Method and circuit for detecting the presence of a spark in internal combustion engine

Abstract: The invention concerns a method of detecting a spark produced by means of a spark coil (L) having a primary circuit (L') connected to a supply voltage generator (Vb) and a secondary circuit (L'') with the spark coil (L) being inserted in an electronic ignition device of an internal combustion motor. The method consists of the following phases: generation of a voltage signal (U) proportional to a voltage (VL) present on the primary circuit of the spark coil (L), comparison of the voltage signal (U) with a first, upper, threshold value (U1) by means of a comparator (C) with hysteresis, comparison by means of said comparator (C) of the voltage signal (U) with a second, lower, threshold value (U2) proportional to the supply voltage (Vb), detection of the duration of a voltage (Vc) output from the comparator (C), and signalling of the presence of the spark if said duration is greater than a reference value (B).

Description: The present invention relates to a method for detecting the presence of a spark in an electronic ignition device for an internal combustion motor and to a related circuit. It is known that in an electronic ignition device for an internal combustion motor there is the need to detect whether during the explosion phase a spark is present or absent on the secondary circuit of a spark coil inserted in the device. If there is no spark it is necessary to proceed with subsequent operations to avoid deterioration of the catalytic muffler. A known technical solution for meeting this necessity is described in European Patent Application 88108869.4. which suggests detecting the presence or absence of spark on the secondary circuit of the spark coil by analyzing the different behaviour or the voltage present on the primary circuit of the coil. To clarify this phenomenon reference is made to FIG. 1. When a controlled switch S inserted in the electronic ignition device of a internal combustion motor closes, in the primary circuit L' of the spark coil L passes a current whose behaviour is linearly increasing in time. But when the controlled switch S opens, the voltage VL present on the primary circuit L' of the spark coil L increases rapidly to then settle after a certain period of time at a value equal to that of the supply voltage Vb supplied by the vehicle's battery. In the latter case it can be noted that in the absence of spark on the secondary circuit L" of the spark coil L the duration of the overvoltage created on the primary circuit L' is approximately 100µs. But in the presence of a spark the duration of the overvoltage is approximately 2ms. The technical problem underlying the present invention is to provide a method and related circuit for correctly and accurately detecting a spark in an electronic ignition device of an internal combustion motor. The technical problem is solved by a method and related circuit of the type indicated above and defined in the characterizing part of the annexed claims. The characteristics and advantages of the method and related circuit in accordance with the present invention are set forth in the description of an embodiment thereof given below by way of non-limiting example with reference to the annexed drawings. In the drawings: FIG. 1 shows a diagram of principle of an electronic ignition device of an internal combustion motor, FIG. 2 shows a block diagram of a circuit provided in accordance with the present invention, and FIGS. 3, 4, 5, 6 and 7 show respective diagrams with the same time base of electrical signals present in the circuit in accordance with the present invention. With reference to the FIGS. reference number 1 indicates as a whole and diagrammatically a circuit provided in accordance with the present invention for detecting the presence of a spark produced by means of a spark coil L inserted in an electronic ignition device of an internal combustion motor. Specifically with reference to FIG. 2 the coil L comprises a primary circuit L' inserted between a power supply voltage generator Vb and a controlled switch S and a secondary circuit L". Again with reference to FIG. 2 the circuit 1 also comprises a first input block I provided e.g. by means of a resistive divider (R1,R2) having an input terminal connected to the primary circuit L' of the coil L and an output terminal. This circuit comprises also a comparator C with hysteresis having a first input terminal connected to the output terminal of the input block I. The comparator also comprises a second and a third input terminals kept respectively at a first U1 and a second U2 reference voltage. It is well to specify that this first reference voltage U1 and second reference voltage U2 represent respectively the upper and lower thresholds of the comparator C. The second reference voltage U2 is obtained preferably by means of a resistive divider (R3,R4) connected to the supply voltage generator Vb. At output the comparator C also comprises a low-pass filter FB. The circuit 1 also comprises a second signalling block SEG having an input terminal connected to an output terminal of the comparator C and an output terminal which is an output terminal OUT in the drawing of the circuit 1. This second signalling block SEG comprises in turn a counter CON having a first input terminal connected to the output terminal of the comparator C and a second input terminal receiving a timing signal Ck. The counter CON has an output connected to an input of a digital comparator CD having a second input terminal connected to a memory element M in which is memorized a binary reference value B. There is now described the operation of the circuit in accordance with the present invention with particular reference to an initial state in which the switch S is open. In this state it is seen that at the ends of the primary circuit L' of the spark coil L is present a voltage VL from which by means of the resistive divider (R1,R2) is obtained a proportional voltage U shown in FIG. 3. The comparator C with hysteresis compares this voltage U with the upper threshold U1 and lower threshold U2. Specifically, from the comparator C is output a voltage Vc which is at a high logic level when the voltage U exceeds the threshold value U1. The voltage Vc by means of the filter FB keeps this logic level high for the entire duration of the transient until the voltage U falls below the lower threshold value U2 as shown in FIG. 4. The duration of the voltage Vc is proportional to the duration of the overvoltage created on the primary circuit L' of the coil L when the controlled switch S opens. It is also well to note that the height of the curve representing the behaviour of the voltage VL if there is a spark on the secondary circuit L" of the coil L depends strictly on the supply voltage Vb as shown in FIG. 5. Consequently even the lower threshold U2 of the comparator C must be correlated with the supply voltage Vb. Indeed, as shown in FIGS. 6 and 7 if the threshold value U2 were constant and hence independent of the supply voltage Vb the duration of the voltage Vc would no longer be proportional to the duration of the overvoltage which there would be on the primary circuit L' of the coil L. Thus, as the counter CON supplies at output a binary value A proportional to the duration of the voltage Vc this value A by means of the digital comparator CD is compared with the reference binary value B. If A>B the digital comparator CD switches to a higher level and signals on the output terminal OUT of the circuit 1 the presence of the spark on the secondary circuit L" of the coil L.

