Abstract:
A fuel injection system for an internal combustion engine has an upstream fuel injector provided upstream from the throttle valve and a downstream fuel injector provided downstream therefrom. A device is provided for determining a fuel injection quantity of the upstream and downstream fuel injectors. A sensor detects the intake temperature TA on the upstream side from an injection area of the upstream fuel injector. A device is provided for seeking an intake temperature correction factor KTA on the basis of the intake temperature TA and a fuel injection quantity of the upstream fuel injector. At least one of the fuel injection quantities due to the upstream and downstream fuel injectors is corrected on the basis of the intake temperature correction factor KTA.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2002-258212, filed in Japan on Sep. 3, 2002, the entirety of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a fuel injection system for an internal combustion engine. More particularly, the present invention relates to a fuel injection system in which injection valves have been provided on the upstream side and on the downstream side thereof, respectively, with a throttle valve interposed therebetween. 
   2. Description of Background Art 
   When the fuel injector is provided upstream from the throttle valve, the volumetric efficiency is improved because heat is taken from intake air when injection fuel vaporizes. Therefore, the engine output can be increased as compared with when the fuel injector is provided downstream from the throttle valve. On the other hand, when the fuel injector is provided on the upstream side, a distance between the fuel injection port of the upstream fuel injector and the combustion chamber inevitably increases. Accordingly, a response lag occurs in fuel transport as compared with when the fuel injector is provided downstream from the throttle valve. This causes the driveability of the engine to deteriorate. 
   In Japanese Patent Laid-Open Nos. 4-183949 and 10-196440, it has been attempted to solve such technical problems, to improve engine output and to ensure that driveability is compatible with the engine output. In the above documents, a fuel injection system has been disclosed in which fuel injectors have been provided on the upstream side and on the downstream side from the intake pipe, respectively, with the throttle valve interposed therebetween. 
     FIG. 7  is a cross-sectional view showing a major portion of an internal combustion engine according to the background art in which two fuel injectors have been arranged with a throttle valve  52  of an intake pipe  51  interposed therebetween. A downstream fuel injector  50   a  has been arranged on a side portion of the downstream side (engine side) of the throttle valve  52  and an upstream fuel injector  50   b  has been arranged on the upstream side (air cleaner side) of the throttle valve  52 . A lower end portion of the intake pipe  51  is connected to an intake passage  52 . An intake port  53  faces a combustion chamber of the intake passage  52  and is opened and closed by an intake valve  54 . 
   The fuel injection quantity of each fuel injector is determined with plural parameters including the throttle opening as a function. However, volumetric efficiency within the combustion chamber is dependent on the intake temperature. Accordingly, an electronic controlled fuel injection system detects the intake temperature TA to control in such a manner that the injection quantity is relatively reduced as the intake temperature TA becomes higher. 
   The intake temperature TA is preferably detected immediately before the combustion chamber. However, when a temperature sensor is provided at the portion concerned, the intake efficiency of an air-fuel mixture into the combustion chamber is deteriorated. Accordingly, in an engine in which two fuel injectors are arranged, the temperature sensor is often provided on the upstream side from the fuel injection area of the upstream fuel injector  50   b.    
   However, the air within the intake pipe is cooled by the fuel injected from the upstream fuel injector  50   b . Accordingly, a difference occurs between the intake temperature to be detected by the temperature sensor and the intake temperature immediately before the combustion chamber. This causes problems in detecting the correct intake temperature TA. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to solve the above-described problems of the background art. Specifically, it is an object to provide a fuel injection system for an internal combustion engine capable of supplying an optimum quantity of fuel for a particular intake temperature, in a structure in which fuel injectors are arranged on the upstream side and on the downstream side of the throttle valve, respectively. 
   In order to achieve the above-described object, there is provided a fuel injection system for an internal combustion engine according to the present invention having an intake pipe equipped with a throttle valve, an upstream fuel injector provided upstream from the throttle valve and a downstream fuel injector provided downstream from the throttle valve. Means are provided for determining the fuel injection quantity of the upstream and downstream fuel injectors. Means are provided for detecting intake temperature TA on the upstream side from an injection area of the upstream fuel injector. Means are provided for determining an intake temperature correction factor KTA on the basis of the intake temperature TA and the fuel injection quantity of the upstream fuel injector. In addition, means are provided for correcting at least one of the fuel injection quantities due of the upstream and downstream fuel injectors on the basis of the intake temperature correction factor KTA. 
