JP2003065141A - Air-fuel ratio detecting system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect an air fuel ratio without using an oxygen sensor. SOLUTION: A three dimensional map is provided for each air fuel ratio having speed mean effective pressure and fluctuation amount thereof filled in lattice points of engine speed and intake pipe pressure. The air fuel ratio of current speed mean effective pressure and fluctuation amount thereof is calculated by linear interpolation of speed mean effective pressure or fluctuation value air fuel ratio given by three dimensional interpolation. An average value when engine speed and intake pipe pressure continuously stay in the same lattice point area for a designated time is used as the speed mean effective pressure and standard deviation thereof is used as the fluctuation amount. An air fuel ratio calculated from standard deviation of speed mean effective pressure is defined as a reference air fuel ration, and an air fuel ratio calculated from average value is defined as a comparison air fuel ratio. When a difference between both values is, for example 0.5 or less, the more correlative reference air fuel ratio calculated form standard deviation is selected as the final air fuel ratio.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio detecting device for detecting the air-fuel ratio of an air-fuel mixture supplied to an engine.

[0002]

2. Description of the Related Art In recent years, with the widespread use of injectors called injectors, it has become easier to control the timing of fuel injection, the amount of injected fuel, that is, the air-fuel ratio, etc., to achieve high output, low fuel consumption, and clean exhaust gas. It has become possible to promote such changes. Of these, particularly with respect to the timing of injecting fuel, strictly speaking, it is common to detect the state of the intake valve, that is, generally the phase state of the camshaft, and inject fuel accordingly.
However, a so-called cam sensor for detecting the phase state of the camshaft is expensive, and cannot be used in many cases, especially in a two-wheeled vehicle because of a problem such as an increase in the size of the cylinder head. Therefore, for example, Japanese Patent Laid-Open No. 10-22
Japanese Patent No. 7252 discloses an engine control device that detects a phase state of a crankshaft and an intake pipe pressure, and detects a stroke state of a cylinder from them. Therefore, by using this conventional technique, it is possible to detect the stroke state without detecting the phase of the camshaft, and it is possible to control the fuel injection timing and the like in accordance with the stroke state.

[0003]

By the way, in order to clean exhaust gas, it is necessary to detect the air-fuel ratio.
For that purpose, an expensive oxygen sensor is required, which is a barrier to cost reduction. The present invention was developed to solve the above problems, and an object of the present invention is to provide an air-fuel ratio detection device capable of detecting an air-fuel ratio without using an oxygen sensor.

[0004]

In order to solve the above-mentioned problems, an air-fuel ratio detecting device according to a first aspect of the present invention comprises an engine rotation state detecting means for detecting at least the rotation state of an engine in an explosion stroke, and In the steady state of the average effective pressure calculation means for calculating the average effective pressure of the engine for each cycle from the engine rotation state detected by the engine rotation state detection means, and the average effective pressure calculated by the average effective pressure calculation means An air-fuel ratio calculating means for calculating an air-fuel ratio based on an average value and a deviation of the average effective pressure in a steady state is provided.

Further, in the air-fuel ratio detecting device according to a second aspect of the present invention, in the invention of the first aspect, the air-fuel ratio calculating means is a steady state of the average effective pressure calculated by the average effective pressure calculating means. When the difference between the air-fuel ratio calculated from the average value in the state and the air-fuel ratio calculated from the deviation in the steady state of the average effective pressure is within a predetermined value, it was calculated from the deviation in the steady state of the average effective pressure. It is characterized by selecting the air-fuel ratio.

The air-fuel ratio detecting device according to a third aspect of the present invention is the air-fuel ratio detecting device according to the first or second aspect of the invention, further comprising intake pipe pressure detecting means for detecting the pressure in the intake pipe. Is a search map for the air-fuel ratio, the engine speed, and the intake pipe pressure obtained in advance, using the engine rotation state detected by the engine rotation state detection means and the intake pipe pressure detected by the intake pipe pressure detection means. It is characterized by calculating the air-fuel ratio.

According to a fourth aspect of the present invention, in the air-fuel ratio detecting apparatus according to the third aspect, the air-fuel ratio calculating means is the engine rotation state detected by the engine rotation state detecting means. And the intake pipe pressure detected by the intake pipe pressure detecting means is in a steady state when the intake pipe pressure is within a preset range continuously for a predetermined time.

