Fuel vapor purging control system with air/fuel ratio compensating system for internal combustion engine

A fuel vapor purging control system for an internal combustion engine is provided. This purging control system incorporates an air/fuel ratio compensating system which includes a purge control valve for controlling a purge flow rate of an air-fuel vapor mixture flowing into an induction system of the engine through a purge passage according to a preselected engine operating condition parameter, a sensor disposed in the purge passage for determining an amount of fuel in the air-fuel vapor mixture, an air/fuel ratio controller for modifying an amount of fuel in an air/fuel mixture injected through a fuel injection system of the engine based on the amount of fuel in the air-fuel vapor mixture determined by the sensor for assuring a desired air/fuel ratio of the air/fuel mixture entering a combustion chamber of the engine.

BACKGROUND OF THE INVENTION 
1. Field of The Invention 
The present invention relates generally to an evaporation control system 
for an internal combustion engine. More particularly, the invention 
relates to a fuel vapor purging control system including an air/fuel ratio 
compensating system which is operable to assure a desired air/fuel ratio 
of an air/fuel mixture entering combustion chambers of the engine whether 
or not fuel vapor purging control is carried out. 
2. Description of The Prior Art 
Japanese Patent First Publication No. 62-233466 discloses a fuel vapor 
purging control system for an internal combustion engine which stores in a 
canister fuel vapors subsequently generated from a fuel tank before they 
can escape to the atmosphere, and supplies the fuel vapors to an induction 
system of the engine according to an engine intake vacuum level. In this 
conventional purging control system, the fuel vapors in the canister are 
purged first at a relatively lower rate. An air/fuel sensor arranged in an 
exhaust system of the engine monitors the oxygen concentration remaining 
in exhaust gas to project an air/fuel ratio which is subjected to change 
caused by the purged fuel vapors. When a deviation of the air/fuel ratio 
from a desired value is less than a preselected threshold value, the purge 
flow rate is increased. Alternatively, when the deviation is greater than 
the threshold value, the relatively lower purge flow rate is maintained as 
is. This prevents the air/fuel ratio from varying, which may be caused by 
the purged fuel vapors introduced into the induction system of the engine. 
The above prior art purging control system, however, rises a drawback in 
that it is impossible to determine a total amount of fuel supplied into 
combustion chambers of the engine as the amount of fuel contained in the 
purged fuel vapors cannot be determined before they go into the engine. 
Therefore, a variation in air/fuel ratio caused by the purged fuel vapors 
is not compensated precisely. 
SUMMARY OF THE INVENTION 
It is therefore a principal object of the present invention to avoid the 
disadvantages of the prior art. 
It is another object of the invention to provide a fuel vapor purging 
control system for an internal combustion engine which is operable to 
modify the amount of fuel injected by a fuel injector based on the amount 
of fuel contained in an air-fuel vapor mixture supplied form a canister to 
an induction system of the engine for assuring a desired air/fuel ratio 
whether or not purging control is carried out. 
According to one aspect of the present invention, there is provided a fuel 
vapor purging control system for an internal combustion engine which 
comprises a purge passage communicating between a fuel tank and an 
induction system of the engine, a purge control means including valve 
means for providing a variable flow restriction in the purge passage to 
control a purge flow rate of an air-fuel vapor mixture flowing into the 
induction system of the engine through the purge passage, a fuel mass 
determining means for determining the purge flow rate of the air-fuel 
vapor mixture in the purge passage based on an opening degree of the valve 
means and a preselected engine operating condition parameter, the fuel 
mass determining means determining fuel density of the air-fuel vapor 
mixture to determine fuel mass based on the purge flow rate and the fuel 
density of the air-fuel vapor mixture, and an air/fuel ratio controlling 
means for modifying an amount of fuel in an air/fuel mixture injected 
through a fuel injection system of the engine based on the fuel mass in 
the air-fuel vapor mixture determined by the fuel mass determining means 
to assure a desired air/fuel ratio of the air/fuel mixture entering a 
combustion chamber of the engine. 
