JP4388735B2 - Lean burn engine - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a lean burn engine, and more particularly to a technology for reducing the exhaust emission.
[0002]
[Prior art]
In a conventional lean burn engine that burns a lean air-fuel mixture, there is one ignition point per combustion chamber. Therefore, a rich air-fuel mixture is formed around the ignition point, and ignition is performed by surrounding the periphery with a thin air-fuel mixture or air. Stratified combustion is performed to facilitate the above.
[0003]
However, in an engine that performs stratified combustion, a curved depression is formed on the top surface of the piston in order to form a stratified mixture, and heat loss increases at high speed regions and high load regions where the air-fuel ratio is high. Could not prevent. In addition, the piston weight is increased, and it is impossible to realize a simple shape of the combustion chamber with high thermal efficiency.
[0004]
On the other hand, when a lean mixture is formed uniformly rather than stratified, even if flame nuclei are ignited, the combustion speed is slow, and the probability of going out is high, which increases rapidly as the mixture dilutes. . However, if the multipoint ignition plug disclosed in Patent Document 1 is used, a plurality of flame nuclei are formed, so that stable operation is possible even at an extremely low air-fuel ratio such as 30, and in this air-fuel ratio region, HC, CO and NOx are also reduced, and clean operation is possible without relying on a catalyst.
[0005]
[Patent Document 1]
JP 2002-70561 A
[Problems to be solved by the invention]
Even when the lean operation is performed using the multipoint ignition plug disclosed in Patent Document 1, it is still necessary to use a catalyst when it is necessary to further reduce the HC, CO, and NOx levels. . However, if the air-fuel ratio is lower than the stoichiometric air-fuel ratio, oxygen is exhausted into the exhaust, the catalyst does not become a reducing atmosphere, and NOx in the exhaust cannot be sufficiently purified.
[0007]
As is the case with conventional lean burn engines, it is conceivable to sometimes return the air-fuel ratio to the stoichiometric air-fuel ratio and supply CO or HC as a reducing agent to the catalyst, but switch from the lean air-fuel ratio to the stoichiometric air-fuel ratio. If the engine torque increases suddenly, a shock may occur, which may impair driving performance.
[0008]
The present invention has been made in view of such technical problems, and it is an object of the present invention to reduce exhaust emissions without impairing drivability in a lean burn engine.
[0009]
[Means for solving problems]
A lean burn engine is provided with a catalyst (three-way catalyst, zeolite catalyst, etc.) having an action of reducing at least NOx and an additional fuel supply means for supplying fuel upstream thereof, and the engine has an air-fuel ratio thinner than the stoichiometric air-fuel ratio. Fuel is supplied into the exhaust gas from the additional fuel supply means, the additional fuel supply means includes a bag-like porous body projecting into the exhaust passage, and a spray inside the porous body. And an injection mechanism for injecting fuel from the nozzle .
[0010]
[Action and effect]
According to the present invention, when the engine is operating at a lean air-fuel ratio, fuel is injected into the exhaust upstream of the catalyst. As a result, the amount of HC in the exhaust in which oxygen is excessive increases, and the catalyst atmosphere is brought close to the stoichiometric air-fuel ratio, so that NOx in the exhaust can be sufficiently reduced in the catalyst.
By supplying the fuel into the exhaust gas through the bag-like porous body, the amount of fuel supplied into the exhaust gas can be smoothed to make the exhaust air-fuel ratio uniform, and the NOx purification efficiency can be increased. . In addition, since the nozzle of the injection mechanism is housed in the bag-like porous body and is not exposed to the exhaust, the nozzle can be protected from fouling and heat loss, and good operability of the injection mechanism can be maintained.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
[0012]
FIG. 1 shows the overall configuration of a lean burn engine according to the present invention. An air cleaner 3, a supercharger 4, an intercooler 5, a throttle valve 6, and a fuel injection valve 7 are provided in the intake passage 2 of the engine 1 in order from the upstream.
[0013]
The air introduced into the intake passage 2 via the air cleaner 3 is compressed by the supercharger 4 and then cooled by the intercooler 5. This is because the temperature of the air compressed by the supercharger 4 has risen, and if supplied to the engine 1 as it is, the charging efficiency may be lowered or knocking may occur.
