JP5220908B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Air-fuel ratio control device for internal combustion engine Download PDF

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JP5220908B2
JP5220908B2 JP2011186683A JP2011186683A JP5220908B2 JP 5220908 B2 JP5220908 B2 JP 5220908B2 JP 2011186683 A JP2011186683 A JP 2011186683A JP 2011186683 A JP2011186683 A JP 2011186683A JP 5220908 B2 JP5220908 B2 JP 5220908B2
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fuel ratio
frequency
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difference signal
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JP2013050028A (en
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暢 関口
淳宏 宮内
健 青木
理範 谷
貴志 柳浦
誠二 渡辺
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Honda Motor Co Ltd
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    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Description

本発明は、内燃機関の空燃比制御装置に関し、特に空燃比制御系における故障を判定する機能を有する空燃比制御装置に関する。   The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine, and more particularly to an air-fuel ratio control apparatus having a function of determining a failure in an air-fuel ratio control system.

特許文献１には、機関排気系に設けられた空燃比センサの出力信号に基づいて、複数気筒のそれぞれに対応する空燃比が許容限度を超えてばらつくインバランス故障を判定する機能を有する空燃比制御装置が示されている。この装置によれば、機関運転中に空燃比を所定周波数で振動させる空燃比振動制御を実行し、その制御実行中における空燃比センサ出力信号に含まれる０．５次周波数成分の強度を、所定周波数成分の強度で除算することにより得られる判定パラメータを用いて、インバランス故障が判定される。０．５次周波数成分は、機関の回転速度に対応する周波数の１／２の周波数成分であり、インバランス故障が発生すると、この０．５次周波数成分の強度が増加し、インバランス度合が増加するほど判定パラメータの値が増加する。したがって、判定パラメータと所定閾値とを比較することによって、インバランス故障を判定することができる。   Patent Document 1 discloses an air-fuel ratio having a function of determining an imbalance failure in which the air-fuel ratio corresponding to each of a plurality of cylinders exceeds an allowable limit based on an output signal of an air-fuel ratio sensor provided in an engine exhaust system. A controller is shown. According to this apparatus, the air-fuel ratio oscillation control for oscillating the air-fuel ratio at a predetermined frequency is executed during engine operation, and the intensity of the 0.5th-order frequency component included in the air-fuel ratio sensor output signal during the execution of the control is determined. An imbalance failure is determined using a determination parameter obtained by dividing by the intensity of the frequency component. The 0.5th order frequency component is a half frequency component corresponding to the rotational speed of the engine. When an imbalance failure occurs, the strength of the 0.5th order frequency component increases, and the degree of imbalance increases. As the value increases, the value of the determination parameter increases. Therefore, an imbalance failure can be determined by comparing the determination parameter with a predetermined threshold value.

特開２０１１−１４４７５４号公報JP 2011-144754 A

上記従来の装置では、０．５次周波数成分あるいは所定周波数成分の強度を算出するために、バンドパスフィルタ処理を用いて必要な周波数成分の抽出が行われる。インバランス故障判定では、所定周波数は０．５次周波数の近傍に設定することが望ましいが、その場合には所定周波数成分を抽出するためのバンドパスフィルタ処理の通過帯域幅ＷＢを狭くすることが必要となる。図８は通過帯域幅ＷＢと、過渡応答特性との関係を示す図であり、図８（ａ）に示す実線Ｌ１，破線Ｌ２，及び一点鎖線Ｌ３で示す特性と、図８（ｂ）に示す実線Ｌ１１，破線Ｌ１２，及び一点鎖線Ｌ１３で示す特性とが対応している。すなわち、通過帯域幅ＷＢを狭くするほど、過渡応答特性における立ち上り時間（バンドパスフィルタ処理の開始時点ｔ０からフィルタ出力ＶＯＵＴが定常出力ＶＳＴに達するまでの時間）が長くなる。図９（ａ）は、バンドパスフィルタ処理の開始時点ｔ０からの、フィルタ出力信号の立ち上り特性の一例を示している。また、入力信号（空燃比センサ出力信号）がステップ的に変化したような場合に、通過帯域幅ＷＢを狭くするほど図９（ｂ）に示すような初期発振が発生し易くなる。   In the above-described conventional apparatus, in order to calculate the intensity of the 0.5th-order frequency component or the predetermined frequency component, a necessary frequency component is extracted using bandpass filter processing. In the imbalance failure determination, it is desirable to set the predetermined frequency in the vicinity of the 0.5th order frequency, but in that case, the passband width WB of the bandpass filter processing for extracting the predetermined frequency component may be narrowed. Necessary. FIG. 8 is a diagram showing the relationship between the passband width WB and the transient response characteristics. The characteristics shown by the solid line L1, the broken line L2, and the alternate long and short dash line L3 shown in FIG. 8A and the characteristics shown in FIG. 8B. The characteristics indicated by the solid line L11, the broken line L12, and the one-dot chain line L13 correspond to each other. That is, the narrower the passband width WB, the longer the rise time in the transient response characteristic (the time from when the bandpass filter processing start time t0 until the filter output VOUT reaches the steady output VST). FIG. 9A shows an example of the rising characteristic of the filter output signal from the start time t0 of the bandpass filter processing. Further, when the input signal (air-fuel ratio sensor output signal) changes stepwise, initial oscillation as shown in FIG. 9B is more likely to occur as the passband width WB is narrowed.

このようにバンドパスフィルタ処理の通過帯域幅ＷＢを狭くすると不都合な点があるため、通過帯域幅ＷＢを比較的広く設定しつつ、抽出信号のＳ／Ｎ（不要周波数成分強度に対する抽出周波数成分強度の比率）を高くすることが望まれていた。   Thus, since it is inconvenient to narrow the passband width WB of the bandpass filter processing, the S / N of the extracted signal (the extracted frequency component intensity relative to the unnecessary frequency component intensity is set while the passband width WB is set relatively wide. It has been desired to increase the ratio).

本発明はこの点に着目してなされたものであり、通過帯域幅が比較的広いバンドパスフィルタ処理を用いて抽出信号の十分なＳ／Ｎを確保し、空燃比制御系の故障判定を短時間で精度良く行うことができる空燃比制御装置を提供することを目的とする。   The present invention has been made paying attention to this point, and ensures sufficient S / N of the extraction signal by using a bandpass filter process having a relatively wide pass band width, thereby making it possible to shorten the failure determination of the air-fuel ratio control system. It is an object of the present invention to provide an air-fuel ratio control device that can be accurately performed in time.

上記目的を達成するため請求項１に記載の発明は、複数気筒を有する内燃機関の排気通路において空燃比を検出する空燃比検出手段を備える内燃機関の空燃比制御装置において、前記空燃比を第１の周波数（ｆ１）で振動させる空燃比振動手段と、前記空燃比検出手段による今回検出値（ＫＡＣＴ(k)）と、前記第１の周波数（ｆ１）とは異なる特定周波数（ｆＩＭＢ，ｆ２）に対応する特定周波数成分（０．５次周波数成分，周波数ｆ２成分）を減衰させるように設定された第１特定期間（ＮＩＭＢ，Ｎｆ２）前の過去値（ＫＡＣＴ(k-NIMB)，ＫＡＣＴ(k-Nf2)）との差分を第１差分信号（ＤＫＡＣＴ１(k)）として生成する第１差分信号生成手段と、前記第１差分信号（ＤＫＡＣＴ１(k)）に含まれる前記第１の周波数（ｆ１）に対応する第１周波数成分を抽出する第１抽出手段と、前記空燃比検出手段による今回検出値（ＫＡＣＴ(k)）と、前記第１周波数成分（ｆ１）を減衰させるように設定された第２特定期間（Ｎｆ１）前の過去値（ＫＡＣＴ(k-Nf1))との差分を第２差分信号（ＤＫＡＣＴ２(k)）として生成する第２差分信号生成手段と、前記第１差分信号（ＤＫＡＣＴ１(k)）に含まれる前記特定周波数成分（０．５次周波数成分，周波数ｆ２成分）を抽出する第２抽出手段と、前記第１抽出手段により抽出される前記第１周波数成分の強度（ＭＰＴｆ１）と、前記第２抽出手段により抽出される前記特定周波数成分の強度（ＭＩＭＢ，ＭＰＴｆ２）との相対関係に基づいて、空燃比制御系の故障を判定する故障判定手段とを備えることを特徴とする。   In order to achieve the above object, according to a first aspect of the present invention, there is provided an air-fuel ratio control apparatus for an internal combustion engine comprising air-fuel ratio detection means for detecting an air-fuel ratio in an exhaust passage of an internal combustion engine having a plurality of cylinders. An air-fuel ratio oscillating means for oscillating at a frequency (f1) of 1, a current detection value (KACT (k)) by the air-fuel ratio detecting means, and a specific frequency (fIMB, f2) different from the first frequency (f1) Past value (KACT (k-NIMB), KACT (k) before the first specific period (NIMB, Nf2) set to attenuate the specific frequency components (0.5th order frequency component, frequency f2 component) corresponding to -Nf2)) as a first difference signal (DKACT1 (k)), and first frequency (f1) included in the first difference signal (DKACT1 (k)). 1st frequency corresponding to A first extraction means for extracting the minute, a current detection value (KACT (k)) by the air-fuel ratio detection means, and a second specific period (Nf1) set so as to attenuate the first frequency component (f1) Second difference signal generating means for generating a difference from the previous past value (KACT (k−Nf1)) as a second difference signal (DKACT2 (k)) and the first difference signal (DKACT1 (k)) Second extraction means for extracting the specific frequency component (0.5th order frequency component, frequency f2 component), the intensity (MPTf1) of the first frequency component extracted by the first extraction means, and the second And a failure determination unit that determines a failure of the air-fuel ratio control system based on a relative relationship with the intensity (MIMB, MPTf2) of the specific frequency component extracted by the extraction unit.

請求項２に記載の発明は、請求項１に記載の内燃機関の空燃比制御装置において、前記特定周波数成分は、前記機関の回転速度に対応する周波数（ｆＮＥ）の１／２の周波数である０．５次周波数成分であり、前記故障判定手段は、前記第１周波数成分の強度（ＭＰＴｆ１）と前記０．５次周波数成分の強度（ＭＩＭＢ）とに基づいて、前記複数気筒のそれぞれに対応する空燃比が許容限度を超えてばらついているインバランス故障を判定することを特徴とする。   According to a second aspect of the present invention, in the air-fuel ratio control apparatus for an internal combustion engine according to the first aspect, the specific frequency component is a frequency that is a half of a frequency (fNE) corresponding to a rotational speed of the engine. The failure determination means corresponds to each of the plurality of cylinders based on the intensity of the first frequency component (MPTf1) and the intensity of the 0.5th order frequency component (MIMB). It is characterized by determining an imbalance failure in which the air-fuel ratio to be dispersed varies beyond an allowable limit.

請求項３に記載の発明は、請求項１に記載の内燃機関の空燃比制御装置において、前記特定周波数成分は、前記第１の周波数の整数倍の周波数（ｆ２）に対応する第２周波数成分であり、前記故障判定手段は、前記第１周波数成分の強度（ＭＰＴｆ１）と前記第２周波数成分の強度（ＭＰＴｆ２）とに基づいて、前記空燃比検出手段の応答特性劣化故障を判定することを特徴とする。   According to a third aspect of the present invention, in the air-fuel ratio control apparatus for an internal combustion engine according to the first aspect, the specific frequency component is a second frequency component corresponding to a frequency (f2) that is an integral multiple of the first frequency. And the failure determination means determines a response characteristic deterioration failure of the air-fuel ratio detection means based on the first frequency component intensity (MPTf1) and the second frequency component intensity (MPTf2). Features.

請求項４に記載の発明は、請求項２または３に記載の内燃機関の空燃比制御装置において、前記第１の周波数（ｆ１）は、前記機関の回転速度が高くなるほど高く設定され、前記第２特定期間（Ｎｆ１）は、前記第１の周波数（ｆ１）が高くなるほど短くなるように設定されることを特徴とする。   According to a fourth aspect of the present invention, in the air-fuel ratio control apparatus for an internal combustion engine according to the second or third aspect, the first frequency (f1) is set higher as the rotational speed of the engine becomes higher. The second specific period (Nf1) is set to be shorter as the first frequency (f1) is higher.

