HK1033770B - Process for detecting a misfire in an internal combustion engine and system for carrying out said process - Google Patents

Description

Method for detecting misfiring in an internal combustion engine and system for implementing said method

Technical Field

The present invention relates to a method of detecting misfire in an internal combustion engine, and in particular to a method of detecting misfire in one or more cylinders of an internal combustion engine. The invention also relates to a system for implementing said method.

Background

It is known that in order to monitor the performance of an internal combustion engine, particularly a racing engine having a large number of cylinders, it is desirable to detect misfires of the fuel mixture occurring in one or more cylinders. A known method of implementing said detection consists in measuring the sudden changes in the crankshaft speed with an electronic sensor arranged close to the flywheel, which method is known from US-5576936 and currently plays an important role in controlling the polluting exhaust gases under more stringent regulations. The sensor is connected to a control device arranged inside the vehicle, which receives all the data relating to the engine and transmitted by the appropriate sensor. By calculating the speed fluctuation from the transmitted torque, it is possible to identify a possible misfire in one cylinder of the engine. However, this method does not allow to identify precisely in which cylinder a misfire occurred, and has a rather high probability of error, particularly if, for example, a traveling car is subjected to violent oscillations caused by road surface defects, which temporarily affect the rotational speed of the crankshaft.

To address these drawbacks, fluctuations in the pressure of the engine exhaust should be measured in time. Although commercially available pressure sensors are very accurate and provide almost real-time response, the known methods of detecting misfire from measurements of pressure fluctuations in the exhaust gas are still very inaccurate and poorly reliable, especially when applied to engines with a large number of cylinders.

It is therefore an object of the present invention to provide a method for detecting misfires without the above-mentioned disadvantages. It is a further object of the invention to provide a system for carrying out said method. These objects are achieved by a method and a system, the main features of which are disclosed in the first and eighth claims, respectively.

Disclosure of Invention

The method according to the invention offers a higher accuracy and reliability than the methods of the prior art, thanks to the sampling and subsequent frequency analysis of the pressure values detected in the exhaust pipe. In fact, if the engine ignition is normal, the periodic opening of the exhaust valves of the cylinders produces pressure pulses in the exhaust pipe with the same period and similar waveform. In contrast, in the case of a misfire of one of the cylinders, the corresponding pressure pulse is changed, thus changing the periodic pattern of pressure values. The reference synchronized with the pulse frequency is easily derived from a sensor detecting the rotational speed of the crankshaft and/or the camshaft.

Another advantage of the method according to the invention is that by frequency analysis of the sampled signal it is possible to determine that only one or more misfires occur during a single engine cycle. In fact, the amplitude of the modes of the different harmonics of the sampled signal depends on the number of cylinders in which misfiring occurs.

A further advantage of the method according to the invention is that by frequency analysis of the sampled signal it is possible to determine not only misfires but also the position of the cylinder in which the misfires occurred. In fact, knowledge of the cylinder firing order and comparison of the phase of the first harmonic of the sampled signal to the phase of the first cylinder provides a phase difference indicative of the location of the cylinder where misfiring occurred.

Drawings

These and other advantages and features of the method and system according to the invention will be apparent to those skilled in the art from the following detailed description of embodiments thereof, which is made with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic diagram of a system according to the present invention;

FIG. 2 shows a flow chart of a method according to the invention;

3a, 3b and 3c show three graphs of pressure as a function of crankshaft rotation;

FIGS. 4a, 4b and 4c show three further graphs of pressure as a function of crankshaft rotation;

FIGS. 5a, 5b and 5c show three graphs of the misfire index as a function of the number of engine cycles;

FIG. 6 shows a Fourier transform of the graph of FIG. 3 a; and

fig. 7a, 7b and 7c show three plots of the polar coordinates of the first harmonic of the pressure in the plots of fig. 3a, 3b and 3 c.

Detailed Description

With reference to fig. 1, it can be seen that the system according to the invention comprises, in a known manner, a control device 1 (indicated with a dashed line) which in turn comprises a pair of interconnected electronic controllers 2, 2 ', each providing control over one of the two rows of cylinders 3, 3' of the engine. In the present embodiment, a V12 engine is described having two banks of six cylinders 3, 3 'each, but in other embodiments the cylinders and/or banks may vary considerably, the controller 2, 2' being connected in known manner to a pair of coolant temperature sensors 4, 4 'and to two pairs of sensors 5, 5' and 6, 6 'which detect the temperature and pressure of the air in the intake manifolds 7, 7' respectively. The controller 2, 2 ' is also connected to a pair of lambda sensors 8, 8 ' for analysing the oxygen content in the exhaust pipes 9, 9 ', two series of injectors 10, 10 ' for injecting fuel into the intake pipes 11, 11 ' of the cylinders 3, 3 ', and a pair of ignition coils 12, 12 '. The exhaust pipes 9, 9 ' are preferably also provided with a pair of temperature sensors 13, 13 ' connected to the controllers 2, 2 '.