   According to the above-described feature, the intake temperature correction factor KTA can be determined as a function of the fuel injection quantity of the upstream fuel injector. Accordingly, if it is arranged in such a manner that the intake temperature correction factor KTA becomes relatively large as the fuel injection quantity of the upstream fuel injector increases, a drop in the intake temperature due to upstream fuel injection will be properly compensated for. Therefore, it becomes possible to supply an optimum quantity of fuel for a particular intake temperature. 
   Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
       FIG. 1  is a general block diagram showing a fuel injection system according to one embodiment of the present invention; 
       FIG. 2  is a functional block diagram for a fuel injection control unit  10 ; 
       FIG. 3  is a view showing one example of an injection rate table; 
       FIG. 4  is a flowchart showing a calculation procedure of a correction factor KTA; 
       FIG. 5  is a view showing an example of an intake temperature correction factor table; 
       FIG. 6  is a flowchart showing a control procedure of fuel injection; and 
       FIG. 7  is a cross-sectional view showing an internal combustion engine according to the background art in which two fuel injectors have been arranged. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, the present invention will be described with reference to the accompanying drawings. It should be noted that the same reference numerals have been used throughout the several views to identify the same or similar elements.  FIG. 1  is a general block diagram showing a fuel injection system according to one embodiment of the present invention. An intake port  22  and an exhaust port  23  open into a combustion chamber  21  of the engine  20 . Each port  22  and  23  is provided with an intake valve  24  and an exhaust valve  25 , respectively. In addition, an ignition plug  26  is provided extending into the combustion chamber  21 . 
   A throttle valve  28  for adjusting intake air quantity in accordance with an opening θTH thereof, a throttle sensor  5  for detecting the opening θTH and a vacuum sensor  6  for detecting intake manifold vacuum PB are provide on an intake passage  27  leading to the intake port  22 . An air cleaner  29  is provided at a terminal of the intake passage  27 . An air filter  30  is provided within the air cleaner  29 . Open air is taken into the intake passage  27  through the air filter  30 . 
   A downstream injection valve  8   b  is arranged in the intake passage  27  downstream from the throttle valve  28 . An upstream injection valve  8   a  is arranged on the air cleaner  29  upstream from the throttle valve  28  so as to point to the intake passage  27 . An intake temperature sensor  2  is provided for detecting intake (atmospheric) temperature TA. 
   An engine speed sensor  4  is provided opposite to a crankshaft  33 , which is coupled to a piston  31  of the engine  20  through a connecting rod  32 , for detecting the engine speed NE on the basis of a rotation angle of a crankshaft  33 . Furthermore, a vehicle speed sensor  7  is arranged opposite to a rotor  34 , such as a gear which is coupled to the crankshaft  33  for rotation, for detecting vehicle speed V. A water temperature sensor  3  is provided on a water jacket formed around the engine  20  for detecting cooling water temperature TW representing the engine temperature. 
   An ECU (Engine Control Unit)  1  includes a fuel injection control unit  10  and an ignition timing control unit  11 . The fuel injection control unit  10  outputs, on the basis of signals (process values) obtained from each of the above-described sensors, injection signals Qupper and Qlower of each injection valve  8   a ,  8   b  on the upstream and downstream sides. Each of these injection signals is a pulse signal having pulse width responsive to the injection quantity. Each injection valve  8   a ,  8   b  is opened for a time corresponding to the pulse width to inject the fuel. The ignition timing control unit  11  controls the ignition timing of the ignition plug  26 . 
     FIG. 2  is a functional block diagram for the fuel injection control unit  10 . A total injection quantity determination unit  101  determines a total quantity Qtotal of fuel to be injected from each fuel injector  8   a ,  8   b  on the upstream and downstream sides on the basis of the engine speed NE, the throttle opening θTH and intake pressure PB. An injection rate determination unit  102  refers to an injection rate table on the basis of the engine speed NE and throttle opening θTH to determine an injection rate Rupper of the upstream injection valve  8   a . An injection rate Rlower of the downstream injection valve  8   b  is determined as (1-Rupper). 
