CN1097673C

CN1097673C - Internal combustion engine

Info

Publication number: CN1097673C
Application number: CN98123689A
Authority: CN
Inventors: 伊藤丈和; 佐佐木静夫
Original assignee: Toyota Motor Corp
Current assignee: Toyota Motor Corp
Priority date: 1997-11-07
Filing date: 1998-10-30
Publication date: 2003-01-01
Anticipated expiration: 2018-10-30
Also published as: CN1217425A; KR100289916B1; KR19990044826A

Abstract

An internal combustion engine comprising an exhaust gas recirculation system, wherein the amount of EGR gas in the combustion chamber is made larger than the amount of EGR gas where the amount of soot produced peaks when the engine load is comparatively low so as to suppress the temperatures of the fuel and gas around the fuel at the time of combustion in the combustion chamber to a temperature lower than the temperature at which soot is produced. This prevents the production of soot and NOx in the combustion chamber.

Description

Internal combustion engine

The present invention relates to an internal combustion engine.

In the past, in an internal combustion engine such as a diesel engine, generation of nitrogen oxides was suppressed by connecting an engine exhaust passage with an engine intake passage using an Exhaust Gas Recirculation (EGR) passage, so that exhaust gas, i.e., EGR gas, was recirculated in the engine intake passage through the exhaust gas recirculation passage. In this case, the exhaust gas recirculation gas has a relatively high specific heat and therefore can absorb a large amount of heat, so the greater the amount of exhaust gas recirculation gas, i.e., the higher the exhaust gas recirculation rate [ exhaust gas recirculation gas amount/(exhaust gas recirculation gas amount + intake air amount)], the lower the combustion temperature in the engine intake passage. When the engine intake passage temperature decreases, the amount of nitrogen oxides produced also decreases, and therefore the higher the exhaust gas recirculation rate, the lower the amount of nitrogen oxides produced.

It follows that, in the past, the higher the exhaust gas recirculation rate, the lower the amount of nitrogen oxides that may be produced. However, when the recirculation rate of the exhaust gas exceeds a certain limit, if the recirculation rate of the exhaust gas is increased, soot generation, that is, the amount of soot starts to increase sharply. In this regard, in the past, it was thought that smoke would increase without limit as the exhaust gas recirculation rate increased. Therefore, the exhaust gas recirculation rate at the time when the smoke starts to increase sharply is considered as the maximum allowable limit of the exhaust gas recirculation rate.

Therefore, in the past, the exhaust gas recirculation rate was set within a range not exceeding the maximum allowable limit. The maximum allowable limit of the exhaust gas recirculation rate is greatly different depending on the type of engine and fuel, but is from 30% to about 50%. Thus, in diesel engines, the exhaust gas recirculation rate is limited to 30% up to 50%.

Since the exhaust gas recirculation rate has been considered to have the maximum allowable limit in the past, the amount of nitrogen oxides and smoke generated in the past when the exhaust gas recirculation rate was set within the range not exceeding the maximum allowable limit was as small as possible. However, even if the exhaust gas recirculation rate is set so that the amounts of nitrogen oxides and smoke generated become as small as possible, there is a limit to the reduction in the amounts of nitrogen oxides and smoke generated. Therefore, in practice, a large amount of nitrogen oxides and smoke continues to be produced.

However, the present inventors have found in the course of studying diesel combustion that if the exhaust gas recirculation rate is set to be larger than the maximum allowable limit, the smoke increases sharply as described above, but there is a peak in the amount of smoke generated, and if the exhaust gas recirculation rate is set higher once this peak is exceeded, the smoke starts to decrease sharply, while if the exhaust gas recirculation rate is set to at least 70% at the time of engine idling or if the exhaust gas recirculation rate is forcibly cooled and set to at least 55%, the smoke disappears almost completely, that is, soot is hardly generated. Further, it was also found that nitrogen oxides are generated very little at this time, and based on this finding, further research in the future has identified the reason why soot is not generated and thus a new combustion system capable of reducing both soot and nitrogen oxides more than in the past has been established. This new combustion system will be described in detail later. But simply it is based on the concept of stopping the hydrocarbon from producing soot at a stage prior to hydrocarbon growth.

From repeated experiments and studies, it was found that the hydrocarbon stops generating soot at a stage before the temperature of fuel and gas around the fuel when burned in the combustion chamber of the engine is lower than a certain temperature, and that the hydrocarbon generates soot as soon as the temperature of fuel and gas around the fuel becomes higher than a certain temperature. In this case, the temperature of the fuel and the gas around the fuel is largely affected by the endothermic effect of the gas around the fuel when the fuel is burned. The temperature of the fuel and the gas around the fuel can be controlled by adjusting the amount of heat absorbed by the gas around the fuel based on the amount of heat generated when the fuel is burned.

Therefore, if the temperature of the fuel and the gas around the fuel at the time of combustion in the combustion chamber of the engine is suppressed to be lower than the temperature in the middle of the stop of the production of hydrocarbons, soot is not generated any more, and the temperature of the fuel and the gas around the fuel at the time of combustion in the combustion chamber can be suppressed to be lower than the temperature in the middle of the stop of the production of hydrocarbons by adjusting the amount of heat absorbed by the gas around the fuel. On the other hand, hydrocarbons stopped in the middle of production are easily purified before becoming soot by post-treatment using an oxidation catalyst or the like, which is a basic idea of the new combustion system herein.

The object of the present invention is to provide an internal combustion engine which operates on a new combustion principle based on a new combustion system.

