A distributorless ignition system.
Figure shows an ignition system for a four-cylinder engine. There are two ignition coils, one for cylinders 1 and 4, and another for cylinders 2 and 3. A spark is produced each time a pair of cylinders reaches the firing point which is near top dead center (TDC). This means that a spark occurs on the exhaust stroke as well as on the power stroke. For this reason, this type of ignition system is sometimes known as the ‘lost spark’ system. There are two sensors at the flywheel: one of these sensors registers engine speed and the other is the trigger for the ignition. They both rely on the variable reluctance principle for their operation.
Details of engine speed and crank position sensors.
An alternative method of indicating the TDC position is to use a toothed ring, attached to the flywheel, which has a tooth missing at the TDC positions. With this type of sensor, the TDC position is marked by the absence of an electrical pulse. This is also a variable reluctance sensor. The other teeth on the reluctor ring, which are often spaced at 10Ž intervals, are used to provide pulses for engine speed sensing.
Engine speed and position sensor that uses a detachable reluctor ring.
Distributorless ignition has all the features of programmed ignition systems but, by using a special type of ignition coil, outputs to the spark plugs without the need for an HT distributor. The system is generally only used on four cylinder engines as the control system becomes too complex for higher numbers. The basic principle is that of the ‘lost spark’. The distribution of the spark is achieved by using double ended coils, which are fired alternately by the ECU. The timing is determined from a crankshaft speed and position sensor as well as load and other corrections. When one of the coils is fired a spark is delivered to two engine cylinders, either 1 and 4, or 2 and 3. The spark delivered to the cylinder on the compression stroke will ignite the mixture as normal. The spark produced in the other cylinder will have no effect, as this cylinder will be just completing its exhaust stroke.
Because of the low compression and the exhaust gases in the ‘lost spark’ cylinder the voltage used for the spark to jump the gap is only about 3kV. This is similar to the more conventional rotor arm to cap voltage. The spark produced in the compression cylinder is therefore not affected. An interesting point here is that the spark on one of the cylinders will jump from the earth electrode to the spark plug centre. Many years ago this would not have been acceptable as the spark quality when jumping this way would not have been as good as when it jumps from the centre electrode. However, the energy available from modern constant energy systems will produce a spark of suitable quality in either direction. The direct ignition system (DIS) consists of three main components, the electronic module, a crankshaft position sensor and the DIS coil. In many systems a MAP sensor is integrated in the module. The module functions in much the same way as has been described for the electronic spark advance system.
The crankshaft position sensor is similar in operation to the one described in the previous section. It is again a reluctance sensor and is positioned against the front of the flywheel or against a reluctor wheel just behind the front crankshaft pulley. The tooth pattern consists of 35 teeth. These are spaced at 10° intervals with a gap where the 36th tooth would be. The missing tooth is positioned at 90° BTDC for numbers 1 and 4 cylinders. This reference position is placed a fixed number of degrees before TDC, in order to allow the timing or ignition point to be calculated as a fixed angle after the reference mark. The low tension winding is supplied with battery voltage to a centre terminal. The appropriate half of the winding is then switched to earth in the module. The high tension windings are separate and are specific to cylinders 1 and 4, or 2 and 3.
DIS Coil.