The air-fuel ratio detecting device according to a fifth aspect of the present invention is the air-fuel ratio detecting device according to any one of the first to fourth aspects of the invention, wherein the air-fuel ratio calculating means uses a standard deviation as a deviation of the average effective pressure. It is characterized by being used.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. FIG. 1 is a schematic configuration showing an example of an engine for a motorcycle and its control device, for example. The engine 1 is a 4-cylinder 4-cycle engine, and includes a cylinder body 2, a crankshaft 3, a piston 4, a connecting rod 14, a combustion chamber 5, an intake pipe 6, an intake valve 7, an exhaust pipe 8, an exhaust valve 9, and a spark plug. 10 and an ignition coil 11 are provided. In addition, a throttle valve 12 that is opened and closed according to the accelerator opening is provided in the intake pipe 6, and the intake pipe 6 downstream of the throttle valve 12 is provided.
In addition, an injector 13 as a fuel injection device is provided. The injector 13 is connected to the filter 18, the fuel pump 17, and the regulator 16 arranged in the fuel tank 19. The regulator 16 regulates the upper limit of the fuel pressure by the fuel pump 17. When the regulator 16 is installed in the fuel tank 19 as described above, the atmospheric pressure is used as the back pressure and is preset from there. The regulator control pressure is set to rise.

The operating state of the engine 1 is controlled by the engine control unit 15. And
As means for detecting the control input of the engine control unit 15, that is, the operating state of the engine 1, a crank angle sensor 20 for detecting the rotation angle of the crankshaft 3, that is, a phase, and an intake pipe pressure in the intake pipe 6 are detected. An intake pipe pressure sensor 24 is provided for this purpose. Then, the engine control unit 15
Inputs the detection signals of these sensors and outputs a control signal to the injector 13.

The engine control unit 15
Is composed of a microcomputer (not shown). FIG. 2 is a block diagram showing an engine control calculation process performed by the microcomputer in the engine control unit 15. In this calculation process, an engine speed calculator 26 that calculates the engine speed from the crank angle signal, an average effective pressure calculator 27 that also calculates the average effective pressure of the engine from the crank angle signal, and an average effective pressure calculator. An air-fuel ratio calculation unit 28 that calculates an air-fuel ratio based on the average effective pressure calculated by the unit 27, the engine speed calculated by the engine speed calculation unit 26, and the intake pipe pressure signal, and the engine speed calculation A target air-fuel ratio calculation unit 33 that calculates a target air-fuel ratio from the engine speed calculated by the unit 26 and the intake pipe pressure signal, and a target air-fuel ratio calculated by the target air-fuel ratio calculation unit 33 and the engine speed calculation A CMA described later from the engine speed calculated by the unit 26, the air-fuel ratio calculated by the air-fuel ratio calculation unit 28, and the intake pipe pressure signal.
A learning amount calculation unit 29 that calculates the learning amount of C, and the learning amount calculated by the learning amount calculation unit 29 are used for CMAC, and the engine speed and the intake pipe pressure calculated by the engine speed calculation unit 26 are used. The intake air amount calculation unit 31 that calculates the intake air amount from the signal, the target air-fuel ratio calculated by the target air-fuel ratio calculation unit 33, and the fuel injection amount from the intake air amount calculated by the intake air amount calculation unit 31 In the fuel injection amount calculation unit 34 for calculating, the fuel injection time calculation unit 44 for calculating the fuel injection time and its timing from the fuel injection amount calculated by the fuel injection amount calculation unit 34, and the fuel injection time calculation unit 44. An injection pulse output unit 30 that outputs an injection pulse to the injector 13 based on the calculated fuel injection time and the crank angle signal.