請求項５に記載の発明は、複数気筒を有する内燃機関の排気通路において空燃比を検出する空燃比検出手段を備える内燃機関の空燃比制御装置において、前記空燃比を第１の周波数（ｆ１）で振動させる空燃比振動手段と、前記空燃比振動手段の作動中に、前記空燃比検出手段による今回検出値（ＫＡＣＴ(k)）と、前記第１の周波数（ｆ１）とは異なる特定周波数に対応する特定周波数成分（０．５次周波数成分）を減衰させるように設定された第１特定期間（ＮＩＭＢ）前の過去値との差分を第１差分信号（ＤＫＡＣＴ１(k)）として生成する第１差分信号生成手段と、前記第１差分信号の今回値（ＤＫＡＣＴ１(k)）と、前記第１の周波数（ｆ１）の整数倍の周波数に対応する第２周波数成分（ＭＰＴｆ２を減衰させるように設定された第２特定期間（Ｎｆ２）前の過去値（ＤＫＡＣＴ１(k-Nf2)）との差分を第１の二次差分信号（ＤＤＫＡＣＴ１(k)）として生成する第１の二次差分信号生成手段と、前記第１の二次差分信号（ＤＤＫＡＴ１(k)）に含まれる前記第１の周波数（ｆ１）に対応する第１周波数成分（周波数ｆ１成分）を抽出する第１抽出手段と、前記空燃比振動手段の作動中に、前記空燃比検出手段による今回検出値（ＫＡＣＴ(k)）と、前記第１周波数成分を減衰させるように設定された第３特定期間（Ｎｆ１）前の過去値（ＫＡＣＴ(k-Nf1)）との差分を第２差分信号（ＤＫＡＣＴ２(k)）として生成する第２差分信号生成手段と、前記第２差分信号の今回値（ＤＫＡＣＴ２(k)）と、前記第２特定期間前の過去値（ＤＫＡＣＴ２(k-Nf2)）との差分を第２の二次差分信号（ＤＤＫＡＣＴ２(k)）として生成する第２の二次差分信号生成手段と、前記第２の二次差分信号（ＤＤＫＡＣＴ２(k)）に含まれる前記特定周波数成分（０．５次周波数成分）を抽出する第２抽出手段と、前記第１差分信号の今回値（ＤＫＡＣＴ１(k)）と、前記第３特定期間（Ｎｆ１）前の過去値（ＤＫＡＣＴ１(k-Nf1)）との差分を第３の二次差分信号（ＤＤＫＡＣＴ３(k)）として生成する第３の二次差分信号生成手段と、前記第３の二次差分信号（ＤＤＫＡＣＴ３(k)）に含まれる前記第２周波数成分を抽出する第３抽出手段と、前記第１抽出手段により抽出される前記第１周波数成分の強度（ＭＰＴｆ１）と、前記第３抽出手段により抽出される第２周波数成分の強度（ＭＰＴｆ２）との相対関係に基づいて、前記空燃比検出手段の応答特性劣化故障の判定を行う第１故障判定手段と、前記第１抽出手段により抽出される前記第１周波数成分の強度（ＭＰＴｆ１）と、前記第２抽出手段により抽出される前記特定周波数成分の強度（ＭＩＭＢ）との相対関係に基づいて、前記複数気筒のそれぞれに対応する空燃比が許容限度を超えてばらついているインバランス故障の判定を行う第２故障判定手段とを備えることを特徴とする。   According to a fifth aspect of the present invention, in the air-fuel ratio control apparatus for an internal combustion engine comprising air-fuel ratio detection means for detecting the air-fuel ratio in an exhaust passage of the internal combustion engine having a plurality of cylinders, the air-fuel ratio is set to the first frequency (f1). When the air-fuel ratio oscillation means is caused to vibrate, and during the operation of the air-fuel ratio oscillation means, the current detection value (KACT (k)) by the air-fuel ratio detection means and the first frequency (f1) are different from each other. A first difference signal (DKACT1 (k)) is generated as a first difference signal (DKACT1 (k)), which is a difference from a past value before the first specific period (NIMB) set to attenuate the corresponding specific frequency component (0.5th order frequency component). 1 difference signal generation means, a current value (DKACT1 (k)) of the first difference signal, and a second frequency component (MPTf2) corresponding to a frequency that is an integral multiple of the first frequency (f1) Second set A first secondary differential signal generating means for generating a difference from a past value (DKACT1 (k−Nf2)) before a fixed period (Nf2) as a first secondary differential signal (DDKACT1 (k)); First extraction means for extracting a first frequency component (frequency f1 component) corresponding to the first frequency (f1) included in one secondary differential signal (DDKAT1 (k)), and the air-fuel ratio oscillation means During operation, the current detection value (KACT (k)) by the air-fuel ratio detection means and the past value (KACT (k−) before the third specific period (Nf1) set to attenuate the first frequency component. Nf1)) as a second difference signal (DKACT2 (k)), a second difference signal generating means for generating a difference between the current value (DKACT2 (k)) of the second difference signal, and the second specific period before The difference from the past value (DKACT2 (k−Nf2)) of the second secondary difference signal (DDK) CT2 (k)) as second secondary difference signal generation means, and the specific frequency component (0.5th-order frequency component) included in the second secondary difference signal (DDKACT2 (k)) is extracted. The difference between the second extraction means, the current value (DKACT1 (k)) of the first difference signal, and the past value (DKACT1 (k−Nf1)) before the third specific period (Nf1) Third secondary difference signal generating means for generating a secondary differential signal (DDKACT3 (k)) and a second frequency component for extracting the second frequency component included in the third secondary differential signal (DDKACT3 (k)) 3 based on the relative relationship between the extraction means, the intensity of the first frequency component extracted by the first extraction means (MPTf1), and the intensity of the second frequency component extracted by the third extraction means (MPTf2). The response characteristic deterioration failure of the air-fuel ratio detection means First failure determination means for performing determination, intensity of the first frequency component extracted by the first extraction means (MPTf1), intensity of the specific frequency component extracted by the second extraction means (MIMB), And a second failure determination means for determining an imbalance failure in which the air-fuel ratio corresponding to each of the plurality of cylinders varies beyond an allowable limit on the basis of the relative relationship.

請求項６に記載の発明は、請求項５に記載の内燃機関の空燃比制御装置において、前記第１の周波数（ｆ１）は、前記機関の回転速度（ＮＥ）が高くなるほど高く設定され、前記第２特定期間（Ｎｆ２）及び第３特定期間（Ｎｆ１）は、前記第１の周波数（ｆ１）が高くなるほど短くなるように設定されることを特徴とする。   According to a sixth aspect of the present invention, in the air-fuel ratio control apparatus for an internal combustion engine according to the fifth aspect, the first frequency (f1) is set higher as the rotational speed (NE) of the engine becomes higher. The second specific period (Nf2) and the third specific period (Nf1) are set to be shorter as the first frequency (f1) is higher.

請求項１に記載の発明によれば、空燃比を第１の周波数で振動させる空燃比振動制御が行われ、空燃比振動制御実行中における空燃比検出手段による今回検出値と、特定周波数に対応する特定周波数成分を減衰させるように設定された第１特定期間前の過去値との差分が第１差分信号として生成され、第１差分信号に含まれる第１の周波数に対応する第１周波数成分が抽出される。さらに空燃比振動制御実行中における空燃比検出手段による今回検出値と、第１周波数成分を減衰させるように設定された第２特定期間前の過去値との差分が第２差分信号として生成され、第２差分信号に含まれる特定周波数成分が抽出される。そして第１抽出手段により抽出される第１周波数成分の強度と、第２抽出手段により抽出される特定周波数成分の強度との相対関係に基づいて、空燃比制御系の故障が判定される。第１差分信号に含まれる特定周波数成分は大きく減衰しており、また第２差分信号に含まれる第１周波数成分は大きく減衰しているため、比較的広い通過帯域幅のバンドパスフィルタ処理を用いて、第１周波数成分及び特定周波数成分を十分なＳ／Ｎで抽出することができる。その結果、空燃比制御系の故障判定を短時間で精度良く行うことができる。   According to the first aspect of the present invention, air-fuel ratio oscillation control is performed to oscillate the air-fuel ratio at the first frequency, and the current detection value by the air-fuel ratio detection means during execution of the air-fuel ratio oscillation control corresponds to the specific frequency. The first frequency component corresponding to the first frequency included in the first difference signal is generated as a first difference signal that is a difference from the previous value before the first specific period set to attenuate the specific frequency component to be Is extracted. Further, a difference between the current detection value by the air-fuel ratio detection means during execution of the air-fuel ratio oscillation control and a past value before the second specific period set so as to attenuate the first frequency component is generated as a second difference signal, A specific frequency component included in the second differential signal is extracted. Then, a failure of the air-fuel ratio control system is determined based on the relative relationship between the intensity of the first frequency component extracted by the first extracting means and the intensity of the specific frequency component extracted by the second extracting means. Since the specific frequency component included in the first differential signal is greatly attenuated and the first frequency component included in the second differential signal is greatly attenuated, a bandpass filter process with a relatively wide pass bandwidth is used. Thus, the first frequency component and the specific frequency component can be extracted with sufficient S / N. As a result, the failure determination of the air-fuel ratio control system can be accurately performed in a short time.

請求項２に記載の発明によれば、機関の回転速度に対応する周波数の１／２の周波数である０．５次周波数の成分強度と、空燃比振動制御の周波数である第１周波数の成分強度とに基づいて、複数気筒のそれぞれに対応する空燃比が許容限度を超えてばらついているインバランス故障が判定される。したがって、０．５次周波数成分強度及び第１周波数成分強度を精度よく算出し、インバランス故障判定を正確に行うことができる。   According to the second aspect of the present invention, the component intensity of the 0.5th order frequency that is a half of the frequency corresponding to the rotational speed of the engine and the component of the first frequency that is the frequency of the air-fuel ratio oscillation control. Based on the strength, an imbalance failure in which the air-fuel ratio corresponding to each of the plurality of cylinders varies beyond an allowable limit is determined. Therefore, the 0.5th order frequency component strength and the first frequency component strength can be accurately calculated, and imbalance failure determination can be performed accurately.

請求項３に記載の発明によれば、空燃比振動制御の周波数である第１の周波数の成分強度と、第１の周波数の整数倍の第２の周波数の成分強度とに基づいて、空燃比検出手段の応答特性劣化故障が判定される。空燃比検出手段の応答特性劣化故障の判定手法は、本出願人による出願の公報（例えば特開２０１０−１０１２８９号公報）に示されており、第１周波数成分強度、及び第１周波数成分強度と第２周波数成分強度との比率に応じて、空燃比検出手段の応答特性劣化故障の判定が可能である。したがって、請求項３の発明により、第１周波数成分強度及び第２周波数成分強度を精度よく算出し、空燃比検出手段の応答特性劣化故障を正確に行うことができる。   According to the third aspect of the present invention, the air-fuel ratio is based on the component intensity of the first frequency that is the frequency of the air-fuel ratio oscillation control and the component intensity of the second frequency that is an integer multiple of the first frequency. A response characteristic deterioration failure of the detection means is determined. The determination method of the response characteristic deterioration failure of the air-fuel ratio detection means is shown in a gazette (for example, Japanese Patent Application Laid-Open No. 2010-101289) of an application by the present applicant, and the first frequency component strength, the first frequency component strength, The response characteristic deterioration failure of the air-fuel ratio detection means can be determined according to the ratio with the second frequency component intensity. Therefore, according to the invention of claim 3, the first frequency component intensity and the second frequency component intensity can be accurately calculated, and the response characteristic deterioration failure of the air-fuel ratio detecting means can be accurately performed.

請求項４に記載の発明によれば、第１の周波数は機関の回転速度が高くなるほど高く設定され、第２特定期間は第１の周波数が高くなるほど短くなるように設定されるので、機関運転状態に応じた周波数で空燃比振動制御を行い、故障判定実行中における排気特性の悪化を抑制することができる。また第２特定期間を第１の周波数が高くなるほど短くなるように設定することにより、第２差分信号に含まれる第１の周波数成分を機関回転速度に拘わらず適切に減衰させることができる。   According to the fourth aspect of the present invention, the first frequency is set to be higher as the rotational speed of the engine is higher, and the second specific period is set to be shorter as the first frequency is higher. By performing air-fuel ratio oscillation control at a frequency corresponding to the state, it is possible to suppress deterioration of exhaust characteristics during execution of failure determination. In addition, by setting the second specific period to be shorter as the first frequency becomes higher, the first frequency component included in the second differential signal can be appropriately attenuated regardless of the engine speed.

