An apparatus for detecting misfiring of an internal combustion engine

Abstract

 An object of the present invention is to provide an apparatus for detecting misfiring of internal combustion engines capable of detecting the misfiring reliably even though an ignition plug fouls.
When a mixture in a cylinder burns normally due to a discharge at an ignition plug 12, a voltage developed across a detection resistor 16 is fetched into a microcomputer 18 during a period other than a period where ions are generated. Thus, an insulation resistance due to fouling of the ignition plug 12 is measured. If the voltage developed across the detecting resistor 16 which is fetched into the microcomputer 18 during the period where ions are generated is equal to or larger than a threshold value, it is determined that mixture has burnt normally. If the voltage is smaller than the threshold value, it is determined that a misfire has occurred. The threshold value is corrected according to the decrease of insulation resistance. Consequently, though the ignition plug fouls, misfiring can be detected reliably.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to an apparatus for detecting misfiring of an internal combustion engine. More particularly, this invention relates an apparatus for detecting misfiring of an internal combustion engines capable of detecting misfiring reliably when an ignition plug fouls.

2. Description of the Related Art

[0002] An apparatus for detecting misfiring of an internal combustion engine based on a phenomena that a current flows via ions generated in the cylinder when mixture in a cylinder burns normally has already been known.

[0003] Note, when a mixture of gas in a cylinder ignites normally due to discharge of an ignition plug, the mixture of gas is ionized, and then a charge accumulated in a capacitor is discharged via ions. Consequently, a voltage develops across a detection resistor.

[0004] Conversely, when the mixture is not ignited although the ignition plug discharges, that is, when a misfire is caused, a voltage does not develop across the detection resistor.

[0005] Therefore, it is possible to detect misfirings by determining with a microcomputer whether or not a voltage higher than a fixed threshold voltage is caused across the detecting resistor.

[0006] However, when fuel or an additive contained in a lubricant carbonizes and adheres to the ignition plug, that is, when fouling happens, insulation of the ignition plug deteriorates and the misfire cannot be detected due to a leakage current although the misfire has actually occurred.

[0007] Figures. 2(A) to 2(D) are diagrams explaining the principles of misfire detection by a known misfire detecting system. Figure. 2(A) shows a waveform generated when the ignition plug does not foul and a mixture is normally ignited. Figure. 2(B) shows a waveform generated when the ignition plug does not foul but a misfire is caused. Figure. 2(C) shows a waveform generated when the ignition plug fouls and a misfire is caused. Figure. 2(D) shows a waveform generated when the ignition plug fouls and the mixture is normally ignited. In each drawing, an upper graph shows the waveform of an ignition command signal, and a lower graph shows the waveform of a voltage supplied to a microcomputer.

[0008] When the ignition plug does not foul, and mixture is normally ignited shown in Figure 2(A), a pulse develops in the secondary winding of an ignition coil at a time t₁, that is, at the leading edge of an ignition command signal, and the ignition plug discharges between a time t₂, that is, at the trailing edge of the ignition command signal, and a time t₃.

[0009] Noise deriving from residual energy develops in the secondary winding of the ignition coil between a time t₃ and a time t₄. After the time t₄, an ion current occurs because the mixture has normally burnt. The ion current has a peak value at a time t₅, and, the ion current gradually decreases.

[0010] Conversely, when the ignition plug does not foul but a misfire is caused as shown in Figure 2(B), no ion current is detected after the time t₄.

[0011] Therefore, when the ignition plug does not foul it is possible to detect whether or not a misfire occurs by determining whether or not an ion current higher than a predetermined fixed threshold α is detected between the time t₄ and a time t₆ later than the time t₅.

[0012] According to the first aspect of the present invention, there is provided an apparatus for detecting misfirings of an internal combustion engine, comprising: a voltage applying means for applying a voltage between an ignition plug and the ground; a current detecting means for detecting a current due to the voltage applied by said voltage applying means; a fouling degree detecting means for detecting a fouling degree of the ignition plug in accordance with the current detected by said current detecting means for a period except a period where ions are generated in mixture when the mixture is normally burnt by discharging of the ignition plug; and a misfire determining means for determining misfiring if the current detected by said current detecting means is smaller than a threshold current determined in accordance with the fouling degree detected said fouling degree detecting means for a period where ions are generated in a mixture when the mixture is normally burnt by a discharge at the ignition plug.

[0013] According to the first invention, a leakage resistance due to fouling is detected for a period where an ion current never flows, and a threshold for detecting a misfiring is varied in accordance with the leakage resistance.

