February 2022

32 AFTERMARKET FEBRUARY 2022 TECHNICAL/SNAP-ON www.aftermarketonline.net Fig.1 Fig.2 Fig.3 Fig.4 Fig.5 current flow switched off. The current was then allowed to dissipate away rather than being switched off quickly. Fig.8 shows the primary circuit voltage. There is a complete absence of self-induction, or back EMF, and there is no activity taking place in the secondary circuit. Once the engine control module was repaired the ignition coil performed correctly. An ignition coil driver was replaced. See Fig.9 for the post-repair image. The current ramp also shows clean and fast switching of the primary circuit. Please refer to Fig.10. Now, with a reasonable understanding of the fundamental principles of ignition systems, we can consider the Renault system. This must abide by the same rules to create a satisfactory spark, but is controlled slightly differently. As previously stated, the system operates on the wasted spark principle. This is not unusual, however normally the companion cylinders share the same secondary winding. Another way of creating this type of system is by having two primary windings in series and a separate secondary circuit for each spark plug. The other side of the secondary winding is connected to the primary winding. The secondary circuit must be complete to allow current to flow. Again, a number of physical laws can be observed by analysing this circuit. Lenz’s law Lenz’s law states the following: “The direction of the induced EMF is always such as to oppose the change producing it.” Please refer to Fig.11, which demonstrates this. The diagram shows us two key points. Firstly, the ignition coil fitted to these vehicles is a 6-volt coil, as both windings have the same resistance and both windings are in series with one another. Kirchhoff’s law Another law can now be proven, namely Kirchoff’s law. In a DC circuit the sum of the voltage drops is equal to the applied voltage. Please refer to Fig.12, where you will find that Vt = V1 + V2 + V3. Synopsis It is evident from this information that there is a lower voltage induced into the primary winding than would normally be observed. 200 volts across each winding in this case, normally 350 to 400 volts. However, this has no effect on the firing voltage or spark duration in the secondary. Returning to the law of transformers, if the applied primary voltage is reduced then the turns ratio must increase proportionally to allow for an increased secondary voltage. Please refer to Fig.13. Referring to Fig.14, the secondary voltage from all four ignition coils is in excess of 40,000 volts, with a spark duration of approximately 1 millisecond when firing on the compression stroke. One final point to note for direct ignition coils that don’t utilise wasted spark; A diode is required to ensure a spark doesn’t take place at the spark plug when current begins to flow through the primary winding. This is known as a switch on spark. Referring to Fig.15, the diode is on series with the secondary winding and the spark plug.