FUEL INJECTION SYSTEM AND CONTROL METHOD THEREFOR

Abstract: A fuel injection system and a control method therefor in which the opening time of a fuel injection valve is controlled by valve opening time determining means (l4) on the basis of a prestored program and in accordance with various operation parameters such as the intake air flow rate (air flow sensor 7), engine speed (engine speed sensor 8) and engine temperature (engine temperature sensor l0). The system has a means (l6) for detecting at least one of the offset of the engine speed from a command speed and the engine speed variation per unit time, the data being used together with the above-mentioned operation parameters in controlling the opening time of the fuel injection value (3).

Description: BACKGROUND OF THE INVENTION The present invention broadly relates to a fuel injection system and a control method therefor, suitable for use in an automotive gasoline engine which is specifically required to operate stably at low speed. Automotive gasoline engines sometimes experience unstable operation when the engine speed is lowered by a release of the accelerator pedal, or when idling. In order to overcome this problem, hitherto, it has been proposed to effect, when the engine speed is lowered, a rich compensation in response to an idle signal, as in Japanese Patent Laid-Open Nos. 23ll44/l984 and 30446/85. Such proposed methods, however, do not contribute to improvement in the operation characteristics after the steady engine operation is achieved. SUMMARY OF THE INVENTION Accordingly, an object of the invention is to provide a fuel injection system and a control method therefor which can ensure a stable engine operation at low speed by elimination of engine speed variation and surging, thereby overcoming the above-described problems of the prior art. To this end, according to the invention, there is provided a fuel injection system and a control method therefor in which the opening time of a fuel injection value is controlled on the basis of a predetermined program and in accordance with various operation parameters such as the intake air flow rate, engine speed and engine temperature. The system has a means for detecting at least one of the offset of the engine speed from a command speed and the engine speed variation per unit time, the data being used together with the above­ mentioned parameters in controlling the opening time of the fuel injection valve BRIEF DESCRIPTION OF THE DRAWINGS Fig. l is a flow chart illustrating the operation of an embodiment of the fuel injection system in accor­ dance with the invention; Fig. 2 is a block diagram of an engine system to which the invention is applied; Fig. 3 is a block diagram of an example of a control unit; Fig. 4 is an illustration of the operation charac­ teristics; Fig. 5 is an illustration of an example of a map table; Fig. 6 is an illustration of a practical example of the map table; Figs. 7, 8 and 9 are illustrations of problems encountered in the conventional arts; and Fig. l0 is a flow chart of another embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the invention will be described hereinunder with reference to the accompany­ ing drawings. Fig. 7 shows air-fuel ratio to torque characteris­ tic as observed in ordinary engines. As will be seen from this Figure, the change in the torque is minimized when the air-fuel ratio is around l3. Actually, however, the air-fuel ratio is set on the leaner side, e.g., l4.7 or greater, in order to meet various requirements such as fuel economization and cleaning of exhaust gas. In consequence, the torque is largely changed even by a slight change in the air-fuel ratio, resulting in an unstable engine operation. Fig. 8 shows an example of speed variation encoun­ tered by a conventional engine. It will be seen that a speed offset ΔN and speed variation dN/dt are caused despite that the engine is controlled to operate at a command speed Nset. It will be understood that the speed offset ΔN and the speed variation dN/dt are minimized in engines which operate stably. To explain in more detail with reference to Fig. 9, in the low-speed engine operation to which the present invention pertains, the throttle valve is fully closed so that the intake air flow rate can be regarded as being materially constant, although the air flow rate through an idle speed control valve detouring the throttle valve is changed. Since air flow rate Qa is substantially constant, the valve opening time of the fuel injection valve, expressed by Tp = Qa/N, is determined in inverse proportion to the engine speed N. To explain about the combustion in the engine, the fuel injected in the suction stroke produces the torque in the explosion stroke which is two strokes after the suction stroke. This means that the informa­ tion signal concerning the combustion control lags by a time length corresponding to two engine strokes. Namely, the fuel is actually injected at a rate (Tp)c, when a piece of injection information Tp is given. In consequence, an error corresponding to the valve opening time ΔTp is caused in the fuel supply rate, with a result that the air-fuel ratio A/F is offset correspondingly, leading to the change in torque as illustrated