[0014] According to the second aspect of the present invention, there is provided an apparatus for detecting misfirings in an internal combustion engine, wherein, said fouling degree detecting means comprises: an on-period current detecting means for detecting a current with said current detection means for a period where a current is applied to the primary coil of an ignition coil contained in a period except a period where ions are generated in mixture when the mixture is normally burnt by discharging of the ignition plug; an off-period current detecting means for detecting a current with said current detecting means for a period except a period where a current is applied to the primary winding of an ignition coil contained in a period except a period where ions are generated in mixture when the mixture is normally burnt by discharging of the ignition plug; and a changing means for changing from said on-period current detecting means to said off-period current detecting means, or vice versa in accordance with fouling degree of the ignition plug.

[0015] According to the second invention, a period for detecting the leakage resistance is changed in accordance with the fouling degree of the ignition plug.

[0016] However, when the ignition plug fouls, a leakage current occurs after the time t₁, that is at the leading edge of the ignition command signal. In addition, if a misfire occurs as shown in Figure 2(C), the leakage current flows after the time t₄. The leakage current is generally larger than the threshold value .

[0017] Note when mixture is ignited, although the ignition plug fouls as shown in Figure 2(D), the ion current is super imposed on the leakage current after the time t₄.

[0018] Therefore when the ignition plug fouls, it is impossible to distinguish igniting from a misfiring.

[0019] For preventing incorrect detection due to fouling, an ion current detecting apparatus based on a phenomena that a leakage current due to fouling flow for a period that an ion current never flows has been proposed. This controls the timing of detecting a current. If a current is detected in the period when an ion current never flows, that is, the period between the time t₁ and time t₂, the apparatus inhibits misfire detection to prevent misjudgment (Japanese Unexamined Patent Publication No. 4-259671).

[0020] However, in the above proposed apparatus, when the ignition plug fouls, a misfire cannot be detected. Besides, it is unavoidable that unburnt gas flows into a catalytic converter, and burns to damage the catalytic converter.

SUMMARY OF THE INVENTION

[0021] Accordingly, an object of the present invention is to provide an apparatus for detecting misfirings in an internal combustion engines capable of detecting misfiring reliably although an ignition plug fouls.

[0022] Therefore, when the ignition plug does not foul it is possible to detect whether or not a misfire occurs by determining whether or not the ion current higher than a predetermined fixed threshold α is detected between the time t₄ and a time t₆ later than the time t₅.

[0023] According to the first aspect of the present invention, there is provided an apparatus for detecting misfirings of an internal combustion engine, comprising: a voltage applying means for applying a voltage between an ignition plug and the ground; a current detecting means for detecting a current due to the voltage applied by said voltage applying means; a fouling degree detecting means for detecting a fouling degree of the ignition plug in accordance with the current detected by said current detecting means for a period except a period where ions are generated in mixture when the mixture is normally burnt by discharging of the ignition plug; and a misfire determining means for determining misfirings if the current detected by said current detecting means is smaller than a threshold current determined in accordance with the fouling degree detected said fouling degree detecting means for a period where ions are generated in mixture when the mixture is normally burnt by a discharge at the ignition plug.

[0024] According to the first invention, a leakage resistance due to fouling is detected in a period when an ion current never flows, and a threshold for detecting a misfiring is varied in accordance with the leakage resistance.

[0025] According to the second aspect of the present invention, there is provided an apparatus, for detecting misfirings in an internal combustion engine, wherein said fouling degree detecting means comprises: an on-period current detecting means for detecting a current with said current detection means for a period where a current is applied to the primary coil of an ignition coil contained in a period except a period where ions are generated in mixture when the mixture is normally burnt by a discharge at the ignition plug; an off-period current detecting means for detecting a current with said current detecting means for a period except a period where a current is applied to the primary winding of an ignition coil contained in a period except a period where ions are generated in mixture when the mixture is normally burnt by a discharge at the ignition plug; and a changing means for changing from said on-period current detecting means to said off-period current detecting means, or vice versa in accordance with the fouling degree of the ignition plug.

[0026] According to the second invention, a period for detecting the leakage resistance is changed in accordance with the fouling degree of the ignition plug.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] 

Figure 1 is a circuit diagram of an apparatus for detecting misfiring of an internal combustion engine in accordance with the present invention;

Figure 2(A) to 2(D) are diagrams explaining the principles of detecting misfirings in a known apparatus for detecting misfirings;

Figure 3 is an equivalent circuit when an ignition plug fouls;

Figure 4 shows the waveform of the monitored voltage;

Figure 5 is a flowchart of a misfire detecting routine;

Figure 6 is a flowchart of a misfire detecting subroutine;

Figure 7 is a graph indicating a relationship between a voltage V₁₆ and an insulation resistance R₂;

Figure 8 is a flowchart of a second misfire detecting routine; and

Figure 9 is a flowchart of a second misfire detecting subroutine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0028] Figure. 1 is a circuit diagram of an apparatus for detecting misfirings according to the present invention. An ignition command is given to an ignition coil 11 by an ignition control system 10.

