Method and apparatus for reducing NOx in internal combustion engine

A method of reducing the amount of nitrogen oxides (NOx) in an internal combustion engine in which the air/fuel ratio is controlled higher than a theoreticl air/fuel ratio during normal operation of a vehicle, characterized in that the air/fuel ratio is controlled lower than the theoretical air-fuel ratio for a predetermined period of time during acceleration from the start of acceleration, the control being effected by means for detecting an accelerated state of the vehicle, means for measuring the lapse of time during acceleration from the start of acceleration and means for performing an acceleration increment correction upon detection of acceleration.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a method and apparatus for reducing the 
amount of nitrogen oxides (hereinafter referred to as "NOx") exhausted 
during acceleration of an internal combustion engine adapted to operate at 
an air/fuel ratio (hereinafter referred to as "A/F") higher than a 
stoichiometric value. 
(2) Description of the Prior Art 
Heretofore, an internal combustion engine control system has been proposed 
in which the A/F is controlled to a value larger than a stoichiometric 
value, namely, to a lean side, during normal operation and normal 
acceleration (hereinafter referred to also as "learn burn engine") mainly 
for the purpose of improving fuel economization. In this system, an 
acceleration increment correction has been suggested so that the A/F 
somewhat decreases to the rich side in comparison with that during normal 
operation in order to improve the drivability during acceleration. 
Lean burn engines of this type permit fuel economization, but as shown in 
FIG. 7 which represents the amount of NOx produced relative to A/F, if an 
acceleration increment correction to set the acceleration increment ratio 
at about 40% is performed in a lean burn engine in which the A/F is set at 
around "22" (region A in the figure), the A/F shifts to around "16" 
(region B in the figure), namely, an A/F region with a larger amount of 
NOx generated. As a result, the amount of NOx exhausted during 
acceleration increases, which may cause environmental pollution. 
SUMMARY OF THE INVENTION 
The present invention has been accomplished in view of the above-mentioned 
problems, and it is the object thereof to reduce the amount of NOx 
exhausted during acceleration of a lean burn engine, with little increase 
of fuel consumption. 
To this end, the method of the present invention reduces the amount of NOx 
by reducing the A/F below the theoretical A/F, namely, to a rich side for 
a predetermined period of time during acceleration from the start of 
acceleration. The occurrence of an accelerated state is detected to start 
measuring the lapse of time during acceleration. During a predetermined 
period, an acceleration increment correction occurs. 
These and other objects, features, and advantages of the invention will be 
apparent from the following detailed description with reference to the 
attached drawings.

DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1 illustrates the basic concept of the present invention. At step s1, 
it is detected whether the vehicle is in a state of acceleration. If it is 
not accelerating, the program ends. If vehicle acceleration is detected, 
then at step S2, it is determined whether acceleration is just starting. 
If so, a counter T is set to zero at step S5 and an acceleration increment 
correction is added to cause the A/F fuel ratio to be rich at step S6. 
The next pass through the program, assuming that acceleration continues, 
the determination at step S1 will be positive and the determination at 
step S2 will be negative. Therefore, the counter T will incremented at 
step S3. Presuming that the counter T has not reached a predetermined 
value of T.sub.0 the acceleration increment correction continues at step 
S6. 
Once the counter T has reaches a predetermined value of T.sub.0, step S4 
causes acceleration increment correction to end even though acceleration 
may be continuing. 
The present invention will be described in more detail hereinunder with 
reference to FIGS. 2 through 8. 
Referring first to FIG. 2, there is illustrated an entire configuration of 
an example of a system for implementing the present invention, in which in 
order to detect an operating condition of an engine 10, an intake system 
is provided with a potentiometer type throttle sensor 18 for detecting the 
opening of a throttle valve 16 and an intake pipe pressure sensor 22 for 
detecting a pressure in an intake pipe 24, and in an exhaust system a lean 
sensor 31 for detecting the oxygen concentration in exhaust gases is 
attached to an exhaust pipe 30. Further, an electromagnetic pickup type 
crank angle sensor 34 for detecting both the number of revolutions of the 
engine 10 and a reference crank angular position is attached to a 
distributor 32 which supplies a high voltage to a spark plug 28. An 
electronic controller 36 receives detected signals from these operating 
condition detecting means, then determines the amount of fuel to be 
injected and an ignition timing according to the operating condition of 
the engine 10 and provides a valve-opening signal to an injector 26 and 
ignition signal to an igniter 20. In addition, Though not shown, a ternary 
catalyst device is provided downstream of the exhaust pipe 30. 