According to the present invention, there is provided an internal combustion engine in which the amount of soot generated gradually increases and then it reaches a peak when the amount of inert gas in a combustion chamber increases, wherein the amount of inert gas is made larger than the amount of inert gas at which the amount of soot generated reaches a peak so that the temperature of fuel and gas around the fuel at the time of combustion in the combustion chamber is lower than the temperature at the time of soot generation, while an aftertreatment device for purifying unburned hydrocarbon discharged from the combustion chamber in the form of soot precursor or its former is provided in an engine exhaust passage.

The present invention may be more fully understood from the description of the preferred embodiments of the invention set forth in conjunction with the following drawings, in which:

fig. 1 is an overall view of a compression ignition type internal combustion engine;

FIG. 2 is a graph of smoke and NOx production;

FIGS. 3A and 3B are combustion pressure profiles;

FIG. 4 is a fuel oil map;

FIG. 5 is a graph of a relationship between a generated amount of smoke and an exhaust gas recirculation rate;

FIG. 6 is a graph showing a relationship between an amount of injected fuel and an amount of mixed gas;

FIG. 7 is a graph showing the relationship between the amount of fuel injected and the amount of mixed gas;

FIG. 8 is a graph showing a relationship between an amount of injected fuel and an amount of mixed gas;

FIG. 9 is a graph showing a relationship between an amount of injected fuel and an amount of mixed gas;

FIGS. 10A and 10B are graphs of ignition timing;

FIG. 11 is a graph of air to fuel ratio sensor output;

FIG. 12 is a flowchart of engine operation control;

fig. 13 is a graph showing the relationship between the amount of injected fuel and the amount of mixed gas;

FIG. 14 is a general view of another embodiment of a compression ignition type internal combustion engine;

FIG. 15 is a side cross-sectional view of an actuator driving an intake valve;

FIG. 16 is a graph of the degree of opening of the throttle valve or the like;

FIG. 17 is a graph of intake valve operating time;

FIG. 18 is a graph of an ignition cycle;

FIG. 19 is a graph showing the amount of cooling water supplied to the cooling device;

FIG. 20 is a general view of still another embodiment of a compression ignition type internal combustion engine; and FIGS. 21A and 21B are diagrammatic views illustrating the effect of absorbing and releasing nitric oxide. Description of the preferred embodiments

Fig. 1 is a view showing a case where the present invention is applied to a four-stroke compression ignition type internal combustion engine.

Referring to fig. 1, 1 denotes an engine body, 2 denotes a cylinder block, 3 denotes a cylinder head, 4 denotes a piston, 5 denotes a combustion chamber, 6 denotes an electrically controlled fuel injector, 7 denotes an intake valve, 8 denotes an intake port, 9 denotes an exhaust valve, and 10 denotes an exhaust port, the intake port 8 being connected to a surge tank 12 through a corresponding intake pipe 11. The pressure equalizing box 12 is connected with an air purifier 14 through an air inlet pipe 13, and a throttle valve 16 driven by a motor 15 is arranged in the air inlet pipe 13; on the other hand, the exhaust port 10 is connected to a catalytic converter 20 incorporating a catalyst 19 having an oxidizing action through an exhaust pipe 17 and an exhaust pipe 18, and an air-fuel ratio sensor 21 is incorporated in the exhaust pipe 17.

The exhaust gas duct 17 and the surge tank 12 are connected to each other through an exhaust gas recirculation passage 22, and an electrically controlled exhaust gas recirculation control valve 23 is provided in the exhaust gas recirculation passage 22. Each fuel nozzle 6 is connected to a fuel tank, i.e., a common oil sump 25, by a fuel supply pipe 24. Fuel is supplied to the common sump 25 from an electrically controlled variable drain pump 26. The fuel supplied into the common oil sump 25 is supplied to the fuel nozzle 6 through each fuel supply pipe 24. A fuel pressure sensor 27 for detecting the fuel pressure in the common oil tank 25 is mounted on the common oil tank 25. The amount of oil discharged from the fuel pump 26 is controlled based on the output signal of the fuel pressure sensor 27, so that the fuel pressure in the common oil reservoir 25 becomes a predetermined fuel pressure.

The electronic control unit 30 includes a digital computer and is provided with a ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, an input port 35, and an output port 36 connected to each other by a bidirectional bus 31. The output signal of the air fuel ratio sensor 21 is input to the input port 35 through a corresponding analog-to-digital converter 37. Further, the output signal of the fuel pressure sensor 27 is input to the input port 35 through the corresponding analog-to-digital converter 37. The accelerator pedal 40 is connected toa load sensor 41 to generate an output voltage proportional to the amount L of compression of the accelerator pedal 40. The output voltage of the load sensor 41 is input to the input port 35 via the corresponding analog-to-digital converter 37. Furthermore, the input port 35 is connected to a crank angle sensor 42 for generating an output pulse every time the crankshaft rotates, for example, 30 °. On the other hand, the output port 36 is connected to the fuel nozzle 6, the motor 15, the exhaust gas recirculation control valve 23, and the fuel pump 26 via corresponding drive circuits 38.

Fig. 2 shows an example of an experiment showing changes in the output torque, the amounts of exhaust smoke, hydrocarbons, carbon monoxide and nitrogen oxides when the air-fuel ratio a/F (abscissa of fig. 2) is changed by changing the opening degree of the throttle valve 16 and the exhaust gas recirculation rate at the time of engine low load operation. As will be understood from fig. 2, in this experiment, the smaller the air-fuel ratio a/F, the larger the egr rate becomes. When the air-fuel ratio a/F is lower than the stoichiometric ratio (≈ 14.6), the exhaust gas recirculation rate becomes more than 65%.

