Patent Publication Number: US-6662787-B2

Title: Method and device for monitoring the fuel/air ratio of the mixture of air and vapor being fed from the outlet of a fuel vapor accumulator

Description:
The present invention relates to a method and device for monitoring the fuel/air ratio of the mixture of air and vapour being fed from the outlet of a fuel vapour accumulator. 
     BACKGROUND OF THE INVENTION 
     It is known that recent antipollution regulations provide for automobiles to be provided with a vapour accumulator (canister) designed to absorb the fuel vapours which are formed, while the vehicle is parked, by the liquid fuel contained in the vehicle&#39;s fuel tank. An accumulator of this type generally comprises a casing containing an activated carbon structure adapted to absorb the fuel vapour. An evaporative system is also provided which is adapted to carry out a vapour desorption stage (or scavenging) of the accumulator, in which the fuel stored in the activated carbon is desorbed and fed to the engine, in particular fed to the intake manifold of the engine. This evaporative system generally comprises a discharge duct which extends between an accumulator outlet and the intake manifold so as to utilise the vacuum created in the intake manifold when the engine is running and to provide a flow of air and vapour towards the intake manifold. The evaporative system further comprises an intake duct designed to allow the intake of air into the interior of said accumulator. 
     The evaporative systems of known type have a disadvantage in that the flow of air and vapour fed from the outlet is of variable and indeterminate composition; in particular, it is not possible to determine the percentage ratio of vapour fed to the manifold in relation to the total of vapour and air aspirated into the accumulator. Therefore, during the scavenging stage of the accumulator, a mixture of air and fuel is fed to the intake manifold, the percentage ratio of which mixture is not known. For this reason, during the aforementioned scavenging stage, the final air and fuel mixture which is fed to the engine may deviate from the stoichiometric ratio, which clearly brings about a deterioration in the emissions from the engine and in the operation of the catalytic converter. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a device for monitoring the fuel/air ratio of the mixture of vapours being fed from the outlet of a fuel vapour accumulator. 
     This object is achieved by the present invention in that it relates to a device for monitoring the fuel/air ratio of the mixture of air and vapour being fed from the outlet of a fuel vapour accumulator of the type described in claim 1. 
     The present invention also relates to a method of monitoring the fuel/air ratio of the mixture of air and vapour being fed from the outlet of a fuel vapour accumulator of the type described in claim 6. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the accompanying drawings which illustrate a preferred non-restrictive embodiment, in which: 
     FIG. 1 illustrates schematically a device for monitoring the fuel/air ratio of the mixture of air and vapour being fed from the outlet of a fuel vapour accumulator designed in accordance with the present invention, and 
     FIG. 2 illustrates a block diagram of the operations carried out by the device in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1 the reference numeral  1  generally denotes a device for monitoring the fuel/air ratio of the mixture of air and vapour being fed from the outlet of a fuel vapour accumulator 
     In particular, the fuel vapour accumulator  3  (of known type—also known as a CANISTER) has a first inlet  3   a  connected, via a duct  5 , to a fuel tank  7  and a second inlet  3   b  connected to an intake duct  8  which, at its free end  8   a,  provides an air intake. Furthermore, the vapour accumulator  3  has an outlet  34  which communicates via a duct  10  with the intake manifold  12  (partly illustrated) of a petrol engine (illustrated schematically). 
     A solenoid valve  14  is provided along the duct  10  to cut off the flow of air and fuel vapour coming from the accumulator  3  and directed towards the intake manifold  12 . In particular, the solenoid valve  14  is controlled according to a mode of operation (of known type) in which opening and closing cycles of said solenoid valve are repeated iteratively; moreover, the opening time period may be controlled continuously so as to regulate the flow of air and vapour directed towards the intake manifold  12 . 
