Inductive system operation

After the positive pulse is received, a dwell control section of the control circuit determines when the primary circuit will be switched on, and how long current will flow in the primary winding. The dwell period can thus be varied according to engine speed, improving coil efficiency. At low engine speeds, the dwell period is short. As engine speed rises the dwell period increases.

Ignition coils employed with electronic systems are referred to as low inductance coils, because their primary winding resistance and number of turns is low. Current flow is much higher than with contact breaker systems and reaches its optimum level sooner. A current limiting device within the control circuit limits maximum current flow to a safe value.

Ignition advance according to engine speed and load is provided by centrifugal and vacuum advance mechanisms.

Overview

In an electronic ignition, a rotating reluctor and magnetic-pickup coil replace the traditional cam, breaker points and condenser in the distributors of cars equipped for electronic ignition. This system reduces the time between tune-ups. The high spots of the reluctor interrupt the magnetic field of the pickup coil and the permanent magnet. These interruptions, or pulses, are transmitted from the pickup to a nearby electronic control unit. There, the pulses signal a transistor to break the low-voltage sub-circuit and release high voltage from the coil to the spark plugs.

Electronic Ignition System Components

The components of a basic electronic ignition system all perform the same functions. Manufacturer has their own preferred terminology and location of the components.

The basic components of an electronic ignition system are as follows:

• TRIGGER WHEEL
  The trigger wheel, also known as a reluctor, pole piece, or armature, is connected to the upper end of the distributor shaft. The trigger wheel replaces the distributor cam. Like the distributor cam lobes, the teeth on the trigger wheel equal the number of engine cylinders.
  
• PICKUP COIL
  The pickup coil, also known as a sensor assembly, sensor coil, or magnetic pickup assembly, produces tiny voltage surges for the ignition systems electronic control unit. The pickup coil is a small set of windings forming a coil.
  
• ELECRTONIC CONTROL UNIT AMPLIFIER
  The ignition system electronic control unit amplifier or control module is an "electronic switch" that turns the ignition coil primary current ON and OFF. The ECU performs the same function as the contact points. The ignition ECU is a network of transistors, capacitors, resistors, and other electronic components sealed in a metal or plastic housing. The ECU can be located
  1. in the engine compartment, or
  2. on the side of the distributor, or
  3. inside the distributor, or
  4. under the vehicle dash.

ECU dwell time (number of degrees the circuit conducts current to the ignition coil) is designed into the electronic circuit of the ECU and is NOT adjustable.

When the distributor shaft rotates, a fluctuating magnetic field is generated due to changes in the air gap between the crank angle sensor and the distributor shaft reluctor.

Source: CDX Global & Wikipedia - en.wikipedia.org