Exhaust purifier for internal combustion engine

An exhaust purifier for an internal combustion engine is capable of properly heating a catalyst in consideration of a deterioration with time and manufacturing variation in an electric heater that heats the catalyst. The exhaust purifier has the catalyst 4 disposed in an exhaust pipe, the electric heater for forcibly heating the catalyst 4, a power source for supplying power to the heater, an estimate unit for estimating the temperature that the catalyst 4 will reach, a comparator for providing the difference between the estimated temperature and a target temperature, and an operation controller for controlling the operating conditions of the engine according to the temperature difference. The estimate unit measures the voltage and current of the heater, calculates the resistance or power of the heater according to the voltage and current, and estimates the temperature. The operation controller adjusts the quantity of intake air and ignition timing, or adjusts ignition timing and then the quantity of intake air to maintain an engine speed, or, if a battery 6 is abnormal, adjusts the quantity of intake air and ignition timing to increase the temperature of exhaust gas.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an exhaust purifier for an internal 
combustion engine, and particularly, to one that heats a catalyst with an 
electric heater in consideration of a deterioration with time and 
manufacturing variation in the heater. 
2. Description of the Related Art 
An exhaust system of an internal combustion engine employs a catalyst in an 
exhaust passage to remove toxic substances such as HC, CO, and NOx from 
the exhaust gas. The catalyst purifies exhaust gas only after it is heated 
above an activation temperature. To heat the catalyst, hot exhaust gas 
from the engine is usually used. If the engine is started from a cold 
state, it will take time to heat the catalyst to the activation 
temperature because the temperature of exhaust gas is low at first. Until 
the catalyst is activated, the exhaust gas is insufficiently purified. 
To solve this problem, an electrically heated catalyst (EHC) has been 
proposed. The EHC has a catalyst and a metal carrier to which a current is 
applied at the start of the engine to quickly heat the catalyst up to an 
activation temperature. An example of the EHC is disclosed in Japanese 
Unexamined Patent Publication No. 5-179939. The EHC, however, consumes 
large amounts of power, dropping the voltage of a battery during an idling 
period. 
To prevent such voltage drop, Japanese Unexamined Patent Publication No. 
6-101459 checks an idling state and a heater operation. If the heater is 
operating while the engine is idling, the disclosure increases the 
quantity of intake air by controlling an electronic throttle or an idling 
speed controller (ISC) disposed in an intake system of the engine. As the 
quantity of intake air increases, the quantity of injected fuel increases 
to increase the torque and idling speed of the engine. This increases the 
power generated by a generator to compensate for the voltage drop of the 
battery. 
This disclosure determines a period for operating the heater according to 
the temperature of cooling water of the engine without considering a 
deterioration with time of the heater resistance and manufacturing 
variation in the heater. Accordingly, the heater operating period may be 
too long or too short for the catalyst. As a result, the heater and the 
heat of exhaust gas will excessively or insufficiently heat the catalyst. 
An object of the present invention is to provide an exhaust purifier for an 
internal combustion engine, capable of properly heating a catalyst in 
consideration of a deterioration with time and manufacturing variation in 
an electric heater that heats the catalyst. 
SUMMARY OF THE INVENTION 
In order to accomplish the object, the present invention, which provides an 
exhaust purifier for an internal combustion engine, includes a catalyst 
disposed in an exhaust pipe, an electric heater for forcibly heating the 
catalyst, and a power source for supplying power to the heater. The 
exhaust purifier is characterized in that it has an estimating unit for 
estimating a temperature the catalyst will reach, a comparator for 
providing the difference between the estimated temperature and a target 
temperature, and an operation controller for controlling the operating 
conditions of the engine according to the temperature difference. 
The present invention also measures the voltage and current of the heater, 
calculates the resistance of the heater according to the voltage and 
current, and estimates the temperature that the catalyst will reach 
according to the resistance. 
The present invention also measures the voltage and current of the heater, 
calculates the power of the heater according to the voltage and current, 
and estimates the temperature that the catalyst will reach according to 
the power. 
The present invention also adjusts the quantity of intake air and ignition 
timing according to the temperature difference between the estimated and 
target temperatures. 
Increasing the quantity of intake air increases the quantity of injecting 
fuel because the air-fuel ratio of the engine is controlled to be 
unchanged. This increases an engine speed and the temperature of exhaust 
gas to quickly heat the catalyst up to an activation temperature. Delaying 
the ignition timing of the engine delays the start of combustion in each 
cylinder and speeds up the opening of each exhaust valve. This also 
increases the temperature of exhaust gas to quickly heat the catalyst up 
to the activation temperature. 