     The device  1  for monitoring the fuel/air ratio further comprises an electronic processor  16  which controls via a driver (not shown) the length of time of the opening/closing cycles of the solenoid valve  14 . In particular, it is possible to control the duty cycle K of the solenoid valve  14 , which is defined as the ratio between the opening time Ton of the valve and the total opening and closing time Ton+Toff, i.e.: 
     
       
           K=T on/( T on+ T off)  
       
     
     A flow rate sensor  18  communicating with the electronic processor  16  is provided along the duct  8  and is adapted to measure the flow of air drawn in by the duct  8  towards the vapour accumulator  3 . The processor  16  further communicates with an engine control processor  19  adapted to control the injection unit  19   i  of the engine  13 . However, it is evident that the processors  16  and  19 , which are shown as separate in FIG. 1, could be integrated with one another. 
     It is known that when a vehicle is parked (not shown) the fuel  20  (petrol) contained in the tank  7  evaporates partially and passes via the duct  5  into the accumulator  3 , in which it is deposited. During the induction stroke of the engine  13  a vacuum is created in the intake manifold  12 , which via the duct  10  returns fuel vapour from the accumulator  3  towards the intake manifold  12 . Moreover, this vacuum takes part in the aspiration of air which passes through the duct  8  and is fed to the inlet  3   b  of the accumulator  3 . 
     In particular, in the following description the reference numeral: 
     Qv 1  denotes the flow rate of fuel vapour coming from the tank  7  (said vapours Qv 1  are fed to the accumulator  3  via the duct  5 ); 
     Qv 2  denotes the flow rate of petrol vapour released (desorbed) by the accumulator  3 ; 
     Qv denotes the vapour fed from the outlet of the accumulator  3 —therefore, Qv is given by the sum of the vapour released by the accumulator and the vapour evaporated from the tank, i.e: Qv=Qv 1 +Qv 2 ; 
     Qa denotes the flow rate of air fed to the accumulator  3  via the intake duct  8  (the flow rate Qa is detected by the sensor  18 ), and 
     Qm denotes the flow rate of the mixture of air and vapour fed to the manifold  12  via the duct  10 ; Qm is equal to Qa+Qv and comprises the air drawn into the accumulator and the fuel vapour released by the accumulator  3 . 
     FIG. 2 illustrates operations performed by the electronic processor  16  operating in accordance with the present invention. 
     Initially, a block  100  is reached which carries out the detection of a plurality of data, including: 
     the flow rate of air Qa aspirated towards the accumulator  3  (this information is obtained by means of the signal generated by the sensor  18 ); 
     the vacuum ΔP which is created in the intake manifold  12  (this information may be obtained by means of a pressure sensor  22  disposed in the intake manifold  12 ); 
     the duty cycle K with which the switching-over of the solenoid valve  14  is controlled. 
     The electronic processor  16  is also provided with a memory (not shown) in which are stored the values of a plurality of parameters, including: 
     the specific weight of the air γa; 
     the specific weight of the fuel vapour γv, and 
     the passage section area A of the solenoid valve  14 . 
     The block  100  is followed by a block  110  which calculates the flow rate of air Qa° which would pass through the solenoid valve  14  (i.e. the flow rate of air at the outlet of the accumulator  3  and directed towards the manifold  12 ) in the absence of vapour coming from the accumulator  3 .                Qa   0     =     KA            Δ                 P       γ                 a                   (   1   )                         
     in which ΔP represents the vacuum in the intake manifold  12 , γa, represents the specific weight of the air, A represents the passage section of the solenoid valve  14  and K takes into account the duty cycle with which the switching-over of the valve  14  is controlled. 
     The block  110  is followed by a block  120  which calculates the ratio between the flow rate of air Qa fed to the accumulator  3  and the flow rate of air Qa° which would pass through the solenoid valve  14  in the absence of vapour coming from the accumulator, i.e.: Qa/Qa°. 
     The block  120  is followed by a block  130  which calculates the percentage P of vapour fed to the manifold  12  in relation to the total of vapour and air drawn into the accumulator, i.e.:              p   =     Qv     Qv   +   Qa               (   2   )                         
     The calculation of p is carried out on the basis of the following quantities: 
     the ratio Qa/Qa° between the rate of flow of air Qa fed to the accumulator  3  and the flow rate of air Qa° which would flow through the solenoid valve  14  in the absence of vapours coming from the accumulator  3 ; 
     the specific weight of the air γa, and 
     the specific weight of the vapour γv. 
