to avoid
`sible to improve the performance such as the power out~ 25 heating of the intake manifold, thus with the improvement
`put. Other disadvantages includes a drop of scavenging
`of so-called mechanical octane number permitting an in-
`efficiency because the recess in the piston crown serves
`crease of the compression ratio to a fairly high level so
`as a dead space.
`that the engine can exhibit performance well comparable
`The present invention has for an object to prefectly
`to conventional engines in full-load operation.
`achieve the purification of exhaust from the combustion 30
`Still another object is to provide an engine of stratified
`chambers of reciprocating, piston-type internal combus-
`charge system but wherein the ignition and combustion of
`tion engines without allowing noxious components to
`fuel mixture are not effected simultaneously with the
`appear in the emissions and also to permit remarkable
`injection of fuel and, notwithstanding the restriction of
`improvement of performance in operation with no sacri-
`octane number, the fuel is injected at a certain crank
`fice of engine output and other features.
`35 angle before ignition with allowance of time for evapora-
`It is another object of the invention to eliminate the
`tion, dispersion and mixing, that is, utilizing the swirling
`disadvantages of ordinary stratified charge systems that
`air stream based on scavenging air in the cylinder the fuel
`hinder the successful commercialization of the concept
`is injected thereagainst from a suitable position at a
`and to enable conventional engines to achieve stratifica-
`suitable angle thereto so as to form a stratified charge
`tion of mixed charge stably and positively at entire load 40 spread over the inner surface of cylinder head by the
`range without the need of any special, expensive attach-
`composite action with the swirling air stream, thereby
`ments, and attain improved ignitability, combustibility
`permitting full use of air and making it possible to avoid
`and controllability which all combine to contribute to the
`a smoke limit, give a configuration insensitive to poor
`betterment of overall engine performance, particularly for
`atomization, provide a timing allowance for fuel injevtion
`the improvement of both performance and durability in 45 and ,firing, improve the durability and controllability, con-
`tribute to noise control by virtue of the omission of valve
`full-load operation.
`Another object is to provide an internal combustion
`mechanism and the driving and transmission means the;,re-
`engine having a single combustion engine which is de-
`for, and also attain improved engine durability and reli-
`fined between the cylinder and piston in which fuel is
`ability.
`directly injected for effective charge stratification, so that 50 Yet another object is to provide an internal combustiOn
`a desirable mode of combustion is easily attained by
`engine of simple and compact construction wherein the
`cleverly taking the advantage of the self-circulating prop-
`mass of atomized fuel particles injected into the combus-
`erty of exhaust without imparting any special directional-
`tion chamber of the engine is combined in optimum con-
`ity to the air and inducing any air swirl around the center
`ditions with the scavenging air stream or swirling air
`axis of the cylinder, and also by utilizing the scavenging 55 stream based thereon so that the mixture containing the
`air stream that is naturally produced in the exhausting,
`fuel particles can be thoroughly spread over the entire
`charging and compression strokes of the piston, thereby
`corners of the inner surface of cylinder head by making
`to simply achieve complete combustion of the fuel in
`most of the correlative action of the particle mass with
`order to permit the simultaneous control of the three
`said air stream, and an extension is formed on the cylin-
`60 der head that protrudes toward the scavenging air stream
`noxious pollutants which pose social hazard.
`Another object is to secure operations over a broad
`in such manner as to provide a thick boundary layer down-
`range of loads at a lean overall air-fuel ratio by attaining
`stream therefrom or to produce a zone of turbulent flow
`stratification of charge which may be spread over the cyl-
`by the partial removal therefrom, so that an appropriate
`inder head by the resulting force of a swirling air stream
`concentration distribution of stratified charge and a gas
`in the combustion chamber of cylinder and the move- 65 stream condition favorable for flame propagation can be
`ment of mist from the spray nozzle fitted to the cylinder
`realized by the correlative action of the air stream with
`head, thereby ensuring further stability of charge, with
`any one or a given combination of the spreading of atom-
`no attempt to create any special swirl stream for the
`ized particles, localized mixing and propagation of burning
`stratification but injecting the fuel against a swirling air
`flame, thereby to obviate the drawbacks of high fuel con-
`stream based on the consequent loop type scavenging air 70 sumption and inadequate flexibility as have been ob-
`stream and its conservation of momentum so as to form
`served with the conventional homogeneous mixture sys-
`stratiform clouds by dispersion, mixing and evaporation
`tern, and achieve remarkable improvements for low fuel
`and, at the same time, to increase the air proportion on
`consumption, high power output and high response.