The present invention also adjusts the ignition timing of the engine and 
then the quantity of intake air to maintain an engine speed. 
Delaying the ignition timing of the engine delays the start of combustion 
in each cylinder and speeds up the opening of each exhaust valve, to 
increase the temperature of exhaust gas. The exhaust gas heats and quickly 
activates the catalyst. This, however, reduces the torque and speed of the 
engine. Accordingly, the fifth aspect increases the quantity of intake air 
to maintain a given engine speed. 
Upon occurrence of an abnormal state in which no power is supplied to the 
heater, the present invention also adjusts the operating conditions of the 
engine to increase the temperature of exhaust gas. 
Power supply to the heater will be cut if, for example, a power source 
voltage drops too low or a break occurs in the heater. In this case, the 
sixth aspect increases the quantity of intake air and delays ignition 
timing, to increase the temperature of exhaust gas.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an exhaust purifier of an internal combustion engine according 
to an embodiment of the present invention. The exhaust purifier involves 
an alternator 2 driven by the engine 1, an intake controller 3, disposed 
in an intake system of the engine 1, to control the quantity of intake 
air, and an electrically heated catalyst (EHC) 4 disposed in an exhaust 
system of the engine 1. The EHC 4 has an electric heater for heating the 
same. A three-way catalyst 4a is disposed in the exhaust system downstream 
from the EHC 4. The three-way catalyst 4a has no heater and is heated by 
the heat of exhaust gas. The intake controller 3 may be an electronic 
throttle that responds to an accelerator or to an electronic control unit 
5, to drive a stepping motor in a forward or reverse direction to increase 
or decrease the quantity of intake air. The intake controller 3 may be an 
idling speed controller (ISC) to be explained later. The control unit 5 
may be a microcomputer having a CPU, a ROM, a RAM, an input interface, an 
output interface, and a bus line to connect the components to one another. 
The control unit 5 controls fuel injection, ignition timing, and the power 
supply to the heater. 
An airflow meter (not shown) is arranged in an intake duct, to provide a 
voltage signal in proportion to the quantity of intake air. A water 
temperature sensor (not shown) is attached to a water jacket (not shown) 
of the engine 1, to provide a voltage signal in proportion to the 
temperature of cooling water of the engine 1. A voltmeter (not shown) is 
connected in parallel with a battery 6, to provide a signal representing a 
terminal voltage of the battery 6. These signals are supplied to the input 
interface of the control unit 5. A terminal voltage of the heater is equal 
to the terminal voltage of the battery 6 and is supplied to the input 
interface of the control unit 5. A current passing through the heater is 
detected as a terminal voltage of a resistor connected in series with the 
heater and is supplied to the input interface of the control unit 5. A 
crank angle sensor (not shown) is attached to a distributor (not shown) of 
the engine 1 and provides a signal representing a crank angle. This signal 
is also supplied to the input interface of the control unit 5 and is used 
to calculate an engine speed. 
The output interface of the control unit 5 provides the intake controller 3 
with a signal to change the quantity of intake air, a regulator 7 with a 
signal to change the field current of the alternator 2, a switch 8 with a 
control signal to turn on/off the heater, and an ignition circuit 9 
connected to an ignition plug with an ignition signal for each cylinder. 
The heater is integral with the EHC (electrically heated catalyst) 4 and is 
connected to a power source. The power source consists of the alternator 2 
and battery 6 connected in parallel with each other. A temperature 
detected by the water temperature sensor and an engine speed NE calculated 
according to the output of the crank angle sensor are used to determine 
whether or not the EHC 4 is active. The regulator 7 changes the magnetic 
field of the alternator 2, to change the generated power. The switch 8 may 
be a control relay that connects the power source to the EHC 4 when the 
EHC 4 is inactive. 
The battery 6 always receives power from the alternator 2 and supplies 
power to the EHC 4, control unit 5, regulator 7, and other parts. The 
regulator 7 increases the field current of the alternator 2 if the voltage 
of the battery 6 drops due to load. The field current of the alternator 2 
is changeable by controlling the duty factor thereof. The switch 8 has a 
resistor (not shown) connected in series with the heater of the EHC 4, to 
detect a current flowing from the power source to the heater. The resistor 
is electrically connected to the control unit 5, which calculates the 
current to the heater according to a terminal voltage of the resistor. The 
battery 6 is electrically connected to the control unit 5, which 
calculates the resistance and power of the heater according to a terminal 
voltage of the battery 6 and the current to the heater calculated 
according to the terminal voltage of the resistor. Alternatively, the 
current to the heater may be calculated with the use of a current 
transformer (CT). 
