CHAPTER 2 - PART 3 - ENGINE AUXILIARY SYSTEMS - IGNITION SYSTEM AND STARTING SYSTEM

IGNITION SYSTEM

The ignition system produces the electric spark needed to ignite the air-fuel mixture in the cylinder at the right time. It is specific to spark-ignition (SI) engines.

In modern cars, given the high engine speeds, power outputs and compression ratios, electronic ignition systems are used, eliminating conventional mechanical components and ensuring more precise and reliable operation.

In most modern engines, ignition is electronically controlled by the engine control unit, and each cylinder may be equipped with its own ignition coil in a “coil-on-plug” system. This eliminates the need for a conventional mechanical distributor.

Electronic ignition systems may use transistor control, and the control signal may come either from a mechanical contact breaker or from contactless sensors, such as photoelectric, inductive or Hall sensors.

THE IGNITION SYSTEM CONSISTS OF:

• battery: supplies the electrical energy needed to operate the ignition system and other electrical loads, particularly when starting the engine or when the generator does not produce enough current;
• ignition switch / start button: allows power to be supplied to the ignition circuit and controls the starting circuit, which activates the starter motor;

• ignition coil: operates on the principle of a transformer and converts the low-voltage current (approximately 12 V) received from the battery into high-voltage current (approximately 20.000 V), needed to produce the electric spark at the spark plug.
• contact breaker-distributor, used in conventional ignition systems in petrol engines, comprises:
  • Contact breaker: closes and interrupts the coil's low-voltage circuit through contacts called “breaker points”. Interrupting the current causes the high voltage needed for the spark to be generated;
  • Distributor: delivers high voltage to the spark plugs in turn, in the cylinders' firing order;
  • Condenser: limits sparking between the breaker points, protecting them, and helps generate high voltage in the coil;
  • Ignition advance mechanisms: adjust spark timing according to engine speed through the centrifugal advance mechanism, and engine load through the vacuum advance mechanism;
  • Ignition advance means producing the spark shortly before the end of compression to allow combustion to take place at the right time.
• spark plug: produces the electric spark needed to ignite the air-fuel mixture.

STARTING SYSTEM

The starting system turns the engine's crankshaft until the engine begins to run on its own.

This requires a minimum engine speed that allows the air-fuel mixture to ignite in spark-ignition engines or provides the conditions needed for fuel injection in compression-ignition engines.

THE STARTING SYSTEM CONSISTS OF:

• battery: supplies the electrical energy needed to operate the starter motor;
• ignition switch / start button: controls the power supply to the starting circuit and the activation of the starter motor;
• starter relay: allows the required current to flow to the starter motor;
• electric starter motor (starter): converts the electrical energy received from the battery into mechanical energy, turning the engine's crankshaft;
• flywheel engagement mechanism: allows the starter pinion to mesh with the flywheel ring gear to transmit motion to the crankshaft.

The engine is started by the starter motor turning the crankshaft until the minimum speed required for engine operation is reached.