JPH07167028A - Misfire detection device for gasoline engine - Google Patents

Abstract

PURPOSE:To accurately detect misfire without being affected by dispersion of engine operations. CONSTITUTION:As for a spark discharge at a continuous specified revolution frequency in a same cylinder, the average value of peak voltage detected by a peak voltage detection circuit 31 is computed for each cylinder by an average value computing section 33. A reference value is determined based on the average value and the degree of slip-off against the average value by a judging level deciding section 34, peak voltage at each cylinder after the specified number of spark discharges are induced since the reference value is determined, is compared with the reference value by a comparison section 35, so that the misfire is judged by the result of this comparison. Since the reference value is properly set in order at each cylinder, and at intervals of the specified number of spark discharges, misfire can be accurately detected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a misfire detection device for a gasoline engine, which detects whether or not a gasoline engine has misfired, based on a peak voltage of a voltage appearing between electrodes of a spark plug after completion of spark discharge.

[0002]

2. Description of the Related Art In a gasoline engine, a voltage due to the residual energy of the secondary circuit of the ignition coil appears between the electrodes of the spark plug after the spark discharge is completed, and the voltage at this time is ignited by the ion density between the electrodes of the spark plug. Or it depends on the misfire. For this reason, as a misfire detection device for a gasoline engine, for example, an insulating coating of each plug cord connecting the distributor and each spark plug is used as a dielectric and a conductor provided corresponding to the conductor at the center of each plug cord. A sensor that detects the voltage between the electrodes of the spark plug by the capacitance between
In some cases, the misfire is detected by comparing the peak voltage of the sensor after completion of the spark discharge with an integral value of an integration circuit that integrates the detection voltage of the sensor within a predetermined period set including the spark discharge period.

[0003]

However, in the gasoline engine, the voltage appearing at the spark plug after the spark discharge is completed is not only different depending on the conditions such as the electrode shape of the spark plug and the gap length, but also the spark plug is mounted. The inventor of the present application has found that it also varies depending on the difference between the transient state and the steady state such as the variation of the engine and the elapsed operating time, such as the relationship with the air flow in each combustion chamber for each cylinder.

Therefore, the reference value for discriminating misfire is
The conventional misfire detection device, which is set uniformly regardless of the individual gasoline engine that actually has the spark plug installed, is adequately compatible with transient and steady state differences such as variations in each gasoline engine and elapsed operating time. Therefore, it is necessary to give a margin to the reference value in order to prevent accidental misfire detection for differences in the detected voltage for each gasoline engine and elapsed driving time. However, there is a problem that accurate misfire detection cannot be performed.

The present invention provides a misfire detection device for a gasoline engine that can accurately detect misfires without being affected by variations in the air flow in each combustion chamber of the gasoline engine and variations in operating conditions. To aim.

[0006]

According to a first aspect of the present invention, the spark discharge termination is achieved in an ignition device in which a spark plug is connected to a secondary circuit of an ignition coil that generates a high voltage for spark discharge due to intermittent primary current. And a peak voltage detecting means for detecting the peak value of the voltage between the discharge electrodes of the spark plug,
In a gasoline engine misfire detection device that detects a misfire state of a gasoline engine based on the detected peak voltage, a spark discharge of the same cylinder of the gasoline engine for every predetermined number of consecutive times is detected by the peak voltage detection means. And an average value calculating means for calculating an average value of the peak voltage for each predetermined number of times, and a deviation for detecting a degree of deviation of the peak voltage for each predetermined number of times used for the calculation of the average value calculating means from the average value. Detecting means, a reference value calculating means for calculating a reference value for determining the presence or absence of misfire from the average value and the degree of deviation, and for the predetermined number of spark discharges after calculating the reference value, the Comparing means for comparing the predetermined number of peak voltages detected by the peak voltage detecting means with the reference value, and misfire when the peak voltage is higher than the reference value. The technical means to and a determining misfire determination means.

