GB2103284A - Noise suppression in spark ignition systems for internal combustion engines - Google Patents

Abstract

Suppression of radio frequency interference radiation in an ignition system is effected by ensuring grounding of the noise current. A capacitor 9 is included in a circuit connecting the ignition coil 3 and the ignition switch 2 and one terminal of the capacitor is connected to an electrical ground reference for the engine by way of wire 9e not exceeding 10mm in length. <IMAGE>

Description

SPECIFICATION Improvements in or relating to noise suppression in spark ignition systems for internal combustion engines The present invention relates to the suppression of spark ignition noise in an internal combustion engine, particularly the suppression of AM radio frequency interference produced by the spark ignition system of an internal combustion engine.

As is known, radio frequency interference is caused by internal combustion engines equipped with spark ignition. Various systems and/or devices have been developed for the suppression of the relatively high frequency radiation in the FM frequency range. For example, in the United States, the regulations of the Society of Automotive Engineers relating to FM radio frequency interference have led to the development of systems for suppressing FM interference arising in spark ignition engines.

It is usual to provide a capacitor, a so-called autocondenser, to suppress low-frequency interference. The magnitude of radio frequency interference is known to be proportional to the physical size of the loop in which noise current can circulate.

Conventional suppression means however, does not always reduce the radio frequency interference from a spark ignition engine by enough to prevent degradation of the performance of a radio system close to the engine, such as in the vehicle powered by the engine.

If the vehicle radio receives a weak signal through being in a low signal strength area or through having an antenna positioned in the windshield, the suppressed radio frequency interference is usually strong enough to cause noise in the radio.

It is an object of the present invention to provide improved means for suppressing radio frequency interference created by a spark ignition engine.

The improved suppression of radio frequency interference in a spark ignition internal combustion engine is accomplished by reducing the size of the current loop which includes the spark gaps of the distributor and ignition plug. This is achieved by preventing the noise creating current from leaking to the ignition switch and by ensuring grounding of the noise creating current. For this purpose, a bypass circuit with a noise-suppression capacitor is arranged to provide a low impedance between the ignition switch and the capacitor compared with the impedance that exists between the ignition switch and the ignition coil in a vehicle powered by a spark ignition engine.

An ignition system for a spark ignition internal combustion engine includes a capacitor arranged for one terminal to be connected to an electrical ground reference point for the engine by way of a short electrical path, and, a supply electrical conductor for delivering low-tension electrical energy to spark generator means for the engine, connected to the said spark generator means by way of the other terminal of the capacitor.

The part of the supply electrical conductor extending between the capacitor and the spark generator means may be shorter than the part of the supply conductor intended for connection to a source of low-tension electrical energy in order to provide increased isolation of the low-tension source from the noise source. The low-tension source may be a battery connectible to the spark generator means by way of an ignition switch.

The capacitor is inserted in a circuit connecting the ignition coil and the ignition switch so that the noise creating current always flows through the capacitor to ground.

A noise suppressing ignition system for an internal combustion engine comprises: a primary circuit including a vehicle battery, an ignition switch, a primary winding in an ignition coil and a circuit breaker for breaking the primary circuit for inducing substantially high voltage for ignition, a secondary circuit including a secondary winding in the ingnition coil, a distributor and ignition plugs, a bypass circuit for grounding a noise creating current produced by spark ignition in the internal combustion engine, the bypass circuit including a capacitor located adjacent a member for grounding the noise creating current and having a first wire connecting the capacitor to the member and being of substantially short length and a second, branched wire connecting the capacitor respectively to the ignition switch and the ignition coil.

A noise-suppression capacitor for a spark ignition system in an internal combustion engine is provided. The capacitor is located adjacent a member suitable for grounding the noise current produced by spark ignition in the internal combustion engine, is connected to the member by a substantially short lead wire, and has a branched second lead wire connected to an ignition switch and an ignition coil.

A method of connecting a capacitor for suppressing noise in a spark ignition engine includes the steps of connecting one terminal of the capacitor to an electrical ground reference point for the engine by way of a short electrical path, and connecting, by way of the other terninal of the capacitor and to spark generator means for the engine, an electrical conductor for delivering low-tension electrical energy to the spark generator means of the engine.