In the preferred mode, the fuel mass determining means is operable to 
determine the oxygen concentration of the air-fuel vapor mixture flowing 
through the purge passage and determines fuel density based on the oxygen 
concentration to project the fuel mass based on the fuel density and the 
purge flow rate. The air/fuel ratio controlling means compensates the 
amount of fuel in the air/fuel mixture injected through the fuel injection 
system by the projected fuel mass. 
Alternatively, the fuel mass determining means may determine fuel 
concentration of the air-fuel vapor mixture and determines the fuel 
density based on the fuel concentration to project the fuel mass based on 
the fuel density and the purge flow rate. 
Additionally, the purge control means may modify the purge flow rate 
according to a value of the fuel density.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, particularly to FIG. 1, there is shown a 
fuel vapor purging control system including an air/fuel ratio compensating 
system according to the present invention. This purging control system 
generally includes a control unit 44, an oxygen sensor 6 for determining 
an air/fuel ratio, a solenoid operated purge control valve 40, a fuel 
vapor collection canister 37, and an oxygen sensor 46 for detecting oxygen 
concentration of fuel vapors purged from the canister. 
An intake manifold 2 and an exhaust manifold 3 are connected to cylinders 
of a multiple-cylinder internal combustion engine 1 respectively. An 
solenoid operated fuel injector 4 is installed in each passage of the 
intake manifold 2. The intake manifold 2 is communicated with a surge tank 
35 which is, in turn, connected to an air cleaner (not shown) through an 
induction passage 60. A throttle valve 5 is disposed within the induction 
passage 60 to modify an amount of fresh air drawn through the air cleaner 
to combustion chambers of the engine 1. The exhaust manifold 3 is 
connected to an exhaust passage 65 in which the oxygen sensor 6 is 
arranged to monitor oxygen concentration in exhaust gas and provides a 
signal indicative thereof to the control unit 44. 
A fuel supply system includes a fuel tank 7, a fuel pump 8, and a fuel 
filter 9. The fuel pump 8 feeds liquid fuel (i.e., gasoline) stored in the 
fuel tank 7 to each fuel injector 4 through the fuel filter 9 while the 
fuel fed to the fuel injector is pressurized to a preselected level by a 
pressure regulator 10. 
The canister 37 is arranged in a purge passage 36 which communicates 
between the fuel tank 7 and the surge tank 35. The canister is filled with 
an absorbing substance such as activated carbon which captures therein 
fuel vapors subsequently generated from the fuel tank 7. An air inlet port 
38 which is exposed to atmospheric pressure is formed in the canister 37 
for introducing fresh air thereinto. The purge control valve 40 is 
arranged in the purge passage 35 between the canister 37 and the surge 
tank 35. The purge control valve 40 includes a spring (not shown), a valve 
boy 41, a valve seat 42, and a solenoid 43. The valve body 41 is 
spring-loaded into constant engagement with the valve seat 42. The 
solenoid 43 is energized by a control signal having a variable duty ratio 
to move the valve 41 away from the valve seat 42 against spring force. 
With this arrangement, the purge control valve 40 is duty ratio modulated 
to establish an effective variable restriction in the purge passage 36 for 
modifying the purge flow rate of an air-fuel vapor mixture purged from the 
canister 37. 
The oxygen sensor 46 is arranged in the purge passage 36 between the 
canister 37 and the purge control valve 40, and monitors oxygen 
concentration in the air-fuel vapor mixture flowing through the purge 
passage 36 to provide a signal indicative thereof to the control unit 44. 
Referring to FIG. 2, the oxygen sensor 46 of galvanic cell type is shown 
which includes a sensor housing 47, an electrode 49 made of a noble metal 
Pt, an electrode 50 made of a base metal Pb, and electrolytic solution 51. 
The electrode 49 may alternatively be provided with Ag. 