According to a second aspect of the present invention, the deviation detecting means has a technical means of detecting the degree of deviation from the difference between the average value and the minimum value of the peak voltage for each predetermined number of times.
According to a third aspect of the present invention, the deviation detecting means has a technical means of detecting the degree of the deviation from the difference between the maximum value and the average value of the peak voltage for each predetermined number of times. In the present invention, the average value calculating unit may set a fixed value that is set to be larger than the average value and the degree of deviation when the degree of deviation detected by the deviation detecting unit is less than or equal to a predetermined value. The technical means is to calculate the reference value from the above.

According to a fifth aspect of the present invention, the misfire determination means further comprises a crank angular velocity sensor for detecting the crank angular velocity of the engine and a crank angular velocity variation detecting means for detecting the degree of variation of the crank angular velocity detected by the crank angular velocity sensor. Provided, even when the degree of variation in the crank angular velocity is a certain value or more, it is determined that the misfire, the average value calculation means, the average value of the peak voltage when the misfire determination means is determined as misfire. The technical means not to use it for calculation.

[0009]

[Action]

<Operation of Claim 1> After a high voltage is applied to the spark plug by the ignition device and the spark discharge is completed, electric charge is generated between the discharge electrodes of the spark plug due to the residual energy of the ignition coil, and between the discharge electrodes of the spark plug. The peak value of the voltage appearing at is detected by the peak voltage detecting means. The peak voltage after the end of the detected spark discharge is calculated by the average value calculation means every time the spark discharge is performed a predetermined number of times, and the average value of the peak voltage of the predetermined number of times is calculated. The degree of deviation from the average value of the peak voltage for each predetermined number of times used in the calculation of the value calculating means is detected, and the reference value calculating means determines that the reference value for determining the presence or absence of misfire is the degree of deviation from the average value. Calculated from

After that, for a predetermined number of spark discharges after the reference value is calculated, the peak voltage after the spark discharge is compared with the calculated reference value. If the peak voltage is higher than the reference value, it is determined that a misfire has occurred. It is determined that the spark discharge did not ignite. In a gasoline engine, since there are variations in air flow in each combustion chamber and variations in operating conditions for each cylinder, the inter-electrode voltage of the spark plug is different and the peak voltage is different, but the reference value for comparing the peak voltages is , The average value of the peak voltage after the spark discharge in the predetermined number of spark discharges immediately before the comparison is performed and the degree of the deviation, so that the reference value can be calculated in consideration of the variation and the operating state in each cylinder. . Therefore,
Since an optimum reference value can be set for the operating state of each cylinder, more accurate misfire detection can be performed.

<Operation of Claim 2> Since the degree of deviation of the peak voltage from the average value is detected by detecting how wide the peak voltage appears a predetermined number of times, the minimum peak voltage is detected. Since the maximum value opposite to the minimum value can be estimated by calculating the difference between the average value and the minimum value using the value, it is possible to detect the approximate degree of variation in the peak voltage. <Operation of Claim 3> Since the degree of deviation of the peak voltage from the average value is detected by detecting how wide the peak voltage appears a predetermined number of times, the maximum value of the peak voltage is used. By calculating the difference between the maximum value and the average value, the minimum value opposite to the maximum value can be estimated, so that the degree of variation in the peak voltage can be detected.

<Operation of claim 4> When the reference value is determined from the average value and the degree of deviation, if the degree of deviation detected by the deviation detecting means is less than a predetermined value, the degree of deviation is small. The standard value is mainly determined by the average value. However, even if the degree of deviation is less than or equal to a predetermined value, it only shows the variation for a limited number of spark discharges, so the variation is not necessarily small as an engine, and a certain number of sparks are required. On the contrary, when the deviation is detected only in the discharge, the engine variation may not appear correctly. In such a case, if the reference value is determined only by the average value, the peak voltage in the case of ignition cannot be set with a margin, so that misfire may be erroneously detected. In the present invention, when the degree of deviation is less than or equal to a predetermined value, a fixed value that is set larger than the degree of deviation is used instead of the degree of deviation, and the reference value is determined from the average value and this fixed value. Therefore, in the case of ignition, it is not mistakenly determined to be misfire.