請求項５に記載の発明によれば、空燃比を第１の周波数で振動させる空燃比振動制御が行われ、空燃比振動制御実行中における空燃比検出手段による今回検出値と、特定周波数に対応する特定周波数成分を減衰させるように設定された第１特定期間前の過去値との差分が第１差分信号として生成され、第１差分信号の今回値と、第１の周波数の整数倍の周波数に対応する第２周波数成分を減衰させるように設定された第２特定期間前の過去値との差分が第１の二次差分信号として生成され、第１の二次差分信号に含まれる第１の周波数に対応する第１周波数成分が抽出される。さらに空燃比振動制御実行中における空燃比検出手段による今回検出値と、第１周波数成分を減衰させるように設定された第３特定期間前の過去値との差分が第２差分信号として生成され、第２差分信号の今回値と、第２特定期間前の過去値との差分が第２の二次差分信号として生成され、第２の二次差分信号に含まれる特定周波数成分が抽出される。また第１差分信号の今回値と、第３特定期間前の過去値との差分が第３の二次差分信号として生成され、第３の二次差分信号に含まれる第２周波数成分が抽出される。そして、第１抽出手段により抽出される第１周波数成分の強度と、第３抽出手段により抽出される第２周波数成分の強度との相対関係に基づいて、空燃比検出手段の応答特性劣化故障が判定される一方、第１抽出手段により抽出される第１周波数成分の強度と、第２抽出手段により抽出される特定周波数成分の強度との相対関係に基づいて、空燃比のインバランス故障が判定される。第１の二次差分信号に含まれる特定周波数成分及び第２周波数成分は大きく減衰し、第２の二次差分信号に含まれる第１及び第２周波数成分は大きく減衰し、第３の二次差分信号に含まれる特定周波数成分及び第１周波数成分は大きく減衰しているので、比較的広い通過帯域幅のバンドパスフィルタ処理を用いて、第１周波数成分、第２周波数成分、及び特定周波数成分を十分なＳ／Ｎで抽出することができる。その結果、空燃比検出手段の応答特性劣化故障及び空燃比のインバランス故障の判定を短時間で精度良く行うことができる。   According to the fifth aspect of the present invention, the air-fuel ratio vibration control is performed to vibrate the air-fuel ratio at the first frequency, and the current detection value by the air-fuel ratio detection means during execution of the air-fuel ratio vibration control and the specific frequency are supported. A difference from a past value before the first specific period set to attenuate the specific frequency component to be generated is generated as a first difference signal, and the current value of the first difference signal and a frequency that is an integral multiple of the first frequency A difference from a past value before the second specific period set to attenuate the second frequency component corresponding to is generated as a first secondary difference signal, and is included in the first secondary difference signal The first frequency component corresponding to the frequency is extracted. Further, a difference between the current detection value by the air-fuel ratio detection means during execution of the air-fuel ratio vibration control and the past value before the third specific period set so as to attenuate the first frequency component is generated as a second difference signal, A difference between the current value of the second difference signal and the past value before the second specific period is generated as a second secondary difference signal, and a specific frequency component included in the second secondary difference signal is extracted. Further, a difference between the current value of the first difference signal and the past value before the third specific period is generated as a third secondary difference signal, and the second frequency component included in the third secondary difference signal is extracted. The Based on the relative relationship between the intensity of the first frequency component extracted by the first extracting means and the intensity of the second frequency component extracted by the third extracting means, the response characteristic deterioration failure of the air-fuel ratio detecting means is detected. On the other hand, an air-fuel ratio imbalance failure is determined based on the relative relationship between the intensity of the first frequency component extracted by the first extracting means and the intensity of the specific frequency component extracted by the second extracting means. Is done. The specific frequency component and the second frequency component included in the first secondary differential signal are greatly attenuated, and the first and second frequency components included in the second secondary differential signal are greatly attenuated, and the third secondary signal Since the specific frequency component and the first frequency component included in the difference signal are greatly attenuated, the first frequency component, the second frequency component, and the specific frequency component are obtained using bandpass filter processing with a relatively wide pass bandwidth. Can be extracted with sufficient S / N. As a result, it is possible to accurately determine the response characteristic deterioration failure of the air-fuel ratio detection means and the air-fuel ratio imbalance failure in a short time.

請求項６に記載の発明によれば、第１の周波数は機関の回転速度が高くなるほど高く設定され、第２特定期間及び第３特定期間は第１の周波数が高くなるほど短くなるように設定されるので、機関運転状態に応じた周波数で空燃比振動制御を行い、故障判定実行中における排気特性の悪化を抑制することができる。また第２及び第３特定期間を第１の周波数が高くなるほど短くなるように設定することにより、第２差分信号及び第３の二次差分信号に含まれる第１の周波数成分を機関回転速度に拘わらず適切に減衰させることができるとともに、第１及び第２の二次差分信号に含まれる第２の周波数成分を機関回転速度に拘わらず適切に減衰させることができる。   According to the sixth aspect of the present invention, the first frequency is set to be higher as the rotational speed of the engine is higher, and the second specific period and the third specific period are set to be shorter as the first frequency is higher. Therefore, air-fuel ratio oscillation control can be performed at a frequency corresponding to the engine operating state, and deterioration of exhaust characteristics during failure determination execution can be suppressed. In addition, by setting the second and third specific periods to be shorter as the first frequency becomes higher, the first frequency component included in the second differential signal and the third secondary differential signal is set to the engine speed. Regardless of being able to be attenuated appropriately, the second frequency component contained in the first and second secondary difference signals can be appropriately attenuated regardless of the engine speed.

本発明の一実施形態にかかる内燃機関及びその空燃比制御装置の構成を示す図である。1 is a diagram illustrating a configuration of an internal combustion engine and an air-fuel ratio control device thereof according to an embodiment of the present invention. インバランス故障の判定手法を説明するための周波数成分強度分布を示す図である。It is a figure which shows frequency component intensity distribution for demonstrating the determination method of an imbalance failure. 特定の周波数成分を減衰させる差分演算処理の特性を示す図である。It is a figure which shows the characteristic of the difference calculation process which attenuates a specific frequency component. インバランス故障判定を行う処理のフローチャートである。It is a flowchart of the process which performs imbalance failure determination. 酸素濃度センサ（空燃比センサ）の応答特性劣化故障判定を行う処理のフローチャートである。It is a flowchart of the process which performs the response characteristic degradation failure determination of an oxygen concentration sensor (air-fuel ratio sensor). インバランス故障判定及び応答特性劣化故障判定を行う処理のフローチャートである。It is a flowchart of the process which performs imbalance failure determination and response characteristic degradation failure determination. インバランス故障判定及び応答特性劣化故障判定を行う処理のフローチャートである。It is a flowchart of the process which performs imbalance failure determination and response characteristic degradation failure determination. バンドパスフィルタ処理の通過帯域幅と過渡応答特性との関係を説明するための図である。It is a figure for demonstrating the relationship between the pass band width of a band pass filter process, and a transient response characteristic. バンドパスフィルタ処理出力の過渡応答例を示すタイムチャートである。It is a time chart which shows the example of a transient response of a band pass filter process output.

以下本発明の実施の形態を図面を参照して説明する。
［第１の実施形態］
図１は、本発明の一実施形態にかかる内燃機関（以下「エンジン」という）及びその空燃比制御装置の全体構成図であり、例えば４気筒のエンジン１の吸気管２の途中にはスロットル弁３が配されている。スロットル弁３にはスロットル弁開度ＴＨを検出するスロットル弁開度センサ４が連結されており、その検出信号は電子制御ユニット（以下「ＥＣＵ」という）５に供給される。 Embodiments of the present invention will be described below with reference to the drawings.
[First Embodiment]
FIG. 1 is an overall configuration diagram of an internal combustion engine (hereinafter referred to as “engine”) and an air-fuel ratio control device thereof according to an embodiment of the present invention. For example, a throttle valve is provided in the middle of an intake pipe 2 of a 4-cylinder engine 1. 3 is arranged. A throttle valve opening sensor 4 for detecting the throttle valve opening TH is connected to the throttle valve 3, and the detection signal is supplied to an electronic control unit (hereinafter referred to as “ECU”) 5.

燃料噴射弁６はエンジン１とスロットル弁３との間かつ吸気管２の図示しない吸気弁の少し上流側に各気筒毎に設けられており、各噴射弁は図示しない燃料ポンプに接続されていると共にＥＣＵ５に電気的に接続されて当該ＥＣＵ５からの信号により燃料噴射弁６の開弁時間が制御される。   The fuel injection valve 6 is provided for each cylinder between the engine 1 and the throttle valve 3 and slightly upstream of the intake valve (not shown) of the intake pipe 2, and each injection valve is connected to a fuel pump (not shown). At the same time, it is electrically connected to the ECU 5 and the valve opening time of the fuel injection valve 6 is controlled by a signal from the ECU 5.

スロットル弁３の上流側には吸入空気流量ＧＡＩＲを検出する吸入空気流量センサ７が設けられている。またスロットル弁３の下流側には吸気圧ＰＢＡを検出する吸気圧センサ８、及び吸気温ＴＡを検出する吸気温センサ９が設けられている。これらのセンサの検出信号は、ＥＣＵ５に供給される。エンジン１の本体には、エンジン冷却水温ＴＷを検出する冷却水温センサ１０が装着されており、その検出信号はＥＣＵ５に供給される。   An intake air flow rate sensor 7 for detecting the intake air flow rate GAIR is provided on the upstream side of the throttle valve 3. An intake pressure sensor 8 for detecting the intake pressure PBA and an intake air temperature sensor 9 for detecting the intake air temperature TA are provided on the downstream side of the throttle valve 3. Detection signals from these sensors are supplied to the ECU 5. A cooling water temperature sensor 10 for detecting the engine cooling water temperature TW is attached to the main body of the engine 1, and the detection signal is supplied to the ECU 5.

ＥＣＵ５には、エンジン１のクランク軸（図示せず）の回転角度を検出するクランク角度位置センサ１１が接続されており、クランク軸の回転角度に応じた信号がＥＣＵ５に供給される。クランク角度位置センサ１１は、エンジン１の特定の気筒の所定クランク角度位置でパルス（以下「ＣＹＬパルス」という）を出力する気筒判別センサ、各気筒の吸入行程開始時の上死点（ＴＤＣ）に関し所定クランク角度前のクランク角度位置で（４気筒エンジンではクランク角１８０度毎に）ＴＤＣパルスを出力するＴＤＣセンサ及びＴＤＣパルスより短い一定クランク角周期（例えば６度周期）で１パルス（以下「ＣＲＫパルス」という）を発生するＣＲＫセンサから成り、ＣＹＬパルス、ＴＤＣパルス及びＣＲＫパルスがＥＣＵ５に供給される。これらのパルスは、燃料噴射時期、点火時期等の各種タイミング制御、エンジン回転数（エンジン回転速度）ＮＥの検出に使用される。   The ECU 5 is connected to a crank angle position sensor 11 that detects a rotation angle of a crankshaft (not shown) of the engine 1, and a signal corresponding to the rotation angle of the crankshaft is supplied to the ECU 5. The crank angle position sensor 11 is a cylinder discrimination sensor that outputs a pulse (hereinafter referred to as “CYL pulse”) at a predetermined crank angle position of a specific cylinder of the engine 1, and relates to a top dead center (TDC) at the start of the intake stroke of each cylinder. A TDC sensor that outputs a TDC pulse at a crank angle position before a predetermined crank angle (every 180 degrees of crank angle in a four-cylinder engine) and one pulse (hereinafter referred to as “CRK”) with a constant crank angle cycle shorter than the TDC pulse (for example, a cycle of 6 °). The CYL pulse, the TDC pulse, and the CRK pulse are supplied to the ECU 5. These pulses are used for various timing controls such as fuel injection timing and ignition timing, and detection of engine speed (engine speed) NE.

排気通路１３には三元触媒１４が設けられている。三元触媒１４は、酸素蓄積能力を有し、エンジン１に供給される混合気の空燃比が理論空燃比よりリーン側に設定され、排気中の酸素濃度が比較的高い排気リーン状態では、排気中の酸素を蓄積し、逆にエンジン１に供給される混合気の空燃比が理論空燃比よりリッチ側に設定され、排気中の酸素濃度が低く、ＨＣ、ＣＯ成分が多い排気リッチ状態では、蓄積した酸素により排気中のＨＣ，ＣＯを酸化する機能を有する。   A three-way catalyst 14 is provided in the exhaust passage 13. The three-way catalyst 14 has an oxygen storage capacity, the air-fuel ratio of the air-fuel mixture supplied to the engine 1 is set to be leaner than the stoichiometric air-fuel ratio, and in the exhaust lean state where the oxygen concentration in the exhaust gas is relatively high, In the exhaust rich state where the air-fuel ratio of the air-fuel mixture supplied to the engine 1 is set richer than the stoichiometric air-fuel ratio, the oxygen concentration in the exhaust gas is low, and the HC and CO components are large. It has the function of oxidizing HC and CO in the exhaust with the accumulated oxygen.

三元触媒１４の上流側であって各気筒に連通する排気マニホールドの集合部より下流側には、比例型酸素濃度センサ１５（以下「ＬＡＦセンサ１５」という）が装着されており、このＬＡＦセンサ１５は排気中の酸素濃度（空燃比）にほぼ比例した検出信号を出力し、ＥＣＵ５に供給する。   A proportional oxygen concentration sensor 15 (hereinafter referred to as “LAF sensor 15”) is mounted on the upstream side of the three-way catalyst 14 and on the downstream side of the collection portion of the exhaust manifold communicating with each cylinder. 15 outputs a detection signal substantially proportional to the oxygen concentration (air-fuel ratio) in the exhaust gas and supplies it to the ECU 5.

ＥＣＵ５には、エンジン１により駆動される車両のアクセルペダルの踏み込み量（以下「アクセルペダル操作量」という）ＡＰを検出するアクセルセンサ２１及び当該車両の走行速度（車速）ＶＰを検出する車速センサ２２が接続されており、それらセンサの検出信号がＥＣＵ５に供給される。スロットル弁３は図示しないアクチュエータにより開閉駆動され、スロットル弁開度ＴＨはアクセルペダル操作量ＡＰに応じてＥＣＵ５により制御される。
なお、図示は省略しているが、エンジン１には周知の排気還流機構が設けられている。 The ECU 5 includes an accelerator sensor 21 for detecting an accelerator pedal depression amount (hereinafter referred to as “accelerator pedal operation amount”) AP of a vehicle driven by the engine 1 and a vehicle speed sensor 22 for detecting a traveling speed (vehicle speed) VP of the vehicle. Are connected, and detection signals from these sensors are supplied to the ECU 5. The throttle valve 3 is driven to open and close by an actuator (not shown), and the throttle valve opening TH is controlled by the ECU 5 in accordance with the accelerator pedal operation amount AP.
Although not shown, the engine 1 is provided with a known exhaust gas recirculation mechanism.

ＥＣＵ５は、各種センサからの入力信号波形を整形し、電圧レベルを所定レベルに修正し、アナログ信号値をデジタル信号値に変換する等の機能を有する入力回路、中央演算処理ユニット（以下「ＣＰＵ」という）、該ＣＰＵで実行される各種演算プログラム及び演算結果等を記憶する記憶回路、燃料噴射弁６に駆動信号を供給する出力回路を備えている。   The ECU 5 shapes input signal waveforms from various sensors, corrects the voltage level to a predetermined level, converts an analog signal value into a digital signal value, etc., and a central processing unit (hereinafter referred to as “CPU”). A storage circuit for storing various calculation programs executed by the CPU and calculation results, and an output circuit for supplying a drive signal to the fuel injection valve 6.

