Ignition Systems And Timing

Ignition Timing

There are several different types of ignition systems which have two basic functions: they must control the spark and timing of the spark plug firing to match varying engine requirements, and they must increase battery voltage to a point where it will overcome the resistance offered by the spark plug gap and fire the plug.

Maintenance & Repair Point type ignition Electronic ignition
Distributorless ignition Timing Maintenance

Point-type ignition

Primary & Secondary Circuit

The primary circuit carries low voltage and operates only on battery current controlled by the breaker points and the ignition switch.

The secondary circuit consists of the secondary windings in the coil, the high tension lead between the distributor and the coil on external coil distributors, the distributor cap, the distributor rotor, the spark plug leads and the spark plugs. The distributor switches the primary current on and off and distributes the current to the proper spark plug each time a spark is needed.

Electronic ignition systems

These systems generate a much stronger spark which is needed to ignite leaner fuel mixtures. The primary circuit of the electronic ignition systems operate on full battery voltage which helps to develop a stronger spark. Spark plug gaps have widened due to the ability of the increased voltage to jump the larger gap. Cleaner combustion and less deposits have led to longer spark plug life. On some systems, the ignition coil has been moved inside the distributor cap. This system is said to have an internal coil as opposed to the conventional external one.


Distributorless ignition systems

The spark plugs are fired directly from the coils. The spark timing is controlled by an Ignition Control Unit (ICU) and the Engine Control Unit (ECU). The distributorless ignition system may have one coil per cylinder, or one coil for each pair of cylinders.

Some popular systems use one ignition coil per two cylinders. This type of system is often known as the waste spark distribution method. In this system, each cylinder is paired with the cylinder opposite it in the firing order (usually 1-4, 2-3 on 4-cylinder engines or 1-4, 2-5, 3-6 on V6 engines). The ends of each coil secondary leads are attached to spark plugs for the paired opposites. These two plugs are on companion cylinders, cylinders that are at Top Dead Center (TDC) at the same time. But, they are paired opposites, because they are always at opposing ends of the 4 stroke engine cycle. When one is at TDC of the compression stroke, the other is at TDC of the exhaust stroke. The one that is on compression is said to be the event cylinder and one on the exhaust stroke, the waste cylinder. When the coil discharges, both plugs fire at the same time to complete the series circuit.

Since the polarity of the primary and the secondary windings are fixed, one plug always fires in a forward direction and the other in reverse. This is different than a conventional system firing all plugs the same direction each time. Because of the demand for additional energy; the coil design, saturation time and primary current flow are also different. This redesign of the system allows higher energy to be available from the distributorless coils, greater than 40 kilovolts at all rpm ranges.

The Direct Ignition System (DIS) uses either a magnetic crankshaft sensor, camshaft position sensor, or both, to determine crankshaft position and engine speed. This signal is sent to the ignition control module or engine control module which then energizes the appropriate coil.

The advantages of no distributor, in theory, is:

  • No timing adjustments
  • No distributor cap and rotor
  • No moving parts to wear out
  • No distributor to accumulate moisture and cause starting problems
  • No distributor to drive thus providing less engine drag

The major components of a distributorless ignition are:

  • ECU or Engine Control Unit
  • ICU or Ignition Control Unit
  • Magnetic Triggering Device such as the Crankshaft Position Sensor and the Camshaft Position Sensor
  • Coil Packs

Ignition timing

Ignition timing is the measurement, in degrees of crankshaft rotation, of the point at which the spark plugs fire in each of the cylinders. It is measured in degrees before or after Top Dead Center (TDC) of the compression stroke.

Because it takes a fraction of a second for the spark plug to ignite the mixture in the cylinder, the spark plug must fire a little before the piston reaches TDC. Otherwise, the mixture will not be completely ignited as the piston passes TDC and the full power of the explosion will not be used by the engine.

Ignition timing on many of today's vehicles is controlled by the engine control computer and is not adjustable. However the timing can be read using a scan tool connected to the data link connector.

The timing measurement is given in degrees of crankshaft rotation before the piston reaches TDC (BTDC). If the setting for the ignition timing is 5° BTDC, the spark plug must fire 5° before each piston reaches TDC. This only holds true, however, when the engine is at idle speed.

As the engine speed increases, the pistons go faster. The spark plugs have to ignite the fuel even sooner if it is to be completely ignited when the piston reaches TDC. To do this, distributors have various means of advancing the spark timing as the engine speed increases. On older vehicles, this was accomplished by centrifugal weights within the distributor along with a vacuum diaphragm mounted on the side of the distributor. Later vehicles are equipped with an electronic spark timing system in which no vacuum or mechanical advance is used, instead all timing changes electronically based on signals from various sensors.

If the ignition is set too far advanced (BTDC), the ignition and expansion of the fuel in the cylinder will occur too soon and tend to force the piston down while it is still traveling up. This causes engine ping. If the ignition spark is set too far retarded, after TDC (ATDC), the piston will have already passed TDC and started on its way down when the fuel is ignited. This will cause the piston to be forced down for only a portion of its travel. This will result in poor engine performance and lack of power.

Ignition system maintenance

Electronic ignitions, of course, do not need distributor maintenance as often as conventional point-type systems; however, nothing lasts forever. The distributor cap, rotor and ignition wires should be replaced at the manufacturer's suggested interval. Also, because of the higher voltages delivered, spark plugs should last anywhere from 30,000 to 60,000 miles