As shown in fig. 2, if the egr rate is increased to decrease the air-fuel ratio a/F, the amount of smoke generated starts to increase when the egr rate becomes close to 40% and the degree of the air-fuel ratio a/F is 30. Secondly, when the exhaust gas recirculation rate is further increased and the air-fuel ratio a/F is made smaller, the amount of smoke generated sharply increases and peaks. Again, when the egr rate is further increased and the air-fuel ratio is decreased, the smoke amount is drastically decreased, and when the egr rate is made to exceed 65% and the air-fuel ratio a/F becomes close to 15.0, the generated smoke amount becomes substantially zero. I.e. almost no soot is produced. At this time, the output torque of the engine slightly decreases, and the amount of nitrogen oxide generated becomes sufficiently low. On the other hand, at this time, the amounts of hydrocarbons and carbon monoxide produced start to increase.

Fig. 3A shows the change in compression pressure in the combustion chamber 5 when the amount of smoke generated maximally approaches the air-fuel ratio a/F18. Fig. 3B shows the change in the compression pressure in the combustion chamber 5 when the amount of smoke generated is substantially zero near the air-fuel ratio a/F13. As can be understood from a comparison of fig. 3A and 3B, the combustion pressure in the case shown in fig. 3B where the amount of generated smoke is substantially zero is lower than that in the case shown in fig. 3A where the amount of generated smoke is large.

The results of the experiments shown in FIG. 2 and FIGS. 3A and 3B can be described as follows. That is, first, when the air-fuel ratio a/F is less than 15.0 and the amount of generated smoke is substantially zero, the amount of generated nitrogen oxide greatly decreases as shown in fig. 2. The fact that the amount of generated nitrogen oxide decreases means that the combustion temperature in the combustion chamber 5 decreases. Therefore, it can be said that when soot is hardly generated, the combustion temperature in the combustion chamber 5 becomes lower, and the same can be explained from fig. 3A and 3B. That is, in the state shown in fig. 3 where soot is hardly generated, the combustion pressure becomes low, and therefore the combustion temperature in the combustion chamber 5 at this time becomes low.

Second, when the amount of smoke generated, that is, the amount of soot generated becomes substantially zero, as shown in fig. 2, the amounts of hydrocarbons and carbon monoxide increase. This means that the hydrocarbons are discharged without producing soot. That is, when the temperature rises in the oxygen deficient state, the straight chain hydrocarbons and aromatic hydrocarbons contained in the fuel oil and shown in fig. 4 are decomposed, causing the formation of the soot precursor structure. Secondly, soot is generated which mainly comprises solid matter of carbon atoms. In this case, the actual process of soot generation is complex. It is not clear how the soot precursor is formed, but in any event the hydrocarbon shown in figure 4 generates soot from the soot precursor. Therefore, as described above, when the soot generation amount becomes substantially zero, the discharge amounts of hydrocarbons and carbon monoxide increase as shown in fig. 2, but at this time the hydrocarbons are in the state of soot precursors or hydrocarbons before them.

Summarizing such an opinion from the experimental results shown in fig. 2 and fig. 3A and 3B, when the combustion temperature in the combustion chamber 5 is low, the amount of soot generated becomes substantially zero. At this time, the state of the soot precursor or the hydrocarbon before it is discharged from the engine intake passage. More detailed experiments and studies were derived therefrom. Therefore, it is known that when the temperature of the fuel and the gas around the fuel in the engine combustion chamber 5 is lower than a certain temperature, the generation process of soot is stopped halfway, that is, soot is not generated at all, and that when the temperature of the fuel and the gas around the fuel in the engine combustion chamber 5 becomes higher than a certain temperature, soot is generated.

When the hydrocarbon production process is stopped in the soot precursor state, the temperature of the fuel and its surrounding gas, i.e., the above-mentioned certain temperature, varies depending on various factors such as the type of fuel, the air-fuel ratio, and the compression ratio, so it cannot be said to what extent, but the certain temperature is extremely related to the amount of nitrogen oxide produced. Therefore, this certain temperature can be determined to some extent from the amount of nitrogen oxides produced. That is, the greater the exhaust gas recirculation rate is, the lower the temperature of the fuel and its surrounding gas at the time of combustion is, and the lower the amount of nitrogen oxides produced at the same time is. At this time, when the amount of nitrogen oxides generated becomes about 10 parts per million (10ppm) or less, soot is hardly generated any more. Therefore, the above certain temperature substantially matches the temperature when the amount of nitrogen oxides generated becomes about 10ppm or less.

Once soot is generated, it cannot be removed by post-treatment using an oxidation catalyst or the like. In contrast, the soot precursor or the hydrocarbon state preceding it can be easily removed by post-treatment using an oxidation catalyst or the like. When considering aftertreatment by an oxidation catalyst or the like, there is a great difference between whether the hydrocarbons are discharged from the engine combustion chamber in the form of soot precursors or in a state preceding them, or are discharged from the engine combustion chamber 5 in the form of soot. The combustion system according to the present invention is based on a concept that the discharge of hydrocarbons from the engine combustion chamber 5 in the form of soot precursors or in a state before them does not allow soot to be generated in the engine combustion chamber 5 and the hydrocarbons to be oxidized by an oxidation catalyst or the like.

Now to stop the production of hydrocarbons in a state before soot is generated, it is necessary to suppress the temperature at which the fuel and its surrounding gas are burned in the engine combustion chamber 5 to be lower than the temperature at which soot is generated. In this case, the endothermic effect of the gas around the fuel at the time of combustion of the fuel has a great influence on the suppression of the temperature of the fuel and the gas around the fuel.