[0029] One terminal of a secondary winding of the ignition coil 11 is connected to an ignition plug 12, and the other terminal thereof is grounded through first and second zener diodes 13 and 14 connected in series with each other in opposite directions.

[0030] A capacitor 15 is connected in parallel with the first zener diode 13, and a detecting resistor 16 is connected in parallel with the second zener diode 14.

[0031] A voltage developed across the detecting resistor 16 is supplied to a microcomputer 18 through an inverting amplifier 17.

[0032] In the foregoing circuitry, when a pulsed ignition command is supplied from the ignition control system 10 to the primary winding of the ignition coil 11, the ignition plug 12 produces a discharge due to a high voltage induced in the secondary winding of the ignition coil 11 at the leading edge of the ignition command. At the same time, a voltage controlled by the first zener diode 13 is applied to the capacitor 15.

[0033] In other words, after the capacitor 15 is charged, an ion current detection circuit is driven using the capacitor 15 as a power supply.

[0034] To begin with, a method of detecting a fouling degree which is implemented in the present invention will be described.

[0035] Fig. 3 shows an equivalent circuit of an ionization current detection circuit when ignition plug 12 fouls. In the circuit, a leakage resistor 21 is connected in parallel with the ignition plug 12.

[0036] When the ignition plug fouls a little, the resistance of the leakage resistor 21 becomes large. When the ignition plug fouls heavily, the resistance of the leakage resistor 21 becomes small.

[0037] An equation below is established for the equivalent circuit on the basis of Ohm's law.

where R₁ is a resistance of the detecting resistor 16, R₂ is a resistance of the leakage resistor 21, C is a capacitance of the capacitor 15, and V₀ is an initial value of a voltage across the capacitor 15. Consequently, a current i flowing though ion current is expressed as follows:

Moreover, a monitoring voltage V_M developing across the detection resistor 16 is expressed as follows:

[0038] Fig. 4 shows the waveform of the monitoring voltage. The abscissa indicates time, while the ordinate indicates the monitoring voltage. A monitoring voltage at a predetermined time t_M is expressed as follows:

[0039] Herein, the resistance R₁ of the detection resistor 16, the capacitance C of the capacitor 15, and the initial value V₀ of the voltage across the capacitor 15 shall be provided as known values. When the time t_M is fixed, the resistance R₂ of the leakage resistor 21 can be calculated by measuring the monitoring voltage V_M(t_M) according to the above expression. The fouling degree of the ignition plug 12 can therefore be assessed quantitatively.

[0040] As mentioned previously, when the ignition plug fouls, an ion current flowing when a mixture has burnt is superimposed on the leakage current due to fouling. If the threshold value for detecting the ion current is corrected in accordance with the fouling degree, a misfire is correctly detected even though the ignition plug is fouling.

[0041] Figure 5 is a flowchart of a misfire detecting routine executed by the microcomputer 18 on the basis of the aforesaid principles. The processing is started at the trailing edge of an ignition command signal outputted from the ignition control system 10 (at time t₂).

[0042] At step 51, a standby state is held until a period T₂₅ has elapsed after the start of this routine period. The period T₂₅ is set as a period between a time when the ignition command signal vanishes and a time when an ion current reaches its peak after noise due to discharging and/or residual energy is suppressed.

[0043] When the time T₂₅ has elapsed, the control proceeds to step 52 where a voltage V(t₅) developing across the detecting resistor 16 is read.

[0044] The control then proceeds to step 53 where a standby state is held until a period T_2M has elapsed after the start of routine. The time T_2M is set as a time longer than the time T₂₅.

[0045] When the period T_2M has elapsed, the determination at step 53 becomes affirmative. At step 54, a voltage V(t_M) developing across the detecting resistor 16 is read at time t_M. This routine is terminated after a misfire detecting subroutine is executed at step 55.

[0046] Fig. 6 is a detailed flowchart of a misfire detecting subroutine executed at step 55. At step 551, the resistance R₂ of the leakage resistor 21 is calculated by rewriting the expression (3) as follows:

[0047] At step 552, the resistance R₂ of the leakage resistor 21 calculated at the previous step is used to calculate a leakage voltage V_L at the time t₅ according to the expression (2).

