February 2022

BY Damien Coleman, Product Manager/ EBI Specialist at Snap-on T he control of the spark ignition system on early 16-valve Renault engines is unique when compared with other system designs currently available. The system utilises direct ignition coils, often referred to as pencil coils. However, the primary windings of the coils fitted to the companion cylinders are wired in series with one another. On a 4-cylinder engine the companion cylinders are one/four and two/three. Please refer to Fig.1 to see this illustrated. This layout results in a ‘wasted spark’ operation of the ignition system. This system not only produces a spark in the cylinder on its compression stroke, but also on the companion cylinder which will be on its exhaust stroke, hence the term wasted spark. The cylinder on compression will have a higher pressure present when compared to a cylinder on the exhaust stroke. This results in a larger firing voltage being evident on the cylinder completing its compression stroke. Please refer to Fig.2. It must also be noted that the polarity of the spark voltage does not change regardless of the operating cycle/stroke of the engine. A number of factors and physical laws must be considered when attempting to gain a full understanding of the ignition system on an internal combustion engine. The following section will outline these in greater detail. For optimum combustion to take place the timing and the intensity of the secondary circuit spark must be precisely controlled. There are three fundamental requirements for a high intensity spark to take place: The amount of current flowing in the primary circuit The turns ratio between the primary winding and secondary winding The rate of change of magnetic flux density A large current flow through the primary winding is required to ensure the winding reaches, or is close to, magnetic saturation. Please refer to Fig.3, and note Points A through C. When the ignition coil driver grounds the ignition coil primary winding, a current begins to flow through the winding. The current flow ramps up due to the inductance of the winding. This is due to the creation of magnetic fields which oppose the current flow. The engine control module breaks the circuit and current flow ceases. This results in a voltage becoming induced into the secondary winding due to mutual inductance and into the primary winding due to self-inductance. This system uses ramp and fire. This is where the primary ground circuit is open just before the primary winding reaches magnetic saturation. This reduces thermal stress on the coil. 30 AFTERMARKET FEBRUARY 2022 TECHNICAL/SNAP-ON www.aftermarketonline.net RENAULT IGNITION COIL CONTROL CIRCUITS This issue, Damien is looking at the particular and specific composition of ignition coil control circuits on some Renault vehicles The secondary circuit activity is mirrored by the primary circuit. The spark duration and oscillations in the secondary circuit can be observed at this point. A vehicle which has an anomaly in any of these key areas will have an ignition system fault present. Now please move onto Fig.4, which was taken from a vehicle which had a shorted winding. It can be seen from Point A that the characteristic ramp-up of current flow is missing and the current has increased sharply. This short circuit has the effect of altering the turns ratio between the primary winding and the secondary winding. As can be observed from Point B, an ignition misfire is present in the secondary circuit. Another fact to note can be found at Point C, a reduction in the induced back EMF in the primary winding. Please also note that the primary circuit voltage waveform mirrors the ignition events in the secondary circuit due to induction. A turns ratio of 100:1 is normal, where the primary winding has 200 turns and the secondary winding has 20,000 turns. If this ratio is altered then the voltage available in the secondary winding will be adversely affected. Secondary voltage = Primary voltage * (Secondary turns)/(primary turns) Now move onto Fig.5, which shows the same vehicle after the faulty ignition coil was replaced. Point A shows the correct current ramp-up trace for a functioning primary winding. At Point B it can be clearly observed that correct secondary circuit operation is evident, and at Point C a larger induced voltage is displayed. Please note that normally an induced voltage of 300-400 volts will be observed on the primary waveform. See the synopsis at the end for more detail. When a large firing voltage is required there is less available energy to have a large spark duration. This limits the available energy to maintain the spark; Burn time. When a small firing voltage is required there is more available energy to have a large spark duration. Please refer to Fig.6. Faraday’s law Another one of the fundamental principles of the ignition system which was previously mentioned is the rate of change of magnetic flux. This phenomenon is called Faraday’s law and is as follows: “The magnitude of the induced EMF is proportional to the rate of change of flux.” The rate of change of magnetic flux can be reduced due to a faulty ignition coil driver or a high resistance in the control circuit. Fig.7 shows a current ramp from a vehicle with such a fault. The ignition coil drivers for this vehicle are located in the engine control module. Point A shows how the primary winding was delayed in having the