That is, if there is only air surrounding the fuel, the volatilized fuel will immediately react with oxygen in the air and combust. In this case, the temperature of the air remote from the fuel does not rise that high. Only the temperature around the fuel locally becomes extremely high. That is, at this time, the air away from the fuel does not absorb much of the heat of the fuel combustion at all. In this case, since the combustion temperature locally becomes extremely high, unburned hydrocarbons receive combustion heat to generate soot.

On the other hand, when fuel is present in a mixed gas of a large amount of inert gas and a small amount of air, the situation is somewhat different. In this case, the volatilized fuel oil is dispersed in the ambient gas and burned by reacting with oxygen mixed in the inert gas. In this case, the heat of combustion is absorbed by the surrounding inert gas, so the combustion temperature no longer rises much. That is, the presence of the inert gas plays an important role in suppressing the combustion temperature, and it is possible to suppress the combustion temperature low by the endothermic action of the inert gas.

In this case, in order to suppress the temperature of the fuel and its surrounding gas below the temperature at which soot is generated, the amount of inert gas is required to be sufficient to absorb heat at a reduced temperature. Therefore, if the amount of fuel is increased, the amount of inert gas required is also the sameAre increased together. It should be noted that the greater the specific heat of the inert gas in this case, the stronger the endothermic effect. Therefore, the inert gas is preferably a gas having a large specific heat. In this respect, carbon dioxide (CO)₂) And the offgas recycle gas have a large specific heat, so it can be said that it is optimal to use the offgas recycle gas as the inert gas.

Fig. 5 shows the relationship between the exhaust gas recirculation rate and the flue gas when the exhaust gas recirculation gas is used as an inert gas and the degree of cooling of the exhaust gas recirculation gas is changed. That is, curve a of fig. 5 represents the case where the egr gas is cooled strongly and the temperature of the egr gas is maintained at about 90 ℃, while curve B represents the case where the egr gas is cooled with a small cooling device and curve C represents the case where the egr gas is not cooled strongly.

When the exhaust gas recirculation gas is strongly cooled as shown by curve a in fig. 5, the amount of soot generated peaks when the exhaust gas recirculation rate is slightly below 50%. In this case, if the exhaust gas recirculation rate is made about 55% or higher, soot is hardly generated any more.

On the other hand, when the exhaust gas recirculation gas is cooled slightly as shown by the curve B in fig. 5, the amount of soot generated peaks when the exhaust gas recirculation rate is slightly higher than 50%. In this case, if the exhaust gas recirculation rate is made higher than about 65%, soot is hardly generated any more.

Further, when the egr gas is not forcibly cooled as shown by curve C in fig. 5, the amount of soot generated peaks at an egr rate close to 55%. In this case, if the exhaust gas recirculation rate is made to exceed about 70%, soot is hardly generated any more.

It should be noted that fig. 5 shows the amount of soot generated when the engine load is relatively high. When the load of the engine becomes smaller, there is some decrease in the exhaust gas recirculation rate at which the amount of soot generated reaches a peak, while there is some decrease in the lower limit of the exhaust gas recirculation rate at which sootis hardly generated any more. Thus, the lower limit of the exhaust gas recirculation rate at which soot is hardly generated changes depending on the degree of cooling of the exhaust gas recirculation gas or the load of the engine.

Fig. 6 shows the amount of the mixture of egr gas and air, the proportion of air in the mixture, and the proportion of egr gas in the mixture, which are required to make the temperature of fuel and its surrounding gas at the time of combustion lower than the temperature at which soot is generated in the case of using egr gas as an inert gas. It should be noted that in fig. 6, the ordinate represents the total amount of intake gas that enters the engine combustion chamber 5, and the chain line Y represents the total amount of intake gas that can enter the engine combustion chamber 5 when supercharging is not performed. Further, the horizontal axis indicates the required load, and Z1 indicates a low-load operation region.

Referring to fig. 6, the proportion of air, that is, the amount of air in the mixture gas, indicates the amount of air required to completely combust the injected fuel. That is, in the case shown in fig. 6, the ratio of the air amount to the injected fuel amount becomes the stoichiometric air-fuel ratio. On the other hand, in fig. 6, the proportion of egr gas, that is, the amount of egr gas in the mixture gas is represented as the minimum amount of egr gas required to make the temperature of the fuel and its surrounding gas lower than the temperature at which soot is generated. The quantity of exhaust gas recirculation gas expressed as exhaust gas recirculation rate is at least 55%; in the embodiment shown in fig. 6, at least 70%. That is, if the total amount of intake gas entering the engine combustion chamber 5 forms the solid line X in fig. 6 while the ratio between the amount of air and the amount of egr gas in the total amount of intake gas X is made to the ratio shown in fig. 6, the temperature ofthe fuel and its surrounding gas becomes lower than the temperature at which soot is generated, and thus soot is not generated at all. Further, the amount of nitrogen oxide generated at this time is 10ppm or less, and therefore the amount of nitrogen oxide generated becomes extremely small.

If the amount of fuel injected is increased, the heat generated during combustion is increased in order to keep the temperature of the fuel and its surrounding gases below that which generates soot, and the heat absorbed by the exhaust gas recirculation gas must be increased. Therefore, as shown in FIG. 6, the greater the amount of fuel injected, the greater the amount of EGR must be. That is, as the required load becomes higher, the amount of egr gas must be increased.