[0014]
The air whose flow rate is adjusted by the throttle valve 6 is mixed with the fuel injected from the fuel injection valve 7, and a uniform air-fuel mixture is introduced into the combustion chamber 10 of the engine 1. Further, an air flow rate sensor 8 for detecting the flow rate of air sucked through the air cleaner 3 is attached upstream of the supercharger 4, and the output of the air flow rate sensor 8 is input to the control unit 30.
[0015]
The fuel injection valve 7 is driven according to an injection pulse sent from the control unit 30 and injects pressurized fuel into the intake manifold 11 at a predetermined timing. This engine is a system in which fuel is injected into the intake manifold 11, but a system in which fuel is directly injected into the cylinder 12 may be used. In any method, it is assumed that a uniform premixed gas is formed in the cylinder 12 immediately before ignition.
[0016]
The engine 1 is operated with a lean air-fuel mixture that is thinner than the stoichiometric air-fuel ratio in the entire operation region. However, when attempting to operate with the lean air-fuel mixture in the entire operation region, the torque is insufficient in the high speed and high load region. Therefore, in this engine, the torque shortage is compensated by supercharging with the supercharger 4. It is noted that the torque shortage may be compensated by increasing the air-fuel ratio in the high speed and high load region as in the conventional lean burn engine.
[0017]
The air-fuel mixture introduced into the combustion chamber 10 is compressed as the piston 13 rises and is ignited by the multipoint spark plug 14. The multi-point spark plug 14 is a rod-shaped spark plug, and a plurality of ignition gaps 14g arranged in a line are formed in the insulator part attached to the combustion chamber 10 so as to cross the cylinder 12 of the engine 1. ing. When a voltage is applied to the terminal 14g of the multi-point spark plug 14, a plurality of spark gaps 14g cause a spark to fly almost simultaneously, a large number of flame nuclei are formed, and the flame rapidly burns and spreads throughout the combustion chamber 11. Thereby, even a very thin air-fuel mixture having an air-fuel ratio of 20 or more, for example, an air-fuel ratio of 30, can be ignited and stably combusted.
[0018]
Exhaust gas from the engine 1 is discharged into the atmosphere through an exhaust manifold 20, a first catalyst 22 provided in the exhaust passage 21, and a second catalyst 23 provided downstream thereof. Further, an exhaust temperature sensor 27 for detecting the temperature of the exhaust is attached downstream of the second catalyst 23, and the output of the exhaust temperature sensor 27 is input to the control unit 30.
[0019]
The first catalyst 22 is a so-called three-way catalyst, and preferably a three-way catalyst having HC adsorption properties is used. Alternatively, as the first catalyst 22, a reduction catalyst having HC adsorption property, for example, a zeolite catalyst having a narrow hole corresponding to the size of the discharged HC molecule may be used. The second catalyst 23 is an oxidation catalyst carrying platinum or the like.
[0020]
An additional fuel supply device 25 that is driven in response to a pulse signal input from the control unit 30 is attached upstream of the first catalyst 22, and fuel is supplied when the engine 1 is operating at a lean air-fuel ratio. Supply into the exhaust. Even when the engine 1 is operating at a lean air-fuel ratio, the additional fuel is supplied so that the relationship between NO and HC in the vicinity of the surface of the catalyst 22 is brought close to the relationship in the stoichiometric air-fuel ratio. Performed to maximize conversion efficiency.
[0021]
In the lean combustion state, the exhaust gas in the exhaust manifold 20 contains a large amount of oxygen. That is, since the oxygen concentration in the air is 21%, if the air-fuel ratio is 29.4, which is twice the stoichiometric air-fuel ratio, the oxygen concentration in the exhaust manifold 20 will be 10.5%, resulting in an excessive oxygen state. It has become. Accordingly, even if the exhaust gas is allowed to flow into the first catalyst 22 as it is, the catalyst atmosphere becomes excessive in oxygen, so that NOx cannot be sufficiently reduced. Therefore, in this engine, fuel is supplied into the exhaust gas in accordance with the exhaust air-fuel ratio so that the relationship between NO and HC on the catalyst surface is close to the relationship in the stoichiometric air-fuel ratio.