Closing and opening the switch 8 will be explained. 
When the control unit 5 determines that the EHC 4 is inactive, the switch 8 
is closed to supply power from the power source to the EHC 4. Since the 
alternator 2 supplies power to the battery 6, the control unit 5 controls 
the regulator 7 to control the field current of the alternator 2 so that 
the output voltage of the alternator 2 is about 14 V or below. Once the 
EHC 4 is activated, the control unit 5 opens the switch 8 to disconnect 
the EHC 4 from the power source. 
The ignition circuit 9 is connected to the ignition plug (not shown) of 
each cylinder (not shown) of the engine 1. The control unit 5 controls the 
ignition circuit 9 to fire each ignition plug at a timing that is 
determined according to the output of the crank angle sensor. 
An example of the idling speed controller (ISC) which may serve as the 
intake controller 3 will be briefly explained with reference to FIG. 2. 
The idling speed controller involves an engine 21, an intake duct 22, a 
throttle valve 23, an airflow meter 24, and a surge tank 25. A bypass 26 
bypasses the throttle valve 23, to connect a part of the intake duct 22 
between the throttle valve 23 and the airflow meter 24 with the surge tank 
25. An idling speed control valve 28 is arranged in the bypass 26 and is 
driven by an actuator 27 such as a stepping motor. The actuator 27 is 
connected to, for example, the output interface of the control unit 5 of 
FIG. 1, so that the control unit 5 controls the opening of the valve 28 
through the actuator 27, to control the quantity of intake air 
independently of the throttle valve 23. 
Subroutines of controlling the heater according to the embodiments of the 
present invention will be explained. 
FIG. 3 is a flowchart showing a subroutine of controlling the heater 
according to the first embodiment. The subroutines mentioned below are 
each carried out at intervals of 100 ms. Step 301 determines whether or 
not conditions to activate the heater are met. If they are met, step 302 
is carried out, and if not, step 303 is carried out. The heater activating 
conditions which must all be met are -10.degree. C.&lt;THW&lt;35.degree. C. 
where THW is the temperature of cooling water of the engine 1, NE&gt;500 rpm 
where NE is an engine speed, BAT&gt;10 V where BAT is the voltage of the 
battery 6, and a heater break flag indicating a break in the heater is 
off. Step 302 checks a heater flag to see if the heater is ON. If the 
heater is ON, the subroutine ends, and if not, step 304 is carried out. 
Step 303 resets the heater flag to OFF because the heater activating 
conditions are not completely met. Step 304 turns on the heater through 
the switch 8 and calculates a heater ON period corresponding to the engine 
speed NE and water temperature THW based on a map (not shown). Step 304 
sets a timer for the heater ON period. Step 305 reads a terminal voltage 
of the heater and a current passing the heater and calculates the 
resistance of the heater accordingly. The resistance or power of the 
heater is used to look up a map to estimate a temperature the EHC 4 will 
attain after the heater ON period. This map will be explained before 
explaining step 306. 
FIG. 4 shows the map used to estimate a temperature the EHC 4 will reach at 
the end of the heater ON period according to the resistance of the heater, 
and FIG. 5 shows the map to estimate the same according to the power of 
the heater. These maps and step 305 correspond to the estimate means of 
the present invention. The map of FIG. 4 is prepared by energizing heaters 
having different resistance values from a cold start and measuring the 
temperature of EHC 4 for each heater after about 20 seconds of 
energization. The map is stored in a ROM. The map of FIG. 5 is prepared by 
energizing heaters having different power levels from a cold start and 
measuring the temperature of the EHC 4 for each heater after about 20 
seconds of energization. The map is stored in the ROM. 
Returning to the flowchart of FIG. 3, steps 306 and 307 correspond to the 
difference providing means of the present invention. 
Step 306 determines whether or not the estimated temperature of the heater 
found from the map shown in FIG. 4 based on the heater resistance 
calculated in step 305 is above a target temperature. If the estimated is 
above the target, step 309 is carried out, and if not, step 307 is carried 
out. The target temperature is equal to the activation temperature of the 
EHC 4, which is usually 400.degree. C. Step 307 determines whether or not 
the estimated temperature is below the target temperature. If the 
estimated is below the target, step 308 is carried out, and if not, i.e., 
if the estimated is equal to the target, step 310 is carried out. Steps 
308 and 309 correspond to the control means of the present invention to 
adjust the quantity of intake air and ignition timing according to the 
difference between the estimated and target temperatures. 