[0048] At step 553, it is determined whether or not the voltage V(t₅) developed across the detecting resistor 16, which is read at step 53, is bigger than a sum of the leakage voltage V_L and a predetermined threshold value α. The threshold value α is set at a suitable value to detect a peak value of an ion current when the ignition plug does not foul.

[0049] When the determination at step 553 is affirmative, it is determined that mixture has burnt normally at step 554. If the determination at step 553 is negative, it is determined that a misfire occurred at step 555. This subroutine is then terminated.

[0050] A vehicle is actually designed so that if misfiring is frequently detected for a given period, that is, if a misfire rate exceeds a given value, an alarm is activated because a catalyst placed in an exhaust pipe in an internal combustion engine may be damaged.

[0051] In the aforesaid embodiment, the degree of fouling of the ignition plug 12 is determined on the basis of the voltage V(t_M) developing across the detecting resistor 16 at the monitoring time t_M. When this voltage drops, the precision for reading it by the microcomputer 18 deteriorates. The precision for determining a fouling degree, that is, the precision for calculating a leakage resistance, deteriorates accordingly. Eventually, it becomes unavoidable that misfiring is incorrectly detected.

[0052] When the ignition plug 12 fouls, a leakage current flows when the ignition control system 10 is outputting an ignition command signal, that is, for period between the time t₁ and time t₂. The leakage current is usually larger than a current proportional to the voltage V(t_M) developed across the detecting resistor 16 at the monitoring time t_M. This fact can be utilized for solving the above-mentioned problem.

[0053] Figure. 7 is a graph indicating the relationship between a voltage V₁₆ developed across the detecting resistor 16 while the ignition control system 10 is outputting an ignition command signal when the ignition plug 12 has been fouling and the insulation resistance of the ignition plug 12. The abscissa indicates the insulation resistance on the logarithmic scale, and the ordinate indicates the voltage. On the graph, black dots indicate measured values, and a solid line is a curve to which the measured values are fitted.

[0054] The leakage resistance R₂ of the ignition plug 12 can be expressed as a function of the voltage V₁₆.

[0055] As seen from this graph, when the leakage resistance is small, that is, the fouling degree is small, the voltage V₁₆ rises and may exceed a maximum voltage readable by the microcomputer. Since a drop of leakage current occurring after time T₄ is small in this case, the precision for reading a voltage at the monitoring time t_M does not deteriorate very much.

[0056] In the second embodiment, the precision for determining the fouling degree is retained by changing the timing of reading a voltage.

[0057] Fig. 8 is a flowchart of a second misfire detecting routine executed by the microcomputer 18. Execution is started at the leading edge of an ignition command signal outputted from the ignition control system 10.

[0058] At step 81, a standby state is held until a period T_1P has elapsed after the start of execution of the second misfire detecting routine. The time T_1P is set as a period where the ignition control system 10 is outputting the ignition command signal.

[0059] When the time T_1P has elapsed, the determination at step 81 becomes affirmative. Control proceeds to step 82 where a voltage V(t_P) developed across the detection resistor 16 is read.

[0060] At step 83, a standby state is held until a predetermined period T₁₅ elapses. The period T₁₅ is set as a period between a time when the ignition command signal vanishes and a time when an ion current reaches its peak after noise due to discharging and/or residual energy is suppressed.

[0061] When the period T₁₅ has elapsed, the determination at step 83 becomes affirmative. Control proceeds to step 84 when a voltage V(t₅) developed across the detecting resistor 16 is then read.

[0062] At step 85, a standby state is held until a predetermined period T_1M has elapsed. The period T_1M is set as a time longer than the period T₁₅.

[0063] When the period T_1M has elapsed, the determination at step 85 becomes affirmative. At step 86, a voltage V(t_M) developed across the detecting resistor 16 is read at time instant t_M. At step 87, the second misfire detecting subroutine is executed and then this routine is terminated.

[0064] Fig. 9 is a detailed flowchart of the second misfire detecting subroutine executed at step 87. At step 871, it is determined whether or not a voltage V(t_P) at time t_P is equal to or larger than a predetermined upper limit voltage V_U. The upper limit voltage V_U is defined as a voltage slightly lower than the maximum voltage readable by the microcomputer 18.

[0065] If the determination at step 871 is negative, control proceeds to step 872 where it is determined whether or not a voltage V(t_M) at time t_M is equal to or lower than a predetermined lower limit voltage V_L. The lower limit voltage V_L is defined as a voltage slightly higher than a minimum voltage readable by the microcomputer 18.

[0066] If the determination at step 871 or 872 is affirmative, the resistance R₂ of the leakage resistor 21 is calculated according to the expression (3) at step 873. Control proceeds to step 875.