The system according to the present embodiment of the invention suitably comprises one sensor 14 for detecting the rotational speed of a flywheel 15 integral with the crankshaft, and another pair of sensors 16, 16' for detecting the rotation of a camshaft 17. These sensors 14, 16 and 16 'are connected to the controllers 2, 2' so that the latter can calculate the rotational speed and the rotational angle of the crankshaft during the engine cycle in real time on the basis of the received data. The fact that the flywheel 15 performs two revolutions per cycle (720 °) in a four-stroke engine necessitates the presence of the sensors 14, 16 and 16 ', so that the reference provided by the sensors 16, 16' allows to distinguish between the first and the second revolution.

In order to implement the method according to the invention, two high-precision pressure sensors 18, 18 ' connected to the controllers 2, 2 ' are suitably arranged in the two exhaust lines 9, 9 ', said sensors delivering in real time an electrical signal whose voltage is proportional to the measured pressure. In addition, the controller 2, 2 'is connected to a pair of warning lights 19, 19' arranged inside the vehicle, a port 20 connected to an external processor, and a sensor 21 detecting the position of an engine throttle 22.

Referring now to fig. 2, it can be seen that the method according to the invention comprises a first step of periodic checking of the engine operating state, for example every second, after a certain time period of engine start has elapsed. In fact, in order to obtain reliable results from the method, it is preferred that the method is only implemented when some engine parameters are within predetermined values. In particular, the method according to the invention is only activated when the coolant temperature measured by the sensors 4, 4 ', the air temperature measured by the sensors 5, 5 ', and the air pressure measured by the sensors 6, 6 ' in the manifolds 7, 7 ' are greater than a certain threshold value stored in the memory of the controller 2, 2 '. WhileAnd, these controllers check that the revolutions per minute (rpm) detected by the sensor 14 is within a predetermined value range. Table 1 below shows an example of values satisfying the conditions of the startup method.

Minimum number of revolutions	990rpm
		Maximum number of revolutions	7550rpm
Status check cycle	1s
		Delay from engine start	10s
Minimum coolant temperature	20℃
		Minimum air temperature	20℃
Minimum absolute pressure in the manifolds 7, 7	250mmHg

Table 1: starting conditions

Another condition of the starting method may be that the throttle valve 22 detected by the sensor 21 reaches a certain opening degree.

If the above conditions are met, at the beginning of the engine cycle, the controller 2, 2 'starts to sample the electric signal delivered by the sensor 18, 18' and proportional to the pressure in the exhaust pipe 9, 9 ', corresponding to a certain position of the camshaft 17 detected by the sensor 16, 16'. These analog signals are converted to digital form in a known manner and then stored in buffer memories within the respective controllers 2, 2'. The sampling frequency is suitably synchronized with the rotational speed of the flywheel 15 detected by the sensor 14 so that at the end of the engine cycle detected by the sensors 16 and 16', a predetermined number, for example 64 pressure samples, are stored. Although the response of the pressure sensor 18, 18 ' is almost instantaneous, the controller 2, 2 ' takes into account the almost constant lag caused by the time required for a pressure pulse to travel along the exhaust pipe 9, 9 ' from the exhaust valve of the cylinder 3, 3 ' to the pressure sensor 18, 18 ' in order to be accurately synchronized with the engine. Due to the temperature sensors 13, 13 'it is possible to compensate for very small fluctuations of the hysteresis due to temperature fluctuations within the tubes 9, 9'.

After sampling, the pressure values corresponding to the engine cycle are processed by the controller 2, 2' while the controller samples another series of pressure values stored in another buffer memory for subsequent processing.

This processing carried out by the respective processors of the controllers 2, 2' suitably comprises an analysis in the frequency domain, in particular a fourier transform of the sampled signal, which is processed in such a way as to obtain two series of coefficients corresponding to the real and imaginary parts of the first harmonic of the signal. In particular, in this embodiment, the coefficients of the first 3 harmonics of the sampled signal are calculated, while in other embodiments it is obviously possible to calculate different numbers of harmonics as required.