`the outer side and increase the exhaust composition on the
`A further object is to provide at low cost an economical
`inner or concentric side, and maintain this relationship, 75 stratified charge engine which can be of throttleless design
`
`Page 6 of 17
`
`

`
`3,572,29S
`
`5
`
`5
`having a cylinder head of semipherical contour for defin(cid:173)
`ing the combustion chamber and also having a recess from
`the inner wall surface for inhibiting wasteful dispersion of
`the atomized fuel particles, wherein charge stratification
`is accomplished by the correlative movements of fuel with
`the scavenging air stream or a swirling air stream based
`thereon, and which is designed to serve as an evaporating
`section thereby to ensure good startability and is provided
`with a piston which is lightweight and adapted for high(cid:173)
`speed operation so that it can combine these features with
`improved thermal efficiency or mean effective pressure to
`contribute to an increase in the specific power output of
`the engine.
`In order to achieve the foregoing objects, the present
`invention in essence provides an internal combustion en(cid:173)
`gine of stratified charge system comprising a reciprocat(cid:173)
`ing piston, a cylinder having an exhaust port and a scav(cid:173)
`enging port adapted to be opened or closed by the move(cid:173)
`ment of the piston, and an injection nozzle provided in the
`cylinder head for the supply of fuel into the cylinder, in
`such manner that the spray from the injection nozzle
`mounted in the cylinder head is directed toward the inner
`surface of said cylinder head against or into a swirling
`stream of fresh air to provide a combined action for
`stratification of the combustible mixture along the wall
`surface of the cylinder.
`An important feature of the present invention is to
`provide an internal combustion engine of stratified charge
`system designed for loop or cross scavenging, including
`means for directing a mass of ·fine atomized particles of
`liquid fuel or a jet of gaseous fuel injected from a fuel
`injection nozzle mounted in the cylinder head constitut(cid:173)
`ing a combustion chamber into the scavenging air stream
`or a swirling air stream based thereon or both in the cylin(cid:173)
`der, and for spreading, by the combined action, the :fine
`particles or gaseous mixture formed iby evaporation
`thereof to a stratified form over the inner surface of the
`cylinder head for subsequent ignition.
`Another important feature is the provision of a re(cid:173)
`ciprocating piston-type internal combustion engine of
`direct injection system, wherein a combustion chamber is
`defined between the cylinder head of a cylinder accom(cid:173)
`modating a piston and the piston head and a squish flow
`formed by the inner surface of the cylinder head and the
`top surface of the piston head is superposed and combined
`with the scavenging air stream or a swirling air stream
`based thereon in the cylinder so that the gaseous mixture
`of fine particles of injected fuel and the gas formed by
`evaporation thereof can be dispersed, rarified or concen(cid:173)
`trated, or the air stream is given a localized flow or tur(cid:173)
`bulence, thereby to attain a desired state of stratified charge
`suitable for combustion.
`Another feature of the invention is that the internal
`combustion engine of loop or cross scavenging type there(cid:173)
`by provided includes means for directing the mass of fine
`injected particles from the fuel injection nozzle provided
`in the cylinder head to a cavity formed in the cylinder
`head thereby dispersing the mass of particles before it is
`fed in and mixed with the scavenging air stream or
`swirling air stream based thereon or •both in the cylinder,
`so that a combustible mixture can be stratified and mixed
`up along the cylinder wall.
`A further feature of the invention is that, in the engine
`thereby provided having an injection nozzle in the
`cylinder head for supply of fuel into the cylinder, the
`cylinder head has an inner wall surface of curved contour,
`e.g., of semispberical section, and is further formed with
`a cavity in the already curved inner surface, and means is
`provided to permit direct injection of -fuel from the in(cid:173)
`jection nozzle against the cavity to avoid wasteful scat(cid:173)
`tering of the atomized fuel particles and effect the disper(cid:173)
`sion and mixing of the fuel with air within that region,
`and stratify the combustible mixture in the combustion
`chamber by any one or a given combination of the actions
`of dispersion and mixing, stratiform stabilization and
`
`6
`carrying of the fuel particles by the scavenging air stream
`or swirling air stream produced by the reciprocating
`piston in the cylinder, and then burn the mixture in strati(cid:173)
`fied state thus permitting complete combustion.
`In accordance with the present invention, fuel supplied
`to the engine is readily spread in the region of cylinder
`head and eventually carried away by the main stream of
`swirling air. Since the main stream possesses a field of
`centrifugal force, the fuel is moved under the mixed action
`10 of centrifugal force and turbulence. Attendant, induced
`stream of localized air flow accompanying the injection
`of the mass of fine atomized fuel particles also contri(cid:173)
`butes effectively to the spreading and turbulent actions.