The electronic controller 36, which is of a known configuration as shown in 
FIG. 3, includes an A/D converter 42 for converting detected analog 
signals provided from the lean sensor 31 which detects an A/F higher than 
a theoretical A/F, the intake pipe pressure sensor 22 and throttle sensor 
18 selectively into digital signals; an engine speed signal forming 
circuit 44 for forming an engine speed signal in accordance with a pulse 
signal provided from the crank angle sensor 34; a central processing unit 
(CPU) 40; a read-only memory (ROM) 48; a random access memory (RAM) 50; a 
clock generation circuit 46; output ports 54 and 60; drive circuits 52 and 
58; and a common bus 56. 
When an ignition switch (now shown) is turned on to apply power, the CPU 40 
starts processings in accordance with a program pre-stored in the ROM 48 
in synchronism with a reference clock signal provided from the clock 
generation circuit 46. Among the processings just mentioned, those related 
to the present invention are shown in FIGS. 4A-4C, 5 and 6. 
Referring to FIG. 4A, there is shown a processing which is executed in a 
main routine. In this processing, steps 101 to 104 are known processing 
steps for determining a basic injection time, TAUbase, and a basic 
ignition timing .theta.base. Step 105 is for calculating an amount of 
variation .DELTA.P in intake pipe pressure P in order to determine a state 
of acceleration of the engine (vehicle). Step 106 is for detecting an 
accelerated state together with later-described flag F and step 116. When 
the amount of variation .DELTA.P is larger than a predetermined value 
.DELTA.Po, a judgment is made as to whether or not the acceleration is at 
a start point on the basis of flag F in step 107. At this time, if the 
flag is "0", it is judged that the acceleration is at a start point, while 
if it is "1", it is judged that the acceleration is not at a start point. 
Where it has been judged that the acceleration is at a start point, the 
flag F is set to "1" in step 108 and then a fuel increment .DELTA.TAU is 
determined in step 109. The fuel increment .DELTA.TAU takes a certain 
value predetermined so that the A/F is on a richer side than the 
theoretical A/F or a value corresponding to the amount of variation 
.DELTA.P. It is stored beforehand in the ROM 48. Next, in step 110, a 
final injection time TAU is determined by adding the fuel increment 
.DELTA.TAU to the basic injection time TAUbase, and then the amount of 
correction .DELTA..theta. of the ignition timing is determined in step 
111. The amount of correction .DELTA..theta. may be a predetermined 
constant value or a value corresponding to the engine condition. Then, in 
step 112, the final ignition timing .theta. is determined by subtracting 
the amount of correction .DELTA..theta. from the basic ignition timing 
.theta. base. 
In the execution of this routine after start of acceleration and while the 
amount of variation .DELTA.P is larger than the predetermined value 
.DELTA.Po, since the flag F has been set to "1" at the beginning of 
acceleration as previously described, the result of judgment in step 107 
is normally "NO", the value of a timer T is continued to be incremented in 
step 113, and the value of the timer T after the increment is compared 
with a predetermined time To. The predetermined time To is preset to a 
suitable time width considering the various possible variations of the 
intake pipe pressure and engine speed appearing later than such pressure 
variations. For example, on the basis of a pattern with the highest 
frequency of occurrence among intake pipe pressure variation patterns 
wherein the amount of variation .DELTA.P is larger than the predetermined 
value .DELTA.Po, there is determined a period (here assumed to be Tp) in 
this pattern, namely, a period in which the amount of variation .DELTA.P 
is larger than the predetermined value .DELTA.Po, and the time To is set 
larger than at least the time Tp. Therefore, during the normal intake pipe 
pressure variation as mentioned above, an elapsed time T from the start 
point of acceleration until the amount of variation .DELTA.P becomes below 
the predetermined value .DELTA.Po does not exceed the predetermined time 
To, so that during this period the result of judgment in step 114 becomes 
"YES" and the route consisting of steps 101 to 107, 113, 114 and 109 to 
112 is executed repeatedly whereby the fuel increment correction and 
ignition timing correction are performed. When the amount of variation 
.DELTA.P becomes below .DELTA.Po, the result of judgment in step 106 turns 
to "NO", and whether the flag F is "1" or not is judged in step 115. 