On the other hand, in the load zone Z2 of fig. 6, the total amount of intake gas X required to suppress the generation of soot exceeds the total amount of intake gas Y that can enter. Thus, in this case, the exhaust gas recirculation gas and the incoming air or exhaust gas recirculation gas need to be pressurized or pressurized in order to suppress the generation of soot by supplying the total amount of intake gas into the engine combustion chamber 5. When the egr gas or the like is not pressurized or pressurized, the total amount X of the intake gas matches the total amount Y of the intake gas that can enter in the load zone Z2. Therefore, in this case, in order to suppress the generation of soot, the amount of air is slightly reduced to increase the amount of exhaust recirculation gas while burning the fuel in a state where the air-fuel ratio is richer.

As described above, fig. 6 shows a case where fuel is burned in a case where the air fuel ratio is stoichiometric. In the region Z1 of the low load operation shown in fig. 6, even if the air-fuel ratio is made smaller than the air amount shown in fig. 6, that is, even if the air-fuel ratio is made richer, it is possible to inhibit the generation of soot and to make the amount of generated nitrogen oxide be about 10ppm or less. Further, in the low load region Z1 shown in fig. 6, even if the air-fuel ratio is made larger than the air amount shown in fig. 6, that is, the average value of the air-fuel ratio tends to be 17 to 18, it is possible to inhibit the generation of soot and to make the amount of generated nitrogen oxide to be about 10ppm or less.

That is, when the air fuel ratio is made rich, the fuel becomes excessive, but the excessive fuel does not generate soot because the fuel temperature is suppressed to a low temperature and the excessive fuel does not generate soot. In addition, only a very small amount of nitrogen oxide is generated at this time. On the other hand, when the average air fuel ratio is leaner or when the air fuel ratio is the stoichiometric air fuel ratio, a small amount of soot is generated if the combustion temperature becomes higher, but in the present invention, the combustion temperature is suppressed to a low temperature, so soot is not generated at all. In addition, only very small amounts of nitrogen oxides are produced.

Thus, in the present invention, in the engine low load operation region Z1, no soot is generated and the amount of nitrogen oxides generated becomes extremely small regardless of the air-fuel ratio, that is, regardless of whether the air-fuel ratio is rich or the stoichiometric air-fuel ratio or the average air-fuel ratio is lean. Therefore, when the fuel consumption is improved, it can be said that it is preferable to make the average air-fuel ratio lean.

Fig. 7 and 8 show the case where the intake gas is made the maximum amount of gas that can enter the combustion chamber 5. It should be noted that fig. 7 shows the case where the egr rate is maintained at a substantially constant egr rate of at least 55%, irrespective of the required load. In this case, the increase of the excess air amount is smaller than the required load. On the other hand, fig. 8 shows the case where the air fuel ratio is maintained at a predetermined richer air fuel ratio or a stoichiometric air fuel ratio, or the average air fuel ratio is maintained at a predetermined leaner air fuel ratio, regardless of the required load. In this case, the increase in the exhaust gas recirculation rate is smaller than the required load. However, in this case, even when the exhaust gas recirculation rate is the lowest, it is at least about 55%. The amount of nitrogen oxides generated while no soot is generated at all is extremely small in any of the cases shown in fig. 7 and 8.

Next, a specific example of the control operation at the time of engine low load operation will be described with reference to fig. 9 to 12.

Fig. 9 shows the amount of fuel injected, the amount of intake air, and the degree of opening of the egr control valve 23 (fig. 1) for obtaining the amount of intake air and the amount of egr with respect to the required load and the opening of the throttle valve 16 (fig. 1). It should be noted that in fig. 9, Y represents the same value as Y in fig. 6. In this example, the exhaust gas recirculation control valve 23 is kept in the fully open state except when the required load is the lowest, that is, when the engine is idling. The throttle valve 16 is gradually opened from the half-open state to the fully-open state with an increase in required load, except when the engine is idling. When the engine load becomes higher, the normal combustion state used in the past is maintained. That is, the throttle valve 16 is fully opened, but the exhaust gas recirculation control valve is closed.

As shown in fig. 9, the throttle valve 16 is closed to a nearly fully closed state at the time of engine idling. At this time, the egr control valve 23 is also closed to a nearly fully closed state, thereby giving the optimum amount of egr gas according to the required load. When the throttle valve 16 is closed to near the fully closed state, the pressure in the combustion chamber 5 at the start of compression becomes lower, so the compression pressure becomes smaller. When the compression pressure becomes smaller, the compression work performed by the piston 4 becomes smaller, so the vibration of the engine body 1 also becomes smaller. That is, at the time of engine idling, the throttle valve 16 is closed to a nearly fully closed state, so that vibration of the engine body 1 is suppressed.

The hatched lines in fig. 10A indicate the duration of fuel injection from the fuel injection nozzles 6. As shown in fig. 10A, the timing at which the injection starts is gradually retarded from about 14 ° before top dead center of the compression stroke as the required load becomes higher. It should be noted that as shown in fig. 10B, the fuel can also be injected in two stages: an initial stage of the suction stroke and a final stage of the compression stroke.

Fig. 11 shows the output of the air fuel ratio sensor 21. As shown in fig. 11, the output current I of the air-fuel ratio sensor 21 varies with the air-fuel ratio a/F. Therefore, the air fuel ratio can be determined from the output current I of the air fuel ratio sensor 21.

Fig. 12 shows a routine for controlling the engine to operate at the time of low load operation. Referring to fig. 12, first, at step 50, the throttle valve 16 is controlled to the opening degree shown in fig. 9. Next, in step 51, the egr control valve 23 is controlled to the opening degree shown in fig. 9. Again, at step 52, the fuel pressure in the common fuel tank 25 is controlled to a predetermined fuel pressure based on the output signal of the fuel pressure sensor 27. Again, at step 53, the injection time is calculated. Again, at step 54, the basicfuel injection amount determined by the compression amount of the accelerator pedal 40 and the engine rotational speed is corrected so that the air fuel ratio becomes a predetermined air fuel ratio in accordance with the output signal of the air fuel ratio sensor 21.