ＥＣＵ５のＣＰＵは、上述の各種センサの検出信号に基づいて、種々のエンジン運転状態を判別するとともに、該判別されたエンジン運転状態に応じて、次式（１）を用いて、ＴＤＣパルスに同期して開弁作動する燃料噴射弁６の燃料噴射時間ＴＯＵＴを演算する。燃料噴射時間ＴＯＵＴは、噴射される燃料量にほぼ比例するので、以下「燃料噴射量ＴＯＵＴ」という。
ＴＯＵＴ＝ＴＩＭ×ＫＣＭＤ×ＫＡＦ×ＫＴＯＴＡＬ （１） The CPU of the ECU 5 discriminates various engine operating states based on the detection signals of the various sensors described above, and synchronizes with the TDC pulse using the following equation (1) according to the discriminated engine operating state. Then, the fuel injection time TOUT of the fuel injection valve 6 that opens is calculated. Since the fuel injection time TOUT is substantially proportional to the amount of fuel injected, it is hereinafter referred to as “fuel injection amount TOUT”.
TOUT = TIM × KCMD × KAF × KTOTAL (1)

ここに、ＴＩＭは基本燃料量、具体的には燃料噴射弁６の基本燃料噴射時間であり、吸入空気流量ＧＡＩＲに応じて設定されたＴＩＭテーブルを検索して決定される。ＴＩＭテーブルは、エンジンにおいて燃焼する混合気の空燃比ＡＦがほぼ理論空燃比になるように設定されている。   Here, TIM is a basic fuel amount, specifically, a basic fuel injection time of the fuel injection valve 6, and is determined by searching a TIM table set according to the intake air flow rate GAIR. The TIM table is set so that the air-fuel ratio AF of the air-fuel mixture combusted in the engine becomes substantially the stoichiometric air-fuel ratio.

ＫＣＭＤはエンジン１の運転状態に応じて設定される目標空燃比係数である。目標空燃比係数ＫＣＭＤは、空燃比Ａ／Ｆの逆数、すなわち燃空比Ｆ／Ａに比例し、理論空燃比のとき値１．０をとるので、以下「目標当量比」という。後述するように、空燃比のインバランス故障判定を行うときは、１．０±ＤＡＦの範囲で時間経過に伴って正弦波状に変化するように設定される。   KCMD is a target air-fuel ratio coefficient set according to the operating state of the engine 1. The target air-fuel ratio coefficient KCMD is proportional to the reciprocal of the air-fuel ratio A / F, that is, the fuel-air ratio F / A, and takes a value of 1.0 at the stoichiometric air-fuel ratio. As will be described later, when the imbalance failure determination of the air-fuel ratio is performed, it is set to change in a sine wave shape with the passage of time in the range of 1.0 ± DAF.

ＫＡＦは、空燃比フィードバック制御の実行条件が成立するときは、ＬＡＦセンサ１５の検出値から算出される検出当量比ＫＡＣＴが目標当量比ＫＣＭＤに一致するようにＰＩＤ（比例積分微分）制御あるいは適応制御器（Self Tuning Regulator）を用いた適応制御により算出される空燃比補正係数である。   KAF performs PID (proportional integral derivative) control or adaptive control so that the detected equivalent ratio KACT calculated from the detected value of the LAF sensor 15 matches the target equivalent ratio KCMD when the execution condition of the air-fuel ratio feedback control is satisfied. This is an air-fuel ratio correction coefficient calculated by adaptive control using a self-tuning regulator.

ＫＴＯＴＡＬは夫々各種エンジンパラメータ信号に応じて演算される他の補正係数（エンジン冷却水温ＴＭに応じた補正係数ＫＴＷ、吸気温ＴＡに応じた補正係数ＫＴＡなど）の積である。   KTOTAL is a product of other correction coefficients (a correction coefficient KTW corresponding to the engine coolant temperature TM, a correction coefficient KTA corresponding to the intake air temperature TA, etc.) calculated according to various engine parameter signals.

ＥＣＵ５のＣＰＵは上述のようにして求めた燃料噴射量ＴＯＵＴに基づいて燃料噴射弁６を開弁させる駆動信号を出力回路を介して燃料噴射弁６に供給する。また、ＥＣＵ５のＣＰＵは、以下に説明するように空燃比のインバランス故障判定を行う。   The CPU of the ECU 5 supplies a drive signal for opening the fuel injection valve 6 to the fuel injection valve 6 via the output circuit based on the fuel injection amount TOUT obtained as described above. Further, the CPU of the ECU 5 performs air-fuel ratio imbalance failure determination as described below.

本実施形態におけるインバランス故障判定手法は、基本的には特許文献１に示される手法と同一のものであり、エンジン運転中に空燃比を振動周波数ｆ１で振動させる空燃比振動制御を実行し、その制御実行中におけるＬＡＦセンサ１５の出力信号ＳＬＡＦに含まれる０．５次周波数成分強度ＭＩＭＢを、周波数ｆ１成分強度ＭＰｆ１で除算することにより得られる判定パラメータＲＴを用いて、インバランス故障が判定される。   The imbalance failure determination method in the present embodiment is basically the same as the method disclosed in Patent Document 1, and executes air-fuel ratio vibration control that vibrates the air-fuel ratio at the vibration frequency f1 during engine operation. An imbalance failure is determined using the determination parameter RT obtained by dividing the 0.5th order frequency component intensity MIMB included in the output signal SLAF of the LAF sensor 15 during the execution of the control by the frequency f1 component intensity MPf1. The

図２は、ＬＡＦセンサ出力信号ＳＬＡＦの周波数成分強度分布の一例を示す図であり、この例は振動周波数ｆ１をエンジン回転数ＮＥ［ｒｐｍ］に対応するエンジン回転周波数ｆＮＥ（＝ＮＥ／６０）の０．４倍に設定し、インバランス故障が発生している状態に対応する。この状態では、図２に示すように周波数ｆ１成分の強度ＭＰＴｆ１と、周波数ｆＩＭＢ（＝０．５×ｆＮＥ）成分、すなわち０．５次周波数成分の強度ＭＩＭＢとがピーク値を示す。   FIG. 2 is a diagram showing an example of the frequency component intensity distribution of the LAF sensor output signal SLAF. In this example, the vibration frequency f1 is an engine rotational frequency fNE (= NE / 60) corresponding to the engine rotational speed NE [rpm]. It is set to 0.4 times and corresponds to a state where an imbalance failure has occurred. In this state, as shown in FIG. 2, the intensity MPTf1 of the frequency f1 component and the frequency fIMB (= 0.5 × fNE) component, that is, the intensity MIMB of the 0.5th-order frequency component show peak values.

このように比較的周波数が近接した成分の強度を精度良く算出するために、本実施形態では、ＬＡＦセンサ出力ＳＬＡＦから算出される検出当量比の今回値ＫＡＣＴ(k)と、所定期間ＴＰ前の過去値ＫＡＣＴ(k-NP)との差分ＤＫＡＣＴ(k)を下記式（２）により算出し、差分ＤＫＡＣＴ(k)を用いてインバランス故障判定を行う。式（２）のＮＰは、所定期間ＴＰを、ＬＡＦセンサ出力ＳＬＡＦのサンプリング周期ＴＰＳＰで離散化した離散遅延期間である。
ＤＫＡＣＴ(k)＝ＫＡＣＴ(k)−ＫＡＣＴ(k-NP) （２） In this embodiment, in order to accurately calculate the intensity of components having relatively close frequencies in this way, in this embodiment, the current value KACT (k) of the detected equivalent ratio calculated from the LAF sensor output SLAF and the time before the predetermined period TP A difference DKACT (k) from the past value KACT (k-NP) is calculated by the following equation (2), and an imbalance failure determination is performed using the difference DKACT (k). NP in Expression (2) is a discrete delay period obtained by discretizing the predetermined period TP with the sampling period TPSP of the LAF sensor output SLAF.
DKACT (k) = KACT (k) −KACT (k-NP) (2)

式（２）により算出される差分ＤＫＡＣＴ(k)のゲイン周波数特性は、一般に図３に示すような特性を示す。図３に示す減衰極周波数ｆＰＯＬＥは、下記式（３）に示すように所定期間ＴＰの逆数に相当する周波数である。
ｆＰＯＬＥ＝１／ＴＰ （３） The gain frequency characteristic of the difference DKACT (k) calculated by the equation (2) generally shows a characteristic as shown in FIG. The attenuation pole frequency fPOLE shown in FIG. 3 is a frequency corresponding to the reciprocal of the predetermined period TP as shown in the following formula (3).
fPOLE = 1 / TP (3)

本実施形態では、０．５周波数成分を抽出するときは、減衰極周波数ｆＰＯＬＥを空燃比の振動周波数ｆ１（＝０．４ｆＮＥ）とするため、検出当量比ＫＡＣＴのサンプリングをクランク角度３０度毎に行い、式（２）の離散遅延期間ＮＰを「３０」に設定する。また周波数ｆ１成分を抽出するときは、減衰極周波数ｆＰＯＬＥを０．５次周波数ｆＩＭＢとするため、式（２）の離散遅延期間ＮＰを「２４」に設定する。このように減衰させたい周波数に応じたサンプリング周期ＴＰＳＰ及び対応する離散遅延期間ＮＰを使用することによって、抽出信号のＳ／Ｎを向上させることができる。   In this embodiment, when extracting the 0.5 frequency component, the attenuation pole frequency fPOLE is set to the vibration frequency f1 (= 0.4 fNE) of the air-fuel ratio, so that the detection equivalent ratio KACT is sampled every 30 degrees of the crank angle. Then, the discrete delay period NP of Expression (2) is set to “30”. When the frequency f1 component is extracted, the discrete delay period NP of the equation (2) is set to “24” in order to set the attenuation pole frequency fPOLE to the 0.5th order frequency fIMB. Thus, by using the sampling period TPSP corresponding to the frequency to be attenuated and the corresponding discrete delay period NP, the S / N of the extracted signal can be improved.

そして、差分ＤＫＡＣＴ(k)について、必要な周波数成分を抽出するバンドパスフィルタ処理を行い、０．５次周波数成分強度ＭＩＭＢ及び周波数ｆ１成分強度ＭＰＴｆ１を算出する。このバンドパスフィルタ処理の通過帯域幅ＷＢを比較的広く設定することにより、インバランス故障判定を短時間で精度良く行うこと可能となる。   Then, for the difference DKACT (k), a bandpass filter process for extracting a necessary frequency component is performed to calculate a 0.5th order frequency component intensity MIMB and a frequency f1 component intensity MPTf1. By setting the passband width WB of this bandpass filter processing to be relatively wide, imbalance failure determination can be performed with high accuracy in a short time.

図４は、本実施形態におけるインバランス故障判定処理のフローチャートである。この処理は、インバランス故障判定の実行条件が成立しているときに、所定クランク角度ＣＡＣＡＬ（例えば３０度）毎にＥＣＵ５のＣＰＵで実行される。   FIG. 4 is a flowchart of the imbalance failure determination process in the present embodiment. This process is executed by the CPU of the ECU 5 at every predetermined crank angle CACAL (for example, 30 degrees) when the imbalance failure determination execution condition is satisfied.

インバランス故障判定実行条件は、例えば下記の条件１）〜１１）がすべて満たされると成立する。
１）エンジン回転数ＮＥが所定上下限値の範囲内にある。
２）吸気圧ＰＢＡが所定圧より高い（判定に必要な排気流量が確保されている）。
３）ＬＡＦセンサ１５が活性化している。
４）ＬＡＦセンサ１５の出力に応じた空燃比フィードバック制御が実行されている。
５）エンジン冷却水温ＴＷが所定温度より高い。
６）エンジン回転数ＮＥの単位時間当たりの変化量ＤＮＥが所定回転数変化量より小さい。
７）吸気圧ＰＢＡの単位時間当たりの変化量ＤＰＢＡＦが所定吸気圧変化量より小さい。
８）燃料の加速増量（急加速時に実行される）が行われていない。
９）排気還流率が所定値より大きい。
１０）ＬＡＦセンサ出力が上限値または下限値に張り付いた状態ではない。
１１）ＬＡＦセンサの応答特性が正常である（応答特性の劣化故障が発生しているとの判定が行われていない）。 The imbalance failure determination execution condition is satisfied when, for example, all of the following conditions 1) to 11) are satisfied.
1) The engine speed NE is within a predetermined upper and lower limit value range.
2) The intake pressure PBA is higher than a predetermined pressure (an exhaust flow rate necessary for determination is secured).
3) The LAF sensor 15 is activated.
4) Air-fuel ratio feedback control according to the output of the LAF sensor 15 is executed.
5) The engine coolant temperature TW is higher than a predetermined temperature.
6) The change amount DNE per unit time of the engine speed NE is smaller than the predetermined speed change amount.
7) The change amount DPBAF per unit time of the intake pressure PBA is smaller than the predetermined intake pressure change amount.
8) Acceleration increase of fuel (executed during sudden acceleration) is not performed.
9) The exhaust gas recirculation rate is larger than a predetermined value.
10) The LAF sensor output is not stuck to the upper limit value or the lower limit value.
11) The response characteristic of the LAF sensor is normal (it is not determined that a deterioration failure of the response characteristic has occurred).