<Operation of Claim 5> In Claim 5, the misfire determination means further includes a crank angular velocity sensor for detecting the crank angular velocity of the engine, and a crank for detecting the degree of variation in the crank angular velocity detected by the crank angular velocity sensor. A misfire which is provided with an angular velocity variation detection means and is determined not to be misfire based on the above-mentioned average value and a reference value calculated from the degree of deviation by determining misfire even when the degree of variation in crank angular velocity is a certain value or more. Even in the case of time, misfire can be detected based on the crank angular velocity of the engine, and the accuracy of misfire detection is improved.
Further, in calculating the average value of the peak voltage by the average value calculating means, the value of the peak voltage when the misfire determining means determines that there is a misfire is not used for calculating the average value. Therefore,
The average value does not deviate due to the value of the peak voltage at the time of misfire that has a large deviation from the average value, and the average value can be calculated from only the more average value at the time of ignition. As a result, it is possible to prevent the misfire from being erroneously determined upon ignition.

[0014]

According to the first aspect of the present invention, the reference value for misfire determination is calculated from the average value of the peak voltage detected after the end of the spark discharge and the degree of deviation from the average value for each cylinder of the gasoline engine. In order to do so, the air flow in each combustion chamber for each cylinder,
Even if the operating conditions vary from cylinder to cylinder,
A reference value can be set in consideration of those differences. As a result, the optimum reference value is sequentially obtained for each cylinder according to the operating state. Therefore, the presence or absence of misfire can be accurately detected by comparing the peak voltage detected for each cylinder with the reference value. .

In the second aspect, the minimum value of the peak voltage is used as the deviation degree with respect to the average value of the peak voltage, and the deviation degree is detected based on the difference between the average value and the minimum value. Therefore, a complicated calculation is performed. It is possible to easily detect the degree of deviation without processing. In claim 3, since the maximum value of the peak voltage is used as the degree of deviation of the peak voltage from the average value, and the degree of deviation is detected based on the difference between the maximum value and the average value, complicated arithmetic processing is unnecessary. Instead, the degree of deviation can be easily detected. According to the present invention, when the degree of deviation is small, the degree of deviation is not used to calculate the reference value, but a fixed value that is set to a large extent is used for the degree of deviation. The misfire is not determined and the misfire can be accurately detected.

According to the present invention, since the value of the peak voltage when the misfire determination means determines the misfire is not used in the calculation of the average value, the average value can be calculated only from the more average ignition value. It is possible to prevent accidental misfires when ignited. Further, the misfire determination means determines misfire even when the degree of variation in crank angular velocity is equal to or greater than a certain value. Therefore, even if misfire is not determined to be misfire based on the reference value based on the above-mentioned peak voltage, Since the misfire can be detected based on the crank angular velocity, the accuracy of misfire detection is improved.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described based on the embodiments shown in the drawings. FIG. 1 shows one embodiment of a misfire device for a gasoline engine according to the present invention. An ignition device 10, a sensor 20 for detecting misfire, and a misfire detection circuit for detecting misfire based on a detection signal from the sensor 20. It is provided in an engine control unit (ECU) 50 that controls the engine E.

In the ignition device 10, 1 is an ignition coil,
2 is a distributor (distributor), 3 is a spark plug of the number of cylinders, 41 is a transistor of a switching element for connecting and disconnecting the primary current of the primary circuit 11 of the ignition coil 1, 42 is a signal generator for issuing an ignition timing signal, and an engine control unit Included within 50. V is a vehicle-mounted power source, and each spark plug 3 has a secondary circuit 1 of the ignition coil 1.
The high voltage from 2 is applied.