In step 308, the target temperature minus the estimated temperature is 
positive, and therefore, the EHC 4 must be heated more quickly than usual. 
Therefore, step 308 finds an increase in the quantity of intake air 
according to the difference between the target and estimated temperatures 
from the map shown in FIG. 6, and the intake controller 3 increases the 
quantity of intake air accordingly. At this time, the quantity of 
injecting fuel is also increased because the air-fuel ratio of the engine 
1 is controlled to be unchanged. Consequently, the engine speed increases 
to increase the temperature of exhaust gas to heat the EHC 4 up to the 
activation temperature more quickly than usual. In addition, step 308 
finds ignition timing from a map of FIG. 7 according to the difference 
between the target and estimated temperatures. The ignition timing to be 
selected is on the delay side of TDC as compared with usual and 
corresponds to, for example, a crank angle of 5 degrees advanced from TDC. 
This results in delaying the start of combustion in each cylinder and 
speeding up the opening of each exhaust valve, thereby increasing the 
temperature of exhaust gas to quickly heat the EHC 4 up to the activation 
temperature. 
In step 309, the target temperature minus the estimated temperature is 
negative, and therefore, it is not necessary to quickly heat the EHC 4. 
Step 309 calculates a reduction in the quantity of intake air by looking 
up the map of FIG. 6 according to the difference between the target and 
estimated temperatures. Also, step 309 calculates normal ignition timing 
by looking up the map of FIG. 7 according to the difference between the 
target and estimated temperatures. The ignition timing corresponds to, for 
example, a crank angle of 10 degrees on the advance side of TDC. As a 
result, the intake controller 3 reduces the quantity of intake air. At 
this time, the quantity of injected fuel is reduced because the air-fuel 
ratio of the engine 1 is controlled to be unchanged. Then, the engine 
speed slows down to decrease the temperature of exhaust gas, to slowly 
heat the EHC 4 up to the activation temperature. Step 310 sets the heater 
flag to ON. In the next cycle of the subroutine, step 301 determines that 
the heater activating conditions are all met, and step 302 determines that 
the heater flag is ON. Namely, steps 304 to 310 are carried out once. Step 
311 determines whether or not the timer set in step 304 has counted the 
heater ON period. If the heater ON period has elapsed, step 312 is carried 
out, and if not, step 301 is again carried out. Step 312 turns off the 
heater and terminates the subroutine. 
In this way, the first embodiment calculates the resistance of the heater 
and estimates a temperature the EHC 4 will attain by looking up the map of 
FIG. 4 according to the resistance. If the estimated temperature is lower 
than a target temperature, the embodiment increases the quantity of intake 
air according to the map of FIG. 6, to increase the engine speed and the 
temperature of exhaust gas. At the same time, the embodiment delays 
ignition timing according to the map of FIG. 7, to increase the 
temperature of exhaust gas. This results in heating the EHC 4 up to the 
activation temperature more quickly than usual. If the estimated 
temperature is higher than the target temperature, the embodiment 
decreases the quantity of intake air according to the map of FIG. 6 and 
selects normal ignition timing according to the map of FIG.7, to decrease 
the engine speed and the temperature of exhaust gas. As a result, the EHC 
4 slowly reaches the activation temperature. 
FIG. 8 is a flowchart showing a subroutine of controlling the heater 
according to the second embodiment of the present invention. Steps 801 to 
812 of FIG. 8 are the same as steps 301 to 312 of FIG. 3, respectively, 
except steps 805 to 807, and therefore, only steps 805 to 807 will be 
explained. Step 805 reads a terminal voltage of the heater as well as a 
current flowing to the heater and calculates the power of the heater 
accordingly. The calculated power is used to estimate a temperature the 
EHC 4 will attain according to the map of FIG. 5. The estimated 
temperature is used in steps 806 and 807. 
If the estimated temperature is lower than a target temperature, the 
embodiment increases the quantity of intake air according to the map of 
FIG. 6, to increase the engine speed and the temperature of exhaust gas. 
At the same time, the embodiment delays ignition timing according to the 
map of FIG. 7, to increase the temperature of exhaust gas. As a result, 
the EHC 4 is heated to the activation temperature more quickly than usual. 
If the estimated temperature is higher than the target temperature, the 
embodiment decreases the quantity of intake air according to the map of 
FIG. 6 and selects normal ignition timing according to the map of FIG. 7, 
to decrease the engine speed and the temperature of exhaust gas. As a 
result, the EHC 4 slowly reaches the activation temperature. 