In contrast, the conventional noise method of connecting a suppression capacitor involves the use of a length of lead wire with an inductance of 1 to 2 ,t4H between the capacitor and the electrical ground reference. This value of inductance represents, for AM frequency, e.g. 0.5 MHz to 6 MHz, an impedance of about 1 OQ to 20s2. The impedance of the ground wire is, therefore, higher than that of the lead wire connecting the ignition coil and the ignition switch, assuming that the length of the ground wire connecting the noise suppression capacitor to ground is 1 50cm, and that the inductance of the ground wire can be obtained from the following equation:: 2h L=2(log-) X1x20-9 (H) r where h is a height of the circuit from a grounding surface which may be, for example, 1 cm; I is a length of the lead wire; r is a diameter of the lead wire, which may be, for example 0.04cm, giving an inductance of 1.2 iLH for 1 50cm of ground wire.

Assuming that the value of the capacitor is 0.5 uF, the impedance of the bypass circuit for grounding is 122 for a frequency of 1.6 MHz. This impedance may cause the noise creating current (high frequency current) to appear at the ignition switch.

The present invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a schematic circuit diagram of an ignition system showing one arrangement of a radio frequency interference suppressing capacitor according to the present invention; Figure 2 is a diagrammatic representation of a wiring arrangemebnt of an automotive vehicle engine which employs the ignition system of Fig. 1; Figure 3 is a perspective view of a capacitor which may be utilized in the ignition system of Fig. 1; Figure 4 is a graph showing the results of experiments with the ignition system of Fig.

1, in which the noise level drop is shown as a function of frequency, Figure 5 is a schematic circuit diagram of another arrangement of an ignition system according to the present invention; Figure 6 is a perspective view of a capacitor suitable for use in the ignition system for Fig.

5; and Figure 7 is a graph showing relationship of the length of the lead wires in the spark ignition system and the noise level.

Referring now to Figs. 1 and 2 of the drawings, there is shown a vehicle battery 1, an ignition switch 2, an ignition coil 3, a distributor 5 and an ignition plug 6. The ignition coil 3 includes a primary winding 31 and a secondary winding 32. The primary winding 31 is connected to the vehicle battery 1 via the ignition switch 2 and an external resistor 4 and to a contact breaker 7. The secondary winding 32 is coupled to the primary winding 31 so as to provide a high voltage when the power supplied to the primary is broken by the opening of the contact breaker 7 in the conventional manner. I ne induced high voltage is applied to the ignition plug 6 to produce a spark across a gap 6 of the ignition plug through the distributor 5.

The distributor 5 also has a gap which exists between a rotor (not shown) and fixed output contacts (not shown). A bypass circuit 9a with a capacitor 9 is inserted to bypass the power to ground at a point upstream of the external resistor. As shown in Fig. 2, the bypass circuit 9a is connected to the metallic housing of the distributor 5.

The bypass circuit 9a comprises a pair of lead wires 9b and 9c and the capacitor 9.

One each of each lead wire 9b and 9c is connected to a terminal 9d of the capacitor 9.

The other end of the lead wire 9b is connected to the ignition switch 2, and the other end of the lead wire 9c is connected to the ignition coil 3 via the external resistor 4. As shown in Fig. 2, the capacitor 9 is located adjacent the distributor 5 and acts to ground noise currents generated by the sparks. Noise suppression is effected by providing a relatively short length of lead wire 9e to connect the capacitor 9 to the distributor 5. The lead wire is preferably about 1 Omm or less. With this arrangement, the inductance of the lead wire 9e and capcitor 9 is approximately 0 (zero).

Preferably, the capacitor 9 is a metallized paper (MP) capacitor having a high breakdown voltage.

In the noise suppression ignition system as shown, the noise currents generated by the sparks flow through the lead wire 9c to the capacitor 9. Since the impedance of the lead wire 9e is quite small, the noise currents flow to ground through the capacitor 9. In tests on the arrangement shown, the noise level drop was found to be as illustrated in Fig. 4, that is, about 3 dB to 5 dB for the AM frequency band of 0.5 MHz to 1.6 MHz. In the arranged ment shown, the lead wire 9b is of length 3.5m and the length of the lead wire 9c is 80cm. Ideally the lead wire 9c should be as short as possible. The resistance value of the external resistor 4 is 2S2 and capacity of the capacitor 9 is 0.5 ,uF.