The sensor 20 for detecting the voltage between the electrodes of each spark plug 3 comprises a voltage dividing circuit for dividing the voltage of the primary circuit 11 of the ignition coil 1 by the two resistors 21 and 22. The high voltage appearing between the electrodes of the resistor 21 is detected through the secondary circuit 12 of the ignition coil 1 and the resistor 21,
22 to lower.

The misfire detection circuit 30 is included in the engine control unit 50 together with the signal generator 42, and includes a peak voltage detection circuit 31 and a misfire determination circuit 32 mainly composed of a microcomputer. The peak voltage detection circuit 31 peak-holds the peak voltage of the detection signal of the sensor 20 after completion of the spark discharge by the peak hold circuit which is reset immediately after the start of the spark discharge according to the ignition timing signal from the signal generator 42.

The misfire determination circuit 32 outputs the peak voltage after the spark discharge, which is peak-held in the peak voltage detection circuit 31, after the spark discharge voltage is applied from the signal generator 42 to each cylinder (cylinder) at a time corresponding to the timing signal. The peak voltage signal for each cylinder is divided into four systems according to the number of cylinders of the engine E (for example, 4), and the peak voltage signal for each cylinder is processed for each cylinder. A reference value for determining misfire is determined from the degree of the variation, and misfire is detected based on the reference value.

As a configuration therefor, the misfire determination circuit 32 is provided.
As shown in FIG. 2, the average value calculation unit 33, the determination level determination unit 34, the comparison unit 35, and the misfire determination unit 36 are functional units, and each corresponds to a peak voltage signal for each cylinder. Process. The average value calculation unit 33 averages the peak voltage signals for a predetermined number of times (for example, 20 times) consecutive for each cylinder to calculate an average value A for each cylinder. The determination level determination unit 34 determines, for each cylinder, the average value A calculated by the average value calculation unit 33 and the minimum value m of the peak voltage signals used to calculate the average value A a predetermined number of times. And from

[0023]

[Number 1] criterion values S ₁ = average value A + a (average A- minimum m) where, a is determined positive number (2> a ≧ 1) by the criterion values S ₁ for each cylinder, respectively .

The comparing unit 35 determines, for each cylinder, the next predetermined number of peak voltage signals following the predetermined number of peak voltage signals used for determining the above-mentioned determination reference value S ₁ for each cylinder. Whether or not the value is larger than the reference value S _{1 is} compared for each cylinder, and a pulse signal corresponding to the comparison result is output until the peak voltage detection circuit 31 is reset. Here, when the voltage of each peak voltage signal given to each cylinder becomes higher than the judgment reference value S _{1 due} to misfire, it becomes a high-level pulse signal, and the voltage of the sensor signal becomes lower than the judgment reference value S ₁ due to ignition. If so, the pulse signal becomes a low level, and each pulse signal is sent out corresponding to the ignition timing of each cylinder.

The misfire discriminating unit 36 discriminates in which cylinder the misfire has occurred from the timing of the high-level pulse signal output from the comparing unit 35, and stores it in an engine computer, a lamp, or the like. Is notified to the driver by lighting.

Next, the operation of the misfire detecting device of the present embodiment having the above-mentioned structure will be described with reference to FIG.
Will be explained with reference to. When the transistor 41 is turned on and off by the ignition timing signal of the signal generator 42, a pulse current flows through the primary circuit 11 of the ignition coil 1. Due to this intermittent pulse current, the ignition coil 1
A secondary voltage is generated in the secondary circuit 12 as shown by voltage waveforms o and p, and in the spark plug 3, spark discharge starts due to the high voltage generated at the end of the pulse current, and then inductive discharge occurs. This spark discharge lasts for a period of time according to the rotational speed of the gasoline engine, and ends when the electric energy of the ignition coil 1 is reduced.