in Fig. 7. According to the invention, the air-fuel ratio A/F is changed in such a manner as to suppress the error ΔTp in the valve opening time. An embodiment of the fuel injection system of the invention will be described in more detail with reference to the drawings. Fig. 2 shows an example of an engine system to which an embodiment of the invention is applied. Referring to this Figure, an engine l is equipped with a plurality of injectors 3 provided on respective intake branch pipes 2. The number of the injectors corresponds to the number of the cylinders of the engine. The intake branch pipes 2 merge at their upstream ends in a common collector 4 which is disposed downstream of a throttle valve 5 for controlling the rate of flow of intake air to the engine. At the same time, an ISC valve 6 for controlling the engine speed is provided in a passage which by-­ passes the throttle valve 5. When the throttle valve is in the fully closed state, the speed of the engine l is controlled by this ISC valve 6. On the other hand, the intake air flow rate of the engine l is detected by an air flow sensor 7 which is disposed upstream of the throttle valve 5, while the engine speed is detected by an engine speed sensor 8. A control unit 9 receives. besides the intake air flow rate signal and the engine speed signal, other various signals such as signals from an engine temperature sensor l0, exhaust gas sensor ll, and so forth. The supply of the fuel to the engine l is conducted by the opening and closing action of the fuel injector 3 to which the fuel is supplied after pressurizing and pressure regulation by a fuel pump l2 and a fuel pressure regulator l3. Fig. 3 is a block diagram of a portion of the control unit 9 for controlling the fuel injector 3. This portion has a valve open time determining means l4 which receives operation parameter signals from various sensors such as the air flow sensor 7, engine speed sensor 8, engine temperature sensor l0, exhaust gas sensor ll, and so forth. The engine speed signal from the engine speed sensor 8, corresponding to the actual engine speed, is delivered to a speed change detecting means l6 which is adapted to detect either oen of the offset of the actual engine speed from the command speed set by a command speed setting means l5 and the varia­ tion of the engine speed per unit time. The data derived from the speed change detecting means l6 is delivered to a correction component generating means l7 which in turn is converted into a component for correcting the opening time of the fuel injector 3, as one of the operation parameters for the operation of the valve opening time determining means l4. The operation of this embodiment will be described hereinunder. In this embodiment, in view of the fact that the variation in the engine speed N and the variation in the air-fuel ratio A/F has a certain correlation, the air-fuel ratio A/F is changed in accordance with a change in the values of the speed offset ΔN and the speed variation dN/dt. That is, the final valve opening time Ti of the injector 3 is determined in accordance with the following formula. Ti = Tp (l + K₁ + K₂ + ... K₃ + K tp ) + Ts (l) In this formula, Tp represents the basic valve open time which is determined by Qa/N, while K₁, K₂ and K₃ are correction coefficients determined in accor­ dance with the engine temperature. Ts represents a coefficient which is used for the purpose of compen­ sation for the delay in the opening of the fuel injector 3. The coefficient K tp is the one which constitutes one of the features in accordance with the invention. A description will be made hereinunder as to the relationship between the air-fuel ratio A/F and the speed offset ΔN from the command engine speed Nset and the engine speed variation dN/dt. During idling and low-speed engine operation, the throttle valve 5 is closed almost fully, so that the intake air flow rate is maintained substantially constant. In this state, there is no reason for any change in the engine speed. Actually, however, a speed variation is inevitably caused by any disturbance, such as a change in the air-fuel ratio. The change in the engine speed can be sorted into two types: namely, static one and dynamic one. The static change appears as the offset Δ N of the mean speed with respect to the command speed Nset. Usually, the offset ΔN is proportional to the air-­ fuel ratio A/F. That is, the richer the air-fuel mixture, the greater the value of the speed offset ΔN. This relationship will be clearly understood from Fig. 4a. On the other hand, the speed variation dN/dt is a dynamic speed change. When the value of this dynamic speed change becomes greater, the driver will feel the occurrence of surging. Both the speed offset ΔN and the speed variation dN/dt are detected by the speed change detecting means l6. In order to improve the drivability, it is necessary that the speed variation dN/dt is reduced. As explained before in connection with Fig. 9, the relationship between the speed varia­ tion dN/dt and the air-fuel ratio A/F is not a simple proportional relationship but the relationship is such that the dN/dt is largely changed even by a small change in the air-fuel ratio A/F. According to the