     In particular, the calculation of p is carried out according to the following formula (3):              p   =       0                   .5        [     2   -       (     1   -       γ                 v       γ                 a         )            (     Qa     Qa   0       )     2         ]         -     0   .   5              [     2   -       (     1   -       γ                 v       γ                 a         )            (     Qa     Qa   0       )     2         ]     2     -     4        [     1   -       (     Qa     Qa   0       )     2       ]                       (   3   )                         
     The block  130  is followed by a block  140  which feeds the previously calculated value of p to the engine control processor  19  which ensures the metering of the quantity of fuel fed by the injectors  19   i,  taking into account the value of p in the following manner. 
     Once the value of P is known, calculated with the expression (3) from block  130 , and of Qa (measured by the sensor  18 ), it is possible to calculate from the expression (2) the value of Qv. Since the total metering of the engine should be stoichiometric, the value of the flow rate QF of petrol fed by the injectors can be calculated by the following formula:          14   .   56     =       Ga   +     Qa                 γ                 a         GF   +     Qv                 γ                 v                         
     in which: 
     Ga is the mass flow rate of air aspirated by the engine and measured by the vehicle&#39;s flow meter, and 
     GF is the mass flow rate of petrol injected into the intake manifold by the injectors. 
     In this way the final mixture of air and fuel which is fed to the engine  13  does not deviate from the stoichiometric ratio even during the scavenging stage of the accumulator  3 . 
     There will now be briefly described the mathematical process which resulted in the definition of the formula for the calculation of p. 
     The flow rate of the mixture of air and vapour which flows towards the manifold  12  via the duct  10  can be expressed in accordance with Bemouilli&#39;s law, with the following formula:              Qm   =     KA            Δ                 P       γ                 m                   (   4   )                         
     in which ΔP represents the vacuum in the intake manifold  12 , γm represents the specific weight of the air and vapour mixture, A represents the passage section of the solenoid valve  14  and K takes into account the duty cycle with which the switching-over of the valve  14  is controlled. 
     Furthermore, the specific weight of the air and vapour mixture can be expressed by way of the following equation:                γ                 m     =         Qa                 γ                 a     +     Qv                 γ                 v         Qa   +   Qv               (   5   )                         
     In turn the rate of air flow Qa° which would flow through the solenoid valve  14  in the absence of vapour coming from the accumulator can be expressed in accordance with Bemouilli&#39;s law as:                Qa   0     =     KA            Δ                 P       γ                 a                   (   6   )                         
     in which ΔP represents the vacuum in the intake manifold  12 , γa represents the specific weight of the air, A represents the passage section of the solenoid valve  14  and K takes into account the duty cycle with which the switching-over of the valve  14  is controlled. 
     By compounding (4) with (6) one arrives at:              Qm   =       Qa   0              γ                 a       γ                 m                   (   7   )                         
     and expressing the definition of p        p   =       Qv     Qv   +   Qa       =       Qv   Qm     =         Qm   -   Qa     Qm     =       1   -     Qa   Qm       =     1   -       Qa       Qa   0          γ        a                  Qa                 γ                 a     +     Qv                 γ                 v       Qm                                   
     namely:              p   =     1   -       Qa       Qa   0            γ                 a                        Qa                 γ                 a     Qm     +     p                 γ                 v                                  (   8   )                         
     from which:              p   =     1   -       Qa       Qa   0            γ                 a                        (     Qm   -   Qv     )        γ                 a     Qm     +     p                 γ                 v                     (   9   )                                     p   =     1   -       Qa       Qa   0            γ                 a                      (     1   -   p     )        γ                 a     +     p                 γ                 v                     (   10   )                                     p   =     1   -       Qa       Qa   0            γ                 a                    γ                 a     -     p        (       γ                 a     -     γ                 v       )                       (   11   )                         
     therefore, from the expression (11) the value of p can be obtained as:        p   =       0   .     5        [     2   -       (     1   -       γ                 v       γ                 a         )            (       Q                 a       Q                   a   0         )     2         ]         -     0   .   5              [     2   -       (     1   -       γ                 v       γ                 a         )            (       Q                 a       Q                   a   0         )     2         ]     2     -     4        [     1   -       (       Q                 a       Q                   a   0         )     2       ]