The sensor housing 47 defines therein a gas passage 48 communicating with 
the purge passage 36 through which fuel vapors purged from the canister 37 
flow into the purge control valve 40. The electrode 49 is exposed to the 
fuel vapors in the gas passage 48 through a teflon film 52. A load 
resistance 53 is arranged between the electrodes 49 and 50. A voltmeter 54 
is provided to measure an electric potential difference between both ends 
of the resistance 53 to provide a signal indicative thereof to the control 
unit 44. 
The noble metal electrode 49 chemically reacts according to the following 
relation. 
EQU O.sub.2 +2H.sub.2 O+4e.sup.- .fwdarw.4OH.sup.- (1) 
Likewise, the base metal electrode 50 chemically reacts according to the 
following relation. 
EQU 2Pb.fwdarw.2Pb.sup.2+ +4e.sup.- (2) 
The above chemical reactions according to the relations (1) and (2) produce 
current flow of a level in proportional to the amount of oxygen 
concentration of air/fuel vapor mixture flowing through the purge passage 
36. 
When there is no fuel in the air-fuel vapor mixture flowing through the gas 
passage 48, the voltmeter 54 provides the maximum voltage which represents 
an oxygen concentration value of 21%. Alternatively, when the air-fuel 
vapor mixture has a fuel density of 2.32g/l, the voltmeter 54 provides the 
minimum voltage which represents an oxygen concentration value of 0%. 
However, the fuel density may be varied due to a variation in ambient 
temperature, (i.e., a variation in volatility of fuel). It is, thus, 
preferable that a fuel density data corresponding to an oxygen 
concentration of 0% be mathematically projected to update a relation 
between fuel density and oxygen concentration. 
Referring to FIG. 3, there is shown relations among fuel density, oxygen 
concentration, and output voltage of the oxygen sensor 46. It will be 
appreciated from the graph that the variation in fuel density causes the 
oxygen concentration and the output voltage to vary in proportion thereto. 
Referring back to FIG. 1, the control unit 44 is provided with a 
microcomputer to which a throttle opening degree indicative signal from a 
throttle sensor 90, an engine speed indicative signal from an engine speed 
sensor 92, an intake air pressure indicative signal from an intake air 
pressure sensor 94, a coolant temperature indicative signal from a coolant 
temperature sensor 96 are input, respectively. 
Additionally, the control unit 44 is responsive to a signal from the oxygen 
sensor 46 to determine fuel density with respect to time of an air-fuel 
vapor mixture flowing through the purge passage 36 according to the 
relation as shown in FIG. 3, and also provides a control signal having a 
variable duty ratio to the purge control valve 40 to modify a purge rate 
of fuel vapor through the canister 37. 
A way of determining fuel weight (or fuel mass) based on the flow rate of 
an air-fuel vapor mixture supplied from the canister 37 to the surge tank 
35 will be discussed hereinbelow. 
In general, the purge flow rate of an air-fuel vapor mixture drawn from the 
canister 37 to the surge tank 35 is dependent upon negative pressure in 
the surge tank 35 (i.e., an intake manifold vacuum level) and an opening 
degree of the purge control valve 40 determined by the duty ratio of a 
control signal (i.e., an effective flow restriction in the purge passage 
36). 
FIG. 4 shows a relation between a duty ratio of a control signal output 
from the control unit 44 to the purge control valve 40 and a controlled 
purge flow rate when pressure in the surge tank 35 is -450 mmHg. The 
control unit 44 is responsive to the intake air pressure indicative signal 
from the intake air pressure sensor 94 (i.e., negative pressure in the 
surge tank 35) to determine a duty ratio for the purge control valve 40 
according to mapped data, as shown in FIG. 4, for deriving a required 
purge flow rate through the canister 37. 
It will be appreciated that the fuel mass in an air-fuel vapor mixture may 
be determined by multiplying a mixture flow rate by fuel density in the 
mixture. Accordingly, the control unit 44 determines the fuel mass of an 
air-fuel vapor mixture drawn from the canister 37 to the surge tank 35, 
based on the fuel density, as derived according to the mapped data in FIG. 