Claims: Method for detecting the presence of a spark produced by means of a spark coil (L) having a primary circuit (L') connected to a supply voltage generator (Vb) and a secondary circuit (L"), said spark coil (L) being inserted in an electronic ignition device of an internal combustion motor and said method being characterized in that it consists of the following phases: generating a first voltage signal (U) proportional to )a voltage (VL) present on the primary circuit of the spark coil (L), comparing said first voltage signal (U) with a first, upper, threshold value (U1) and in response producing a second voltage signal (Vc) at a first logic level when the first voltage signal (U) exceeds the first, upper, threshold value (U1), maintaining said second voltage signal (Vc) at the first logic level, comparing said first voltage signal (U) with a second, lower, threshold value (U2), proportional to the supply voltage (Vb), and in response producing the second voltage signal (Vc) at a second logic level when the first voltage signal (U) falls below the second, lower, threshold value (U2), detecting the time duration of the second voltage signal (Vc), and signalling the presence of a spark on the secondary winding if said time duration exceeds a reference value (B). Method in accordance with claim 1 and characterized in ;that the phase of detecting the time duration of the second voltage signal (Vc) includes the phase of: generating a value (A) proportional to the time duration of the second voltage signal (Vc). Method in accordance with claim 2 and characterized in that the phase of signalling the presence of a spark includes the phase of: producing an output value (OUT) at a high logic level when the time duration of the second voltage signal (Vc) exceeds the reference value (B). Method in accordance with claim 1 and characterized in that the first, upper, threshold value (U1) is constant. Method in accordance with claim 1 and characterized in that the first logic level is high and the second logic level is low.

Inventors: DI CICCO BEN, PALARA SERGIO, DI CICCO, BEN, PALARA, SERGIO, Di Cicco, Ben, c/o SGS-Thomson Microelectr. Srl
Applicants: DAIMLER CHRYSLER CORP, ST MICROELECTRONICS SRL, DAIMLERCHRYSLER CORPORATION, STMICROELECTRONICS S.R.L.
Application year: 1995
Main classification: F02P1
IPC codes: G01R31, G01R29, F02P17