Having covered engine fuel control in the last article we now move on to the ignition system.
Prior to the acceptance of ignition electronics the generation and distribution of the high voltage required for the spark plugs was primarily the role of the distributor in conjunction with the ignition coil. By and large it did a reasonable job back suffered from three main drawbacks, 1) high maintenance the contact breaker set (points) had to be checked and/or replaced at regular intervals, 2) damp the exposed high voltage contacts in the distributor cap were always in close proximity to earth points and a small build up of moisture guaranteed a misfire or at worse a non-start, and lastly, 3) ignition timing controls to allow the ignition to respond to engine demand two mechanical systems were built into the distributor body, both were susceptible to wear leading to poor performance.
With the advent of electronic ignition drawbacks 1) and 3) disappeared as there were no longer any mechanical parts requiring service and the need for ignition timing adjustment through various engine speeds was addressed by programs on a microprocessor. Until the advent of distributorless systems, drawback 2) was always present, however, because engine bays became increasingly full weather resistance also increased reducing the effects of a damp atmosphere.
So what do we require of the ignition system? The main requirements are to produce a high enough voltage to generate and maintain a spark at the spark plug sufficient to set the combustion process going and secondly, to provide a system of ignition timing adjustment depending on engine speed and load.
Unfortunately, the spark created by the use of 12volts is of no use whatsoever in the extremely harsh environment of the combustion chamber so a much higher energy is needed for an effective burn to take place. The joint efforts of the ignition control unit and the ignition coil work to produce the high energy required generating voltages in excess of 40,000volts. The generation of such a high voltage relies on the transformer principle; basically if two coils of wire (known as the primary and secondary) are wound around a central core any change in the magnetic field generated by the passing of a current through the primary will induce a voltage in the secondary. The change in the magnetic field of the primary is simply achieved by switching the supply on and off. If the number of turns in the secondary winding is greater than those in the primary then a higher voltage than the primary supply will be generated. With this principle in mind, three main factors will have a direct effect on the secondary voltage produced; 1) the primary current, 2) the ratio of turns between the primary and secondary windings, and 3) the speed at which the magnetic field changes. Points 1 and 3 are usually controlled by the ignition control unit or engine management ECU and point two is the result of the collaboration between the vehicle manufacturer and the manufacturer of the ignition components. Obviously the presence of the voltage required is of little use unless it is channelled to the spark plugs at the correct time to meet the fresh fuel/air charge in the combustion chamber; again this is the job of the control unit which will take into account engine operating conditions.
The most effective burn process producing the most efficient combustion pressure inside the combustion chamber occurs at approximately 10 degrees after top dead centre. As the engine speed increases the time taken by the piston to cover the same distance reduces so the spark has to be created earlier (ignition advance), under higher load conditions more fuel is introduced to produce more power, this richer mixture burns quicker so the presence of a spark can occur later (ignition retard). Ignition control is much like a balancing act, constantly changing to give the most efficient burn and this, along with fuel control has led to the adoption of electronic engine management.
With thousands of volts being produced at the ignition coil and the same being required at the spark plugs a special wiring requirement exists possessing insulating properties sufficient to prevent leakage to earth which will cause misfiring, a construction keeping radio frequency interference to an absolute minimum which can have a detrimental effect on radio reception and on the sensitive workings of some engine management components plus an outer protective covering to enable long service life and a high resistance to the harsh under bonnet environment. One of the most common types of ignition lead comprises six elements; a central non-conducting fibre surrounded by two layers of a conductive material e.g., latex and silicone, a layer of insulation covered by a braid and finally an outer jacket.
Once the high voltage passes through the ignition lead it reaches the spark plug the device screwed into the cylinder head just protruding into the combustion chamber. Basically the plug is designed to initiate combustion by forcing the ignition voltage to jump a carefully engineered gap to earth causing a spark thus starting the burn process. The construction of a typical spark plug usually consists of a terminal which connects to the ignition lead followed by a conductive core which includes a resistor to provide interference suppression and finally the exposed centre electrode. Surrounding this core is a ceramic shell which provides a high degree of electrical insulation and these two areas are sealed by a special packing into a gas tight assembly which is then encased in a steel alloy shell the outer surface of which carries the threads enabling the plug to be screwed into the cylinder head. There are literally hundreds of variations on this basic theme but they all exist to provide an effective spark over a wide range of engine and combustion temperatures and pressures. For those interested, a spark plug working for 20,000 miles will produce well over 20 million sparks, will endure voltages of around 30,000v, pressures at certain points of the four stroke cycle of 100bar (1470 psi) and have to work in an environment made up of hot fuel vapour, combustion products and fuel/oil residues.
Briefly returning to the ignition leads which have been the bane of many a motorist over many a year, high voltage running through the cabling close to good earth points has always been a recipe for misfires as electricity, like water, will always take the easy route, so why not get rid of the ignition leads altogether? In recent years manufacturers have utilized the good old primary/secondary coil technology but used one coil per plug and with many spark plugs being so deeply recessed into the cylinder head this provides a very sheltered environment for the high voltage and also allows each coil/spark plug assembly to be individually controlled by the engine management system.