Since at this time, the flag F is already set to "1", the result of 
judgment in step 115 becomes "YES", and a judgment is made in the next 
step 116 as to whether the amount of variation .DELTA.P is larger than the 
other predetermined value -.DELTA.Po. Even if the amount of variation 
.DELTA.P becomes below the predetermined value .DELTA.Po as mentioned 
above, the intake pipe pressure continues to increase or becomes an almost 
constant value, so at this time point the result of judgment in step 116 
becomes "YES" and the timer T is incremented in the next step 113. Then in 
step 114 it is judged that the value of the timer T after the increment is 
below the predetermined time To and fuel increment correction and ignition 
timing correction are performed. Thereafter, when it is judged in step 114 
that the time of lapse T from the start point of acceleration has exceeded 
the predetermined time To, the result of judgment in step 114 turns to 
"NO" and the timer is cleared in the next step 117. Then in step 118 the 
flag F is reset, and in the following step 119 there is performed an 
ordinary injection amount calculation not involving fuel increment 
correction. Then in the next step 120 there is performed an ordinary 
ignition timing calculation not involving ignition timing correction. If 
deceleration is made before exceeding the predetermined time To from the 
start point of acceleration and the amount of variation .DELTA.P becomes 
below the predetermined value -.DELTA.Po, the result of judgment in step 
116 turns to "NO" and steps 117 to 120 are executed, whereby there are 
performed ordinary injection amount calculation and ignition timing 
calculation. 
Referring now to FIGS. 5 and 6, there are shown respectively a fuel 
injection routine and an ignition routine, whose executions are started at 
predetermined crank angle positions. In these routines, a pulse signal, or 
a valve-opening signal, corresponding to the final injection time TAU 
obtained by the main routine is provided to the injector 26, and an 
ignition signal corresponding to the final ignition timing .theta. is 
provided to the igniter 20. 
FIG. 7 is a diagram showing the amount of NOx produced relative to A/F. As 
previously noted, in a lean burn engine in which the A/F is set at around 
"22" (region A in the figure), the conventional acceleration increment 
correction with the acceleration increment ratio set at about 40% results 
in the A/F becoming "16" or so (region B in the figure) in which a larger 
amount of NOx is produced. On the other hand, if the acceleration 
increment correction is made according to the present invention, the A/F 
becomes around "14" (region C in the figure) in which the amount of NOx 
produced, and thus the amount of NOx produced can be reduced. Also as to 
the percent NOx purification with a reducing catalyst, it can be improved 
to a large extent because the A/F is within the region C in FIG. 8 which 
region corresponds to the range of NOx purification, and consequently 
little NOx is exhausted. 
As to the configuration of the main routine, in the flow chart of FIG. 4A, 
step 115 may be deleted and a step having the same processing contents as 
step 115 may be added just after step 108, whereby the number of times of 
clearing the timer T can be reduced. 
According to the present invention, as set forth hereinabove, since the A/F 
at the beginning of acceleration is controlled to a rich side relative to 
a theoretical A/F, the amount of NOx produced becomes smaller; besides, 
the amount of NOx exhausted can be reduced to a large extent because of 
improvement in the percent NOx purification with a reducing catalyst. In 
this case, the fuel consumption changes little because the period of 
controlling the A/F to a rich side relative to the theoretical or 
stoichiometric A/F is limited to the early period of acceleration. In 
addition, when this fuel increment correction is combined with the 
ignition timing correction, the amount of NOx exhausted can be further 
reduced. 
Although in the above embodiment the amount of variation AP in the intake 
pipe pressure P was determined for detecting an accelerated state, there 
may be obtained for the same purpose the amount of variation in the 
throttle valve opening .DELTA.TA or the amount of variation in the intake 
air volume .DELTA.Q, as shown in FIGS. 4B and 4C, respectively. 
While the invention has been described in its preferred embodiment, it is 
to be understood that the words which have been used are words of 
description rather than limitation and that various changes and 
modifications within the purview of the appended claims may be made 
without departing from the true scope and spirit of the invention in its 
broader aspects.