These coefficients are used to calculate, in a known manner, the first harmonic, for example the first 3 modes, and then by combining the values of these modes, an index is obtained which allows detection of misfiring in one or more of the cylinders 3, 3'. This misfire index can be obtained in various ways, for example by adding or multiplying the modes of the harmonics. Before this addition or multiplication it is possible to multiply or square the harmonics with different coefficients in order to obtain a weighted addition or multiplication. In the present embodiment, the misfire index is calculated by simply adding the modes of the first third harmonic.

Once the indicator is calculated, it is compared with a predetermined threshold value stored in the controller 2, 2'. Table 2 below shows the threshold values for the experimentally obtained misfire index as a function of the engine rpm sensed by sensor 14 and the pressure in the manifolds 7, 7 'sensed by sensors 6, 6'.

mmHg↓300450600760	rpm↓1100 2000 3000 4000 5000 6000 7000 7500
									110	110	124	130	144	148	156	168
	115	120	148	156	180	188	196	204
									130	140	180	188	236	248	256	268
	150	162	224	264	292	300	312	320

Table 2: threshold value of misfire index

The controller 2 or 2 ' detecting that the threshold is exceeded indicates, by means of the warning lamp 19 or 19 ', that a misfire occurred in the corresponding bank of cylinders 3 or 3 '.

At this point, the controller 2 or 2 'detecting misfires preferably compares the respective modes of the first 3 harmonics to predetermined thresholds, which are also stored as a function of engine rpm and pressure in the corresponding manifold 7 or 7'. If all three modes are within the range of values between the minimum threshold and the maximum threshold, a single misfire is detected, i.e. only one of the cylinders 3 or 3 'is misfired, otherwise a multiple misfire is detected, i.e. at least two of the cylinders 3 or 3' belonging to a certain bank are misfired.

Tables 3.1, 3.2, 4.1, 4.2, 5.1 and 5.2 below show examples of the minimum values and amplitudes of the threshold ranges for the modes of the first 3 harmonics.

mmHg↓300450600760	rpm↓1100 2000 3000 4000 5000 6000 7000 7500
									8	28	48	40	56	84	84	84
	8	24	56	40	80	96	96	96
									12	36	60	72	116	104	104	104
	24	44	72	108	160	144	144	144

Table 3.1: minimum threshold of mode of first harmonic

mmHg↓300450600760	rpm↓1100 2000 3000 4000 5000 6000 7000 7500
									128	108	92	180	144	188	192	196
	120	140	148	184	168	192	196	200
									96	128	176	144	192	200	224	244
	68	144	196	188	244	224	228	232

Table 3.2: range amplitude of mode of first harmonic

mmHg↓300450600760	rpm↓1100 2000 3000 4000 5000 6000 7000 7500
									20	8	8	4	12	16	24	28
	24	12	8	8	16	16	28	36
									20	12	8	16	24	16	24	32
	24	16	8	28	40	36	36	36

Table 4.1: minimum threshold of second harmonic mode

mmHg↓300450600760	rpm↓1100 2000 3000 4000 5000 6000 7000 7500
									48	64	72	96	80	72	56	52
	48	80	112	92	104	68	52	48
									72	108	140	124	136	96	80	60
	96	124	172	168	160	136	88	72

Table 4.2: range amplitude of mode of second harmonic

mHg↓300450600760	rpm↓1100 2000 3000 4000 5000 6000 7000 7500
									0	0	0	0	0	0	0	0
	0	0	0	0	0	0	0	0
									8	4	4	4	8	4	0	0
	4	4	4	12	12	8	4	4

Table 5.1: minimum threshold of third harmonic mode

mmHg↓300450600760	rpm↓1100 2000 3000 4000 5000 6000 7000 7500
									92	72	52	124	40	132	144	152
	92	88	84	88	64	84	64	60
									88	104	112	68	96	48	28	24
	88	124	160	136	160	80	80	80

Table 5.2: range amplitude of third harmonic mode

If a misfire is detected in only one of the six cylinders 3 or 3 ', the associated controller 2 or 2' can determine the position of the cylinder in which the misfire occurred by first calculating the phase of the first harmonic in a known manner. Thereafter, a phase difference approximately corresponding to the phase of the cylinder in which misfiring occurred is obtained by subtracting the phase of the first harmonic from the phase of the first cylinder of the engine cycle, which is tabulated in the controller 2, 2' as a function of engine rpm.