[0022]
Specifically, an air-fuel ratio sensor 26 that generates an output voltage as a monovalent function of the oxygen concentration in the exhaust manifold 20 is attached to the exhaust manifold 20, and its output is input to the control unit 30. Based on this signal, the control unit 30 feedback-controls the pulse width supplied to the additional fuel supply device 25 to a predetermined value. For example, as the air-fuel ratio detected by the air-fuel ratio sensor 26 becomes leaner, the oxygen concentration in the exhaust gas becomes higher. Therefore, the pulse width is increased and the amount of fuel supplied from the additional fuel supply device 25 is increased.
[0023]
The fuel supplied from the additional fuel supply device 25 does not burn because the exhaust temperature decreases when the air-fuel ratio becomes thin. The fuel reaches the first catalyst 22 while being decomposed, and the first catalyst 22 adsorbs HC. If the catalyst has the property, it is adsorbed by the first catalyst 22. The NOx in the exhaust gas is reduced by the HC as a reducing agent in the first catalyst 22 and converted into nitrogen (N ₂ ) and water vapor (H ₂ O).
[0024]
The ratio of C to H constituting gasoline is about 1 to 1.9, and nitrogen oxides in the exhaust are almost NO (when exhausted from the tailpipe, it is oxidized by oxygen in the air and most of it is NO. ₂ ), the reduction characteristics of NO by hydrocarbons (HC) are
_{CH 1.9 + 2.95NO = CO 2 +} 0.95H 2 O + 1.475N 2
It becomes. Since the molecular weight is 1 for H, 12 for C, and 14 for N, if 12 + 1 × 1.9 = 13.9 g of fuel is supplied from the additional fuel supply device 25, 2.95 × (14 + 16) = 88.5 g NOx can be reduced.
[0025]
When additional fuel is supplied more than necessary, even if all of the NOx is reduced, HC remains and surplus HC passes through the first catalyst 22, but this surplus HC is exhausted by the second catalyst 23. Since it is oxidized by excess oxygen in the inside, the HC level released into the atmosphere does not increase. Even when a catalyst having no HC adsorptivity is used as the first catalyst 22, some HC flows out downstream of the first catalyst 22. In this case as well, HC is oxidized in the second catalyst 23. Therefore, an increase in the amount of HC released into the atmosphere can be suppressed.
[0026]
Note that the amount of fuel supplied from the additional fuel supply device 25 is very small. For example, when the NO discharged from the engine into the exhaust manifold 20 every 1 km travels is 0.885 g, 0.139 g of fuel per 1 km travel may be supplied from the additional fuel supply device 25. This amount is negligible compared to the fuel consumed to travel, even if a slight margin is provided, and the effect on fuel efficiency is negligible.
[0027]
FIG. 2 shows the detailed structure of the additional fuel supply device 25.
[0028]
The valve body 51 that opens and closes the nozzle hole 50 of the additional fuel supply device 25 is integrated with an armature that moves in the coil unit 52 like a normal fuel injector. When the coil unit 52 is energized, fuel is sprayed from the nozzle and injected into the bag-like porous body case 53.
[0029]
The porous body case 53 is a porous body case for vaporizing fuel made of ceramic or sintered alloy having continuous cell-like pores. When fuel is injected into the porous body case 53, the fuel oozes from the surface. ing. By supplying the fuel into the exhaust gas through the porous body case 53 in this way, the amount of fuel supplied in the exhaust gas is smoothed to make the exhaust air-fuel ratio uniform, and the NOx purification efficiency is increased. Can do. Moreover, since the nozzle hole 50 and the valve body 51 are housed in the porous body case 53 and are not exposed in the exhaust gas, the nozzle hole 50 and the valve body 51 can be protected from fouling and heat loss, and good operability can be maintained.
[0030]
As described above, when the air-fuel ratio becomes thin, the exhaust temperature becomes low, and when the air-fuel ratio becomes close to 30, it is about 600 ° C. Therefore, the fuel that has oozed out from the porous body case 53 burns on its surface. Rather, it reaches the first catalyst 22 while being thermally decomposed.
[0031]
The porous body case 53 is elastically attached to the valve body via a gasket 54. The additional fuel supply device 25 is attached to the exhaust manifold 20 with bolts 60. In order to suppress heat conduction from the exhaust manifold 20 to the coil unit 52, the trunk portion 55, which is a connecting portion between the valve body 51 and the coil unit 52, is a coil unit. Narrower than 52. Pressurized fuel is supplied to the nipple 56 of the additional fuel supply device 25, and fuel is injected into the porous body case 53 according to the injection pulse width applied to the socket 57.