インバランス故障判定実行条件が成立すると、目標当量比ＫＣＭＤを下記式（４）によって振動させる空燃比振動制御を開始する。空燃比振動制御実行中は、空燃比補正係数ＫＡＦを「１．０」または「１．０」以外の特定の値に固定される。式（４）のＫｆ１は、例えば振動周波数ｆ１を０．４ｆＮＥとする場合には、「０．４」に設定される第１周波数係数であり、「ｋ」は目標当量比ＫＣＭＤの算出周期ＣＡＣＡＬで離散化した離散化時刻である。
ＫＣＭＤ＝ＤＡＦ×ｓｉｎ（Ｋｆ１×ＣＡＣＡＬ×ｋ）＋１ （４） When the imbalance failure determination execution condition is satisfied, air-fuel ratio oscillation control for oscillating the target equivalent ratio KCMD by the following equation (4) is started. During execution of the air-fuel ratio oscillation control, the air-fuel ratio correction coefficient KAF is fixed to a specific value other than “1.0” or “1.0”. For example, when the vibration frequency f1 is set to 0.4 fNE, Kf1 in Expression (4) is a first frequency coefficient set to “0.4”, and “k” is a calculation cycle CACAL of the target equivalent ratio KCMD. It is the discretization time discretized by.
KCMD = DAF × sin (Kf1 × CACAL × k) +1 (4)

図４のステップＳ１１では、空燃比振動制御フラグＦＰＴが「１」であるか否かを判別する。空燃比振動制御フラグＦＰＴは、空燃比振動制御の開始時点から所定安定化時間ＴＳＴＢＬが経過すると「１」に設定される。ステップＳ１１の答が否定（ＮＯ）であるときは直ちに処理を終了する。   In step S11 of FIG. 4, it is determined whether or not the air-fuel ratio oscillation control flag FPT is “1”. The air-fuel ratio vibration control flag FPT is set to “1” when a predetermined stabilization time TSTBL has elapsed from the start time of the air-fuel ratio vibration control. If the answer to step S11 is negative (NO), the process immediately ends.

ステップＳ１１の答が肯定（ＹＥＳ）であるときは、検出当量比ＫＡＣＴの今回値ＫＡＣＴ(k)を取得し、メモリに格納する処理を行う（ステップＳ１２）。メモリには、上述した式（２）による差分ＤＫＡＣＴの算出に必要な数の過去値が格納される。   If the answer to step S11 is affirmative (YES), the current value KACT (k) of the detected equivalent ratio KACT is acquired and stored in the memory (step S12). The memory stores the number of past values necessary for calculating the difference DKACT by the above-described equation (2).

ステップＳ１３では、下記式（５）により、第１差分ＤＫＡＣＴ１(k)を算出する。式（５）のＮＩＭＢは、０．５次周波数成分を減衰させるための第１離散遅延期間であり、本実施形態では「２４」に設定される。
ＤＫＡＣＴ１(k)＝ＫＡＣＴ(k)−ＫＡＣＴ(k-NIMB) （５） In step S13, the first difference DKACT1 (k) is calculated by the following equation (5). NIMB in Expression (5) is a first discrete delay period for attenuating the 0.5th-order frequency component, and is set to “24” in the present embodiment.
DKACT1 (k) = KACT (k) −KACT (k−NIMB) (5)

ステップＳ１４では、第１差分ＤＫＡＣＴ１(k)について、周波数ｆ１成分を抽出するバンドパスフィルタ処理を行い、ステップＳ１５ではステップＳ１４で得られるバンドパスフィルタ処理出力の絶対値（振幅）を積算することにより、周波数ｆ１成分強度ＭＰＴｆ１を算出する。   In step S14, bandpass filter processing for extracting the frequency f1 component is performed on the first difference DKACT1 (k). In step S15, the absolute value (amplitude) of the bandpass filter processing output obtained in step S14 is integrated. The frequency f1 component strength MPTf1 is calculated.

ステップＳ１６では、下記式（６）により第２差分ＤＫＡＣＴ２(k)を算出する。式（６）のＮｆ１は、周波数ｆ１成分を減衰させるための第２離散遅延期間であり、本実施形態では「３０」に設定される。
ＤＫＡＣＴ１(k)＝ＫＡＣＴ(k)−ＫＡＣＴ(k-Nf1) （６） In step S16, the second difference DKACT2 (k) is calculated by the following equation (6). Nf1 in Expression (6) is a second discrete delay period for attenuating the frequency f1 component, and is set to “30” in the present embodiment.
DKACT1 (k) = KACT (k) −KACT (k−Nf1) (6)

ステップＳ１７では、第２差分ＤＫＡＣＴ３(k)について、０．５次周波数成分を抽出するバンドパスフィルタ処理を行い、ステップＳ１８ではステップＳ１７で得られるバンドパスフィルタ処理出力の絶対値（振幅）を積算することにより、０．５次周波数成分強度ＭＩＭＢを算出する。   In step S17, bandpass filter processing for extracting the 0.5th order frequency component is performed on the second difference DKACT3 (k), and in step S18, the absolute value (amplitude) of the bandpass filter processing output obtained in step S17 is integrated. By doing so, the 0.5th-order frequency component intensity MIMB is calculated.

ステップＳ１９では、周波数成分強度の算出開始時点から所定積算時間ＴＩＮＴが経過したか否かを判別し、その答が否定（ＮＯ）である間は直ちに処理を終了する。ステップＳ１９の答が肯定（ＹＥＳ）となると、下記式（７）により判定パラメータＲＴを算出する。
ＲＴ＝ＭＩＭＢ／ＭＰＴｆ１ （７） In step S19, it is determined whether or not the predetermined integration time TINT has elapsed since the calculation start time of the frequency component intensity, and the process is immediately terminated while the answer is negative (NO). If the answer to step S19 is affirmative (YES), the determination parameter RT is calculated by the following equation (7).
RT = MIMB / MPTf1 (7)

ステップＳ２１では、判定パラメータＲＴが判定閾値ＲＴＴＨより大きいか否かを判別し、その答が肯定（ＹＥＳ）であるときは、インバランス故障が発生していると判定する（ステップＳ２２）。一方、ステップＳ２１の答が否定（ＮＯ）であるときは、気筒毎の空燃比の差は許容限度内にある（正常）と判定する（ステップＳ２３）。   In step S21, it is determined whether or not the determination parameter RT is larger than a determination threshold value RTTH. If the answer is affirmative (YES), it is determined that an imbalance failure has occurred (step S22). On the other hand, when the answer to step S21 is negative (NO), it is determined that the difference in air-fuel ratio for each cylinder is within an allowable limit (normal) (step S23).

以上のように本実施形態では、空燃比を周波数ｆ１で振動させる空燃比振動制御が行われ、空燃比振動制御実行中における検出当量比の今回検出値ＫＡＣＴ(k)と、０．５次周波数ｆＩＭＢに対応する０．５次周波数成分を減衰させるように設定された第１離散遅延期間ＮＩＭＢ前の過去値ＫＡＣＴ(k-NIMB)との差分が第１差分ＤＫＡＣＴ１(k)として算出され、第１差分ＤＫＡＣＴ１(k)に含まれる周波数ｆ１に対応する周波数ｆ１成分がバンドパスフィルタ処理により抽出され、バンドパスフィルタ処理出力の絶対値の積算演算によって周波数ｆ１成分強度ＭＰＴｆ１が算出される。一方空燃比振動制御実行中における検出当量比の今回値ＫＡＣＴ(k)と、周波数ｆ１成分を減衰させるように設定された第２離散遅延期間Ｎｆ１前の過去値ＫＡＣＴ(k-Nf1)との差分が第２差分ＤＫＡＣＴ２(k)として算出され、第２差分ＤＫＡＣＴ２(k)に含まれる０．５次周波数成分がバンドパスフィルタ処理により抽出され、バンドパスフィルタ処理出力の絶対値の積算演算によって０．５次周波数成分強度ＭＩＭＢが算出される。そして周波数ｆ１成分強度ＭＰＴｆ１で０．５次周波数成分強度ＭＩＭＢを除算することにより算出される判定パラメータＲＴと、判定閾値ＲＴＴＨとを比較することにより、インバランス故障が判定される。第１差分ＤＫＡＣＴ１(k)に含まれる０．５次周波数成分は大きく減衰しており、また第２差分ＤＫＡＣＴ２(k)に含まれる周波数ｆ１成分は大きく減衰しているため、比較的広い通過帯域幅のバンドパスフィルタ処理を用いて、周波数ｆ１成分及び０．５次周波数成分を十分なＳ／Ｎで抽出することができる。その結果、インバランス故障判定を短時間で精度良く行うことができる。   As described above, in the present embodiment, air-fuel ratio oscillation control is performed to oscillate the air-fuel ratio at the frequency f1, and the currently detected value KACT (k) of the detected equivalent ratio during execution of the air-fuel ratio oscillation control and the 0.5th order frequency A difference from the previous value KACT (k-NIMB) before the first discrete delay period NIMB set so as to attenuate the 0.5th order frequency component corresponding to fIMB is calculated as the first difference DKACT1 (k), A frequency f1 component corresponding to the frequency f1 included in one difference DKACT1 (k) is extracted by bandpass filter processing, and a frequency f1 component strength MPTf1 is calculated by integration of absolute values of bandpass filter processing outputs. On the other hand, the difference between the current value KACT (k) of the detected equivalent ratio during execution of the air-fuel ratio oscillation control and the past value KACT (k−Nf1) before the second discrete delay period Nf1 set so as to attenuate the frequency f1 component. Is calculated as the second difference DKACT2 (k), the 0.5th-order frequency component included in the second difference DKACT2 (k) is extracted by bandpass filter processing, and 0 is obtained by integrating the absolute values of the bandpass filter processing outputs. The fifth-order frequency component intensity MIMB is calculated. Then, the imbalance failure is determined by comparing the determination parameter RT calculated by dividing the 0.5th-order frequency component strength MIMB by the frequency f1 component strength MPTf1 with the determination threshold value RTTH. Since the 0.5th order frequency component included in the first difference DKACT1 (k) is greatly attenuated and the frequency f1 component included in the second difference DKACT2 (k) is greatly attenuated, a relatively wide passband is obtained. The frequency f1 component and the 0.5th order frequency component can be extracted with sufficient S / N by using the band-pass filter processing of the width. As a result, the imbalance failure determination can be accurately performed in a short time.

また本実施形態では、空燃比振動制御の周波数ｆ１は、エンジン回転周波数ｆＮＥの０．４倍に設定されるので、エンジン回転数ＮＥが高くなるほど周波数ｆ１が高くなる。これにより、故障判定実行中における排気特性の悪化及び判定精度の低下を抑制することができる。すなわち、一定の振動周波数で振動させる場合には、エンジン回転数ＮＥと特定の関係となり（エンジン回転数ＮＥに対応する周波数の１／２倍、１倍などに該当し）、空燃比振動制御による空燃比変動と、エンジン回転に起因するノイズによる空燃比変動とが区別できずに、判定精度が低下する可能性があるが、本実施形態においては、周波数ｆ１はエンジン回転周波数ｆＮＥの０．４倍に設定されるので、そのような事態を確実に回避することが可能である。また、故障判定に例えば２０振動周期の期間を要する場合、エンジン回転数ＮＥが高くなるほど故障判定時間が短縮され、空燃比振動制御を行うことに起因する排気特性の悪化を抑制することができる。   In the present embodiment, the frequency f1 of the air-fuel ratio vibration control is set to 0.4 times the engine speed fNE, so that the frequency f1 increases as the engine speed NE increases. Thereby, it is possible to suppress the deterioration of the exhaust characteristics and the decrease of the determination accuracy during the execution of the failure determination. That is, when vibrating at a constant vibration frequency, it has a specific relationship with the engine speed NE (corresponding to 1/2 times, 1 time, etc. of the frequency corresponding to the engine speed NE). Although the air-fuel ratio fluctuation and the air-fuel ratio fluctuation due to noise caused by engine rotation cannot be distinguished from each other, there is a possibility that the determination accuracy is lowered. In this embodiment, the frequency f1 is 0.4 of the engine rotation frequency fNE. It is possible to reliably avoid such a situation because it is set twice. Further, when a period of 20 vibration cycles is required for the failure determination, for example, the failure determination time is shortened as the engine speed NE increases, and the deterioration of the exhaust characteristics due to the air-fuel ratio vibration control can be suppressed.

本実施形態では、ＬＡＦセンサ１５が空燃比検出手段に相当し、燃料噴射弁６が空燃比振動手段の一部に相当し、ＥＣＵ５が空燃比振動手段の一部、第１差分信号生成手段、第１抽出手段、第２差分信号生成手段、第２抽出手段、及び故障判定手段を構成する。具体的には、図４のステップＳ１３が第１差分信号生成手段に相当し、ステップＳ１４が第１抽出手段に相当し、ステップＳ１６が第２差分信号生成手段に相当し、ステップＳ１７が第２抽出手段に相当し、ステップＳ１５，Ｓ１８，及びＳ２０〜Ｓ２３が故障判定手段に相当する。   In the present embodiment, the LAF sensor 15 corresponds to the air-fuel ratio detecting means, the fuel injection valve 6 corresponds to a part of the air-fuel ratio oscillating means, the ECU 5 is a part of the air-fuel ratio oscillating means, the first difference signal generating means, First extraction means, second difference signal generation means, second extraction means, and failure determination means are configured. Specifically, step S13 in FIG. 4 corresponds to the first difference signal generation means, step S14 corresponds to the first extraction means, step S16 corresponds to the second difference signal generation means, and step S17 corresponds to the second difference signal generation means. It corresponds to extraction means, and steps S15, S18, and S20 to S23 correspond to failure determination means.