`-Moreover, the fine particles injected out of the nozzle is
`15 exposed to the air stream flowing at a relatively high
`speed, which combines with the gradual increase of air
`temperature due to the progress of compression stroke to
`give a good fuel and air mixture. This mixture rides on
`the swirl in the cylinder and is stratified with concentra-
`20 tions such that the mixture is relatively uniform along
`the inner wall surface of the cylinder head but becomes
`gradually leaner away from the cylinder head surface.
`Charge stratification is thus accomplished effectively and
`positively, which provides remarkably good adaptability
`25 to the air drawn in the engine. To cite the region outside
`of the scavenging air stream, such as the region near the
`cylinder, as an example, a good rate of scavenging is at(cid:173)
`tained close to the inner wall surface of the head while
`the rate drops toward the center of the cylinder where an
`30 increased amount of residual gas components of the
`preceding stroke is left. When charge stratification is car(cid:173)
`ried out in such manner as to form a rich mixture in
`the region outside of the main stream of the swirl, the
`charge composition is stable and the air supply is well
`35 balanced with the air consumption.
`Also, because it is possible under the invention that the
`combustible mixture is stably spread over an extensive
`region on the inner wall surface of the cylinder head and
`the mixture is retained for a relatively long period in the
`40 cavity space, the allowance for change of spark angle rela(cid:173)
`tive to the timing for fuel injection is rendered large
`enough to permit fairly free selection of the spark timing
`independently of the injection timing. This tendency be(cid:173)
`comes more pronounced with the temperature rise of the
`45 head. Fuel particles injected from the orifice of the in(cid:173)
`jection nozzle are spread and evaporated while approach(cid:173)
`ing the inner wall surface of the bead except for the state
`in the very early period and, as a whole, the swirling
`velocity inside the cylinder is adequately slowed down
`50 to permit charge stratification of a desired intensity
`and richness to be realized with ease.
`The air-fuel mixture containing the injected fuel
`particles and the fuel evaporated from the inner wall sur(cid:173)
`face of the cylinder spreads well to every corner of the
`55 combustion space and from surface of particles and the
`densities of the particles and mixture per unit volume do
`not reach any appreciably high values. These factors com(cid:173)
`bine together to preclude the possibility of misfiring due
`to a too high concentration around the spark plug upon
`60 the introduction of excessive fuel. Even in part-load
`operation with meager fuel injection, a sufficient air-fuel
`ratio is maintained in the combustion ignition zone for
`preclusion of misfire, delay in combustion or other
`trouble with a fairly lean overall mixture. The fact that
`65 good combustion is thus obtained over the no-load to full(cid:173)
`load range and particularly in part-load operation with
`excess air supply affords tremendous practical benefits be(cid:173)
`cause, for automotive engines, the specific fuel consump-
`70 tion in the part-load region where cars are mostly driven
`is considerably improved and, moreover, purification of
`exhaust -gases is achieved positively to solve the problem
`of public hazard due to air-polluting emissions easily and
`without resort to any complicate, expensive additional
`75 attachment for that purpose. Still more, direct fuel in-
`
`Page 7 of 17
`
`

`
`3,572,298
`
`7
`jection provides powerful acceleration, good fuel distribu(cid:173)
`tion and adaptability to high-output performance.
`According to the present invention, fuel can be readily
`fired either by the mass of fine atomized particles of in(cid:173)
`jected fuel or by the gaseous mixture consisting mainly 5
`of the vapor from the atomized fuel and the liquid film
`deposited on the inner wall surface of the cylinder, or
`by the mixture of the two. The firing flame considerably
`promotes further evaporation. After firing, the combustion
`gas which is light in weight naturally runs toward the 10
`center of the main stream of swirl, and is replaced by
`fresh air, which in turn takes part in the formation of a
`mixture and in subsequent combustion. With such mecha(cid:173)
`nism of combustion, it is possible to accomplish the fuel
`injection with somewhat high pressure in order that firing 15
`and combustion be primarily initiated by the mass of fine
`atomized particles at the time of starting and imme(cid:173)
`diately after the starting when the cylinder temperature is
`low, and to decrease the injection pressure upon the rise
`of the cylinder temperature to a suitable level thereby to 20
`lessen the burden on the injection pump, and then to
`enable the mixture of vaporized fuel and air to take over
`the task of firing and combustion from the mass of fine
`atomized particles so as to ensure satisfactory combustion
`and operation.
