Ignition system (only petrol engines)

10.4.1 Ignition system - description

The ignition system is designed for igniting the fuel-air mixture in gasoline and gas-power internal combustion engines. Ignition is carried out due to the electric discharge between the electrodes of the spark plug when applying a voltage of 18,000-20,000 V.

The main components of the ignition system (each of the elements is described in detail below):

• ignition starter switch;
• ignition coil;
• distributor and contact-breaker unit;
• automatic spark timers;
• spark plugs;
• wires that connect the above elements.

10.4.2 Ignition system with distributor

Figure 10.6 shows a typical layout of an ignition system with a distributor and contact-breaker unit.

Figure 10.6 Engine ignition contact combination with distributor.

10.4.3 Ignition switch

The ignition switch is assembled together with the ignition lock. The main function of this switch is to supply consumers with electricity from power supplies. The ignition system as a whole is a consumer of electricity as well. As can be seen from the diagram below, the primary winding of the ignition coil is supplied from the power supply via a switch.

10.4.4 Ignition coil

In essence, the ignition coil is a transformer, which converts the low voltage from the on-board power supplies (12 V) into a voltage, which is sufficient for producing a powerful spark between the electrodes of the spark plug needed to ignite the fuel-air mixture inside the engine cylinder. Sufficient voltage is 20-30, or even 60 thousand volts.

For this type of conversion in there are two windings - primary and secondary, as well as the core the body of the coil. Each winding has a different number of threads and wire cross sections. When you turn the key and turn on the ignition from the battery pack, an electric current is applied to the primary winding and through the contacts it is closed to "ground". When a current pass through the primary winding, an electromagnetic field is created around the coil. As soon as the contacts open and the current flowing through the primary coil stops abruptly, the required voltage and current will appear in the secondary coil. And already current in 30 and more thousand volts from a secondary winding of the ignition coil will flow through the distributor to a spark plug.

10.4.5 Interrupter-distributor

Distributor and contact-breaker unit (in common terms - "trembler") is designed for interrupting and distributing: interrupt - current flowing through the primary winding of the ignition coil, distribute - current from the secondary ignition coil between the spark plugs in the sequence, which is provided by the engine. A high-voltage wire from the secondary winding of the ignition coil is connected to the center of the distributor cover. Terminals that are connected to the spark plugs via high-voltage wires are located around the perimeter of the cover. 

The circuit breaker can be contact and non-contact. In the contact breaker, the circuit of the primary winding of the ignition coil is due to the contacts, which is very unreliable.

Note
The reason for the unreliability of the contacts is that the disappearing magnetic field crosses the threads of not only the secondary but also the primary winding. This results in a self-induction current and a voltage of about 250-300 V. This causes the contacts to spark and burn out, and the current interruption in the primary winding is slowed down. This reduces the voltage in the secondary winding. Of course, this is solved by installing a condenser (usually with a capacity of 0.25 μf). However, there is such a thing as erosion. This is a gradual destruction of the contact surface, as a result of which they are loose and the voltage arising in the secondary winding of the ignition coil is reduced.

A special device called a Hall sensor was installed instead of contacts in order to eliminate the mechanical component of the breaker. There are no contacts - only control pulses that control the operation of the ignition coil.

10.4.6 Ignition timing regulators

In order for the fuel-air mixture to burn while the piston moves from the top dead center to the bottom, it must be ignited a little earlier. Ignition advance angle is the main indicator of the ignition moment. It indicates how many degrees before TDC on the compression stroke there will be a breakdown between the electrodes of the spark plug. 

The change of the ignition advance angle is carried out mechanically - by turning the contacts relative to the drive shaft in one direction or another in distributors of the type described above.

10.4.7 Spark plug

The element, which ignites the fuel-air mixture inside the cylinder, is called the spark plug. The arrangement of this element is very simple (see figure 10.7): the casing with a cut thread and an electrode (negative as contacts with "ground" – cylinder head), the insulator in which the positive electrode passes. A high-voltage wire of the ignition system is connected to this electrode on one side through a tip. The positive electrode is located next to the negative electrode (the air gap between them is 0.8-1.2 mm - depending on the model of the spark plug). When a high-voltage discharge is applied to the positive electrode from the ignition distributor, an air gap breaks through, that is, a spark appears - powerful enough to ignite the fuel-air mixture.

Figure 10.7 Spark plug.

10.4.8 Microprocessor ignition system

As it has been already said many times, the automotive industry is developing rapidly. Thus, the ignition system with a distributor has been replaced by microprocessor systems. They do not have any rotating and moving elements (see Figure 10.8), but there are ignition coils (increasingly on the coil per cylinder), an electronic control unit (with integrated ignition unit) and a switch (if the ignition coil unit is one) or switches (if there are several ignition coils).

Figure 10.8 Microprocessor spark timing system.

Electronic control unit collects data from a number of sensors. When processing them, the ECU outputs a control signal to the switch (or switches), which determines at what point to ignite the fuel-air mixture inside the cylinder. Each spark discharge is obtained by electronic signals with very high accuracy and without the use of any moving parts. In many engines, a spark is generated not only during the compression stroke (this means that each spark plug generates a spark discharge each time the piston reaches TDC). The content of harmful components in the exhaust gases is somewhat reduced.