Control system for internal combustion engines

Abstract: A control system for an internal combustion engine (1) corrects an amount of supply fuel to the engine (1) based upon operating conditions of the engine (1) as well as an estimated amount of adherent fuel adhering to the inner surface of the intake passage (2) and an estimated amount of fuel carried off from fuel adhering to the intake passage (2) inner surface and carried into combustion chambers. The correction of the supply fuel amount is limited when the engine (1) is operating in an idling condition. According to a further aspect of the invention, the adherent fuel amount and the carried-off fuel amount are corrected based upon a calculated amount of exhaust gases (recirculated by recirculation means 20) and a detected operating mode or valve timing (changeable from low to high speed valve timing by means 19) of at least one of intake valves and exhaust valves.

Description: This invention relates to a control system for internal combustion engines, and more particularly to a control system which controls the supply of fuel injected into an intake pipe in a manner compensating for a fuel amount adhering to the inner surface of the intake pipe. In conventional internal combustion engines of the type that fuel is injected into an intake pipe, there is a problem that some of injected fuel adheres to the inner surface of the intake pipe, so that a required amount of fuel cannot be drawn into the combustion chamber, resulting in a difficulty of accurately controlling the air-fuel ratio of a mixture supplied to the engine. To solve this problem, there has been proposed a fuel supply control method which estimates a fuel amount which is to adhere to the inner surface of the intake pipe and one which is to be drawn into the combustion chamber by evaporation from the fuel adhering to the intake pipe, and determines a fuel injection amount in dependence on the estimated fuel amounts (Japanese Provisional Patent Publication (Kokai) No. 61-126337). To improve this fuel supply control method, a supply fuel amount correction method has been proposed by the assignee of the present application, which calculates a fuel amount adhering to the inner surface of the intake pipe by the use of coefficients Ae and Be which are obtained by correcting a direct supply ratio (the ratio of a fuel amount injected into the intake pipe and directly drawn into the combustion chamber to the whole amount of fuel injected into the intake pipe) and a carry-off ratio (the ratio of a fuel amount carried off the inner surface of the intake pipe and drawn into the combustion chambers to the amount of fuel adhering to the inner surface), in dependence on the rotational speed of the engine, etc., the two ratios being determined based upon the temperature of engine coolant and pressure within the intake pipe, and corrects the fuel amount to be supplied to the combustion chamber by the use of the amount of fuel adhering to the inner surface and the coefficients Ae, Be (Japanese Patent Application No. 3-283694, and corresponding U.S. Serial No. 07/945,489 filed September 16, 1992). However, the above mentioned prior art has a problem that when the engine is idling, the pressure within the intake pipe varies due to variations or fluctuations in the intake air amount, and as a result, the values of the coefficients Ae, Be for correcting the supply fuel amount vary so that the actual fuel amount supplied to the engine fluctuates, resulting in the idling condition being unstable. Particularly, in an internal combustion engine provided with an auxiliary air amount control valve for controlling the idling speed of the engine, the auxiliary air amount control valve is much inferior in dynamic characteristics such as a response time lag to fuel being supplied to the engine, and accordingly, during idling, the control valve cannot quickly respond to a change in the supply fuel amount, resulting in hunting of the rotational speed of the engine and unstable engine idling. US-4852538 discloses a fuel injection control system which calculates the fuel injection amount corrected by a transient correction amount. The transient correction amount is partly calculated in accordance with a difference value between an equilibrium amount of adhering and floating fuel in steady state in an intake system and a predicted variable amount of the adhering and floating fuel at a predetermined point of time. The system uses a correction rate with a different weighting factor during idling. However, the correction rate is not limited to a specific limit during idling. It is an object of the invention to provide a control system for an internal combustion engine, which is capable of accurately controlling the air-fuel ratio of a mixture supplied to combustion chambers of the engine, while ensuring stable idling condition of the engine. According to the present invention, there is provided a control system for an internal combustion engine having at least one combustion chamber, and an intake passage having an inner surface, comprising operating condition detecting means for detecting operating conditions of the engine, supply fuel amount