Ignition system design and components


Ignition systems have come a long way since the early days of the automobile.

From early points-style setups to modern coil-on-plug configurations, the ignition system has evolved.

There are five primary types of ignition systems. Each has advantages and disadvantages. We turned to our friends at Autolite to explain what each ignition system type brings to the table. They broke it all down for us, and now we’re sharing the five types with you here.

1. Breaker-Point Ignition System

This “points-style” system is the oldest type of ignition system. It is completely mechanical and electrical—the most complex mechanism in this system is the distributor, which is driven off the engine’s camshaft. It uses:

  • Breaker points to trigger the ignition coil to generate a pulse of high-voltage energy.
  • A cap and spinning rotor to distribute high voltage to each spark plug at the appropriate time.

Advantages: It’s relatively easy to diagnose and repair.

Disadvantages: It contains many moving parts and requires frequent maintenance. Breaker point deterioration cannot provide maximum spark energy for every spark event throughout the life of the engine (frequent misfires are possible, increasing emissions). The ignition timing cannot be precisely controlled.

2. High Energy (Electronic) Ignition System

This system replaces the breaker points and condenser with a transistorized switch within an ignition module that handles the same task—triggering the ignition coil to generate high-voltage current. The distributor cap and rotor still perform the same job of distributing current to the spark plugs.

Advantages: It has fewer moving parts than a breaker-point ignition system and is still relatively easy to diagnose and repair. It can also consistently provide high voltage for every spark throughout the life of the engine (minimal misfires).

Disadvantages: It still relies on a conventional distributor, which eventually wears and requires replacement. Ignition timing cannot be as precisely controlled as with more sophisticated systems.

3. Distributorless Ignition System

This system eliminates the distributor entirely, and uses multiple ignition coils—one for each pair of cylinders. Using engine sensors to determine crankshaft position and sometimes camshaft position, an Electronic Control Unit triggers the appropriate ignition coil and directs the distribution of electrical current to the spark plugs.

The trick behind this system is the use of “waste spark” for one of the paired cylinders. This setup pairs two pistons that will be at top dead center at the same time—one will be at the end of its compression stroke, and the other will be at the end of its exhaust stroke. Each of the spark plugs in these cylinders will fire at the same time using the high voltage from one coil. The piston at the end of its compression stroke will generate power from the ignition of the air/fuel mixture. Igniting the spark plug for the piston at the end of its exhaust stroke will not perform any function—that is the waste spark cylinder.

Advantages: There are no moving parts, so there are typically lower maintenance costs. It can be designed to generate high voltage, and ignition timing can be precisely controlled for low emissions.

Disadvantages: A distributorless ignition system is more difficult to diagnose and more expensive than a traditional system and still requires high voltage wires from the coils to the spark plugs, like a traditional system.

4. Coil-on-Plug (Direct) Ignition

This most sophisticated of all ignition systems places an ignition coil directly on top of each spark plug. All of the ignition timing is handled by the Engine Control Unit, based on input from various sensors. Because each spark plug has its own dedicated coil, high-voltage spark plug wires are completely eliminated.

Advantages: The are no moving parts—which means lower maintenance costs. It can be designed to generate high voltage, and ignition timing can be precisely controlled for low emissions. The coil-on-plug setup is ideal for high-rpm engines.

Disadvantages: It can be more difficult to diagnose and more expensive to repair than a traditional system.

5. Capacitor Discharge Ignition (CDI) System For Small Engines

CDI systems are typically used on small engines—lawn mowers, chain saws, outboard boat motors or motorcycles, including two-stroke and four-stroke engines. Configurations vary widely, and may include a battery and alternator, or a magneto and no battery. The basic system described below uses the engine’s flywheel as both a magneto, to generate the initial voltage, and a triggering device, like the rotor of a distributor.

  • Permanent magnets embedded in the flywheel rotate around stationary source coils, creating the initial voltage.
  • Voltage flows to a capacitor, which builds the electrical charge to about 250+ volts.
  • A triggering device, mounted near the flywheel, signals the CDI control unit’s transistorized switching device (also known as a thyristor) to stop charging the capacitor.
  • At that point, the capacitor discharges its voltage into the coil primary winding. The coil secondary winding steps up the voltage so it can jump the gap at the spark plug.

Advantages: It’s relatively easy to diagnose and repair. Its short charging times and short spark duration are suitable for high-speed operation.

Disadvantages: The short spark duration may be too short for reliable ignition with leaner air/fuel mixtures. It can be hard to start and is generally not suitable for automotive applications.

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Author: David Fuller

David Fuller is OnAllCylinders' managing editor. During his 20-year career in the auto industry, he has covered a variety of races, shows, and industry events and has authored articles for multiple magazines. He has also partnered with mainstream and trade publications on a wide range of editorial projects. In 2012, he helped establish OnAllCylinders, where he enjoys covering all facets of hot rodding and racing.