On the sources of radio interference in the ignition system. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Automobile. Ignition

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The sources of radio interference from the ignition system are considered, methods of interference suppression, ignition systems are shown, a filter is recommended for the power supply circuit of radio equipment.

There are four main sources of electromagnetic oscillations in the ignition system of a gasoline internal combustion engine: the primary circuit, the breaker contact circuit, the distributor circuit, and the spark plug. The last three refer to high-frequency sources of electromagnetic radiation that fall within the broadcast range. For clarity, consider the so-called. "classical" ignition system, invented 100 years ago and little changed until now, shown in Fig.1.

Rice. 1: 1 - battery; 2 - ignition coil; 3 - capacitor "sparking"; 4 - contact-breaker (now instead of it, a transistor is most often used); 5 - additional resistance (to limit the coil current in normal operation); 6 - contact for shunting additional resistance 5 to increase the ignition voltage for the period of starting the engine; 7 - ignition distributor; 8 - spark plugs (according to the number of cylinders).

The first source of radio interference is contact-breaker 4 at the moment when it opens. The second source is the discharge gap in the ignition distributor 7 during its breakdown. The third source is a breakdown in candlesticks 8.

The radiation of electromagnetic oscillations into space is determined not only by the power of the generator, but also by the radiating circuit, galvanic and inductive connections. For example, a battery, being a common source of power for all consumers in a car, leads to the emergence of a current connection between consumers. The classic ideology of ensuring noise immunity predetermines the need to suppress interference, first of all at the source of interference itself, and then at the inputs of consumers.

Based on this concept, in order to exclude the circulation of high-frequency electromagnetic oscillations in the circuit ignition coil - battery - ignition system switching circuit, it is necessary to install a shunt capacitor for the input circuit of the ignition coil, as shown in Fig. 2.

As can be seen, the presence of a shunt capacitor will create a "short" path for electromagnetic oscillations of the fundamental frequency, which will simultaneously improve the conditions for the formation of a high-voltage pulse and will act as a filter for the high-frequency part of the interference spectrum from the contact of the interrupter and high-voltage circuits.

The value of the shunt capacitance is chosen from the condition: Ssh "Ci. Since the capacitance rating usually lies in the range of 0,15 ... 0,3 microfarads, then 2 microfarads will be quite enough.

In existing ignition systems in the high-voltage part, certain measures have been taken to suppress radio interference. So, for example, a resistor is installed in the distributor, and in some systems the connecting high-voltage wires have a conductive core made of an alloy with high ohmic resistance. All these measures significantly reduce radio interference by increasing the attenuation of electromagnetic oscillations. A very effective way to suppress interference is to shield high-voltage circuits instead of introducing damping resistors into them. The fact is that initially high-voltage circuits were shielded. Now in aircraft systems and cars with radio communication equipment, only shielded high-voltage wires are still used in ignition systems.

It can be assumed that events unfolded like this. At first, when the interference was still not very annoying, someone suggested saving money by removing the screens from the wires. Then, when interference began to annoy and people became wiser in the field of electromagnetic oscillations, an inventor was again found who introduced damping resistances, then improvements began on how to make resistances and where to place them.

Screening of high-voltage, especially spark plug wires, leads to a significant increase in the efficiency of ignition of the fuel-air mixture by increasing the energy of the electric discharge in the spark plug gap with its transformation into a multi-spark high-frequency pulse.

Now about the filter in the power supply circuit of the radio. Known filters based on various combinations of inductors and capacitors. The proposed filter is called "diode" and is a half-wave rectifier with a capacitive filter, as shown in Fig.3.

Here: Ak - battery; VD - any diode for a current of 0,5 ... 1,0 A and a reverse voltage of at least 50 V; C - electrolytic capacitor of any brand with a capacity of at least 500 microfarads for a voltage of at least 25 V.

The radio receiver is connected to the battery with a shielded wire with the shield connected to ground at the point of connection of the negative terminal of the battery. The screened wire must be insulated on the outside (PK type cable), and the radio receiver housing must not touch the metal elements of the car body structure in the seat.

Author: ATBelyavsky, Cherkasy

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