For example, if the phase of the first cylinder of an engine cycle is 210 ° at a given engine rpm, misfires occur in the first, second, third, fourth, fifth or sixth cylinder in the firing order when the phase of the first harmonic is between 180 ° and 240 °, 120 ° and 180 °, 60 ° and 120 °, 0 ° and 60 °, 300 ° and 360 °, or 240 ° and 300 °, respectively.

Table 6 below shows the relationship between engine rpm and phase of the first cylinder in order to determine the position of the cylinder where misfire occurred.

rpm	Phase position
		510	164°
990	140°
		1500	106°
2010	80°
		2490	58°
3000	36°
		3510	16°
3990	0°
		3990	360°
4500	348°
		5010	338°
5490	328°
		6000	320°
6510	312°
		6990	302°
7500	292°

Table 6: relationship between engine rpm and phase of first cylinder

The detection of a misfire in one of the engine cylinders, and in the case of a single misfire, the corresponding cylinder position, are stored in a suitable counter in the memory of the controller 2, 2'. This memory can be read by an external processor through the port 20 during operation of the vehicle in order to diagnose possible engine faults.

Referring now to fig. 3a to 3c, it can be seen how the signal delivered by the sensor 18, 18 'varies as a function of misfiring in one of the cylinders 3, 3' as measured by tests for misfiring in the test engine. In particular, FIG. 3a shows that the voltage at the terminals of the pressure sensors 18, 18 ', which is proportional to the pressure in the exhaust pipes 9, 9' (given volts) oscillates almost regularly for six cycles during the engine cycle (indicated by crank angles of-180 to 540) at about 2000rpm at an engine load of about 15%. This voltage is indicated by a thin line, while the thick line indicates the voltage in the case of a first cylinder misfire. In this case, it is clearly seen that the voltage pattern has a first irregularity before and after 240 ° and a second irregularity before and after 480 °. However, figure 3b shows that at about 4000rpm, the engine load is about 100%, and the voltage profile is more complex in the case of regular ignition than in the former case. However, in the case of the first cylinder misfire, the voltage pattern (still indicated by the thick line) deviates from the normal ignition voltage pattern (still indicated by the thin line) around 400 °. Also figure 3c shows that at about 6000rpm the engine load is about 100% and the voltage pattern of the pressure sensor 18, 18' is different from that of misfiring in the first cylinder, in particular around 470 °.

Similarly, with reference to fig. 4a to 4c, and again through experimentally obtained measurements, it can be seen how the signal delivered by the pressure sensor 18, 18 'varies as a function of the misfiring of one of the cylinders 3, 3', independently of the misfiring caused by the absence of fuel injection or ignition in the cylinder. In fact, it can be seen that the voltage profile in the absence of injection (indicated by a bold line) is substantially equal to the voltage profile in the absence of ignition (indicated by a dashed line). This correspondence can be found both at low rpm, about 15% engine load, i.e. about 2000rpm (fig. 4a), at medium rpm, about 55% engine load, i.e. about 4000rpm (fig. 4b), and at high rpm, about 100% engine load, i.e. about 6000rpm (fig. 4 c).

Referring now to fig. 5a to 5c, the misfire index measured as a function of engine cycle (represented by the horizontal axis) represents easily detectable peaks corresponding to the instants when misfire is experimentally induced in one of the engine cylinders. This peak can be found both at low rpm, about 15% engine load, i.e. about 1000rpm (fig. 5a), at medium rpm, about 55% engine load, i.e. about 3000rpm (fig. 4b), and at high rpm, about 100% engine load, i.e. about 5000rpm (fig. 4 c).

Referring to fig. 6, it can be seen that the modes of the first 10 th harmonic of the signal delivered by the sensors 18, 18' (in volts) vary considerably for regular firing of all cylinders (represented by the white bars) from misfiring of the first cylinder (represented by the gray bars). The graph represents the calculated mode for the first 10 th harmonic for the case shown in fig. 3a and 4a with the engine at 2000rpm and a load of about 15%. The figure clearly shows that in the case of regular ignition the modes of the sixth harmonic are higher than all other modes, while in the case of a first cylinder misfire there is also a considerable contribution of the modes of the first harmonic, in particular the first 3 harmonic. It is clear that the composition of the modes of the harmonics depends on certain factors which must be taken into account when setting the threshold for the misfire index. These factors include, for example, the shape of the exhaust pipes 9, 9 ', the number of cylinders 3, 3' in each bank and the firing order.