determining means responsive to operating conditions of the engine detected by the operating condition detecting means, for determining an amount of fuel to be supplied to the engine, adherent fuel amount estimating means for estimating an amount of fuel adhering to the inner surface of the intake passage, carried-off fuel amount estimating means for estimating an amount of fuel carried off from fuel adhering to the inner surface of the intake passage and carried into the combustion chamber, supply fuel amount correction means for correcting the supply fuel amount determined by the supply fuel amount determining means, based upon operating conditions of the engine detected by the operating condition detecting means, and the adherent fuel amount estimated by the adherent fuel amount estimating means and the carried-off fuel amount estimated by the carried-off fuel amount estimating means, and fuel supply means for supplying the supply fuel amount corrected by the supply fuel amount correcting means into the intake passage. The control system is characterised by an improvement comprising correction limiting means for limiting the correction of the supply fuel amount based upon the estimated adherent amount and the estimated carried-off fuel amount by the supply fuel amount correcting means, when the operating condition detecting means detects that the engine is operating in an idling condition. Preferably, the adherent fuel amount estimating means and the carried-off fuel amount estimating means estimate the adherent fuel amount and the carried-off fuel amount, respectively, based upon operating parameters of the engine including pressure within the intake passage. Preferably, the correction limiting means inhibits the correction of the supply fuel amount based upon the estimated adherent amount and the estimated carried-off fuel amount by the supply fuel amount correcting means. Also preferably, the adherent fuel amount estimating means continues the estimation of the adherent fuel amount even when the engine is detected to be operating in the idling condition. The above and other objects, features, and advantages of the invention will be more apparent from the following detailed description, given by way of example only, when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a block diagram showing the whole arrangement for an internal combustion engine and a control system therefor; Fig. 2 is a flowchart of a program for calculating a fuel injection period (Tout); Fig. 3 is a flowchart of a program for calculating an amount of fuel adhering to the inner surface of the intake pipe (TWP (N)); Fig. 4 shows tables for determining correction coefficients KA, KB for correcting a direct supply ratio A and a carry-off ratio B; Referring first to Fig. 1, there is illustrated the whole arrangement of an internal combustion engine, and a control system therefor, according to a preferred embodiment of the invention. In the figure, reference numeral 1 designates an internal combustion engine for automotive vehicles. The engine is a four-cylinder type, for instance. Connected to the cylinder block of the engine 1 is an intake pipe 2 across which is arranged a throttle body 3 accommodating a throttle valve 3' therein. A throttle valve opening ( TH ) sensor 4 is connected to the throttle valve 3' for generating an electric signal indicative of the sensed throttle valve opening and supplying same to an electronic control unit (hereinafter called "the ECU") 5. An auxiliary air passage 27 opens at one end 27a thereof into the intake pipe 2 at a location downstream of the throttle valve 3' and communicates with the atmosphere at the other end thereof through an air cleaner 22. Arranged across the auxiliary air passage 27 is an auxiliary air amount control valve (hereinafter merely referred to as "the AIC control valve") 26, which controls the idling rotational speed of the engine 1 in response to a control signal from the ECU 5 and has its valve opening (the opening area of the passage 27) controlled by the above control signal. In the illustrated embodiment, the AIC control valve 26 is formed by a linear solenoid valve which is comprised of a solenoid 26a connected to the ECU 5, and a valve element 26b displaceable in response to energization of the solenoid 26a to vary the opening area of the auxiliary air passage 27 by an amount proportional to an amount of current applied to the solenoid 26a. Fuel injection valves 6, only one of which is shown, are inserted into the interior of the intake pipe 2 at locations intermediate between the cylinder block of the engine 1 and the throttle valve 3' and slightly upstream of respective intake valves, not shown. The fuel injection valves 6 are connected to a fuel pump, not shown, and electrically connected to the ECU 5 to have their valve opening periods controlled by signals therefrom. On the other hand, an intake pipe absolute pressure (PBA) sensor 8 is provided in communication with the interior of the intake pipe 2 via a conduit 7 at a location immediately downstream of the throttle valve 3' for supplying an electric signal indicative of the sensed absolute pressure PBA within the intake pipe 2 to the ECU 