After the spark discharge is completed, the secondary voltage of the ignition coil 1 starts to increase due to the electric energy remaining in the ignition coil 1, and the high voltage corresponding to the rotation speed of the gasoline engine (2 to 3 kilovolts at low speed operation, high speed at high speed) In operation, the voltage rises to a maximum value q, r of 5 to 8 kilovolts and then drops, and the voltage appearing between the electrodes of the spark plug 3 is detected by the sensor 20 via the ignition coil 1.

In the misfire detection circuit 30, after the detection voltage of the sensor 20 is peak-held after the spark discharge is completed, it is divided as a peak voltage signal for each cylinder corresponding to the ignition timing signal, and a predetermined number of times is performed for each cylinder. The average value A of the peak voltage signal for each spark discharge is calculated, and the determination reference value S ₁ for each cylinder is determined based on the calculated average value A. In the comparison unit 35, the peak voltage after completion of the spark discharge is compared with the determination reference value S _{1 for} each cylinder in a predetermined number of spark discharges after the average value A and the determination reference value S ₁ are calculated. It When the cylinder is normally ignited, the detected voltage of the sensor 20 after the spark discharge ends becomes longer, and as a result, the residual energy becomes smaller, so that the peak voltage becomes lower, whereas the misfire occurs. In this case, the spark discharge time becomes short, and as a result, the residual energy becomes large, so that the peak voltage becomes high.

When a misfire occurs in a cylinder and the voltage of the peak voltage signal is higher than the judgment reference value S ₁ , a high-level pulse signal is sent from the comparison unit 35 at a timing corresponding to the cylinder, and the normal condition is obtained. When the ignition is on, a low level signal is transmitted. The misfire determination unit 36 determines whether or not the cylinder has misfired, depending on whether the pulse signal at the timing corresponding to the cylinder is high level or low level, and the engine control unit 50 performs a predetermined process.

As described above, in this embodiment, the average value A calculated from the peak voltage after completion of the spark discharge in the spark plug 3 and the minimum value m of the peak voltage are obtained every predetermined number of spark discharges.
Since the judgment reference value S ₁ is sequentially calculated from the difference between the difference and the judgment reference value S ₁
Can be determined, and there is no possibility of accidentally determining a misfire at the time of ignition.

Next, a second embodiment of the present invention will be described. In the above embodiment, the determination level determining unit 34 calculates the determination reference value S ₁ from the average value A and the minimum value m of the peak voltage signal for a predetermined number of times used to calculate the average value A, by using the above mathematical expression. In this embodiment, the determination reference value S ₂ determined by the determination level determination unit 34 is the average value A and the peak voltage signal of the predetermined number of times used for calculating the average value A. The maximum value M and the maximum value M are used for the determination.

[0032]

[Number 2] criterion value S ₂ = average value A + a (maximum value M- average A) where, a is a positive number (2> a ≧ 1) in this embodiment, depending on the degree of variation of the peak voltage The determination reference value S ₂ can be appropriately determined, and it is possible to prevent accidental misfire determination during ignition.

Next, a third embodiment of the present invention will be described. In each of the above embodiments, the determination level determination unit 34 determines the average value A and the minimum value m of the peak voltage signal used to calculate the average value A when determining the determination reference values S ₁ and S _2. Coefficient (average value A−minimum value m) or the difference between the maximum value M and the average value A of the peak voltage signal used to calculate the average value A (maximum value M−average value A). The determination reference value S is determined by adding the values D1 and D2, which are determined by
Here, only when a (average value A-minimum value m) ≧ b, the determination reference value S ₁ is determined by using the above mathematical formula 1, and when a (average value A-minimum value m) <b, , A determination reference value S ₁₁ given by the following mathematical formula 3 is used.

[0034]

Equation 3] criterion value S ₁₁ = average value A + b where, b is larger set fixed values for a (mean A- minimum m).

