Fuel Control Apparatus for Engine

This invention relates to a fuel control apparatus for an engine in a vehicle for burning fuel at an optimum air-fuel ratio.

FIG. 5 shows a prior-art fuel control apparatus for an engine. In FIG. 5, numeral 1 designates an engine, numeral 2 an intake manifold, numeral 3 a fuel injection valve mounted in the intake manifold 2 to surround the vicinity of the intake port of the engine 1, numeral 4 a surge tank of intake air pressure provided between the intake manifold 2 and an intake conduit 5, numeral 6 a throttle valve provided in the intake conduit 5, and numeral 7 an air flow sensor provided near the upstream end of the intake conduit 5 and provided, for example to be disposed in a ring-shaped air duct. The air flow sensor 7 is an air flow rate measuring instrument for measuring, on the basis of a heat dissipating principle, the weight, the temperature and the density of the intake air and provides the same as output data. Numeral 8 indicates a controller which calculates and decides the optimum fuel injection amount in accordance with the output of a rotary sensor 9 for detecting the rotating speed of the engine 1 and the output of the air flow sensor 7. The controller 8 generates a signal having a pulse width corresponding to the optimum fuel injection amount so as to operate the fuel injection valve 3 in accordance therewith.

The controller 8 comprises, as shown in FIG. 6, of a computer. More specifically, numeral 81 designates an analog/digital converter (hereinafter referred to as "an A/D converter") for converting the analog output of the air flow sensor 7 into a digital signal for calculation processing, numeral 82 an interface circuit for inputting the digital output of the rotary sensor 9, numeral 83 a microprocessor (hereinafter referred to as "a CPU") for calculating an optimum fuel supply amount in accordance with the outputs of the A/D converter 81 and the interface circuit 82, numeral 84 a memory (hereinafter referred to as "a RAM") for temporarily storing various data (including the abovementioned outputs) used at the calculating time, numeral 85 a memory (hereinafter referred to as "a ROM") for storing data such as calculating sequence, and numeral 86 an amplifier for amplifying a fuel supply amount signal output from the microprocessor 83. Next, the operation will be described.

When the engine 1 is operated in any operating state except the vicinity of full open (WOT) of the throttle valve 6, the output from the air flow sensor 7 becomes a waveform which includes a normal ripple as shown by a curve (a) in FIG. 7. When the area covered by the waveform is calculated, the true intake air weight can be obtained. Thus, when the microprocessor 83 controls the drive pulse width of the fuel injection valve 3 in accordance with the value produced by dividing the intake air amount by the rotating speed of the engine, it can provide a desired air-fuel ratio.

However, in an engine having less than four cylinders, the output waveform of the air flow sensor 7 becomes as shown by a curve (b) in FIG. 7 due to the reverse-flow from the engine 1 in the special rotating speed range (generally in a range of 1000 to 3000 r.p.m.) near the WOT, and the area indicated by the hatched portion is excessively added to the true intake air weight.

This is due to the fact that the hot-wire type air flow sensor 7 detects and outputs as the intake air amount a value irrespective of the air flowing direction.

The detecting error of the sensor 7 caused by the reverse-flow depends, as shown in FIG. 8, upon the rotating speed of the engine, and normally occurs from when the vacuum in the intake conduit is near -50 mmHg and arrives at 50% of the maximum in the WOT range.

When the fuel supply amount is calculated and injected with respect to a value which contains such a large error, the air-fuel ratio becomes very rich, the combustion in the engine becomes unstable, thereby becoming practically impossible to use. Heretofore, as shown in FIG. 9, the upper limit value (designated by a broken line) is set in the maximum air amount determined for the engine in the area a that the error occurs by reason of the reverse-flow, and stored in the ROM 85, and the detected value of the air flow sensor 7 exceeding this limit value is clipped by the upper limit value as shown by (b) in FIG. 7, thereby suppressing the excessively dense air-fuel ratio.

Since the prior-art fuel control apparatus for the engine is composed as described above, the upper limit value of the intake air amount must be set to match the intake air amount characteristic of the engine to be countermeasured at ambient temperature, and the upper limit value must become the upper limit of the mass flow rate at the ambient temperature.

However, if the engine is operated, for example, with a high load in the state that the intake air temperature is high, the output level of the air flow sensor 7 does not reach the average value at the predetermined upper limit value as shown by (c) in FIG. 7 due to the reduction in the air density. Thus, the average value of the output level which contains the reverse-flow is used in the calculation of fuel as it is, with the result that the air-fuel ratio is shifted to the rich side. On the other hand, when the temperature of the intake air is low, the air density increases. Thus, the actual intake air amount of the engine is increased to become larger than the upper limit value as shown by (d) in FIG. 7, and the air fuel ratio is shifted to the lean side. Therefore, the air-fuel ratio varies with respect to the intake air temperature as shown in FIG. 10. In other words, when the upper limit value of the intake air amount is determined by the engine near the ambient temperature, there arises a problem that the error of the air-fuel ratio increases with the increase in atmospheric temperatures.