5. An intake air temperature (TA) sensor 9 is inserted into a wall portion of the intake pipe 2 downstream of the conduit 7, for supplying an electric signal indicative of the sensed intake air temperature TA to the ECU 5. An engine coolant temperature (TW) sensor 10, which is formed of a thermistor or the like, is mounted in the cylinder block of the engine 1, for supplying an electric signal indicative of the sensed engine coolant temperature TW to the ECU 5. A cylinder-discriminating (CYL) sensor 11, a TDC sensor 12, and a crank angle (CRK) sensor 13 are arranged in facing relation to a camshaft or a crankshaft of the engine 1, not shown. The cylinder-discriminating (CYL) sensor 11 generates a pulse at a predetermined crank angle of a particular cylinder of the engine, and the TDC sensor 12 generates a pulse as a TDC signal pulse at each of predetermined crank angles whenever the crankshaft rotates through 180 degrees, both the pulses being supplies to the ECU 5. The CRK sensor 13 generates a pulse as a CRK signal pulse at each of predetermined crank angles whenever the crankshaft rotates through a predetermined angle (e.g. 45 degrees) much smaller than 180 degrees, and supplies the CRK signal pulse to the ECU 5. Output signal pulses from the CYL sensor 11, the TDC sensor 12, and the CRK sensor 13 are used for controlling the timing of execution of fuel injection timing, ignition timing, etc. and for detecting the engine rotational speed NE. An oxygen concentration sensor (hereinafter referred to as "the O 2 sensor") 15 is arranged in an exhaust pipe 14 of the engine 1, for sensing the concentration of oxygen contained in the exhaust gases from the engine 1 and supplying an electric signal indicative of the sensed oxygen concentration to the ECU 5. The ECU 5 comprises an input circuit 5a having the functions of shaping the waveforms of input signals from various sensors, shifting the voltage levels of sensor output signals to a predetermined level, converting analog signals from analog-output sensors to digital signals, and so forth, a central processing unit (hereinafter referred to as "the CPU") 5b, memory means 5c having a ROM storing various operational programs which are executed in the CPU 5b, various maps and tables, etc., and a RAM for storing results of calculations therefrom, etc., and an output circuit 5d which outputs driving signals to the fuel injection valves 6, the AIC control valve 26, etc. Fig. 2 shows a program for calculating a valve opening period of the fuel injection valves 6, i.e. a fuel injection amount Tout. This program is executed upon generation of each TDC signal pulse and in synchronism therewith. At a step S1, a direct supply ratio A and a carry-off ratio B are calculated. The direct supply ratio A is defined as a ratio of a fuel amount injected into the intake pipe 2 and directly or immediately drawn into a combustion chamber to the whole fuel amount injected in a cycle, the direct supply ratio including a fuel amount carried off the inner surface of the intake pipe 2 by evaporation etc. and drawn into the combustion chamber, in the same cycle. The carry-off ratio B is defined as a ratio of a fuel amount carried off the inner surface of the intake pipe and drawn into the combustion chamber in the present cycle to the whole fuel amount which adhered to the inner surface of the intake pipe 2 in the last or immediately preceding cycle. The direct supply ratio A and the carry-off ratio B are read, respectively, from an A map and a B map set in accordance with coolant temperature TW and intake pipe absolute pressure PBA, in response to the detected TW and PBA values. The direct supply ratio A and the carry-off ratio B may be calculated by interpolation, if required. At the next step S2, correction coefficients KA and KB, which correct the direct supply ratio A and the carry-off ratio B, are calculated. The correction coefficients KA and KB are determined in response to the engine rotational speed NE, by tables shown in Fig. 4. The correction coefficients KA, KB are set such that they increase as the engine rotational speed Ne increases. The reason why the correction coefficients KA and KB are thus increased as the engine rotational speed NE increases is that the direct supply ratio A and the carry-off ratio B apparently increase as the intake air flow speed in the intake pipe increases with an increase in the engine rotational speed NE. Next, at a step S3, corrected values Ae and Be of the direct supply ratio and the carry-off ratio are calculated by the use of the following equations (1) and (2). Further, values (1 - Ae) and (1 - Be) are calculated at a step S4, followed by the program proceeding to a step S5: Ae = A x KA Be = B x KB The values Ae, (1 - Ae) and (1 - Be) thus calculated are stored into the RAM of the ECU 5 for use in executing a program shown in Fig. 3, which will be described hereinafter. At a step S5, it is determined whether or not the engine is being started. If the answer is affirmative (YES), the fuel injection amount Tout is calculated based upon a basic fuel amount Ti for use at the start of the engine, and then the program is terminated. If the answer to the question of the