     FIG. 3  is a view showing an example of the injection rate table. In the present embodiment, an injection rate map includes 15 items (Cne 00  to Cne 14 ) as a reference for the engine speed NE and 10 items (Cth 0  to Cth 9 ) as a reference for the throttle opening θTH. The injection rate Rupper of the upstream injection valve  8   a  is registered in advance at each combination of engine speed NE and the throttle opening θTH. The injection rate determination unit  102  determines an injection rate Rupper corresponding to the engine speed NE and the throttle opening θTH that have been detected by means of a four-point interpolation of the injection rate map. 
   Referring again to  FIG. 2 , a correction factor calculation unit  103  refers to a data table on the basis of the intake temperature TA and the cooling water temperature TW that have been detected to seek various correction factors including an intake temperature correction factor KTA and a cooling water temperature correction factor KTW. 
   Referring to the flowchart of  FIG. 4 , a description will now be made in detail of a calculation method for the intake temperature correction factor KTA according to the present embodiment. 
   In a step S 11 , a TA/KTAL table to be described later is referred to and a correction factor KTAL for a light load corresponding to the intake temperature TA is calculated. In a step S 12 , a TA/KTAH table to be described later is referred to, and a correction factor KTAH for a heavy load corresponding to the intake temperature TA is calculated. In a step S 13 , a TA/KTA 2  table to be described later is referred to, and a correction factor KTA 2  for upstream and downstream injection corresponding to the intake temperature TA is calculated. 
     FIG. 5  is a view showing the contents of each of the above-described tables schematically and superimposed. For each intake temperature TA, each correction factor KTAL, KTAH and KTA 2  corresponding thereto has been registered. In the present embodiment, each correction factor for the intake temperature TA has been selected so as to indicate a tendency of KTAL&lt;KTAH&lt;KTA 2 . A relationship between the intake temperature TA and each correction factor has been registered only with nine items of the intake temperature TA. Any other relationship can be determined by interpolation. 
   Referring again to  FIG. 4 , in a step S 14 , the engine speed NE is compared with a predetermined reference speed. In the present embodiment, the engine speed NE is compared with an idle speed. When the engine speed NE becomes lower than the idle speed, the sequence will proceed to a step S 15 . In the step S 15 , the throttle opening θth is compared with a predetermined reference opening. In the present embodiment, the throttle opening θth is compared with the idle opening. When the throttle opening θth becomes lower than the idle opening, the sequence will proceed to a step S 16 . In the step S 16 , the correction factor for a light load KTAL determined in the step S 11  will be adopted as the intake temperature correction factor KTA. A light load flag FL will be set. 
   On the other hand, when either of the steps S 14 , S 15  is negative, the sequence will proceed to a step S 17  to refer to the light load flag FL. If the light load flag FL has been set, the sequence will proceed to a step S 18 , and the correction factor for a heavy load KTAH determined in the step S 12  will be adopted as the intake temperature correction factor KTA. The light load flag FL will then be reset. 
   In the step S 17 , if the light load flag FL has not been set, the sequence will proceed to a step S 19 . An upstream injection quantity Qupper which is determined by an upstream injection quantity determination unit  1051  to be described later will be compared with a predetermined reference injection quantity Qref. If Qupper≦Qref, the sequence will proceed to a step S 20  because a drop in intake temperature due to the upstream injection is low. A correction factor for a heavy load KTAH determined in the step S 12  will be registered to a target correction factor KTAtg. In contrast to this, if Qupper&gt;Qref, the sequence will proceed to a step S 21  because a drop in the intake temperature due to the upstream injection becomes high. A correction factor for upstream and downstream injection KTA 2  determined in the step S 13  will be registered to the target correction factor KTAtg. 
   In a step S 22 , a differential between the target correction factor KTAtg and the present intake temperature correction factor KTA is determined. The differential is compared with the maximum correction quantity ΔKTAmax. If the differential is smaller than the maximum correction quantity ΔKTAmax, the target correction factor KTAtg will be adopted as the intake temperature correction factor KTA in a step S 26 . 