`In the combustion mechanism in accordance with the
`present invention, the rate and highest temperature of
`combustion can be controlled partly because the mixture
`contains some residual gas (i.e., inactive ·gas) already
`burnt in the previous stroke and partly because the resid- 30
`ual gas is more densely distributed in the central zone
`than in the rest. Consequently, the pressure rise rate
`may be kept lower than in the conventional 4-cycle en(cid:173)
`gines which burn homogeneous mixtures, and the maxi(cid:173)
`mum pressure inside the cylinder may also be reduced 35
`so as to provide a lighter engine. It is further possible
`to minimize the development of noise and thereby expect
`smooth engine operation. The arrangement is very con(cid:173)
`tributory to the control of NOx emission which is a
`source of public hazard. Since the fuel is fed by direct 40
`injection into the cylinder and the residence time in the
`cylinder in the form of unburnt mixture is shortened, the
`anti-knock property of the fuel is strengthened. Further,
`because there is no exhaust valve and other member
`which can serve as heat source, for example in the 45
`vicinity of the cylinder head, and because the combus(cid:173)
`tion chamber is formed to a semispherical shape which
`is compact and close to an ideal configuration, the me(cid:173)
`chanical octane number of the fuel is increased and an
`improved thermal efficiency is realized by dint of an in- 50
`crease in the compression ratio. The moderate pressure
`rise rate and relatively low maximum pressure permit the
`use of lightweight moving parts and a quiet high-speed
`operation, thereby rendering it possible to expect an in(cid:173)
`crease of the specific power output of the engine.
`Under the invention, combustion is achieved by par(cid:173)
`ticles and mixture spread over the entire region along the
`inner wall surface of the cylinder head, and therefore
`the heat load on the engine is heavy for the cylinder head
`and relatively light for the piston, the moving part. This 60
`is highly beneficial for the cooling of the piston and for
`the lubrication of the sliding surface of the piston with
`respect to the surrounding wall of the cylinder. On the
`other hand, the stationary head that is subjected to a
`great heat load can be cooled with relative ease from the 65
`outside, and thus the temperature control is made easy.
`Ample allowance for ignition timing brings about a stable
`and practically useful stratified charge, and permits op(cid:173)
`eration at a lean overall air-fuel ratio. With the foregoing
`merits the engine may dispense with the throttle at re- 70
`markable advantages of improved thermal efficiency and
`reductions of noxious contents in exhaust emission in the
`part-load operation.
`Although excess air used in accordance with the in(cid:173)
`vention decreases the combustion temperature, it on the 75
`
`8
`other hand improves the specific heat ratio of the active
`gas, decreases the cooling heat loss to the outside and
`reduces head dissociation due to combustion. These lead
`to an improvement of thermal efficiency and a sharp re(cid:173)
`duction of the carbon monoxide, CO, percentage in the
`exhaust by virtue of excess air. Furthermore, the pres-
`ence of excess air, no possibility of wasteful scattering
`of the fuel particles, and stable charge stratification all
`combine to avoid the extension of the zone wherein the
`mixture becomes too lean to burn. This in turn reduces
`to an appreciable extent the emission of hydrocarbons,
`HC, in the exhaust due to incomplete combustion, main(cid:173)
`tains the combustion temperature at a low level, and
`keeps the combustion temperature and the maximum pres(cid:173)
`sure during combustion at moderate levels with an air(cid:173)
`fuel ratio generally lean enough to fall in the misfiring
`zone. By dint of the self-circulating property of the ex(cid:173)
`haust which is inherent to the inactive gas and also by
`the fact that the distribution factor in the flow pattern
`of the exhaust is high in the central region of the com(cid:173)
`bustion chamber, the production of nitrogen oxides, NOx,
`(especially in the high-temperature zone in the center
`of the combustion chamber) can be reduced adequately.
`It is particularly worthy of note that, in the engine ac-
`25 cording to the present invention, the combustion tem(cid:173)
`perature in the central region of the combustion chamber
`is effectively kept low because of the relatively high per(cid:173)
`centage of the residual gas already burned during the
`course of the previous stroke in the central region of
`the cylinder and consequently an overall purification of
`the exhaust is made possible without an attendant drop
`of the engine output.
`Further, because it is contemplated in accordance with
`the invention that the cylinder head region of the en(cid:173)
`gine not only accomplishes the charge stratification but
`serves also as an evaporating region, the temperature con-
`trol is made extremely easy, and there is no burden in
`the least that should be borne by the piston, but rather
`more of the stratified charge is spread and burned along
`the inner wall surface of the cylinder head, thereby
`lessening the heat load upon the piston. Thus, an internal
`combustion engine generally of the 2-cycle design where(cid:173)
`in the reciprocating parts have no extra weight and are
`light enough for high-speed operation and are there