This invention has the objective of overcoming the disadvantage of the prior-art fuel control apparatus as described above, and has for its main object to provide a fuel control apparatus for an engine in which an error of an air-fuel ratio due to the intake air temperature is removed to obtain a stable combustion state for all operating conditions of the engine.

In a fuel control apparatus for an engine according to this invention, correction value data is obtained by calculation for cancelling the error of the air-fuel ratios due to differences in the intake air temperatures corresponding to the atmospheric temperatures, and a fuel injection amount from the fuel injection valve is determined by a microprocessor based upon the output data considered with the correction value data and the fuel injection amount is controlled accordingly. Thus, a stable constant air-fuel ratio can always be obtained from the fuel injection valve irrespective of the temperature of the intake air, the combustion of mixture gas can be stabilized, and the output of the engine can also be stabilized.

FIG. 1 is a schematic view of the construction of a fuel control apparatus for an engine according to an embodiment of this invention;

FIG. 2 is a block circuit diagram showing the essential portion of the control apparatus;

FIG. 3 is a graphical diagram showing a temperature correction used in this invention;

FIG. 4 is a flowchart showing the calculating process by a microprocessor;

FIG. 5 is a schematic view of the construction of a prior-art fuel supply controller;

FIG. 6 is a block circuit diagram of the controller in FIG. 5;

FIG. 7 is a graphical diagram of an air flow sensor;

FIG. 8 is a graphical diagram of the detecting error of the air flow sensor;

FIG. 9 is a graphical diagram of the output of the air flow sensor versus the rotating speed of the engine; and

FIG. 10 is a graphical diagram of the error of the air-fuel ratio.

An embodiment of this invention will be described with reference to the drawings. In FIG. 1, numeral 10 designates a temperature sensor for detecting the temperature of intake air, which is formed, for example, of a thermistor which provides a variation in the resistance value thereof in response to the temperature of the intake air, and is provided in the intake conduit 5. The temperature sensor 10 provides detected temperature data of the intake air to the controller 8. Other elements are equivalent to those shown in FIG. 5, and the corresponding parts are denoted by the same symbols, and will not be repeatedly explained.

When the engine 1 is operated, the intake air is fed through an air cleaner and the intake conduit 5 into the intake manifold 2, and fuel injection valves 3 provided in the intake manifolds 2 of the respective cylinders inject fuel at a predetermined timing to feed mixture gas of preset air-fuel ratio into the combustion chambers of the respective cylinders. The temperature of the intake air is detected by the temperature sensor 10, the output of which is input to the A/D converter 81 in the controller 8, which converts it into a digital signal, which is, in turn, input to the microprocessor 83.

The calculating process to be executed by the microprocessor 83 will be described by using the temperature data detected of the intake air as described above in accordance with the flowchart of FIG. 4.

The air flow sensor 7 first reads out the intake air amount Qa in step 100, and the temperature sensor 10 then reads out the temperature AT of the intake air in step 101. Then, the intake air temperature correction coefficient (CAT) in FIG. 3 set in advance in the memory is multiplied by the clipped value Q.sub.N (CLIP) of the intake air amount determined in response to the rotating speed of the engine at the ambient temperature to obtain a corrected clipped value Qc in step 102. Subsequently, whether the measured intake air amount Qa is larger than the corrected clipped value Qc or not is judged in step 103. In case of Qa.ltoreq.Qc, Q=Qa is set in step 104, and in case of Qa>Qc, Q=Qc is set in step 105. Then, the rotary sensor 9 reads out the rotating speed Ne in step 106, and the ratio Q/Ne is calculated to determine the pulse width corresponding to the optimum fuel injection amount to be utilized by the fuel injection valve 3 in step 107.

Since the upper limit value of the intake air amount is always corrected by the intake air temperature AT by the abovementioned calculating process, the error of the air-fuel ratio due to the difference of the temperature of the intake air in the operating range near the full open state of the throttle valve 6 can be eliminated to stably burn the mixture gas and to perform the stable operation of the engine.

According to this invention as described above, a temperature sensor for detecting the temperature of the intake air of the engine is provided to correct the upper limit value of the intake air amount by the output of the temperature sensor in the operating range of the engine where the air flow sensor does not exhibit the true value of the intake air amount. Therefore, a stable air-fuel ratio can be provided irrespective of the temperature of the intake air, the formation of a stable gas mixture and a stable combustion state of the engine can be provided.