invention, therefore, the correction coefficient K tp is given from the correction component generating means l7 in such a manner as to negate the change, in accordance with Fig. 4. More practically, this correction is effected by executing a process as shown in Fig. l, by a CPU of the control unit 9, by making use of a map table as shown in Fig. 5. The map table shown in Fig. 5 determines the coefficient K tp using the speed offset ΔN and the speed variation dN/dt as variables. Referring back to Fig. l, the pieces of data N and Qa are picked up in Step Sl and, in Step S2, a judgement as to whether the ISC (Idle Speed Control) is conducted. If the answer is YES, the process proceeds to Step S3 in which the data ΔN and the data dN/dt are determined and, in Step S4, the data K tp is determined through a search over the map table. Then, the valve open time Ti is computed in the process in step S5 and, in Step S6, a signal representing the valve open time Ti is delivered to the injector 3, thereby completing the process. On the other hand, when the answer to the inquiry in Step S2 is NO, i.e., when ISC is not conducted, the process directly proceeds to Step S6 in which the above-described operation is conducted to obtain the output data Ti. Fig. 6 shows an example of the data content shown in Table, as obtained through a test conducted using an automobile having a 2,000 cc engine. It will be seen that the speed offset ΔN is +84 rpm (2%) while the speed variation dN/dt is +84 rpm/40 mS (-0.07%). The use of this Table enables, even when a surging, i.e., a large speed variation dN/dt, is caused, a correction is effected by using the coefficient K tp , so that the engine operation. is converged towards the state of dN/dtk = 0 and ΔN = 0, whereby the surging is suppressed sufficiently. Although the embodiment has been described with reference to the case where the engine system has an ISC function, i.e., the case of an engine system which operates in accordance with a command speed Nset, this is not exclusive and the invention may be applied to the case where the data ΔN is not available. In such a case, the searching over the map table can be conducted solely by means of the data dN/dt. Such a modification will be explained with reference to a flow chart shown in Fig. l0. Pieces of data N and Qa are picked up in Step Sl, and, in Step S3, the speed variation dN/dt is determined. In a subsequent Step S4, a search over the map is conducted to determine the data K tp . Unlike the map shown in Fig. 5 which makes use of both the speed variation dN/dt and speed offset ΔN, the map used in this modification makes use of the speed variation dN/dt as a sole variable. Then, the valve open time Ti is computed in Step S5, and the signal representing the valve open time Ti is outputted to the injector 3, thereby completing the processing. As has been described, according to the invention, the air-fuel ratio is controlled in accordance with the speed offset and the seed variation, so as to enable the control of the engine speed such that the speed converges to the level of the command speed. It is thus possible to avoid unfavourable operating conditions such as surging and others, thus enabling superior drivability.

Claims: 1. A control method for a fuel injection system having a valve opening time determining means which determines the opening time of a fuel injection valve in accordance with operation parameters which include intake air flow rate, engine speed and engine temperature, comprising the following steps: detecting at least one of the data concerning the offset of the actual rotational speed of the engine from a command speed and the data concerning the rotational speed variation per unit time of the engine and generating a component for correcting the open time in accordance with the data, said component being supplied to said valve opening time determining means as one of the operation parameters. 2. A control method according to claim l, wherein the value of said correction component is increased as the amounts of said speed offset or said speed variation are increased. 3. A fuel injection system having a valve opening time determining means which determines the opening time of a fuel injection valve in accordance with operation parameters which include intake air flow rate,-engine speed and engine temperature, said fuel injection system comprising: a speed change detecting means capable of detecting at least one of the data concerning the offset of the actual rotational speed of the engine from a command speed and the data concerning the rotational speed variation per unit time of the engine; and a correction component generating means for generating a component for correcting the open time in accordance with the data, said component being supplied to said valve opening time determining means as one of the operation parameters. 4. A fuel injection system according to claim 3, wherein the value of said correction component is increased as the amounts of said speed offset or said speed variation are increased.

Inventors: ATAGO, TAKESHI, MANAKA, TOSHIO, MOURI, YASUNORI
Applicants: HITACHI, LTD.
Application year: 1986
Main classification: F02D41
IPC codes: F02D41, F02D31, F02B1