step S5 is negative (NO), i.e. if the engine is not being started, a required fuel amount TCYL(N) for each cylinder, which does not include an additive correction term Ttotal, referred to hereinafter, is calculated by the use of the following equation (3), at a step S7: Tcyl(N) = TiM x Ktotal(N) where (N) represents a number allotted to the cylinder for which the required fuel amount Tcyl is calculated. TiM represents a basic fuel amount to be applied when the engine is under normal operating conditions (i.e. operating conditions other than the starting condition) and is calculated in response to the rotational speed NE and the intake pipe absolute pressure PBA. Ktotal(N) represents the product of all correction coefficients (e.g. a coolant temperature-dependent correction coefficient KTW and a leaning correction coefficient KLS) which are calculated based upon engine operating parameter signals from various sensors excluding an air fuel ratio correction coefficient KO2 which is calculated based on an output signal from the O 2 sensor 15. At the following step S8, it is determined whether or not the throttle valve opening TH is lower than a predetermined value THIDL, that is, whether the engine is idling. If the answer to this question is affirmative (YES), the fuel injection period Tout is calculated by the use of the following equation (4): Tout = (Tcyl(N) + Ttotal) x KO2 + TV where Ttotal represents the sum of all additive correction terms which are calculated in response to engine operating parameters from various sensors (e.g. a fuel increasing correction term TRUN applied immediately after the start of the engine), KO2 an air-fuel ratio correction coefficient calculated in response to the output from the O2 sensor 15, and TV an ineffective time correction term dependent upon an output voltage from a battery. If the answer to the question of the step S8 is negative (NO), i.e. if the engine is not idling, a fuel amount TNET to be supplied to the combustion chamber of the cylinder into which the fuel injection is to be made in the present loop, is calculated by the use of the following equation (5): TNET = Tcyl(N) + Ttotal - Be x TWP(N) where TWP(N) represents an intake pipe-adherent fuel amount (estimated value), which is calculated by the program of Fig.3. (Be x TWP(N)) corresponds to an amount of fuel, which is evaporated from fuel adhering to the inner surface of the intake pipe 2 and carried into the combustion chamber. A fuel amount corresponding to the fuel amount carried off the intake pipe inner surface need not be injected, and therefore, is to be subtracted from the value Tcyl(N) in the equation (5). At the next step S11, it is determined whether or not the value TNET calculated by the equation (5) is larger than a value of 0. If the answer is negative (NO), i.e. if TNET ≦ 0, the fuel injection amount Tout is set to 0 at a step S12, followed by terminating the program. If the answer to the question of the step S11 is affirmative (YES), i.e. if TNET > 0, the Tout value is calculated by the use of the following equation (6): Tout = TNET(N)/Ae x KO2 + TV where KO2 is the aforementioned air-fuel ratio correction coefficient calculated in response to the output from the O 2 sensor 15. TV is the ineffective time correction term. Thus, a fuel amount corresponding to (TNET(N) x KO2 + Be x TWP(N)) is supplied to the combustion chamber by opening the fuel injection valve 6 over the time period Tout calculated by the equation (6). Fig. 3 shows the program for calculating the intake pipe-adherent fuel amount TWP(N), which is executed upon generation of each CRK signal pulse which is generated whenever the crankshaft rotates through a predetermined angle (e.g. 45 degrees). At a step S21, it is determined whether or not the present loop of execution of this program falls within a time period after the start of the calculation of the fuel injection amount Tout and before the fuel injection is completed (hereinafter referred to as the injection control period). If the answer is affirmative (YES), a first flag FCTWP(N) is set to a value of 0 at a step S32, followed by terminating the program. If the answer at the step S21 is negative (NO), i.e. if the present loop is not within the injection control period, it is determined at a step S22 whether or not the first flag FCTWP(N) is equal to 1. If the answer is affirmative (YES), that is, if FCTWP(N) = 1, the program jumps to a step S31, whereas if the answer is negative (NO), i.e. if FCTWP(N) = 0, it is determined at a step S23 whether or not the engine is under fuel cut (interruption of the fuel supply). If the engine is not under fuel cut, the intake pipe-adherent fuel amount TWP(N) is calculated at a step S24 by the use of the following equation (7), then a second flag FTWPR(N) is set to a value of 0, and the first flag FCTWP(N) is set to a value of 1 at steps S30 and S31, followed by terminating the program: TWP(N) = (1 - Be) x TWP(N)(n - 1) + (1 - Ae) x (Tout (N) - TV) (7) where TWP(N)(n - 1) represents a value of TWP(N) obtained on the last occasion, and Tout(N) an updated or new value of the fuel injection amount Tout which has been calculated by the program of Fig. 2. The first