During low engine load operation, no soot is produced at all. Nitrogen oxides are hardly generated. When the exhaust gas contains hydrocarbons in a state of soot precursors or before them, the hydrocarbons are oxidized by the catalyst 19.

As the catalyst 19, an oxidation catalyst, a three-way catalyst, or a nitrogen oxide absorbent may be used. The nitrogen oxide absorbent has a function of absorbing nitrogen oxides when the average air fuel ratio is leaner in the combustion chamber 5, and releasing nitrogen oxides when the average air fuel ratio is richer in the combustion chamber 5.

The nitrogen oxide adsorbent includes, for example, alumina as a carrier, and on the carrier, for example, at least potassium K, sodium Na, lithium Li, cesium Cs, and other alkali metals, barium Ba, calcium Ca, and other alkaline earth metals, one of lanthanum La, yttrium Y, and other rare earth elements plus platinum Pt or another noble metal.

Of course, the oxidation catalyst, and also the three-way catalyst and the nitrogen oxide adsorbent have an oxidation function, and therefore, the three-way catalyst and the nitrogen oxide adsorbent can be used as the catalyst 19 as described above.

Fig. 13 shows another specific example of the control operation at the time of engine low load operation. In this example, the degree of opening of the throttle valve 16 and the degree of opening of the egr control valve 23 increase with increasing required load, and therefore the amount of intake air and the amount of egr gas increase with increasing amount of injected fuel.

Fig. 14to 19 show different embodiments which can be used to further reduce the amount of soot generated at the time of engine low load operation, and can be used to expand the operating region where the amount of soot generated becomes substantially zero toward the high load side.

Referring to fig. 14, a cooling device 60 is provided around the exhaust gas recirculation passage 22. The cooling device 60 has a cooling water suction port 61 and a cooling water discharge port 62. The cooling water suction port 61 is connected to a cooling water outlet of a radiator (not shown) through a cooling water supply pipe 63. The cooling water discharge port 62 is connected to an inlet of, for example, a water pump (not shown) through a cooling water discharge pipe 64. A flow control valve 65 is provided in the cooling water supply pipe 63 to control the amount of cooling water supplied to the cooling device 60.

Further, a driver 70 for driving the suction valve is provided at the top of the suction valve 7. Fig. 15 shows an enlarged view of the actuator 70 for actuating the suction valve. Referring to fig. 15, 71 shows a disk-shaped member made of iron installed on the top of the suction valve 7, 72 and 73 are electromagnetic coils installed on both sides of the iron 71, and 74 and 75 are compression springs installed on both sides of the iron 71. When the solenoid coil 73 is biased, the iron 71 rises while the suction valve 7 is closed. In contrast, when the helical coil 72 is biased, the iron 71 falls while the suction valve 7 is opened. Therefore, by controlling the timing of biasing the solenoids 72 and 73, the suction valve 7 can be opened and closed at any time.

If the dual use is made of the effect of reducing the combustion temperature by another means in addition to the effect of reducing the combustion temperature by the endothermic effect of the inert gas, for example, the exhaust gasrecirculation gas, the combustion temperature can be further reduced and therefore no further soot can be generated. Further, if the combustion temperature is further lowered in this way, the operating region where soot is hardly generated can be expanded toward the high load side. In this case, it is necessary to enhance the effect of lowering the combustion temperature, that is, the more heat is generated at the time of combustion at a higher load.

Fig. 16 shows a case where the combustion temperature is lowered by reducing the opening degree of the throttle valve 16. That is, if the opening degree of the throttle valve 16 is decreased, the pressure in the combustion chamber 5 becomes low at the start of compression, so the pressure in the combustion chamber 5 becomes low at the end of the compression stroke, and therefore the combustion temperature becomes low. It should be noted that in the example shown in fig. 16, as the required load becomes higher, the effect of lowering the combustion temperature is intensified by making the engine load higher by reducing the opening degree of the throttle valve 16, in addition to the engine idling period.

Fig. 17 shows a case where the combustion temperature is lowered by delaying the closing time of the intake valve 7. That is, if the closing time of the intake valve 7 is delayed, the actual compression ratio becomes smaller, and therefore the combustion temperature falls. It should be noted that in the example shown in fig. 17, the higher load required enhances the effect of lowering the combustion temperature by gradually delaying the closing time of the intake valve 7 by the driver 70 so that the higher the engine load. It should be noted that various types of variable compression devices are known for changing the compression ratio. The compression ratio can be reduced using these variable compression devices as the engine load becomes higher.

Fig. 18 shows a casewhere the combustion temperature is lowered by delaying the injection time. That is, if the injection time is made close to the top dead center of the compression stroke or made after the compression stroke, the combustion pressure is lowered while the combustion temperature is lowered. It should be noted that in the example shown in fig. 18, as the engine load becomes higher, the effect of lowering the combustion temperature is enhanced by gradually retarding the injection time from the compression stroke front top dead center to the compression stroke rear top dead center to make the engine load higher.

Fig. 19 shows a case where the combustion temperature is lowered by cooling the egr gas by the cooling device 60. It should be noted that in the example shown in fig. 19, when the engine load is less than the predetermined load, the cooling action by the cooling device 60 is suspended, and when the engine load becomes the predetermined load or higher, the action of lowering the combustion temperature is intensified by controlling the flow rate control valve 65 so that the required load becomes high, that is, the cooling capacity of the cooling device 60 becomes high, and the amount of cooling water supplied to the cooling device increases.