Referring finally to fig. 7 a-7 c, it can be seen that the phase of the first harmonic varies as a function of the position of the cylinder where misfiring occurs. In fact, it is also possible to identify six separate regions, each corresponding to one engine cylinder, in which the polar coordinates of the mode and phase of the misfiring transient first harmonic are concentrated. In particular, it can be seen that the polar coordinates are concentrated in six sectors each extending 60 °, the order of which is defined by the cylinder firing order, which in the present embodiment is 1-4-2-6-3-5 for the row of cylinders 3. Considering the engine phase, this correspondence can be found both at low rpm, i.e. at about 2000rpm, at about 15% engine load (fig. 7a), at medium rpm, i.e. at about 4000rpm, at about 100% engine load (fig. 7b), and at high rpm, i.e. at about 6000rpm, at about 100% engine load (fig. 4 c).

It will be apparent to those skilled in the art that possible additions and/or modifications may be made to the embodiments described and illustrated above without departing from the scope of the present invention. In fact, depending on the type of engine being monitored, it is clear that the type of sampling, the frequency analysis, and in particular the method of calculating the misfire index, can vary. Similarly, the threshold value may also be changed according to tests implemented for various types of engines.

Finally, it is clear that the method according to the invention can be used in combination with one or more prior art methods.

Claims (15)

1. A method for detecting misfiring in one or more cylinders (3, 3') of an internal combustion engine, characterized in that it comprises the following operating steps:

-sampling the exhaust pressure value during at least one engine cycle, the sampling frequency being proportional to the crankshaft speed;

-analyzing the sampled signal in the frequency domain;

-calculating a misfire index as a function of the analysis result;

-comparing said indicator with one or more threshold values.

2. A method according to the preceding claim, characterized in that said frequency domain analysis comprises a fourier transform of the sampled signal.

3. A method according to the preceding claim, characterized in that the calculation of the misfire index comprises combining the modes of some harmonics of the sampled signal.

4. The method of claim 1 wherein the calculation of the misfire index comprises modulo addition of at least the first 3 harmonics of the sampled signal.

5. The method of claim 1, wherein the sampling of the pressure value begins at the beginning of an engine cycle.

6. A method according to claim 1, characterized in that it comprises comparing the modulus of at least one harmonic of the sampled signal with one or more threshold values.

7. A method according to claim 1, characterized in that it comprises calculating the phase of the first harmonic of the sampled signal and calculating the difference between said phase and the phase of at least one engine cylinder (3, 3').

8. A system for implementing the method according to one of the preceding claims, characterized in that it comprises at least one sensor (18, 18 ') for detecting the pressure in the exhaust pipe (9, 9'), and at least one sensor (14) for detecting the rotation of the crankshaft, said sensor (14, 18, 18 ') being connected to at least one control device (1, 2, 2'), the control device comprises means for analog-to-digital conversion of an electrical signal delivered by a sensor (18, 18 ') detecting the pressure in the exhaust duct (9, 9'), means for sampling the signal converted into digital form, wherein the sampling frequency is proportional to the rotational speed of the crankshaft, memory means for storing the sampled signal, and means for analyzing the sampled signal in the frequency domain, calculating an indicator of misfire as a function of the analysis, and comparing the indicator to one or more threshold values.

9. A system according to the preceding claim, characterized in that it comprises at least one sensor (16, 16') detecting the rotation of the crankshaft (17).

10. A system according to claim 8 or 9, characterized in that it comprises means for controlling the sampling frequency of said sampling means on the basis of a signal delivered by a sensor (14) detecting the rotation of the crankshaft.

11. A system according to claim 8, characterized in that it comprises at least one sensor (4, 4 ') for detecting the temperature of the coolant and at least two sensors (5, 5', 6, 6 ') for detecting the temperature and pressure of the air in the intake manifold (7, 7'), respectively, said sensors (4, 4 ', 5, 5', 6, 6 ') being connected to said control means (1, 2, 2').

12. A system according to claim 8, characterized in that it comprises at least one warning lamp (19, 19 ') indicating misfiring in at least one engine cylinder, said warning lamp (19, 19 ') being connected to said control means (1, 2, 2 ').

13. A system according to claim 8, characterized in that it comprises a sensor (21) for detecting the position of the engine throttle (22), said sensor (21) being connected to said control means (1, 2, 2').

14. System according to claim 8, characterized in that it comprises at least one sensor (13, 13 ') for detecting the temperature in the exhaust pipe (9, 9'), said sensor (13, 13 ') being connected to said control device (1, 2, 2').

15. A motor vehicle, characterized in that it comprises a system according to one of claims 8 to 14 for detecting misfires in one or more engine cylinders (3, 3').