term on the right side corresponds to a fuel amount remaining on the inner surface if the intake pipe 2 without being carried into the combustion chamber, out of the fuel previously adhering to the inner surface of the intake pipe 2, and the second term on the right side corresponds to a fuel amount newly adhering to the inner surface of the intake pipe 2 out of newly injected fuel. If the answer at the step S23 is affirmative (YES), i.e. if the engine is under fuel cut, it is determined at a step S25 whether or not the second flag FTWPR(N) has been set to a value of 1. If the answer is affirmative (YES), i.e. if FTWPR(N) = 1, the program jumps to the step S31. If the answer is negative (NO), i.e. if FTWPR(N) = 0, the adherent fuel amount TWP(N) is calculated by the use of the following equation (8) at a step S26, and then the program proceeds to a step S27: TWP(N) = (1 - Be) x TWP(N)(n - 1) The equation (8) is identical with the equation (7), except that the second term on the right side is omitted. The reason for the omission is that there is no fuel newly adhering to the intake pipe inner surface, due to fuel cut. At the step 27, it is determined whether or not the calculated TWP(N) value is larger than a very small predetermined value TWPLG. If the answer is affirmative (YES), i.e. if TWP(N) > TWPLG, the program proceeds to the next step S30. If the answer is negative (NO), i.e. if TWA (N) ≦ TWPLG, the TWP(N) value is set to a value of 0 at a step S28, and then the second flag FTWPR(N) is set to a value of 1 at a step S29, followed by the program proceeding to the step S31. According to the program of Fig. 3 described above, the intake pipe-adherent fuel amount TWP(N) can be accurately calculated. Moreover, an appropriate fuel amount can be supplied to the combustion chamber of each cylinder, which reflects the fuel amount adhering to the inner surface of the intake pipe 2 as well as the fuel amount carried off the adherent amount. Further, according to the present embodiment, when the engine is idling, the correction of the supply fuel amount dependent upon the adherent fuel amount and the carried-off fuel amount is inhibited, to thereby prevent variations in the supply fuel amount due to fluctuations in the intake pipe absolute pressure PBA and hence secure a stable idling condition of the engine.

Claims: A control system for an internal combustion engine (1) having at least one combustion chamber, and an intake passage (2) having an inner surface, comprising operating condition detecting means (8,9) for detecting operating conditions of said engine (1), supply fuel amount determining means responsive to operating conditions of said engine (1) detected by said operating condition detecting means (8,9), for determining an amount of fuel to be supplied to said engine (1), adherent fuel amount estimating means for estimating an amount of fuel adhering to said inner surface of said intake passage (2), carried off fuel amount estimating means for estimating an amount of fuel carried off from fuel adhering to said inner surface of said intake passage (2) and carried into said combustion chamber, supply fuel amount correction means for correcting said supply fuel amount determined by said supply fuel amount determining means, based upon operating conditions of said engine (1) detected by said operating condition detecting means (8,9), and said adherent fuel amount estimated by said adherent fuel amount estimating means and said carried-off fuel amount estimated by said carried-off fuel amount estimating means, and fuel supply means for supplying said supply fuel amount corrected by said supply fuel amount correcting means into said intake passage (2); characterised in that the control system further comprises correction limiting means for limiting said correction of said supply fuel amount based upon said estimated adherent amount and said estimated carried-off fuel amount by said supply fuel amount correcting means, when said operating condition detecting means detects that said engine (1) is operating in an idling condition. A control system as claimed in claim 1, wherein said adherent fuel amount estimating means and said carried-off fuel amount estimating means estimate said adherent fuel amount and said carried-off fuel amount, respectively, based upon operating parameters of said engine (1) including pressure within said intake passage (2). A control system as claimed in claim 1 or 2, wherein said correction limiting means inhibits said correction of said supply fuel amount based upon said estimated adherent amount and said estimated carried-off fuel amount by said supply fuel amount correcting means. A control system as claimed in any preceding claim, wherein said adherent fuel amount estimating means continues said estimation of said adherent fuel amount even when said engine (1) is detected to be operated in said idling condition.

Inventors: MACHIDA KEI, OGAWA KEN, MACHIDA, KEI, OGAWA, KEN, Machida, Kei, c/o Honda R &amp; D Co., Ltd., Ogawa, Ken, c/o Honda R &amp; D Co., Ltd.
Applicants: HONDA MOTOR CO LTD, HONDA GIKEN KOGYO KABUSHIKI KAISHA
Application year: 1993
Main classification: F01L1
IPC codes: F01L1, F02D13, F02D41