Again, a case where a nitrogen oxide absorbent is used as the catalyst 19 will be explained. It should be noted that, as shown in fig. 20, it is also possible to use an oxidation catalyst as the catalyst 19 and to place the nitrogen oxide adsorbent 80 in the discharge passage downstream of the oxidation catalyst 19. If the ratio of air to fuel (hydrocarbon) supplied to the engine intake passage, the combustion chamber 5, and the exhaust passage upstream of the nitrogen oxide adsorbent 19 (fig. 1) or 80 (fig. 20) is referred to as the air-fuel ratio of the exhaust gas flowing into the nitrogen oxide adsorbent 19, 80. Since the nitrogen oxide adsorbent 19, 80 has the function ofadsorbing and releasing nitrogen oxides, that is, adsorbing nitrogen oxides when the air-fuel ratio of the inflow exhaust gas is lean, while releasing nitrogen oxides when the air-fuel ratio of the inflow exhaust gas is stoichiometric or rich.

If the nitrogen oxide adsorbent 19, 80 is placed in the engine exhaust passage, the nitrogen oxide adsorbent 19, 80 has the ability to absorb and release nitrogen oxide. The detailed mechanism of this absorption and release action is partially unclear. However, this absorption and release effect can be considered to be the result of the mechanism shown in fig. 21A and 21B. Next, this mechanism will be described by taking a case where platinum Pt and barium Ba are placed on a support as an example, but the same mechanism is applied even if other noble metals, alkali metals or rare earth elements are used.

In the compression ignition type internal combustion engine shown in fig. 1 and 20, combustion is performed in a relatively lean air-fuel ratio. When combustion is performed in this manner in a relatively lean air-fuel ratio state, the concentration of oxygen in the exhaust gas is high. At this time, as shown in FIG. 21A, oxygen O₂With O₂ ^-Or O₂ ^-Is adhered on the surface of platinum Pt. On the other hand, nitrogen monoxide NO and O in the inflowing exhaust gas₂ ^-Or O^2-Reaction to nitrogen dioxide NO₂( ). Secondly, part of the nitrogen dioxide NO produced₂Oxidized on platinum Pt and absorbed in the adsorbent, while binding with barium oxide BaO to form nitrate ion NO₃ ^-The form of (a) diffuses in the adsorbent as shown in fig. 21A. In this way the nitrogen oxides are adsorbed in the adsorbents 19, 80. Nitrogen dioxide NO is generated on the surface of platinum Pt as long as the oxygen concentration in the inflowingexhaust gas is high₂. As long as nitrogen oxide is adsorbedThe capacity of the adsorbent to adsorb nitrogen oxides is not saturated, nitrogen dioxide NO₂Is adsorbed in the adsorbent and produces nitrate ions NO₃ ^-。

On the other hand, at the time of low-speed engine operation, the combustion temperature is lowered to prevent the generation of soot. At this time, the air-fuel ratio becomes richer. When the air-fuel ratio becomes richer, the oxygen concentration in the inflowing exhaust gas decreases, the amount of nitrogen oxides produced also decreases, and the reaction proceeds in the reverse direction (NO)₃ ^-→NO₂) Thus, nitrate ion NO in the adsorbent₃ ^-With nitrogen dioxide NO₂Is released from the adsorbent. At this time, the nitrogen oxides released from the nitrogen oxide absorbents 19, 80 are reduced by reacting with a large amount of unburned hydrocarbons or carbon monoxide contained in the inflowing exhaust gas, as shown in fig. 21B. So that NO nitrogen dioxide NO is present on the surface of the platinum Pt₂Nitrogen dioxide NO₂Is continuously released from the adsorbent. Therefore, when the air-fuel ratio becomes richer, nitrogen oxides are released from the nitrogen oxide adsorbents 19, 80 in a short time, and the released nitrogen oxides are reduced, so that the emission of nitrogen oxides into the atmosphere can be prevented.

Thus, in this embodiment, the nitrogen oxide adsorbent 19, 80 functions to adsorb and release nitrogen oxide to hinder the release of nitrogen oxide into the atmosphere. Further, even when the air fuel ratio becomes richer at the time of engine low load operation so as to release nitrogen oxides from the nitrogen oxide adsorbents 19, 80, soot is hardly generated.

It should be noted that it is also possible to make the air fuelslightly leaner than at low speed and make the air fuel richer only when nitrogen oxides are released from the nitrogen oxide sorbents 19, 80. It should be noted that since the nitrogen oxide adsorbents 19, 80 also have the function of a reduction catalyst, even when the air fuel ratio becomes the stoichiometric air fuel ratio, the nitrogen oxides released from the nitrogen oxide adsorbents 19, 80 are reduced when the nitrogen oxides are to be released. However, when the air-fuel ratio becomes the stoichiometric air-fuel ratio, nitrogen oxides are released only gradually from the nitrogen oxide adsorbents 19, 80, so it takes a little long time for all the nitrogen oxides absorbed in the nitrogen oxide adsorbents 19, 80 to be released.

The above description is an example in which the present invention is applied to a compression ignition type internal combustion engine, but the present invention can also be applied to a gasoline engine.

Thus, according to the present invention, it becomes possible to prevent the generation of soot and nitrogen oxides at the same time.

While the invention has been described by reference to specific embodiments chosen for purposes of illustration, it should be apparent that numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention.