FIG. 9 is a flowchart showing a subroutine of controlling the heater 
according to the third embodiment of the present invention. The steps of 
FIGS. 3 and 9 basically correspond to each other. Steps specific to the 
third embodiment will be explained. Steps 908 and 909 only adjust the 
ignition timing without adjusting the quantity of intake air. Steps 911 
and 912 are additional, and therefore, steps 913 and 914 correspond to 
steps 311 and 312, respectively. Step 911 calculates .DELTA.NEi=NEi-1-NEi 
where NEi-1 is an engine speed in the preceding cycle and NEi is an engine 
speed of this cycle. Step 912 controls the intake controller 3 to increase 
the quantity of intake air if .DELTA.NEi is positive and decrease the same 
if .DELTA.NEi is negative. Step 913 determines whether or not the timer 
set in step 904 has counted a heater ON period. If the heater ON period 
has elapsed, step 914 is carried out, and if not, step 901 is repeated. 
Step 914 turns off the heater and terminates the subroutine. In this way, 
the third embodiment calculates the resistance of the heater and estimates 
a temperature the EHC 4 will attain by looking up the map of FIG. 5 
according to the resistance. If the estimated temperature is lower than a 
target temperature, the third embodiment delays ignition timing according 
to the map of FIG. 7, to increase the temperature of exhaust gas, so that 
the EHC 4 may reach the activation temperature more quickly than usual. If 
the estimated temperature is higher than the target temperature, the 
embodiment selects normal ignition timing according to the map of FIG. 7 
to maintain the combustion state of the engine 1 and the temperature of 
exhaust gas. As a result, the EHC 4 reaches the activation temperature as 
usual. When delaying the ignition timing, the embodiment controls the 
quantity of intake air not to change the engine speed NE. Namely, the 
embodiment delays only the ignition timing to increase the temperature of 
exhaust gas, and corrects the quantity of intake air not to change the 
engine speed NE. 
FIG. 10 is a flowchart showing a subroutine of controlling the heater 
according to the fourth embodiment of the present invention. This 
embodiment differs from the third embodiment of FIG. 8 in steps 1002, 
1002a, 1002b, and 1002c. Step 1002 determines whether or not the battery 6 
is normal according to whether or not the voltage BAT of the battery 6 is 
above 11 V. If the battery 6 is normal, step 1002a is carried out, and if 
not, step 1002b is carried out. Step 1002a checks the heater flag to see 
if the heater is ON. If the heater is ON, step 1011 is carried out, and if 
not, step 1004 is carried out. Step 1002b turns off the heater through the 
switch 8 because the battery 6 is abnormal. Step 1002c increases the 
quantity of intake air to increase the engine speed and the temperature of 
exhaust gas. At the same time, step 1002c delays ignition timing to 
increase the temperature of exhaust gas. As a result, the EHC 4 is heated 
to the activation temperature more quickly than usual. 
As explained above in detail, the exhaust purifier of the present invention 
has the estimate unit for estimating the temperature that the electrically 
heated catalyst will reach, the comparator for providing the difference 
between the estimated temperature and a target temperature, and the 
operation controller for controlling the operating conditions of the 
engine according to the temperature difference. The invention properly 
heats the catalyst in consideration of a deterioration with time and 
manufacturing variation in the heater. If the heater has high resistance, 
it will insufficiently heat the catalyst. Accordingly, the invention 
controls the operating conditions of the engine to increase the 
temperature of exhaust gas, to supplement the heater and properly and 
quickly heat the catalyst. If the heater has low resistance, it will 
excessively heat the catalyst. Accordingly; the invention controls the 
operating conditions of the engine to decrease the temperature of exhaust 
gas, to moderate the heater and properly heat the catalyst. The invention 
prevents the unnecessary use of the heater and battery, thereby extending 
the durability of the catalyst, heater, and battery. The present invention 
also estimates the temperature that the catalyst will attain according to 
the resistance of the heater, estimates the same according to the power of 
the heater, and adjusts the quantity of intake air and ignition timing to 
properly heat the catalyst. 
The present invention also delays the ignition timing of the engine and 
then increases the quantity of intake air to maintain a given engine 
speed, so that the driver may not feel uncomfortable. 
If an abnormal state of supplying no power to the heater occurs, the 
present invention also increases the quantity of intake air and delays 
ignition timing, to increase the temperature of exhaust gas and quickly 
heat the catalyst. 
Although the present invention has been disclosed and described by way of 
embodiments, it will be apparent to those skilled in the art that other 
embodiments and modifications of the present invention are possible 
without departing from the spirit or essential features thereof.