3, and the purge flow rate, as derived according to the mapped data in 
FIG. 4. 
Referring to FIG. 5, there is shown a flowchart of a program or sequence of 
logical steps performed by the control unit 44. This program is carried 
out at a predetermined time interval. 
After entering the program, the routine proceeds to step 100 wherein it is 
determined whether preselected purging control executing requirements has 
been met or not. For example, it may be determined whether the engine 1 is 
in operation other than idling or fuel cut modes or not. If a NO answer is 
obtained concluding that the engine 1 is undergoing idling or a fuel cut, 
the routine then proceeds directly to step 111. Alternatively, if a YES 
answer is obtained in step 100, the routine proceeds to step 101 wherein 
it is determined whether a flag F is one or not (F=1 ?). The flag F is 
initialized to zero in response to activation of an ignition switch. If a 
YES answer is obtained (F=1) concluding that the purging control has been 
carried out from a previous program cycle, the routine then proceeds 
directly to step 104. Alternatively, if a NO answer is obtained 
(F.noteq.1) concluding that the flat F is not one, that is, that the 
purging control is not yet initiated after the engine 1 starts, the 
routine then proceeds to step 102 wherein a control signal having a duty 
ratio .alpha. (e.g., 0.2 %/sec.) is output to the purge control valve 40 
so that it is energized to be opened to a degree according to the duty 
ratio .alpha.. The routine then proceeds to step 103 wherein it is 
determined whether a preselected period of time Tt has expired or not 
after the purge control valve is energized. The time Tt is set to an 
interval required for fuel vapors purged from the canister 37 to reach the 
oxygen sensor 46 through the purge passage 36. 
If a YES answer is obtained in step 103, the routine then proceeds to step 
104 wherein the control unit 44 monitors the oxygen concentration in an 
air-fuel vapor mixture following through the purge passage 36 which is 
detected by the oxygen sensor 46. The routine then proceeds to step 105 
wherein the fuel density of the air-fuel vapor mixture is determined 
according to the mapped data, as shown in FIG. 3. The routine then 
proceeds to step 106 wherein a flow rate of the air-fuel vapor mixture 
(i.e., a purge flow rate) is determined using the intake air pressure 
detected by the intake air pressure sensor 94 and the duty ratio for the 
purge control valve 40 according to mapped data, as shown in FIG. 4. The 
routine then proceeds to step 107 wherein the fuel mass Q.sub.EVP is 
determined by multiplying the purge flow rate and the fuel density 
together. 
The routine then proceeds to step 108 wherein the fuel mass Q.sub.EVP is 
subtracted from the amount of fuel Q.sub.FUEL to be injected by the fuel 
injector 4 to determine a target fuel injection amount Q.sub.FUEL-TARGET 
(Q.sub.FUEL-TARGET =Q.sub.FUEL -Q.sub.EVP). The control unit 44 then 
controls the fuel injector 4 so that the corrected amount 
Q.sub.FUEL-TARGET of fuel is injected into a combustion chamber of the 
engine 1. With this fuel injection amount correction, a desired air/fuel 
ratio is assured even under the fuel vapor purging control. Usually, the 
control unit 44 determines the fuel injection amount Q.sub.FUEL by 
correcting a reference fuel injection amount based on coolant temperature 
and a throttle opening degree. The reference fuel injection amount is 
derived based on intake air pressure and engine speed. 
