Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
2002-11-05
2004-08-31
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
By means producing a chemical reaction of a component of the...
C060S274000, C060S278000
Reexamination Certificate
active
06782696
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a diesel engine, particularly to a control of fuel injection timing and EGR recirculation quantity.
2. Description of the Related Art
In a diesel engine that performs a lean operation mainly caused by diffusion combustion, NO
x
is likely to occur by a high excess air ratio. As its countermeasure, an EGR control is used in which a part of exhaust gas is recirculationed into an intake air system and a combustion temperature is decreased to suppress the generation of NO
x
. Though the NO
x
suppression effect is enhanced along with an increased EGR ratio, in the diffusion combustion in which combustion is caused on a combustible gas mixture layer at the boundary between injection fuel and compressed air, a cinder of fuel occurs if the EGR ratio is excessively increased, resulting in a problem that the smoke emission amount is rapidly increased. Therefore, to avoid this problem, the EGR ratio had an upper limit value, so that the NO
x
suppression could not be fully realized.
In JP-A-8-218920 (U.S. Pat. No. 5,732,554), a technique for discharging NO
x
from the NO
x
occluded substance provided in an exhaust system of the diesel engine was disclosed. Fuel is injected in a suction stroke and a large quantity of EGR gas is introduced to lower the excess air ratio at or below 1.0 thereby exhausting the NO
x
. The combustion state is changed to a premix combustion in which an injected fuel is diffused and vaporized before the top dead center of compression, premixed with the air and combusted, whereby the cinder of fuel is less likely to arise, causing the upper limit value of the EGR ratio to be raised. Thus, it is presumed that the smoke and NO
x
can be both reduced.
On the other hand, JP-B-3116876 (U.S. Pat. No. 5,937,639) discloses a technique for controlling the EGR ratio when the smoke emission amount tends to decrease in a predetermined area such as a low load condition in order to reduce both of the smoke and NO
x
. It is noted that if the EGR ratio is increased above the upper limit value, the smoke emission amount tends to decrease after rapidly increased to the peak.
However, with the first technique, since there is a long interval from fuel injection in a suction stroke to ignition in a compression stroke, there is a problem that the ignition timing is varied, likely causing an ignition failure such as preignition or ignition delay and a degradation of stability. Also, a part of the fuel injected in the suction process is diffused within a cylinder and deposited on a cylinder wall to cause an oil dilution, and is not captured within a cavity of a piston like the fuel injected at a timing of the top dead center of compression, resulting in a problem that hydrocarbon (HC) or carbon monoxide (CO) is rapidly increased.
Also, with the second technique, when the control mode is switched between the predetermined area and the other areas, the peak of the smoke emission amount appears. Thus, there is a problem that the smoke emission amount is necessarily rapidly increased transiently at every time of mode switching.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a diesel engine that can reduce both of smoke and NO
x
by increasing an upper limit value of EGR ratio for suppressing emission of smoke while preventing oil dilution or increase in HC or CO from occurring.
In order to achieve the above object, the present invention provides a control device for a diesel engine wherein a fuel injection timing by a fuel injection part for injecting a fuel into a combustion chamber of the engine is set at an advance side from an injection timing when the smoke emission characteristic indicates a decreasing tendency after indicating an increasing tendency in accordance with an increase in an EGR ratio, and a lag side from an injection timing when the injected fuel gets out of a cavity formed in a piston and gets to a cylinder wall face, and an EGR ratio adjusting part for adjusting a recirculation amount of exhaust gas exhausted from the engine into an intake air system is operated to reduce the smoke emission amount and the NO
x
emission amount from the engine at the same time.
The smoke emission characteristic for the EGR ratio is changed depending on the injection timing.
FIG. 5
shows an example of the results of a test in which the fuel injection timing is changed in a certain operating range. If the injection timing is advanced to 20° BTDC as compared with 10° BTDC that is applicable to the typical diesel engine, the smoke emission amount for an increase in the EGR ratio indicates an increasing tendency up to a peak, and then a decreasing tendency. In this case, if the excess air ratio is changed with the EGR ratio, the smoke emission amount is over the peak, whereby it is impossible to avoid a rapid transient increase of the smoke emission amount. On the contrary, at 36° BTDC to which the injection timing is further advanced, the smoke emission amount is suppressed to the low value, even if the EGR ratio is increased, and the NO
x
emission amount is suppressed by the high EGR ratio, whereby the smoke and NO
x
can be reduced at the same time, and the transient increase of smoke is suppressed because no peak of the smoke emission amount is formed.
In this manner, the smoke emission amount is suppressed by the advanced injection timing, because the period from fuel injection to ignition is extended to promote the premix between injected fuel and intake air. In a suction stroke injection, for example, the injection timing is not extremely advanced, so that the injected fuel is surely ignited at a predetermined timing neat the top dead center of compression, and the stable operation is enabled without ignition failure.
On the other hand,
FIG. 6
shows the test results of measuring the THC emission amount and the dilution fuel amount by changing the injection timing in the same operation range as in FIG.
5
. If the injection timing is advanced, THC is increased from around 40° BTDC, and the amount of fuel mixed into the engine oil is increased, bringing about a danger of oil dilution.
In view of the above main causes, if the fuel injection timing is set at an advance side from the injection timing when the smoke emission characteristic indicates an increasing tendency and then a decreasing tendency for an increase in the EGR ratio, and a lag side from the injection timing when the injected fuel gets out of a cavity formed in a piston and gets to a cylinder wall face, the problem with the increased THC or oil dilution due to excessive advance is prevented, and the upper limit value of the EGR ratio to suppress the exhaust of smoke can be increased. And since the recirculation amount of exhaust gas is controlled on the basis of this EGR ratio by the EGR adjusting means, the smoke emission amount and the NO
x
emission amount can be reduced at the same time.
In a preferred form, the injection timing is advanced along with the increase in the engine rotational speed.
In this case, if the piston speed is increased with the engine rotational speed, the timing when the injected fuel get into the cavity occurs relatively earlier. Therefore, to bring the injected fuel into the cavity at an appropriate timing, it is required to expedite the fuel injection up to the time when the piston position is still low. Hence, if the injection timing is advanced with the increase of the engine rotational speed, the injected fuel is always brought into the cavity of the piston at appropriate timing, irrespective of the engine rotational speed, whereby a combustible state consistent with the smoke and NO
x
is stably realized.
Also, the injection timing is desirably kept almost constant for a change in the load.
In this case, since the dependency of the injection timing on the engine load is lower than on the engine rotational speed, if the injection timing is kept almost constant for a change in the load, the combustible state consistent with the smoke and NO
x
can be stably realized wit
Hata Michihiro
Koga Noriyuki
Morita Toshinori
Shigahara Kei
Yanagawa Yuji
Denion Thomas
Mitsubishi Jidosha Kogyo Kabushiki Kaisha
Tran Diem
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