Similarly, when the maximum value M of the peak voltage is used, the judgment reference value S ₂ is determined using the above formula 2 only when a (maximum value M-average value A) ≧ b, and a When (maximum value M−average value A) <b, the judgment reference value S ₂₁ given by the following mathematical formula 4 is used.

[0036]

Equation 4] criterion value S ₂₁ = average value A + b where, b is larger set fixed values for a (mean A- minimum m).

As in the first and second embodiments described above, the judgment reference value S ₁ is calculated from the average value A and the deviation values D1 and D2.
If S ₂ is determined and the value of the detected deviation is less than a predetermined value, the judgment reference value S
₁ and S ₂ are mainly determined by the average value A of the peak voltage. However, even if the degree of deviation is less than or equal to a predetermined value, it only shows the variation for a limited number of spark discharges, so the variation is not necessarily small as an engine, and only a certain number of spark discharges are required. On the contrary, in the detection of the deviation of the engine, the variation of the engine may not appear correctly. In such a case, if the average value A is mainly determined as the determination reference value, the peak voltage in the case of ignition cannot be set with a margin, and thus misfire may be erroneously detected.

Corresponding to such a case, when the deviation degree values D1 and D2 are equal to or less than a predetermined value, the following equations 3 and 4 are used to determine the deviation degree value, not the deviation degree value itself. When a fixed value b set to be larger than the value is used, the judgment reference values S ₁₁ and S are calculated from the average value A and the fixed value b.
_{Since 21} is determined, there is no possibility of accidentally determining a misfire in case of ignition. By doing so, the determination reference value becomes large when the variation is small, and the determination reference value can be set relatively small even when the operating conditions are constant and the variation such as the transient state is large. Therefore, accurate misfire determination can be performed in all operating regions.

Next, a fourth embodiment of the present invention will be described with reference to FIG. In this embodiment, the misfire detection circuit 32 in each of the above embodiments further uses a known sensor that uses a magnetic or photoelectric element or the like to detect the crank angular velocity of the engine E, such as a shaft that interlocks with the crankshaft. The crank angular velocity sensor 8 for detecting the rotational position or angular velocity of the gear provided in the above, and the crank angular velocity variation detection circuit 9 for detecting the variation of the crank angular velocity from the signal thereof.

With these configurations, the crank angular velocity variation detection circuit 9 uses the crank angular velocity sensor 8 to determine, for example, the time required for one rotation of the crankshaft measured at each top dead center of the piston of each cylinder to be T360, and for each cylinder. When the time from the spark discharge to the constant crank angle (for example, 90 ° CA) is T90 and R = T90 / T360, the R value at the time of measurement Ri and the R value of the constant engine before this time of measurement Misfire is determined when the difference Ri-Rh from the average value Rh is a certain value or more. The misfire determination based on the detection by the crank angular velocity variation detection circuit 9 is valid only when the engine E is rotating at low speed, for example, 4000 rpm, and is not determined when the engine E is rotating at high speed. .

The misfire detection circuit 30 determines misfire by the crank angular velocity variation detection circuit 9 and the misfire determination by the misfire determination unit 36. Further, the average value calculation unit 33 does not use the peak voltage signal when it is determined that there is a misfire in the calculation of the average value A, but uses only the peak voltage signal when it is determined that there is an ignition. As a result, the average value A does not deviate due to the value of the peak voltage signal at the time of misfire, which has a large deviation from the average value A, and the average value A can be calculated only from the more average value at the time of ignition. In this way, by performing the misfire determination based on the peak voltage between the electrodes of the spark plug 3 and the misfire determination based on the crank angular velocity,
It is possible to detect misfire more accurately without accidentally detecting misfire at the time of ignition.

As described above, in the present invention, the determination reference value for determining misfire is changed with the operating time in accordance with the degree of the peak voltage deviation after the end of the spark discharge which changes according to the operating state. Since it is possible to set the optimum determination reference value during the operation, it is possible to detect misfire more accurately without being mistakenly determined to be misfire at the time of ignition.