First, the principle of detecting the stroke state of the engine from the crank angle signal will be described. As in the stroke determining device described in Japanese Unexamined Patent Publication No. 10-227252 described above, in a four-cycle engine, the crankshaft and the camshaft are constantly rotating with a predetermined phase difference, and therefore, for example, as shown in FIG. Thus, when the crank pulse is being read, the crank pulse shown by "4" is either in the exhaust stroke or the compression stroke. As is well known, in the exhaust stroke, the exhaust valve is closed and the intake valve is closed, so the intake pipe pressure is high, and at the beginning of the compression stroke, the intake pipe pressure is low because the intake valve is still open, or the intake valve pressure is low. Even if is closed, the intake pipe pressure is low in the preceding intake stroke. Therefore, when the intake pipe pressure is low, the crank pulse indicated by "4" indicates that the second cylinder is in the compression stroke, and when the crank pulse indicated by "3" is obtained, the intake pulse of the second cylinder is lower. It becomes a dead point. In this way, if the stroke state of any of the cylinders can be detected, each cylinder is rotating with a predetermined phase difference, so that, for example, the crank pulse of "3" in the figure, which is the intake bottom dead center of the second cylinder, is detected. The next crank pulse of "9" in the figure is the intake bottom dead center of the first cylinder, the next crank pulse of "3" is the intake bottom dead center of the third cylinder, and the next "9" in the figure.
Crank pulse of is the intake bottom dead center of the fourth cylinder. By interpolating the crankshaft rotation speed during this stroke, the current stroke state can be detected more finely.

The engine speed calculator 26 calculates the rotation speed of the crankshaft, which is the output shaft of the engine, as the engine speed from the time change rate of the crank angle signal. The intake air amount calculation unit 31 calculates the cylinder air mass from the intake pipe pressure signal and the engine speed calculated by the engine speed calculation unit 26.
As is well known, since the air mass in the cylinder is uniquely obtained from the engine speed and the intake pipe pressure, it can be calculated, for example, by a map search created by a relatively simple experiment. Correction may be necessary depending on the operating conditions. Therefore, in this embodiment, a learning arithmetic device, CMAC (Cerebller Model Arithmetic) is used.
Computer) while calculating the air mass in the cylinder. CMAC expresses a high-order vector by a low-order vector by mapping, thereby performing a learning by replacing a complicated operation with a plurality of simple operations.
It is possible to suppress complication of processing due to multidimensional input and increase in the number of intermediate layers. Of course, it is possible to use another learning arithmetic device instead of CMAC.

As shown in FIG. 4, the target air-fuel ratio calculating unit 33 is a three-dimensional unit for calculating a target air-fuel ratio from the intake pipe pressure signal and the engine speed calculated by the engine speed calculating unit 26. It has a map. This three-dimensional map can be set to some extent on the desk. The air-fuel ratio generally correlates with the torque, and when the air-fuel ratio is small, that is, the amount of fuel is large and the amount of air is small, the torque increases while the efficiency decreases. On the contrary, when the air-fuel ratio is large, that is, the amount of fuel is small and the amount of air is large, the torque decreases but the efficiency improves. The state where the air-fuel ratio is small is called rich, and the state where the air-fuel ratio is large is called lean. The leanest state is called the so-called ideal air-fuel ratio or stoichiometric, and the air-fuel ratio at which gasoline completely burns, that is, 14.
7

The engine speed is the operating state of the engine. Generally, the air-fuel ratio is increased on the high rotation side and decreased on the low rotation side. This is because the torque response is enhanced on the low rotation side and the rotation state response is enhanced on the high rotation side.
Further, the intake pipe pressure is an engine load state such as a throttle opening.In general, when the engine load is large, that is, when the throttle opening is large and the intake pipe pressure is large, the air-fuel ratio is reduced and the engine load is small. That is, the air-fuel ratio is increased when the throttle opening is small and the intake pipe pressure is also small. This is because the torque is emphasized when the engine load is large, and the efficiency is emphasized when the engine load is small.

As described above, the target air-fuel ratio is a numerical value whose physical meaning can be easily understood, and therefore, the target air-fuel ratio can be set to some extent in accordance with the required output characteristics of the engine. Of course, it goes without saying that tuning may be performed according to the engine output characteristics of the actual vehicle. Further, the target air-fuel ratio calculation unit 33 detects a transitional period of the engine operating state from the intake pipe pressure signal, specifically, an acceleration state or a deceleration state, and corrects the target air-fuel ratio accordingly. The unit 32 is provided. For example, as shown in FIG. 5, since the intake pipe pressure is also the result of the throttle operation, when the intake pipe pressure becomes large, it is understood that the throttle is opened and acceleration is required, that is, the acceleration state. . When such an acceleration state is detected, for example, the target air-fuel ratio is temporarily set to the rich side, and then the original target air-fuel ratio is restored. As a method of returning to the target air-fuel ratio, an existing method such as gradually changing the weighting coefficient of the weighted average of the air-fuel ratio set on the rich side in the transition period and the original target air-fuel ratio can be used. On the contrary, when the deceleration state is detected, it may be set to be leaner than the original target air-fuel ratio, and the efficiency may be emphasized.

