16.6.
Direct Ignition
16.6.1.
General Description
Direct ignition is in a way the extension of the distributorless ignition. This system has an inductive coil for each cylinder. Figure 16.58 shows a cross section of the direct ignition coil. These coils are mounted directly on the spark plugs. The use of an individual coil for each plug provides a very fast rise time for the low inductance primary winding, which ensures a very high voltage and high energy spark. This voltage can be in excess of 40 kV, due to which efficient initiation of the combustion process takes place under cold starting conditions and with weak mixtures. Some direct ignition systems such as the SAAB system use capacitor discharge ignition.
Igniter units are used to switch the ignition coils and these units can control up to three coils. These in fact are simply the
power stages of the control unit, but located in a separate container to avoid interference in the main ECU due to heavy current switching.
16.6.2.
Control of Ignition
Ignition timing and dwell are controlled in the same manner as described in previous systems. One important additional feature in this system is a camshaft sensor used to identify the cylinder, which is on the compression stroke. The Bosch Motronic 1.8 system also uses information from the automatic transmission control unit. This helps in retardation of timing in order to assist gear changes.
A system is available, which does not require a sensor such as a crankshaft sensor to determine the cylinder that is on compression. To do this initially all of the coils are fired. A voltage is then applied across the plugs. The measurement of the current of each spark indicates the cylinder that is on its combustion stroke. The cylinder with the highest current at this point is the cylinder on the combustion stroke, because a burning mixture has a lower resistance. An additional feature is provided in some systems where the engine is cranked over for an excessive time, causing flooding. The plugs are all fired with multi-sparks for a period of time after the ignition is left in the on position for five seconds. This burns away any excess fuel.
In difficult starting conditions, multi-sparking is also used by some systems (SAAB) during 70 degrees of crank rotation before TDC. This assists with starting and then, once the engine is running, the timing retrns to its normal calculated position.
