Internal-combustion engines – Combustion chamber means having fuel injection only – Using multiple injectors or injections
Reexamination Certificate
2000-02-03
2001-12-04
Yuen, Henry C. (Department: 3747)
Internal-combustion engines
Combustion chamber means having fuel injection only
Using multiple injectors or injections
C123S305000
Reexamination Certificate
active
06325041
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a control apparatus for a spark ignition type direct injection engine, which can remove nitrogen oxides (NOx) in exhaust gas even at lean air-fuel ratios by inserting an NOx adsorbing catalyst that adsorbs NOx in exhaust gas in an excess oxygen atmosphere into the exhaust path of the engine.
2. Description of the Related Art
Conventionally, as for a control apparatus for an engine of this type, the following control technique is known. That is, the air-fuel ratio of a combustion chamber is controlled to be a predetermined lean state (e.g., A/F≧20) to improve fuel economy, and NOx in exhaust gas that has become an excess oxygen atmosphere at that time is adsorbed and removed by an NOx adsorbing catalyst. Before the adsorbed NOx amount of that NOx adsorbing catalyst becomes excessive and the adsorption performance drops, the air-fuel ratio of the combustion chamber of the engine is switched to a rich one substantially equal to or higher than the stoichiometric air-fuel ratio, thereby releasing NOx from the NOx adsorbing catalyst (to be also referred to as refresh or regeneration of the catalyst).
The technique disclosed in Japanese Patent Laid-Open No. 10-274085 exploits certain characteristics of the aforementioned NOx adsorbing catalyst; it oxidizes NOx in exhaust gas in an excess oxygen atmosphere and adsorbs it in the forms of nitrates, and releases NOx by substituting the adsorbed nitrates by carbon monoxide (CO) in exhaust gas when the oxygen concentration decreases. That is, the oxygen concentration in exhaust gas is decreased upon releasing NOx from the NOx adsorbing catalyst, and the CO concentration in exhaust gas is increased by injecting and re-combusting additional fuel in an expansion or exhaust stroke of each cylinder, thus promoting release and deoxidization/purification of NOx.
Upon releasing NOx from the NOx adsorbing catalyst, the air-fuel ratio of the combustion chamber is normally controlled to stay rich (A/F=12 to 13) so as to decrease the oxygen concentration, and to increase the concentrations of reducible, uncombusted hydrocarbons (HC) and CO. In this manner, release of NOx from the NOx adsorbing catalyst can be promoted, as well as deoxidization/purification of NOx. On the other hand, since extra fuel is consumed to only refresh the catalyst, fuel economy suffers.
In the conventional control apparatus (Japanese Patent Laid-Open No. 10-274085) that injects additional fuel in the expansion or exhaust stroke of each cylinder, the oxygen concentration in exhaust gas can be decreased and the HC and CO concentrations can be increased without any engine output drift. However, since the additional fuel has nearly no contribution to the engine output, fuel economy may deteriorate considerably.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the above situation, and has as its object to sufficiently improve the refresh effect of a catalyst while minimizing an increase in fuel consumption by devising the control sequence upon refreshing the catalyst in a spark ignition type direct injection engine, exploiting the fact that it is effective for the purpose of promoting refresh of an NOx adsorbing catalyst not only to increase the HC and CO concentrations in exhaust gas but also to especially increase a CO to NOx concentration ratio CO/NOx.
In order to achieve the above object, according to the present invention, upon refreshing an NOx adsorbing catalyst inserted in the exhaust path of an engine, the air-fuel ratio of a combustion chamber is controlled to be slightly leaner than the stoichiometric air-fuel ratio, fuel is divisionally injected in two injections, i.e., leading and trailing injections, and a predetermined amount of exhaust gas is recirculated to an intake system.
More specifically, the invention according to the first aspect is premised on an engine control apparatus which comprises a fuel injection valve
7
for directly injecting and supplying fuel into a combustion chamber
4
in each cylinder of an engine
1
, and an NOx adsorbing catalyst
25
which is inserted in an exhaust path
22
that communicates with the combustion chamber
4
, adsorbs NOx in an excess oxygen atmosphere with high oxygen concentration in exhaust gas, and releases the adsorbed NOx as the oxygen concentration decreases, as shown in
FIG. 1
, and which controls the air-fuel ratio of the combustion chamber to stay lean (excessive air ratio &lgr;≧1.3) when the engine
1
falls within a low-load range after warming up the NOx adsorbing catalyst
25
.
The control apparatus comprises exhaust gas recirculation means
26
and
27
for recirculating some portion of the exhaust gas from the exhaust path
22
to an intake system
10
of the engine
1
, a determination means
40
a
for determining an NOx release timing from the NOx adsorbing catalyst
25
, an air-fuel ratio control means
40
b
for controlling the air-fuel ratio of the combustion chamber
4
to fall within the range
1
<excessive air ratio &lgr;≦1.1 when the determination means
40
a
determines that the NOx release timing has been reached, an injection timing control means
40
c
for controlling the fuel injection valve
7
to divisionally inject fuel in two injections including leading injection that falls within the range from an intake stroke to a former period of a compression stroke of a cylinder
2
, and trailing injection which starts after a middle period of the compression stroke and before an ignition timing when the determination means
40
a
determines that the NOx release timing has been reached, and an exhaust gas recirculation control means
40
d
for controlling the exhaust gas recirculation means
26
and
27
to recirculate a predetermined amount of exhaust gas when the determination means
40
a
determines that the NOx release timing has been reached. Note that the middle period of the compression stroke means that of former, middle, and latter periods obtained by equally dividing the compression stroke of each cylinder into three periods.
With the aforementioned arrangement, when the adsorbed NOx amount of the NOx adsorbing catalyst
25
has become excessive during combustion of the engine
1
, and the determination means
40
a
determines that the NOx release timing has been reached, the air-fuel ratio control means
40
b
controls the air-fuel ratio of the combustion chamber
4
of the engine
1
to fall within the range 1<excessive air ratio &lgr;≦1.1, thus decreasing the oxygen concentration in exhaust gas, and releasing NOx from the NOx adsorbing catalyst
25
. At this time, under the control of the injection timing control means
40
c,
fuel is divisionally injected in two injections including leading injection which falls within the range from the intake stroke to the former period of the compression stroke of the cylinder
2
, and trailing injection which starts after the middle period of the compression stroke and before the ignition timing. The leading injected fuel is uniformly scattered into the combustion chamber to form an air-fuel mixture leaner than the stoichiometric air-fuel ratio, and the trailing injected fuel forms an over-enriched air-fuel mixture slightly richer than the stoichiometric air-fuel ratio around a spark plug
6
(this state will be referred to as a weakly stratified state hereinafter).
Combustion in this weakly stratified state has greatly high combustion stability since the over-enriched air-fuel mixture has very high ignitability and also very high initial burning velocity after that, and at the same time, CO is easily produced in that state. On the other hand, the surrounding lean air-fuel mixture burns slowly, and since some mixture is exhausted before it completely burns, the CO concentration in this exhaust gas becomes very high. Furthermore, since the number of times of opening of the fuel injection valve per combustion cycle increases, the ratio of coarse fuel droplets that are injected in an early valve opening sta
Imada Michihiro
Mamiya Kiyotaka
Tetsuno Masayuki
Yamauchi Takeo
Gimie Mahmoud
Mazda Motor Corporation
Nixon & Peabody LLP
Studebaker Donald R.
Yuen Henry C.
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