Referring to Figs. 5 and 6, there is shown an arrangement in which the capacitor 10 has a common lead wire 1 ova. The lead wire 10a is connected to the ignition switch 2 via a lead wire 1 Ob and to the ignition coil 3 via a lead wire 1 Oc. The lead wire 1 Oa is of relatively short length to minimize its impedance to eiectrical noise. As before, the capacitor 10 is located adjacent to the distributor metal housing which is used for grounding the capa citor 10. A ground wire 1 Od connecting the capacitor 10 to the distributor 5 has a length of 1 Omm or less.As explained previously, the noise currents produced in the secondary circuit are coupled into the primary circuit and flow through the lead wire 1 or. In order to ensure that the noise currents flow through the capacitor 1 0, it is necessary to have an impedance 43 at the point P in the path into the capacitor 10 which is less than the impedance Zwi of the path into the battery. This condition, is satisfied when Zw3 is less than one tenth of ZW1, Zw3 being the impedance of the lead wire 1 Oa. Assuming that the value of the capacitor 10 is 0.5 yF, the impedance of the capacitor is about 1 Q at the frequencies of interest. Usually, the length of the lead wire lOb is about 3 to 4m. Therefore, the length of the lead wire 1 Oa may be less than or equal to lOmm as is apparent from Fig. 7.

Thus, a capacitor connected as described in a spark ignition engine causes the noise currents to flow to ground and prevents the noise currents from leaking into the low-tension supply. This produces a significantly reduced noise level in the AM radio band.

Claims (11)

CLAIMS.

1. An ignition system for a spark ignition internal combustion engine, including, a capacitor arranged for one terminal to be connected to an electrical ground reference point for the engine by way of a short electrical path, and, a supply electrical conductor for delivering low-tension electrical energy to spark generator means of the engine, connected to the said spark generator means by way of the other terminal of the capacitor.

2. An ignition system for a spark ignition internal combustion engine, as claimed in claim 1, wherein the part of the supply electrical conductor extending between the capacitor and the spark generator is shorter than the part of the supply conductor intended for connection to a source of low-tension electrical energy.

3. An ignition system for a spark ignition internal combustion engine, as claimed in claim 1 or claim 2, wherein the electrical path between the capacitor and the electrical ground reference point for the engine is no more than lOmm in length.

4. A noise suppressing ignition system for an internal combustion engine of a vehicle having an audio system comprising: An ignition coil having a housing, a primary winding and a secondary winding, a primary circuit including a vehicle battery, an ignition switch, the primary winding of said ignition coil and a circuit breaker for breaking said primary circuit for inducing high voltage ignition pulses in said secondary winding; a secondary circuit including the secondary winding of said ignition coil, a distributor and ignition plugs;; a bypass circuit for grounding noise creating current produced by spark ignition in the internal combustion engine, said bypass circuit including a capacitor located adjacent a grounding member, a first wire connecting said capacitor to said grounding member and a second, branched wire connecting said capacitor respectively to said ignition switch and said ignition coil, said first wire having an impendance substantially less than the impedance of said second wire connecting said ignition switch in the noise creating frequency range.

5. An ignition system as claimed in claim 4, wherein the second wire is branched at a terminal of the capacitor.

6. An ignition system as claimed in claim 4, wherein the second wire is connected to the capacitor by a third wire having a subtantially short length.

7. A noise suppressive capacitor circuit for a spark ignition system in an internal combustion engine comprising a capacitor located adjacent a member for grounding noise creating current produced by spark ignition in the internal combustion engine, a substantially short, first lead wire connecting said capacitor and member and a branched second lead wire respectively connected to an ignition switch and an ignition coil.

8. A noise suppressive capacitor circuit as claimed in claim 7, wherein the second wire is branced as a terminal of the capacitor.

9. A noise suppressive capacitor circuit as claimed in claim 7, wherein the second wire is connected to the capacitor by a substantially short third wire which has lower impedance than that of the branched second wire connected to the ignition switch.

10. A noise suppressive capacitor circuit as claimed in claim 7, wherein the first lead wire is less than or equal to lOmm.

11. An ignition system for a spark ignition internal combustion engine substantially as herein described with reference to and as illustrated by Figs. 1 to 3, or Figs. 5 and 6 of the accompanying drawings.

1 2. A method of connecting a capacitor for suppressing noise in a spark ignition engine, including the steps of connecting one terminal of the capacitor to an electrical ground reference point for the engine by way of a short electrical path, and connecting by way of the other terminal of the capacitor and to spark generator means for the engine, an electrical conductor for delivering low-tension electrical energy to the spark generator means of the engine.