Afterwards, the routine then proceeds to step 109 wherein the flag F is set 
to one. The routine then proceeds to step 110 wherein it is determined 
whether the fuel density, as determined in step 105, is smaller than a 
preselected threshold value .beta. or not. If a YES answer is obtained 
concluding that an amount of fuel absorbed in the canister 37 is small, 
the routine then proceeds to step 111 wherein the purge control valve 40 
is closed at the duty ratio .alpha. (%/sec.) for reducing fresh air 
introduced from the air inlet port 38 of the canister 37 to the intake 
manifold 2 through the purge passage 36. Alternatively, if a NO answer is 
obtained in step 110 concluding that the amount of fuel absorbed in the 
canister 37 is great, the routine then proceeds to step 112 wherein the 
purge control valve 40 is opened at the duty ratio .alpha. (%/sec.), 
thereby reducing the amount of the fuel absorbed in the canister 37. The 
threshold value .beta. in step 110 may be set to a fuel density value 
which establishes a desired air/fuel ratio (i.e., a stoichiometric ratio) 
of air contained in fuel vapors supplied form the canister 37 to the surge 
tank 35 to fuel contained in the fuel vapors. 
Referring to FIG. 6, there is shown a fuel concentration sensor 66 which 
may be utilized in place of the oxygen sensor 46, as shown in FIG. 1. This 
fuel concentration sensor 66 includes a sensor housing 47, a crystal 
oscillator 69, a lead wire 71, a gas absorbing film 70, a base plate 72, 
and a cover 73. The sensor housing 47 defines therein a gas passage 48 
communicating with the purge passage 36 through which fuel vapors purged 
from the canister 37 follow to the purge control valve 40. The gas 
absorbing film 50 is arranged on the crystal oscillator 69 so as to be 
exposed to the fuel vapors passing through the gas passage 48. The lead 
wire 71 is connected to a terminal 74 which is, in turn, connected to a 
frequency-voltage converter 75 incorporating an oscillator. A voltmeter 76 
is provided to measure a voltage level of a signal output from the 
frequency-voltage converter 75 and provides a signal indicative thereof to 
the control unit 44. 
With the above arrangement, the crystal oscillator 69 always oscillates 
whether or not fuel is contained in an air-fuel vapor mixture flowing 
through the gas passage 48, and provides a sensor signal having a 
reference frequency when fuel concentration of the air-fuel vapor mixture 
is 0%. The frequency-voltage converter 75 is responsive to the sensor 
signal having the reference frequency to provide a signal of 0 mV. 
When the air-fuel vapor mixture flows through the gas passage 48, fuel 
contained therein is absorbed in the gas absorbing film 70 so that the 
crystal oscillator 69 provides a signal having frequency corresponding to 
the fuel concentration of the air-fuel vapor mixture. The 
frequency-voltage converter 75 then provides a signal having voltage 
corresponding to the frequency (i.e., a frequency variation from the 
reference frequency) output from the crystal oscillator 69. 
In addition, the control unit 44 is responsive to the output voltage from 
the fuel concentration sensor 66 to determine the fuel density of the 
air-fuel vapor mixture in the purge passage 36 according to the relation 
as shown in FIG. 7, and provides a control signal having a variable duty 
ratio to the purge control valve 40 to modify a purge rate of fuel vapor 
through the canister 37. 
Referring to FIG. 8, there is shown an alternative embodiment of a program 
carried out by the control unit 44. This program is different from the 
first embodiment, as shown in FIG. 5, in steps 204 and 205 and thus 
explanation of other steps in detail will be omitted here. 
After step 103, the routine proceeds to step 204 wherein the control unit 
44 monitors a voltage level from the voltmeter 76 which represents the 
frequency variation detected by the fuel concentration sensor 66. The 
routine then proceeds to step 205 wherein the fuel density is determined 
according to the mapped data, as shown in FIG. 7. Thereafter, similar to 
the first embodiment, the routine proceeds to step 106 wherein the purge 
flow rate of fuel vapors through the canister 37 is determined. 
While the present invention has been disclosed in terms of the preferred 
embodiment in order to facilitate better understanding thereof, it should 
be appreciated that the invention can be embodied in various ways without 
departing from the principle of the invention. Therefore, the invention 
should be understood to include all possible embodiments and modification 
to the shown embodiments which can be embodied without departing from the 
principle of the invention as set forth in the appended claims.