The fuel injection amount calculation unit 34 divides the cylinder air mass calculated by the intake air amount calculation unit 31 by the target air-fuel ratio calculated by the target air-fuel ratio calculation unit 33, and uses it for the cylinder internal writing. The mass of fuel is calculated. The fuel injection time calculation unit 44 adds the injector 13 to the required fuel mass in the cylinder calculated by the fuel injection amount calculation unit 34.
The fuel injection time is calculated by multiplying the flow rate characteristic of the above, and the fuel injection timing is calculated from this.

Then, the injection pulse output section 30 sets the fuel injection start timing from the crank angle signal and outputs an injection pulse to the injector 13 based on the fuel injection time calculated by the fuel injection time calculation section 44. . On the other hand, the average effective pressure calculating unit 27 calculates the in-cylinder average effective pressure as follows. First, the torque T required to rotate an object having an inertia moment I is
It is expressed by a formula.

[0019]

[Equation 1]

The work P when rotating at the angular velocity ω with the torque T is expressed by the following two equations.

[0021]

[Equation 2]

When this is integrated for one cycle, it appears as the following three expressions.

[0023]

[Equation 3]

When ω (t) = ω is set, the above equation 3 is expressed as the following equation 4.

[0025]

[Equation 4]

Since (ω ² ) ′ dt = 2ωω′dt, the above equation 4 is expressed as the following equation 5.

[0027]

[Equation 5]

However, ω _b and ω _a are crank angular velocities at a predetermined crank angle and are differential values of the crank angle. Then, the total work amount expressed by the above equation 5 is divided by the stroke volume V to obtain the average effective pressure Pmi expressed by the following equation 6.

[0029]

[Equation 6]

Originally, the average effective pressure Pmi is obtained by dividing the total work amount for one cycle by the stroke volume V as described above, but the integration result for one cycle is integrated between arbitrary crank angles. It can be considered that it is proportional to the result, and in particular, it has a strong correlation with the explosion (expansion) stroke nose mood result. Therefore, in the present embodiment, a value integrated between the crank angle of 30 deg. And 80 deg. After the compression top dead center is referred to as a rotational average effective pressure.

Then, in the air-fuel ratio calculating section 28,
Rotational average effective pressure calculated by the average effective pressure calculation unit 27
And the engine speed calculated by the engine speed calculator 26.
Calculation processing of FIG. 6 using the gin rotation speed and the intake pipe pressure signal
The air-fuel ratio is calculated according to. This calculation process is in steady state
The rotating average effective pressure and its fluctuation at
According to the principle of correlation with the air-fuel ratio of
There is a strong correlation with the fluctuation of the rotating average effective pressure. For example,
In FIG. 7a, grid points are created by the engine speed and the intake pipe pressure.
Rotation average effective pressure at the lattice points
Ω_b ²−ω_a ²This is the
There are multiple three-dimensional maps for each air-fuel ratio.
For example, the rotation calculated by the average effective pressure calculation unit 27.
Average effective pressure (ω_b ²−ω_a ²) Is 1600,
Engine speed is 4500 rpm, intake pipe pressure is 45 kP
When it is a, when each 3D map is 3D interpolated,
Rotational mean effective pressure (ω at fuel ratio 13.0_b ²−ω_a
²) Is a rotation average at 1712.5 and an air-fuel ratio of 14.0.
Rotation at effective pressure of 1612.5 and air-fuel ratio of 15.0
The average effective pressure is 1512.5. These air-fuel ratios and times
The rolling average effective pressure is converted into a two-dimensional map as shown in Fig. 7b.
Converted and linearly interpolated by rotating average effective pressure (ω_b ²−
ω_a ²) Is 1600, the air-fuel ratio is calculated to be 14.1
25. Similar map search, more correlated rotation
A more accurate air-fuel ratio can be obtained by changing the average effective pressure.
Can be obtained. Steady state means intake pipe pressure and
Engine rotation speed is any of all lattice points of the three-dimensional map
Existing in the grid and staying within the grid point for a predetermined time
State.

This calculation process is executed as a timer interrupt process which is carried out every predetermined sampling time set to, for example, 10 msec. First, at step S1, the intake pipe pressure and the engine speed are set in all grid point areas of the map. It is determined whether or not they exist, and if they are in all the grid point areas of the map, the process proceeds to step S2, and if not, the process proceeds to step S3.