[0032]
FIG. 3 is a diagram showing an example of injection pulses sent from the control unit 30 to the additional fuel supply device 25. As shown in FIG. 3, the ejection pulse has a constant rise time interval (t ₀ ), that is, the pulse width (duty, t) is changed while the frequency is constant. For example, the additional fuel supply device 25 basically turns on and off the valve body 51 at 25 Hz, and changes only the pulse width t corresponding to the valve opening time. When it is not necessary to inject, the pulse width t may be set to zero as shown by the dotted line in the figure.
[0033]
As for the pulse width t, for example, the NO amount discharged per unit time is stored in the memory of the control unit 30 in advance in the engine control map, and 13.9 / 88.5 times the weight, that is, 0.16 times the weight. The above fuel is preset to be injected from the additional fuel supply device 25.
[0034]
As mentioned above, although embodiment of this invention was described, the said embodiment is only what showed an example of the engine to which this invention is applied, In the meaning which limits the technical scope of this invention to the structure of the said embodiment. Absent. For example, the present invention is preferably applied to a lean burn engine that performs multi-point ignition such as the above-mentioned engine, and the effect thereof is great. However, the present invention is similarly applied to a lean burn engine that performs conventional stratified combustion. Can do. Further, although the engine performs lean combustion in the entire operation region, it goes without saying that the present invention can be applied to an engine that performs lean combustion only in a specific operation region.
[Brief description of the drawings]
FIG. 1 is a diagram showing an overall configuration of a lean burn engine according to the present invention.
FIG. 2 is a partial cross-sectional view showing a structure of an additional fuel supply device.
FIG. 3 is an explanatory diagram of injection pulses applied to the additional fuel supply device.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Engine 2 Intake passage 3 Air cleaner 4 Supercharger 5 Intercooler 7 Fuel injection valve 8 Air flow sensor 14 Multipoint spark plug 14g Ignition gap 14t Terminal 21 Exhaust passage 22 First catalyst (three-way catalyst, etc.)
23 Second catalyst (oxidation catalyst)
25 Additional fuel supply device 26 Air-fuel ratio sensor 30 Control unit 50 Injection hole 51 Valve body 52 Coil unit 53 Porous body case

Claims (7)

In a lean burn engine that operates at an air-fuel ratio lower than the theoretical air-fuel ratio,
A first catalyst provided in the exhaust passage of the engine and having an action of reducing at least NOx;
Additional fuel supply means that is also provided in the exhaust passage of the engine and supplies fuel upstream of the first catalyst;
When the engine is operating at an air / fuel ratio that is lower than the stoichiometric air / fuel ratio, fuel is supplied into the exhaust gas from the additional fuel supply means,
The additional fuel supply means includes
A bag-like porous body protruding into the exhaust passage;
An injection mechanism for injecting fuel from a nozzle so as to form a spray inside the porous body;
Composed of,
A lean burn engine characterized by that. The lean burn engine according to claim 1, wherein the first catalyst is an HC adsorbing catalyst. The lean burn engine according to claim 1 or 2, further comprising a second catalyst that is an oxidation catalyst downstream of the first catalyst. The said injection mechanism is comprised by the valve body and the valve body drive part which drives the said valve body, The space | interval between the said valve body and valve body drive part is thinner than another site | part, It is characterized by the above-mentioned. Item 4. The lean burn engine according to any one of Items 1 to 3. A multipoint spark plug having a plurality of ignition gaps;
5. The lean burn engine according to claim 1, wherein the lean burn engine is a lean burn engine that operates at an air-fuel ratio of 20 or more by igniting a uniform air-fuel mixture using the multipoint ignition plug. The lean burn engine as described in one. In an additional fuel supply device that is provided in an exhaust passage of a lean burn engine having a catalyst having an action of purifying at least NOx in an exhaust passage and supplies fuel upstream of the catalyst,
A bag-like porous body protruding into the exhaust passage;
An injection mechanism for injecting fuel from a nozzle so as to form a spray inside the porous body;
An additional fuel supply device comprising: The injection mechanism includes a valve body and a valve body drive unit that drives the valve body, and a space between the valve body and the valve body drive unit is narrower than the valve body drive unit. The additional fuel supply device according to claim 6.

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