In each of the embodiments described above, the spark plug 3
The sensor 20 for detecting the voltage between the electrodes of the ignition coil 1
The one provided in the primary circuit 11 of the ignition coil 1 is shown.
In place of the sensor 20 on the secondary circuit 12 side, a capacitance sensor for detecting the inter-electrode voltage of the spark plug 3 is provided by the capacitance between the power distribution record 2 or the high voltage cable to the spark plug 3. Alternatively, the peak voltage may be detected by signal-processing the divided voltage value divided by the capacitor.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an ignition device to which a misfire detection device of the present invention is applied.

FIG. 2 is a functional block diagram showing an embodiment of a misfire determination circuit of the present invention.

FIG. 3 is a signal waveform diagram for explaining the operation of the misfire detection circuit of the present invention.

FIG. 4 is a functional block diagram showing another embodiment of the misfire determination circuit of the present invention.

[Explanation of symbols]

1 Ignition coil 3 Spark plug 10 Ignition device 30 Misfire detection circuit (gasoline engine misfire detection device) 31 Peak voltage detection circuit (peak voltage detection means) 33 Average value calculation unit (average value calculation means) 34 Judgment level determination unit (deviation Detection unit, reference value calculation unit) 35 Comparison unit (comparison unit) 36 Misfire determination unit (misfire determination unit) 8 Crank angular velocity sensor 9 Crank angular velocity variation detection circuit (crank angular velocity variation detection unit)

Claims (5)

[Claims] 1. A peak of the voltage between the discharge electrodes of the spark plug after the spark discharge is completed in an ignition device in which a spark plug is connected to a secondary circuit of an ignition coil that generates a high voltage for spark discharge by interrupting a primary current. Provided with a peak voltage detecting means for detecting a value, in a misfire detection device of a gasoline engine for detecting a misfire state of a gasoline engine based on the detected peak voltage, a spark of the same cylinder of the gasoline engine every predetermined number of times consecutively. Regarding discharge, an average value calculating means for calculating an average value of the peak voltage detected by the peak voltage detecting means for each of the predetermined number of times, and a peak voltage for each of the predetermined number of times used in the calculation of the average value calculating means. Deviation detecting means for detecting the degree of deviation from the average value, and a reference value for determining the presence or absence of misfire from the average value and the degree of deviation Reference value calculation means, for the predetermined number of spark discharge after calculating the reference value, a comparison means for comparing the predetermined number of peak voltage detected by the peak voltage detection means with the reference value, the peak A misfire detection device for a gasoline engine, comprising: misfire determination means for determining a misfire when the voltage is higher than the reference value. 2. The misfire of a gasoline engine according to claim 1, wherein the deviation detecting means detects the degree of the deviation from the difference between the average value and the minimum value of the peak voltage for each predetermined number of times. Detection device. 3. The misfire of a gasoline engine according to claim 1, wherein the deviation detecting means detects the degree of the deviation from the difference between the maximum value and the average value of the peak voltage for each predetermined number of times. Detection device. 4. The average value calculating means, when the degree of deviation detected by the deviation detecting means is less than or equal to a predetermined value, a fixed value set to be larger than the average value and the degree of deviation. The misfire detection device for a gasoline engine according to claim 1, 2 or 3, wherein the reference value is calculated from 5. The misfire determining means further includes a crank angular velocity sensor for detecting a crank angular velocity of the engine, and a crank angular velocity variation detecting means for detecting a variation degree of the crank angular velocity detected by the crank angular velocity sensor, Even when the degree of variation in crank angular velocity is equal to or greater than a certain value, it is determined that misfire has occurred, and the average value calculation means uses the value of the peak voltage when the misfire determination means determines misfire to calculate the average value. The misfire detection device for a gasoline engine according to claim 1, wherein the misfire detection device is not provided.