In the step S2, it is judged whether or not the intake pipe pressure and the engine speed are in the same grid point area as the previous time. If they are in the same grid point area as the previous time, the step S4 is performed. If not, the process proceeds to step S5. In step S4, the counter n is incremented and then the process proceeds to step S6.

In step S6, the rotation average effective pressure calculated by the average effective pressure calculating section 27 is stored in a predetermined data area such as a shift register, and then the process proceeds to step S7. In step S7, it is determined whether or not the counter n is equal to or more than a predetermined value n ₀ corresponding to a preset predetermined time. If the counter n is equal to or more than the predetermined value n ₀ , the process proceeds to step S8. , If not, it returns to the main program.

In step S8, the average value of the rotational average effective pressure for the predetermined value n ₀ is calculated, and the map search is performed from the average value of the rotational average effective pressure as described above.
After calculating the comparative air-fuel ratio, the process proceeds to step S9. In step S9, standard deviations thereof are calculated as numerical values showing fluctuations in the rotation average effective pressure for the predetermined value n ₀ , and a map search is performed from the standard deviation of the rotation average effective pressure in the same manner as in step 8. After calculating the reference air-fuel ratio, the process proceeds to step S10.

In the step S10, the step S
It is determined whether the difference between the reference air-fuel ratio calculated in 9 and the comparative air-fuel ratio calculated in step S8 is less than or equal to a predetermined value set to, for example, about 0.5, and the reference air-fuel ratio and the comparison air-fuel ratio are compared. If the difference from the fuel ratio is less than or equal to the predetermined value, step S1
If not, the process proceeds to step S12.

In the step S11, the step S
The reference air-fuel ratio calculated in 9 is selected as the final air-fuel ratio, and then the process returns to the main program. On the other hand, in step S12, the air-fuel ratio is set to zero before returning to the main program. Further, in step S3, the data area of the rotation average effective pressure is cleared and then the process proceeds to step S13.

In step S13, the counter n is cleared and then the main program is resumed. Further, in the step S5, the counter n is cleared and then the process returns to the main program. According to this processing, the steady state as described above, i.e., the intake pipe pressure and the engine speed map of the same grid point area, if continued for a predetermined time corresponding to the predetermined value n _0, the predetermined value n ₀ The standard air-fuel ratio is calculated from the standard deviation of the rotational average effective pressure of the minutes, and the comparative air-fuel ratio is calculated from the average value of the rotational average effective pressure of the predetermined value n _{0. When} the difference between the two is less than the predetermined value, The reference air-fuel ratio is selected as the final air-fuel ratio. Since there is a strong correlation between the standard deviation indicating the fluctuation of the rotating average effective pressure and the air-fuel ratio as described above, the reference air-fuel ratio calculated from the standard deviation of the rotating average effective pressure is the actual air-fuel ratio at that time. Very close. Therefore, it is possible to detect the air-fuel ratio without using an oxygen sensor as in the conventional case. Further, since the air-fuel ratio is estimated using the rotation average effective pressure when the steady state continues for a predetermined time, the estimation accuracy of the air-fuel ratio can be improved.

Then, in the learning amount calculation section 29, the air-fuel ratio calculated by the air-fuel ratio calculation section 28 and the target air-fuel ratio calculation section 3 at the same timing as when the air-fuel ratio is calculated.
The target air-fuel ratio calculated in 3 is compared, and the CMAC teacher data of the intake air amount calculation unit 31 is created from the difference between the two. In the above embodiment, the intake pipe injection type engine is described in detail, but the engine control device of the present invention can be similarly applied to the direct injection type engine.

In the above embodiment, the two-wheeled vehicle of the so-called multi-cylinder type engine having four cylinders has been described in detail, but the invention can be similarly applied to a single cylinder engine having one cylinder. Further, the engine control unit can be replaced with various arithmetic circuits instead of the microcomputer.

[0041]

As described above, according to the air-fuel ratio detecting device of the first aspect of the present invention, the average effective pressure of the engine for each cycle is calculated from at least the engine rotation state in the explosion stroke, Since the air-fuel ratio is calculated based on the average value of the average effective pressure in the steady state and the deviation of the average effective pressure in the steady state, an expensive oxygen sensor is not required and the cost can be reduced.