［第２の実施形態］
本実施形態は、本発明をＬＡＦセンサ１５の応答特性劣化故障判定に適用したものである。以下に説明する点以外は第１の実施形態と同一である。 [Second Embodiment]
In the present embodiment, the present invention is applied to the response characteristic deterioration failure determination of the LAF sensor 15. Except for the points described below, the second embodiment is the same as the first embodiment.

応答特性劣化故障の判定手法は、基本的には上述した特開２０１０−１０１２８９号公報に示されたものと同一であり、判定に必要な周波数成分の抽出に本発明が適用される。すなわち、本実施形態では、周波数ｆ１（以下「第１の周波数ｆ１」という）の空燃比振動制御を実行し、ＬＡＦセンサ１５の出力信号から算出される検出当量比ＫＡＣＴに含まれる周波数ｆ１の成分強度ＭＰＴｆ１と、周波数ｆ１の２倍の周波数である周波数ｆ２（以下「第２の周波数ｆ２」という）に対応する周波数ｆ２の成分強度ＭＰＴｆ２とを用いて、応答特性劣化故障が判定される。   The determination method of the response characteristic deterioration failure is basically the same as that described in Japanese Patent Application Laid-Open No. 2010-101289, and the present invention is applied to extraction of frequency components necessary for the determination. That is, in the present embodiment, the air-fuel ratio oscillation control at the frequency f1 (hereinafter referred to as “first frequency f1”) is executed, and the component of the frequency f1 included in the detected equivalent ratio KACT calculated from the output signal of the LAF sensor 15. A response characteristic deterioration failure is determined using the intensity MPTf1 and the component intensity MPTf2 of the frequency f2 corresponding to the frequency f2 (hereinafter referred to as “second frequency f2”) that is twice the frequency f1.

図５は本実施形態におけるＬＡＦセンサ故障判定処理のフローチャートである。この処理は、ＬＡＦセンサ故障判定の実行条件が成立しているときに、所定クランク角度ＣＡＣＡＬ（例えば３０度）毎にＥＣＵ５のＣＰＵで実行される。   FIG. 5 is a flowchart of the LAF sensor failure determination process in the present embodiment. This process is executed by the CPU of the ECU 5 at every predetermined crank angle CACAL (for example, 30 degrees) when the execution condition for the LAF sensor failure determination is satisfied.

ＬＡＦセンサ故障判定実行条件は、第１の実施形態においけるインバランス故障判定実行条件と同一である。   The LAF sensor failure determination execution condition is the same as the imbalance failure determination execution condition in the first embodiment.

図５のステップＳ３１では、空燃比振動制御フラグＦＰＴが「１」であるか否かを判別し、その答が否定（ＮＯ）であるときは直ちに処理を終了する。ステップＳ３１の答が肯定（ＹＥＳ）であるときは、検出当量比ＫＡＣＴの今回値ＫＡＣＴ(k)を取得し、メモリに格納する処理を行うステップＳ３２）。   In step S31 of FIG. 5, it is determined whether or not the air-fuel ratio oscillation control flag FPT is “1”. If the answer to step S31 is negative (NO), the processing is immediately terminated. If the answer to step S31 is affirmative (YES), the current value KACT (k) of the detected equivalent ratio KACT is acquired and stored in the memory (step S32).

ステップＳ３３では、下記式（１１）により、第１差分ＤＫＡＣＴ１ａ(k)を算出する。式（１１）のＮｆ２は、周波数ｆ２成分を減衰させるための第３離散遅延期間であり、本実施形態では「１５」に設定される。
ＤＫＡＣＴ１ａ(k)＝ＫＡＣＴ(k)−ＫＡＣＴ(k-Nf2) （１１） In step S33, the first difference DKACT1a (k) is calculated by the following equation (11). Nf2 in Expression (11) is a third discrete delay period for attenuating the frequency f2 component, and is set to “15” in the present embodiment.
DKACT1a (k) = KACT (k) −KACT (k−Nf2) (11)

ステップＳ３４では、第１差分ＤＫＡＣＴ１ａ(k)について、周波数ｆ１成分を抽出するバンドパスフィルタ処理を行い、ステップＳ３５ではステップＳ３４で得られるバンドパスフィルタ処理出力の絶対値（振幅）を積算することにより、周波数ｆ１成分強度ＭＰＴｆ１を算出する。   In step S34, bandpass filter processing for extracting the frequency f1 component is performed on the first difference DKACT1a (k), and in step S35, the absolute value (amplitude) of the bandpass filter processing output obtained in step S34 is integrated. The frequency f1 component strength MPTf1 is calculated.

ステップＳ３６では、前記式（６）により第２差分ＤＫＡＣＴ２(k)を算出する。ステップＳ３７では、第２差分ＤＫＡＣＴ２(k)について、周波数ｆ２成分を抽出するバンドパスフィルタ処理を行い、ステップＳ３８ではステップＳ３７で得られるバンドパスフィルタ処理出力の絶対値（振幅）を積算することにより、周波数ｆ２成分強度ＭＰＴｆ２を算出する。   In step S36, the second difference DKACT2 (k) is calculated by the equation (6). In step S37, bandpass filter processing for extracting the frequency f2 component is performed on the second difference DKACT2 (k), and in step S38, the absolute value (amplitude) of the bandpass filter processing output obtained in step S37 is integrated. The frequency f2 component strength MPTf2 is calculated.

ステップＳ３９では、周波数成分強度の算出開始時点から所定積算時間ＴＩＮＴが経過したか否かを判別し、その答が否定（ＮＯ）である間は直ちに処理を終了する。ステップＳ３９の答が肯定（ＹＥＳ）となると、ステップＳ４０に進み、周波数ｆ１成分強度ＭＰＴｆ１が強度判定閾値ＭＰＴｆ１ＴＨより小さいか否かを判別する。   In step S39, it is determined whether or not the predetermined integration time TINT has elapsed from the start of calculation of the frequency component intensity, and the process is immediately terminated while the answer is negative (NO). If the answer to step S39 is affirmative (YES), the process proceeds to step S40, and it is determined whether or not the frequency f1 component intensity MPTf1 is smaller than an intensity determination threshold MPTf1TH.

ステップＳ４０の答が肯定（ＹＥＳ）であるときは、ＬＡＦセンサ出力のリッチ側の応答特性及びリーン側の応答特性がほぼ同様に劣化する第１故障パターンの故障が発生していると判定する（ステップＳ４１）。ステップＳ４０の答が否定（ＮＯ）であるときは、周波数ｆ１成分強度ＭＰＴｆ１及び周波数ｆ２成分強度ＭＰＴｆ２を下記式（１２）に適用し、より判定パラメータＲＴＬＡＦを算出する（ステップＳ４２）。
ＲＴＬＡＦ＝ＭＰＴｆ１／ＭＰＴｆ２ （１２） If the answer to step S40 is affirmative (YES), it is determined that a failure of the first failure pattern in which the response characteristic on the rich side and the response characteristic on the lean side of the LAF sensor output deteriorate in substantially the same manner has occurred ( Step S41). If the answer to step S40 is negative (NO), the frequency f1 component strength MPTf1 and the frequency f2 component strength MPTf2 are applied to the following equation (12) to calculate the determination parameter RTLAF (step S42).
RTLAF = MPTf1 / MPTf2 (12)

ステップＳ４３では、判定パラメータＲＴＬＡＦが判定閾値ＲＴＬＡＦＴＨより大きいか否かを判別し、その答が否定（ＮＯ）であるときは、ＬＡＦセンサ出力のリッチ側の応答特性及びリーン側の応答特性が非対称に劣化する第２故障パターンの故障が発生していると判定する（ステップＳ４４）。ステップＳ４３の答が肯定（ＹＥＳ）であるときは、ＬＡＦセンサ１５は正常である（応答特性劣化故障は発生していない）と判定する（ステップＳ４５）。   In step S43, it is determined whether or not the determination parameter RTLAF is larger than a determination threshold value RTLAFTH. If the answer is negative (NO), the rich response characteristic and the lean response characteristic of the LAF sensor output are asymmetric. It is determined that a failure of the second failure pattern that deteriorates has occurred (step S44). If the answer to step S43 is affirmative (YES), it is determined that the LAF sensor 15 is normal (no response characteristic deterioration failure has occurred) (step S45).

以上のように本実施形態では、空燃比を第１の周波数ｆ１で振動させる空燃比振動制御が行われ、空燃比振動制御実行中における検出当量比ＫＡＣＴの今回検出値ＫＡＣＴ(k)と、第２の周波数ｆ２に対応する周波数ｆ２成分を減衰させるように設定された第３離散遅延期間Ｎｆ２前の過去値ＫＡＣＴ(k-Nf2)との差分が第１差分ＤＫＡＣＴ１ａ(k)として算出され、第１差分ＤＫＡＣＴ１ａ(k)に含まれる周波数ｆ１に対応する周波数ｆ１成分がバンドパスフィルタ処理により抽出され、バンドパスフィルタ処理出力の絶対値の積算演算によって周波数ｆ１成分強度ＭＰＴｆ１が算出される。一方空燃比振動制御実行中における検出当量比ＫＡＣＴの今回値ＫＡＣＴ(k)と、周波数ｆ１成分を減衰させるように設定された第２離散遅延期間Ｎｆ１前の過去値ＫＡＣＴ(k-Nf1)との差分が第２差分ＤＫＡＣＴ２(k)として算出され、第２差分ＤＫＡＣＴ２(k)に含まれる周波数ｆ２成分がバンドパスフィルタ処理により抽出され、バンドパスフィルタ処理出力の絶対値に積算演算によって周波数ｆ２成分強度ＭＰＴｆ２が算出される。そして周波数ｆ１成分強度ＭＰＴｆ１及び周波数ｆ２成分強度ＭＰＴｆ２とを用いて、ＬＡＦセンサ１５の応答特性劣化故障判定が行われる。第１差分ＤＫＡＣＴ１ａ(k)に含まれる周波数ｆ２成分は大きく減衰しており、また第２差分ＤＫＡＣＴ２(k)に含まれる周波数ｆ１成分は大きく減衰しているため、比較的広い通過帯域幅のバンドパスフィルタ処理を用いて、周波数ｆ１成分及び周波数ｆ２成分を十分なＳ／Ｎで抽出することができる。その結果、応答特性劣化故障判定を短時間で精度良く行うことができる。   As described above, in the present embodiment, air-fuel ratio vibration control is performed to oscillate the air-fuel ratio at the first frequency f1, and the detected value KACT (k) of the detected equivalent ratio KACT during the execution of the air-fuel ratio vibration control is The difference from the previous value KACT (k−Nf2) before the third discrete delay period Nf2 set so as to attenuate the frequency f2 component corresponding to the frequency f2 of 2 is calculated as the first difference DKACT1a (k), The frequency f1 component corresponding to the frequency f1 included in the one-difference DKACT1a (k) is extracted by bandpass filter processing, and the frequency f1 component strength MPTf1 is calculated by integration of absolute values of the bandpass filter processing output. On the other hand, the current value KACT (k) of the detected equivalent ratio KACT during execution of the air-fuel ratio oscillation control and the past value KACT (k−Nf1) before the second discrete delay period Nf1 set so as to attenuate the frequency f1 component. The difference is calculated as the second difference DKACT2 (k), the frequency f2 component included in the second difference DKACT2 (k) is extracted by bandpass filter processing, and the frequency f2 component is calculated by integrating the absolute value of the bandpass filter processing output. The intensity MPTf2 is calculated. Then, the response characteristic deterioration failure determination of the LAF sensor 15 is performed using the frequency f1 component strength MPTf1 and the frequency f2 component strength MPTf2. Since the frequency f2 component included in the first difference DKACT1a (k) is greatly attenuated and the frequency f1 component included in the second difference DKACT2 (k) is greatly attenuated, a band having a relatively wide pass bandwidth. By using the pass filter processing, the frequency f1 component and the frequency f2 component can be extracted with sufficient S / N. As a result, the response characteristic deterioration failure determination can be accurately performed in a short time.

本実施形態では、図５のステップＳ３３が第１差分信号生成手段に相当し、ステップＳ３４が第１抽出手段に相当し、ステップＳ３６が第２差分信号生成手段に相当し、ステップＳ３７が第２抽出手段に相当し、ステップＳ３５，Ｓ３８，及びＳ４０〜Ｓ４５が故障判定手段に相当する。   In the present embodiment, step S33 in FIG. 5 corresponds to the first difference signal generation unit, step S34 corresponds to the first extraction unit, step S36 corresponds to the second difference signal generation unit, and step S37 is the second step. Steps S35, S38, and S40 to S45 correspond to extraction means, and correspond to failure determination means.