[0067] Conversely, if the determination at step 872 is negative, the graph shown in Fig. 7 is used to obtain the leakage resistance R₂ of 21 in accordance with the voltage V(t_P). Control proceeds to step 875.

[0068] At step 875, the leakage resistance R₂ obtained at step 873 or 874 is assigned to the expression (2). Thus, the leakage voltage V_L at time t₅ is calculated.

[0069] At step 876, it is determined whether or not the voltage V(t₅) at time t₅ is larger than a sum of the leakage voltage V_L and a predetermined threshold value α. The threshold value α is set as a value enabled to detect a peak value of an ion current when the ignition plug does not foul.

[0070] If the determination at step 876 is affirmative, it is determined that a mixture has burnt normally at step 877. If the determination at step 876 is negative, it is determined a misfire occurred at step 878. The processing is then terminated.

[0071] According to the second embodiment, a drop in insulation resistance can be calculated accurately. The precision for determining misfiring can be improved.

[0072] An object of the present invention is to provide an apparatus for detecting misfiring of internal combustion engines capable of detecting the misfiring reliably even though an ignition plug fouls.

[0073] When a mixture in a cylinder burns normally due to a discharge at an ignition plug 12, a voltage developed across a detection resistor 16 is fetched into a microcomputer 18 during a period other than a period where ions are generated. Thus, an insulation resistance due to fouling of the ignition plug 12 is measured. If the voltage developed across the detecting resistor 16 which is fetched into the microcomputer 18 during the period where ions are generated is equal to or larger than a threshold value, it is determined that mixture has burnt normally. If the voltage is smaller than the threshold value, it is determined that a misfire has occurred. The threshold value is corrected according to the decrease of insulation resistance. Consequently, though the ignition plug fouls, misfiring can be detected reliably.

Claims

1. An apparatus for detecting misfiring of an internal combustion engine, comprising:

a voltage applying means for applying a voltage between an ignition plug and the ground;

a current detecting means for detecting a current due to the voltage applied by said voltage applying means;

a fouling degree detecting means detecting a fouling degree of the ignition plug in accordance with the current detected by said current detecting means for a period except a period where ions are generated in mixture when the mixture is burnt normally by a discharge at the ignition plug; and

a misfire determining means for determining misfiring if the current detected by said current detecting means is smaller than a threshold current determined in accordance with the fouling degree detected said fouling degree detecting means for a period where ions are generated in mixture when the mixture is normally ignited by a discharge at the ignition plug.

2. An apparatus for detecting misfiring of an internal combustion engine of claim 1, wherein,
   said fouling degree detecting means comprises:

an on-period current detecting means for detecting a current with said current detecting means for a period where a current is applied to the primary winding of an ignition coil contained in a period except a period where ions are generated in a mixture when the mixture is ignited normally by a discharge at the ignition plug;

an off-period current detecting means for detecting a current with said current detecting means for a period except a period where a current is applied to the primary winding of an ignition coil contained in a period except a period where ions are generated in a mixture when the mixture is ignited normally by a discharge at the ignition plug; and

a changing means for changing from said on-period current detecting means to said off-period current detecting means, or vice versa in accordance with fouling degree of the ignition plug.

3. A method for detection misfiring of an internal combustion engine, comprising steps of:

a voltage applying step for applying a voltage between an ignition plug and the ground;

a current detecting step for detecting a current due to the voltage applied at said voltage applying step;

a fouling degree detecting step for detecting a fouling degree of the ignition plug in accordance with the current detected at said current detecting step for a period except a period where ions are generated in a mixture when the mixture is ignited normally by a discharge at the ignition plug; and

a misfire determining means for determining misfiring if the current detected by said current detecting means is smaller than a threshold current determined in accordance with the fouling degree detected said fouling degree detecting means for a period where ions are generated in a mixture when the mixture is burnt normally by a discharge at the ignition plug.

4. A method for detecting misfiring of an internal combustion engine of claim 3, wherein,
   said fouling degree detecting step comprises steps of:

an on-period current detecting step for detecting a current at said current detecting step for a period where a current is applied to the primary winding of an ignition coil contained in a period except a period where ions are generated in the mixture when the mixture is ignited normally by a discharge at the ignition plug;

an off-period current detecting step for detecting a current with said current detecting means for a period except a period where a current is applied to the primary winding of an ignition coil contained in a period except a period where ions are generated in the mixture when the mixture is normally ignited by a discharge at the ignition plug; and

a changing step for changing from said on-period current detecting step to said off-period current detecting step, or vice versa in accordance with the fouling degree of the ignition plug.

Drawing