According to the air-fuel ratio detecting device of the second aspect of the present invention, the air-fuel ratio calculated from the average value of the average effective pressure in the steady state and the deviation of the average effective pressure in the steady state are calculated. When the difference with the air-fuel ratio is within a predetermined value, the air-fuel ratio calculated from the deviation in the steady state of the average effective pressure is selected. It is possible to calculate an accurate air-fuel ratio.

According to the third aspect of the present invention, according to the air-fuel ratio detecting device, a map of the air-fuel ratio, the engine speed, and the intake pipe pressure obtained in advance is used to detect the rotational state of the engine and the intake pipe. Since the air-fuel ratio is calculated by searching by pressure, it is possible to easily calculate the air-fuel ratio from three parties having a complicated relationship. Further, according to the air-fuel ratio detecting device of the fourth aspect of the present invention, when the detected engine rotation state and the intake pipe pressure are within a preset range continuously for a predetermined time respectively, Since the steady state is assumed, the air-fuel ratio calculated by searching the map can be made more accurate.

According to the fifth aspect of the present invention, since the standard deviation is used as the deviation of the average effective pressure, the deviation of the average effective pressure can be easily understood. .

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a motorcycle engine and its control device.

FIG. 2 is a block diagram showing an embodiment of an engine control device of the present invention.

FIG. 3 is an explanatory diagram for detecting a stroke state from a crankshaft phase and an intake pipe pressure.

FIG. 4 is a map for calculating a target air-fuel ratio stored in a target air-fuel ratio calculation unit.

FIG. 5 is a diagram illustrating the operation of the transition period correction unit.

FIG. 6 is a flowchart showing a calculation process performed by an air-fuel ratio calculation unit.

FIG. 7 is an explanatory diagram of a map used in the arithmetic processing of FIG.

[Explanation of symbols]

1 is the engine 3 is the crankshaft 4 is a piston 5 is a combustion chamber 6 is an intake pipe 7 is an intake valve 8 is an exhaust pipe 9 is an exhaust valve 10 is a spark plug 11 is an ignition coil 12 is a throttle valve 13 is an injector 14 is a connecting rod 15 is an engine control unit 16 is a fuel pump 17 is a regulator 20 is a crank angle sensor 24 is an intake pipe pressure sensor 26 is an engine speed calculation unit 27 is an average effective pressure calculation unit 28 is an air-fuel ratio calculation unit 30 is an injection pulse output unit 31 is an intake air amount calculation unit 33 is a target air-fuel ratio calculation unit 34 is a fuel injection amount calculation unit 44 is a fuel injection time calculation unit

Claims (5)

[Claims] 1. An engine rotation state detecting means for detecting at least the rotation state of the engine during an explosion stroke, and an average effective pressure of the engine for each cycle is calculated from the rotation state of the engine detected by the engine rotation state detecting means. An average effective pressure calculating means, and an air-fuel ratio calculating means for calculating an air-fuel ratio based on the average value in the steady state of the average effective pressure calculated by the average effective pressure calculating means and the deviation in the steady state of the average effective pressure. An air-fuel ratio detection device characterized by being provided. 2. The air-fuel ratio calculating means is calculated from the deviation of the average effective pressure calculated by the average effective pressure calculating means from the average value in the steady state in the steady state. The air-fuel ratio detection device according to claim 1, wherein when the difference from the air-fuel ratio is within a predetermined value, the air-fuel ratio calculated from the deviation in the steady state of the average effective pressure is selected. 3. An intake pipe pressure detecting means for detecting a pressure in the intake pipe, wherein the air-fuel ratio calculating means detects a map of an air-fuel ratio, an engine speed and an intake pipe pressure, which are obtained in advance, for detecting the engine rotation state. The air-fuel ratio is calculated by searching with the engine rotation state detected by the means and the intake pipe pressure detected by the intake pipe pressure detection means.
Alternatively, the air-fuel ratio detecting device according to item 2. 4. The air-fuel ratio calculation means is configured such that the engine rotation state detected by the engine rotation state detection means and the intake pipe pressure detected by the intake pipe pressure detection means are continuously for a predetermined time, respectively. The air-fuel ratio detection device according to claim 3, wherein the air-fuel ratio detection device is in a steady state when it is within a preset range. 5. The air-fuel ratio detecting device according to claim 1, wherein the air-fuel ratio calculating means uses a standard deviation as a deviation of the average effective pressure.