Claims

1. A method of controlling an internal combustion engine including an exhaust gas recirculation EGR apparatus (22, 23), in which a soot generation amount is gradually increased and then it reaches a peak when an amount of inert gas in a combustion chamber (6) is increased, characterized in that when the soot reaches the peak amount, a value of EGR also reaches a peak; making the amount of inert gas largerthan the amount at which the amount of soot generation reaches a peak, so that the temperature at which fuel oil and gas around the fuel oil are burned in the combustion chamber (6) is lower than the temperature at which soot is generated; an aftertreatment device (19, 80) for cleaning unburned hydrocarbons which are discharged from the combustion chamber in the form of soot precursors or in the form of precursors thereof is installed in the engine exhaust passage (18).

2. A method of controlling an internal combustion engine according to claim 1, wherein the temperature at which the fuel and the gas around the fuel are combusted in the combustion chamber (6) is a temperature at which the amount of nitrogen oxides (NOx) in the exhaust gas is 10ppm or less.

3. A method of controlling an internal combustion engine according to claim 1, characterized in that the temperature at which the fuel and the gas around the fuel are burned in the combustion chamber (6) is made lower than the temperature at which soot is generated only when the engine load is lower than a predetermined load.

4. A method of controlling an internal combustion engine according to claim 1, wherein said aftertreatment device (19, 80) includes a catalyst (19) having an oxidation function for oxidizing unburned hydrocarbons discharged from the combustion chamber (6).

5. The method of controlling an internal combustion engine according to claim 4, wherein the catalyst (19) includes at least one of an oxidation catalyst, a three-way catalyst, and a nitrogen oxide adsorbent.

6. A method of controlling an internal combustion engine according to claim 1, wherein the combustion heat of the fuel is mainly absorbed by the inert gas in the combustion chamber (6) while the amount of the inert gas is preset.

7. A method of controlling an internal combustion engine as claimed in claim 1, characterized in that the majority of the inert gas is fed into the combustion chamber (6) during the intake stroke.

8. The method of controlling an internal combustion engine of claim 7, wherein an Exhaust Gas Recirculation (EGR) rate is at least about 55%.

9. The method of controlling an internal combustion engine according to claim 1, wherein the air-fuel ratio is made a stoichiometric air-fuel ratio, a leaner air-fuel ratio slightly leaner than the stoichiometric air-fuel ratio, or a richer air-fuel ratio.

10. A method of controlling an internal combustion engine according to claim 1, wherein combustion temperature control means are provided for maintaining the temperature at which the fuel and the gas around the fuel are combusted in the combustion chamber (6) lower than the temperature at which soot is generated, irrespective of the amount of fuel fed into the combustion chamber (6) when the engine load is lower than a predetermined load.

11. The method of controlling an internal combustion engine according to claim 10, wherein the combustion temperature control means includes exhaust gas recirculation means for recirculating exhaust gas discharged from an exhaust passage of the engine into an intake passage of the engine; while the more the amount of fuel that enters the combustion chamber increases, the more recirculated exhaust gas that is recirculated into the engine intake passage increases.

12. A method of controlling an internal combustion engine according to claim 10, wherein the combustion temperature control means includes cooling means for cooling recirculated exhaust gas recirculated from the engine exhaust passage (10) into the engine intake passage (9); and the more the amount of fuel entering the combustion chamber increases, the more the cooling capacity of the cooling device increases.

13. The method of controlling an internal combustion engine according to claim 10, wherein the combustion temperature control means controls a compression ratio of the engine; while the more the amount of fuel entering the combustion chamber (6) increases, the lower the compression ratio of the engine.

14. A method of controlling an internal combustion engine according to claim 10, wherein the combustion temperature control means includes an intake control valve (7) installed in an engine intake passage (9); and the more fuel enters the combustion chamber (6), the smaller the degree of opening of the intake control valve (7).

15. A method of controlling an internal combustion engine according to claim 10, wherein the combustion temperature control means comprises an actuator for actuating the intake valve (7); the more fuel is added to the combustion chamber at the same time, the later the time the intake valve (7) is opened by the actuator (70).

16. A method of controlling an internal combustion engine according to claim 10, wherein the combustion temperature control means controls the fuel injection time to be injected into the combustion chamber (6), and when the amount of fuel introduced into the combustion chamber is small, the fuel injection time is made in the compression stroke end stage, and the more the amount of fuel introduced into the combustion chamber increases, the later the fuel injection time.

17. A method of controlling an internal combustion engine according to claim 1, wherein a nitrogen oxide adsorbent (80) is disposed in an exhaust passage of the engine, the nitrogen oxide adsorbent adsorbing nitrogen oxides contained in the exhaust gas when an air-fuel ratio of the inflow exhaust gas is leaner, and releasing the adsorbed nitrogen oxides when the air-fuel ratio of the inflow exhaust gas is stoichiometric or richer; simultaneously burning fuel and gas around the fuel in the combustion chamber (6) at a temperature lower than that at which soot is generated in the low load engine operating state; and the air-fuel ratio in the combustion chamber (6) is made stoichiometric or richer when nitrogen oxides are to be released from the nitrogen oxide adsorbent (80).

18. A method of controlling an internal combustion engine according to claim 17, wherein the oxidation catalyst (19) is disposed upstream of the nitrogen oxide adsorbent (80) in the engine exhaust passage.

19. The method of controlling an internal combustion engine according to claim 1, wherein the internal combustion engine includes a diesel engine; fuel is injected into the interior of the combustion chamber (6) at least during the end of the compression stroke.

20. A method of controlling an internal combustion engine according to claim 19, wherein the fuel is injected into the interior of the combustion chamber (6) in an initial stage of an intake stroke except in a final stageof a compression stroke.