［第３の実施形態］
本実施形態は、上述したインバランス故障判定及びＬＡＦセンサの応答特性劣化故障判定をともに実行し、その際必要となる各周波数成分を抽出するバンドパスフィルタ処理の前に、不要な２つの周波数成分を減衰させる差分演算を行うようにしたものである。以下に説明する点以外は、第１及び第２の実施形態と同一である。 [Third Embodiment]
In the present embodiment, both the above-described imbalance failure determination and LAF sensor response characteristic deterioration failure determination are executed, and before the band-pass filter processing for extracting each frequency component required at that time, two unnecessary frequency components are extracted. The difference calculation which attenuates is performed. Except for the points described below, the present embodiment is the same as the first and second embodiments.

図６及び図７は、インバランス故障及びＬＡＦセンサの応答特性劣化故障の判定を行う処理のフローチャートである。図６の処理は、図４の処理にステップＳ１３ａ及びＳ１６ａを追加し、ステップＳ１４及びＳ１７をそれぞれステップＳ１４ａ及びＳ１７ａに代えたものであり、図７の処理に続く。図７の処理は、図５のステップＳ３１〜Ｓ３５を削除し、ステップＳ３６及びＳ３７をそれぞれステップＳ３６ａ及びＳ３７ａに代えたものである。   6 and 7 are flowcharts of processing for determining imbalance failure and response characteristic deterioration failure of the LAF sensor. The process of FIG. 6 is obtained by adding steps S13a and S16a to the process of FIG. 4 and replacing steps S14 and S17 with steps S14a and S17a, respectively. The processing of FIG. 7 is obtained by deleting steps S31 to S35 of FIG. 5 and replacing steps S36 and S37 with steps S36a and S37a, respectively.

図６のステップＳ１３ａでは、第１差分ＤＫＡＣＴ１の今回値及び第３離散遅延期間Ｎｆ２前の過去値を下記式（２１）に適用し、第１の二次差分ＤＤＫＡＣＴ１(k)を算出する。
ＤＤＫＡＣＴ１(k)＝ＤＫＡＣＴ１(k)−ＤＫＡＣＴ１(k-Nf2) （２１） In step S13a of FIG. 6, the current value of the first difference DKACT1 and the past value before the third discrete delay period Nf2 are applied to the following equation (21) to calculate the first secondary difference DDKACT1 (k).
DDKACT1 (k) = DKACT1 (k) −DKACT1 (k−Nf2) (21)

ステップＳ１４ａでは第１の二次差分ＤＤＫＡＣＴ１(k)について周波数ｆ１成分を抽出するバンドパスフィルタ処理を行い、ステップＳ１５でステップＳ１４ａで得られるバンドパスフィルタ処理出力の絶対値（振幅）を積算することにより、周波数ｆ１成分強度ＭＰＴｆ１を算出する。   In step S14a, bandpass filter processing for extracting the frequency f1 component is performed on the first secondary difference DDKACT1 (k), and in step S15, the absolute value (amplitude) of the bandpass filter processing output obtained in step S14a is integrated. Thus, the frequency f1 component intensity MPTf1 is calculated.

ステップＳ１６ａでは、第２差分ＤＫＡＣＴ２の今回値及び第３離散遅延期間Ｎｆ２前の過去値を下記式（２２）に適用し、第２の二次差分ＤＤＫＡＣＴ２(k)を算出する。
ＤＤＫＡＣＴ２(k)＝ＤＫＡＣＴ２(k)−ＤＫＡＣＴ２(k-Nf2) （２３）
ステップＳ１７ａでは第２の二次差分ＤＤＫＡＣＴ２(k)について０．５次周波数成分を抽出するバンドパスフィルタ処理を行い、ステップＳ１８でステップＳ１７ａで得られるバンドパスフィルタ処理出力の絶対値（振幅）を積算することにより、０．５次周波数成分強度ＭＩＭＢを算出する。 In step S16a, the current value of the second difference DKACT2 and the past value before the third discrete delay period Nf2 are applied to the following equation (22) to calculate the second secondary difference DDKACT2 (k).
DDKACT2 (k) = DKACT2 (k) −DKACT2 (k−Nf2) (23)
In step S17a, bandpass filter processing for extracting a 0.5th order frequency component is performed on the second secondary difference DDKACT2 (k), and in step S18, the absolute value (amplitude) of the bandpass filter processing output obtained in step S17a is obtained. By integrating, the 0.5th order frequency component intensity MIMB is calculated.

図７のステップＳ３６ａでは、第１差分ＤＫＡＣＴ１の今回値及び第２離散遅延期間Ｎｆ１前の過去値を下記式（２３）に適用し、第３の二次差分ＤＤＫＡＣＴ３(k)を算出する。
ＤＤＫＡＣＴ３(k)＝ＤＫＡＣＴ１(k)−ＤＫＡＣＴ１(k-Nf1) （２３） In step S36a of FIG. 7, the current value of the first difference DKACT1 and the past value before the second discrete delay period Nf1 are applied to the following equation (23) to calculate the third secondary difference DDKACT3 (k).
DDKACT3 (k) = DKACT1 (k) −DKACT1 (k−Nf1) (23)

ステップＳ３７ａでは第３の二次差分ＤＤＫＡＣＴ３(k)について周波数ｆ２成分を抽出するバンドパスフィルタ処理を行い、ステップＳ３８でステップＳ３７ａで得られるバンドパスフィルタ処理出力の絶対値（振幅）を積算することにより、周波数ｆ２成分強度ＭＰＴｆ２を算出する。   In step S37a, bandpass filter processing for extracting the frequency f2 component is performed on the third secondary difference DDKACT3 (k), and in step S38, the absolute value (amplitude) of the bandpass filter processing output obtained in step S37a is integrated. Thus, the frequency f2 component strength MPTf2 is calculated.

本実施形態によれば、空燃比を第１の周波数ｆ１で振動させる空燃比振動制御が行われ、空燃比振動制御実行中における検出当量比の今回値ＫＡＣＴ(k)と、０．５次周波数成分を減衰させるように設定された第１離散遅延期間ＮＩＭＢ前の過去値ＫＡＣＴ(k-NIMB)との差分が第１差分ＤＫＡＣＴ１(k)として算出され、第１差分の今回値ＤＫＡＣＴ１(k)と、第２の周波数ｆ２に対応する周波数成分を減衰させるように設定された第３離散遅延期間Ｎｆ２前の過去値ＤＫＡＣＴ１(k-Nf2)との差分が第１の二次差分ＤＤＫＡＣＴ１(k)として算出され、第１の２次差分ＤＤＫＡＣＴ１(k)に含まれる周波数ｆ１成分が抽出される。さらに空燃比振動制御実行中における検出当量比の今回値ＫＡＣＴ(k)と、周波数ｆ１成分を減衰させるように設定された第２離散遅延期間Ｎｆ１前の過去値ＫＡＣＴ(k-Nf1)との差分が第２差分ＤＫＡＣＴ２として算出され、第２差分の今回値ＤＫＡＣＴ２(k)と、第３離散遅延期間Ｎｆ２前の過去値ＤＫＡＣＴ２(k-Nf2)との差分が第２の二次差分ＤＤＫＡＣＴ２(k)として算出され、第２の二次差分ＤＤＫＡＣＴ２(k)に含まれる０．５次周波数成分が抽出される。また第１差分の今回値ＤＫＡＣＴ１(k)と、第２離散遅延期間Ｎｆ１前の過去値ＤＫＡＣＴ１(k-Nf1)との差分が第３の二次差分ＤＤＫＡＣＴ３(k)として算出され、第３の二次差分ＤＤＫＡＣＴ３(k)に含まれる周波数ｆ２成分が抽出される。そして、周波数ｆ１成分強度ＭＰＴｆ１と、周波数ｆ２成分強度ＭＰＴｆ２との相対関係に基づいて、ＬＡＦセンサ１５の応答特性劣化故障が判定される一方、周波数ｆ１成分強度ＭＰＴｆ１と、０．５次周波数成分強度ＭＩＭＢとの相対関係に基づいて、空燃比のインバランス故障が判定される。第１の二次差分ＤＤＫＡＣＴ１(k)に含まれる０．５次周波数成分及び周波数ｆ２成分は大きく減衰し、第２の二次差分ＤＤＫＡＣＴ２(k)に含まれる周波数ｆ１成分及び周波数ｆ２成分は大きく減衰し、第３の二次差分ＤＤＫＡＣＴ３(k)に含まれる０．５次周波数成分及び周波数ｆ１成分は大きく減衰しているので、比較的広い通過帯域幅のバンドパスフィルタ処理を用いて、周波数ｆ１成分、周波数ｆ２成分、及び０．５次周波数成分を十分なＳ／Ｎで抽出することができる。その結果、ＬＡＦセンサ１５の応答特性劣化故障及び空燃比のインバランス故障の判定を短時間で精度良く行うことができる。   According to this embodiment, air-fuel ratio oscillation control is performed to oscillate the air-fuel ratio at the first frequency f1, and the current value KACT (k) of the detected equivalent ratio during execution of the air-fuel ratio oscillation control and the 0.5th order frequency The difference from the past value KACT (k-NIMB) before the first discrete delay period NIMB set to attenuate the component is calculated as the first difference DKACT1 (k), and the current value DKACT1 (k) of the first difference. And the past value DKACT1 (k−Nf2) before the third discrete delay period Nf2 set so as to attenuate the frequency component corresponding to the second frequency f2 is the first secondary difference DDKACT1 (k) And the frequency f1 component included in the first secondary difference DDKACT1 (k) is extracted. Further, the difference between the current value KACT (k) of the detected equivalent ratio during execution of the air-fuel ratio oscillation control and the past value KACT (k−Nf1) before the second discrete delay period Nf1 set so as to attenuate the frequency f1 component. Is calculated as the second difference DKACT2, and the difference between the current value DKACT2 (k) of the second difference and the past value DKACT2 (k−Nf2) before the third discrete delay period Nf2 is the second secondary difference DDKACT2 (k ) And the 0.5th order frequency component included in the second second order difference DDKACT2 (k) is extracted. The difference between the current value DKACT1 (k) of the first difference and the past value DKACT1 (k−Nf1) before the second discrete delay period Nf1 is calculated as a third secondary difference DDKACT3 (k), The frequency f2 component included in the secondary difference DDKACT3 (k) is extracted. Based on the relative relationship between the frequency f1 component strength MPTf1 and the frequency f2 component strength MPTf2, the response characteristic deterioration failure of the LAF sensor 15 is determined, while the frequency f1 component strength MPTf1 and the 0.5th order frequency component strength are determined. An air-fuel ratio imbalance failure is determined based on the relative relationship with MIMB. The 0.5th order frequency component and the frequency f2 component included in the first secondary difference DDKACT1 (k) are greatly attenuated, and the frequency f1 component and the frequency f2 component included in the second secondary difference DDKACT2 (k) are large. Since the 0.5th-order frequency component and the frequency f1 component that are attenuated and included in the third second-order difference DDKACT3 (k) are greatly attenuated, the bandpass filter processing with a relatively wide pass bandwidth is used, and the frequency It is possible to extract the f1 component, the frequency f2 component, and the 0.5th order frequency component with sufficient S / N. As a result, it is possible to accurately determine the response characteristic deterioration failure of the LAF sensor 15 and the air-fuel ratio imbalance failure in a short time.

本実施形態では、ステップＳ１３ａが第１の二次差分信号生成手段に相当し、ステップＳ１４ａが第１抽出手段に相当し、ステップＳ１６ａが第２の二次差分信号生成手段に相当し、ステップＳ１７ａが第２抽出手段に相当し、ステップＳ３６ａが第３の二次差分信号生成手段に相当し、ステップＳ３７ａが第３抽出手段に相当し、ステップＳ１５，Ｓ１８，及びＳ２０〜Ｓ２３が第２故障判定手段に相当し、ステップＳ１５，Ｓ３８，及びＳ４０〜Ｓ４５が第１故障判定手段に相当する。   In the present embodiment, step S13a corresponds to the first secondary difference signal generation means, step S14a corresponds to the first extraction means, step S16a corresponds to the second secondary difference signal generation means, and step S17a. Corresponds to the second extracting means, step S36a corresponds to the third secondary difference signal generating means, step S37a corresponds to the third extracting means, and steps S15, S18, and S20 to S23 are the second failure determination. Steps S15, S38, and S40 to S45 correspond to first failure determination means.

なお本発明は上述した実施形態に限るものではなく、種々の変形が可能である。例えば、上述した実施形態では、第１の周波数ｆ１をエンジン回転周波数ｆＮＥの定数倍の値（エンジン回転に同期した周波数）に設定したが、例えば４Ｈｚ程度の固定周波数に設定するようにしてもよい。ただし、固定周波数とする場合には、故障判定の実行条件におけるエンジン回転数ＮＥの範囲を比較的狭い範囲に限定することが望ましい。   The present invention is not limited to the embodiment described above, and various modifications can be made. For example, in the embodiment described above, the first frequency f1 is set to a value that is a constant multiple of the engine rotation frequency fNE (frequency synchronized with the engine rotation), but may be set to a fixed frequency of about 4 Hz, for example. . However, in the case of a fixed frequency, it is desirable to limit the range of the engine speed NE in the failure determination execution condition to a relatively narrow range.