   In contrast to this, if the differential is larger than the maximum correction quantity ΔKTAmax, the sequence will proceed to a step S 23  to compare the target correction factor KTAtg with the present intake temperature correction factor KTA. If the target correction factor KTAtg is smaller than the intake temperature correction factor KTA, in a step S 24 , a value obtained by deducting the maximum correction quantity ΔKTAmax from the present intake temperature correction factor KTA will be adopted as a new intake temperature correction factor KTA. If the target correction factor KTAtg is larger than the intake temperature correction factor KTA, in a step S 25 , a sum of the present intake temperature correction factor KTA and the maximum correction quantity ΔKTAmax will be adopted as a new intake temperature correction factor KTA. 
   As described above, in the present embodiment, the intake temperature correction factor is switched depending on the injection quantity due to the upstream injection valve. Accordingly, it becomes possible to accurately control the fuel injection even if the intake temperature varies in response to the injection quantity of the upstream injection valve. 
   Referring again to  FIG. 2 , the injection quantity correction unit  104  corrects the injection quantity of each injection valve  8   a ,  8   b  during acceleration, when the throttle opening θth is abruptly closed and at other times. In the injection quantity determination unit  105 , the upstream injection quantity determination unit  1051  determines a basic injection quantity of the upper injection valve  8   a  on the basis of the injection rate Rupper and the total injection quantity Qtotal, and multiplies this basic injection quantity by various correction factors including the correction factor KTA, KTW to determine the injection quantity Qupper of the upstream injection valve  8   a . A downstream injection quantity determination unit  1052  determines the injection quantity Qlower of the downstream injection valve  8   b  on the basis of the upstream injection quantity Qupper and the total injection quantity Qtotal. 
   Referring to the flowchart of  FIG. 6 , a description will now be made in detail of an operation of the fuel injection control unit  10 . This handling is executed by interruption due to a crank pulse in a predetermined stage. 
   In a step S 10 , the engine speed NE, the throttle opening θTH, the manifold air pressure PB, the intake temperature TA and the cooling water temperature TW are detected by each of the above-described sensors. In a step S 11 , in the total injection quantity determination unit  101 , total quantity Qtotal of fuel to be injected from each fuel injector  8   a ,  8   b  on the upstream side and on the downstream side is determined on the basis of the engine speed NE, the throttle opening θTH and the intake pressure PB. 
   In a step S 12 , in the injection rate determination unit  102 , an injection rate table is referred to on the basis of the engine speed Ne and the throttle opening θTH. An injection rate Rupper of the upstream injection valve  8   a  is determined. In a step S 13 , the injection rate Rupper is corrected on the basis of the following expression (1):
 
Rupper=Rupper× KTW×KTA    (1) 
 
   In a step S 14 , the upstream injection quantity determination unit  1051  calculates an injection quantity Qupper of the upstream injection valve  8   a  on the basis of the following expression (2):
 
Qupper=Qtotal×Rupper   (2) 
 
   In a step S 15 , the downstream injection quantity determination unit  1052  calculates the injection quantity Qlower of the downstream injection valve  8   b  on the basis of the following expression (3):
 
Qlower=Qtotal−Qupper   (3) 
 
   When the injection quantity Qupper of the upstream injection valve  8   a  and the injection quantity Qlower of the downstream injection valve  8   b  are determined as described above, an injection signal having a pulse width responsive to each of the injection quantity Qupper, Qlower is outputted to each injection valve  8   a ,  8   b  at predetermined timing synchronized to the crank angle to inject fuel from each injection valve  8   a ,  8   b.    
   In this respect, in the above-described embodiment, the description has been made of a case where the injection quantity of the upstream injection valve  8   a  is reduced when the throttle valve is at low temperature. However, the injection may be completely stopped. 
   According to the present invention, the intake temperature correction factor KTA can be determined as a function of the fuel injection quantity of the upstream fuel injector. Accordingly, if it is arranged in such a manner that the intake temperature correction factor KTA becomes relatively large as the fuel injection quantity of the upstream fuel injector increases, a drop in the intake temperature due to upstream fuel injection will be properly compensated for. Therefore, it becomes possible to supply an optimum quantity of fuel for a particular intake temperature. 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.