また第１の周波数ｆ１を例えば４Ｈｚに設定する場合には、周波数ｆ１成分及び周波数ｆ２成分を減衰させるための処理に適用する検出当量比ＫＡＣＴのサンプリング周期ＴＰＳＰは、例えば１２．５ミリ秒に設定する。この場合、故障判定処理は、サンプリング周期ＴＰＳＰで実行し、第２離散遅延期間Ｎｆ１を「２０」とし、第３離散遅延期間Ｎｆ２を「１０」とする。第１離散時間ＮＩＭＢは、０．５次周波数成分を減衰させるべくエンジン回転数ＮＥの逆数に比例するように設定するとともに、０．５次周波数成分を抽出するバンドパスフィルタ処理のフィルタ係数はエンジン回転数ＮＥに応じて設定する。あるいは、検出当量比ＫＡＣＴを所定時間毎にサンプリングした一定時間間隔データと、所定クランク角度毎にサンプリングした一定クランク角度間隔データをともにメモリに記憶し、一定時間間隔データを用いて周波数ｆ１成分を抽出するバンドパスフィルタ処理を実行し、一定クランク角度間隔データを用いて０．５次周波数成分を抽出するバンドパスフィルタ処理を実行するようにしてもよい。   When the first frequency f1 is set to 4 Hz, for example, the sampling period TPSP of the detection equivalent ratio KACT applied to the process for attenuating the frequency f1 component and the frequency f2 component is set to 12.5 milliseconds, for example. To do. In this case, the failure determination process is executed at the sampling period TPSP, and the second discrete delay period Nf1 is set to “20” and the third discrete delay period Nf2 is set to “10”. The first discrete time NIMB is set to be proportional to the reciprocal of the engine speed NE so as to attenuate the 0.5th order frequency component, and the filter coefficient of the bandpass filter processing for extracting the 0.5th order frequency component is the engine coefficient. Set according to the rotational speed NE. Alternatively, a fixed time interval data obtained by sampling the detected equivalent ratio KACT every predetermined time and a constant crank angle interval data sampled every predetermined crank angle are stored in a memory, and the frequency f1 component is extracted using the fixed time interval data. The band pass filter process may be executed to extract the 0.5th order frequency component using the constant crank angle interval data.

また上述した実施形態では、第２の周波数ｆ２を第１の周波数ｆ１の２倍とする例を示したが、前述した特開２０１０−１０１２８９号公報に示されるように、第２の周波数ｆ２を第１の周波数ｆ１の２倍以外の整数倍に設定してもよい。   In the above-described embodiment, an example in which the second frequency f2 is set to be twice the first frequency f1 has been described. However, as described in Japanese Patent Application Laid-Open No. 2010-101289, the second frequency f2 is It may be set to an integer multiple other than twice the first frequency f1.

また周波数成分強度の算出処理（対象周波数成分の抽出するバンドパスフィルタ処理及びバンドパスフィルタ処理出力の絶対値の積算処理）は、故障判定処理とは別に最適の実行周期で実行するようにしてもよい。その場合には、故障判定処理では周波数成分強度算出を行わず、並行して実行される周波数成分強度算出処理で算出された周波数成分強度（０．５次周波数成分強度ＭＩＭＢ，第１周波数成分強度ＭＰＴｆ１，第２周波数成分強度ＭＰＴｆ２）を読み込んで、判定処理を行う。   The frequency component intensity calculation process (the band pass filter process for extracting the target frequency component and the process for integrating the absolute value of the band pass filter process output) may be executed at an optimal execution cycle separately from the failure determination process. Good. In that case, the frequency component intensity is not calculated in the failure determination process, but the frequency component intensity (0.5th-order frequency component intensity MIMB, first frequency component intensity) calculated in the frequency component intensity calculation process executed in parallel is performed. MPTf1, second frequency component intensity MPTf2) is read and a determination process is performed.

また本発明は、クランク軸を鉛直方向とした船外機などのような船舶推進機用エンジンなどの空燃比制御装置にも適用が可能である。   The present invention can also be applied to an air-fuel ratio control device such as a marine vessel propulsion engine such as an outboard motor having a vertical crankshaft.

１ 内燃機関
５ 電子制御ユニット（空燃比振動手段、第１差分信号生成手段、第１抽出手段、第２差分信号生成手段、第２抽出手段、故障判定手段、第３抽出手段、第１の二次差分信号生成手段、第２の二次差分信号生成手段、第３の二次差分信号生成手段）
６ 燃料噴射弁（空燃比変動手段）
１５ 比例型酸素濃度センサ（空燃比検出手段） DESCRIPTION OF SYMBOLS 1 Internal combustion engine 5 Electronic control unit (Air-fuel ratio oscillation means, 1st difference signal generation means, 1st extraction means, 2nd difference signal generation means, 2nd extraction means, failure determination means, 3rd extraction means, 1st 2nd Secondary difference signal generation means, second secondary difference signal generation means, third secondary difference signal generation means)
6 Fuel injection valve (Air-fuel ratio fluctuation means)
15 Proportional oxygen concentration sensor (air-fuel ratio detection means)

Claims (6)

複数気筒を有する内燃機関の排気通路において空燃比を検出する空燃比検出手段を備える内燃機関の空燃比制御装置において、
前記空燃比を第１の周波数で振動させる空燃比振動手段と、
前記空燃比振動手段の作動中に、前記空燃比検出手段による今回検出値と、前記第１の周波数とは異なる特定周波数に対応する特定周波数成分を減衰させるように設定された第１特定期間前の過去値との差分を第１差分信号として生成する第１差分信号生成手段と、
前記第１差分信号に含まれる前記第１の周波数に対応する第１周波数成分を抽出する第１抽出手段と、
前記空燃比振動手段の作動中に、前記空燃比検出手段による今回検出値と、前記第１周波数成分を減衰させるように設定された第２特定期間前の過去値との差分を第２差分信号として生成する第２差分信号生成手段と、
前記第２差分信号に含まれる前記特定周波数成分を抽出する第２抽出手段と、
前記第１抽出手段により抽出される前記第１周波数成分の強度と、前記第２抽出手段により抽出される前記特定周波数成分の強度との相対関係に基づいて、空燃比制御系の故障を判定する故障判定手段とを備えることを特徴とする内燃機関の空燃比制御装置。 In an air-fuel ratio control apparatus for an internal combustion engine comprising air-fuel ratio detection means for detecting an air-fuel ratio in an exhaust passage of an internal combustion engine having a plurality of cylinders,
Air-fuel ratio oscillation means for oscillating the air-fuel ratio at a first frequency;
During the operation of the air-fuel ratio oscillating means, a value detected by the air-fuel ratio detecting means and a specific frequency component corresponding to a specific frequency different from the first frequency before the first specific period is set. First difference signal generating means for generating a difference from the past value as a first difference signal;
First extraction means for extracting a first frequency component corresponding to the first frequency included in the first differential signal;
During the operation of the air-fuel ratio oscillating means, a difference between a current detection value by the air-fuel ratio detecting means and a past value before a second specific period set so as to attenuate the first frequency component is a second difference signal. Second differential signal generating means for generating
Second extraction means for extracting the specific frequency component included in the second differential signal;
Based on the relative relationship between the intensity of the first frequency component extracted by the first extraction means and the intensity of the specific frequency component extracted by the second extraction means, a failure of the air-fuel ratio control system is determined. An air-fuel ratio control apparatus for an internal combustion engine, comprising: a failure determination unit. 前記特定周波数成分は、前記機関の回転速度に対応する周波数の１／２の周波数である０．５次周波数成分であり、
前記故障判定手段は、前記第１周波数成分の強度と前記０．５次周波数成分の強度とに基づいて、前記複数気筒のそれぞれに対応する空燃比が許容限度を超えてばらついているインバランス故障を判定することを特徴とする請求項１に記載の内燃機関の空燃比制御装置。 The specific frequency component is a 0.5th order frequency component that is a half of the frequency corresponding to the rotational speed of the engine,
The failure determination means is an imbalance failure in which the air-fuel ratio corresponding to each of the plurality of cylinders varies beyond an allowable limit based on the intensity of the first frequency component and the intensity of the 0.5th-order frequency component. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein: 前記特定周波数成分は、前記第１の周波数の整数倍の周波数に対応する第２周波数成分であり、
前記故障判定手段は、前記第１周波数成分の強度と前記第２周波数成分の強度とに基づいて、前記空燃比検出手段の応答特性劣化故障を判定することを特徴とする請求項１に記載の内燃機関の空燃比制御装置。 The specific frequency component is a second frequency component corresponding to an integer multiple of the first frequency,
The said failure determination means determines the response characteristic deterioration failure of the said air-fuel ratio detection means based on the intensity | strength of the said 1st frequency component and the intensity | strength of the said 2nd frequency component. An air-fuel ratio control apparatus for an internal combustion engine. 前記第１の周波数は、前記機関の回転速度が高くなるほど高く設定され、前記第２特定期間は、前記第１の周波数が高くなるほど短くなるように設定されることを特徴とする請求項２または３に記載の内燃機関の空燃比制御装置。   The first frequency is set to be higher as the rotational speed of the engine is higher, and the second specific period is set to be shorter as the first frequency is higher. 4. An air-fuel ratio control apparatus for an internal combustion engine according to 3. 複数気筒を有する内燃機関の排気通路において空燃比を検出する空燃比検出手段を備える内燃機関の空燃比制御装置において、
前記空燃比を第１の周波数で振動させる空燃比振動手段と、
前記空燃比振動手段の作動中に、前記空燃比検出手段による今回検出値と、前記第１の周波数とは異なる特定周波数に対応する特定周波数成分を減衰させるように設定された第１特定期間前の過去値との差分を第１差分信号として生成する第１差分信号生成手段と、
前記第１差分信号の今回値と、前記第１の周波数の整数倍の周波数に対応する第２周波数成分を減衰させるように設定された第２特定期間前の過去値との差分を第１の二次差分信号として生成する第１の二次差分信号生成手段と、
前記第１の二次差分信号に含まれる前記第１の周波数に対応する第１周波数成分を抽出する第１抽出手段と、
前記空燃比振動手段の作動中に、前記空燃比検出手段による今回検出値と、前記第１周波数成分を減衰させるように設定された第３特定期間前の過去値との差分を第２差分信号として生成する第２差分信号生成手段と、
前記第２差分信号の今回値と、前記第２特定期間前の過去値との差分を第２の二次差分信号として生成する第２の二次差分信号生成手段と、
前記第２の二次差分信号に含まれる前記特定周波数成分を抽出する第２抽出手段と、
前記第１差分信号の今回値と、前記第３特定期間前の過去値との差分を第３の二次差分信号として生成する第３の二次差分信号生成手段と、
前記第３の二次差分信号に含まれる前記第２周波数成分を抽出する第３抽出手段と、
前記第１抽出手段により抽出される前記第１周波数成分の強度と、前記第３抽出手段により抽出される第２周波数成分の強度との相対関係に基づいて、前記空燃比検出手段の応答特性劣化故障の判定を行う第１故障判定手段と、
前記第１抽出手段により抽出される前記第１周波数成分の強度と、前記第２抽出手段により抽出される前記特定周波数成分の強度との相対関係に基づいて、前記複数気筒のそれぞれに対応する空燃比が許容限度を超えてばらついているインバランス故障の判定を行う第２故障判定手段とを備えることを特徴とする内燃機関の空燃比制御装置。 In an air-fuel ratio control apparatus for an internal combustion engine comprising air-fuel ratio detection means for detecting an air-fuel ratio in an exhaust passage of an internal combustion engine having a plurality of cylinders,
Air-fuel ratio oscillation means for oscillating the air-fuel ratio at a first frequency;
During the operation of the air-fuel ratio oscillating means, a value detected by the air-fuel ratio detecting means and a specific frequency component corresponding to a specific frequency different from the first frequency before the first specific period is set. First difference signal generating means for generating a difference from the past value as a first difference signal;
A difference between a current value of the first difference signal and a past value before a second specific period set to attenuate a second frequency component corresponding to a frequency that is an integral multiple of the first frequency is a first value. First secondary difference signal generation means for generating a secondary difference signal;
First extraction means for extracting a first frequency component corresponding to the first frequency included in the first secondary difference signal;
During the operation of the air-fuel ratio oscillating means, a difference between a current detection value by the air-fuel ratio detecting means and a past value before a third specific period set so as to attenuate the first frequency component is a second difference signal. Second differential signal generating means for generating
Second secondary difference signal generation means for generating a difference between a current value of the second difference signal and a past value before the second specific period as a second secondary difference signal;
Second extraction means for extracting the specific frequency component included in the second secondary difference signal;
Third secondary difference signal generation means for generating a difference between a current value of the first difference signal and a past value before the third specific period as a third secondary difference signal;
Third extraction means for extracting the second frequency component contained in the third secondary difference signal;
Response characteristic deterioration of the air-fuel ratio detecting means based on the relative relationship between the intensity of the first frequency component extracted by the first extracting means and the intensity of the second frequency component extracted by the third extracting means First failure determination means for determining failure;
Based on the relative relationship between the intensity of the first frequency component extracted by the first extraction means and the intensity of the specific frequency component extracted by the second extraction means, the sky corresponding to each of the plurality of cylinders. An air-fuel ratio control apparatus for an internal combustion engine, comprising: second failure determination means for determining an imbalance failure in which the fuel ratio varies beyond an allowable limit. 前記第１の周波数は、前記機関の回転速度が高くなるほど高く設定され、前記第２特定期間及び第３特定期間は、前記第１の周波数が高くなるほど短くなるように設定されることを特徴とする請求項５に記載の内燃機関の空燃比制御装置。   The first frequency is set to be higher as the rotational speed of the engine is higher, and the second specific period and the third specific period are set to be shorter as the first frequency is higher. The air-fuel ratio control apparatus for an internal combustion engine according to claim 5.
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