Tea break: Ignition systems
How ignition evolved over time.
Without a dependable ignition source, the Belgian engineer, Jean Joseph Étienne Lenoir, would not have succeeded in creating the first successful internal combustion engine in 1860, which ran on domestic piped gas. While he utilised trembler coil ignition, he was not its inventor – several scientists have been credited with that honour, from Irish cleric, Nicholas Callan, to German-born instrument-maker, Heinrich Rühmkorff. The trembler coil used the principles of electromagnetism that were established by British scientist, Michael Faraday, and was improved gradually by other engineers.
Early pioneers
The heart of the trembler coil system is two twists of wire wrapped around an iron core. When a voltage is passed through one coil, voltage is induced within the other one; the amount generated depends on the number of wire turns. Several turns of wire in the primary coil can induce larger relative voltages in a more tightly-wound secondary coil, but only when the power supply is switched on and off.
Rühmkorff integrated a ‘trembler relay’ – an electromagneticallyoperated switch to the primary coil, which disconnected and reconnected the power. All that Étienne Lenoir had to do on his engine was to connect a rudimentary spark plug to the secondary coil and supply a power source.
While this operating principle was used for most ignition systems until the 1970s, it was not accepted universally. Hot bulb/hot tube ignition involved inserting a red-hot tube into the cylinder to ignite the mixture, similar to a modern glow plug. Hot wire ignition, invented by Scottish engineer, Sir Dugald Clerk, replaced this with a red-hot coil of platinum. While these fairly inaccurate and inefficient alternatives were used for the first petrol engines, some innovators preferred the original trembler coil ignition, including Carl Benz and Henry Ford. The Ford Model T, for example, used one trembler coil per cylinder.
Magneto ignition was pioneered by Germany’s Robert Bosch and Britain’s Frederick Simms. Both men were known to pool their expertise, which resulted in several different versions of the technology being developed, each of which sought to improve spark quality and reliability. On all types, instead of requiring a battery, electrical energy was generated by the engine spinning a coil within a magnet (later examples had magnets inside a coil), which operated a contact breaker connected to an internal cam that disconnected the primary coil. Unlike an earlier French design, Bosch and Simms added a condenser (capacitor), which not only assisted with developing higher voltages but also supressed the electrical arc and increased the life of the contact breaker.
Very early magneto systems had no provision for altering the ignition timing according to engine speed and load. However, as petrol-powered engines gained popularity in motor cars, variable timing not only possessed power benefits but also made the engine easier and less dangerous to start, when starter-handle kick-back could break a wrist. Simms is credited for producing a mechanical means of varying the timing, by moving the cam via a cable that was connected to a lever, mounted to the steering column that the driver could adjust manually.
Other companies sought to improve the technology, including ML and Thompsonhouston, but the advent of reliable batteries rendered magneto ignition obsolete for passenger cars, although they continued to be used for certain aviation applications, motorcycles, racing cars and for smaller engines that powered
lawnmowers, chainsaws, etc. One of the most recent developments was Briggs & Stratton replacing the mechanical contact breaker points with transistors in the 1980s, making them almost entirely maintenance-free.
Coil ignition
Poor spark quality, due to the low voltages produced at both idle and cranking speeds, was one of the major criticisms of the magneto, which prompted Charles Kettering of General Motors (GM) to design a batterypowered system. While he used contact breaker points as well, they cut and re-established the power to a separate induction coil. Although some vehicles had both coil and magneto systems (including certain Rolls-royce and Panhard models), the coil system was to prevail in mainstream production cars.
While numerous improvements resulted over time, the most userengine friendly change was removing the ignition advance/retard lever completely. Taking note of the centrifugal method that limited maximum speeds of compression-ignition (diesel) engines, Delco Remy used a pair of spring-loaded bob-weights on a coupling between the contact breaker and the ignition system or distributor’s drive spindle. As engine speed increased, the bob-weights were thrown outwards, moving the cam and advancing the timing; when engine revs reduced, the weights would be drawn in again, retarding the timing.
Unfortunately, this system made no adjustments for load, so damaging detonation (pinking) could result within the engine cylinder from overadvanced timing under full throttle. The answer was to utilise the vacuum in the inlet manifold, which would rotate the plate upon which the contact breaker was mounted. Flooring the throttle, therefore, would decrease the ‘suction’ available and retard the ignition.
Electronics take over
The coil ignition system remained in use for many years. However, contact breaker points were prone to burning, especially if the condenser failed, and required regular adjustment and periodic renewal. Similarly, eroded rotor arm and distributor cap contacts also reduced output gradually, until they were replaced at service time. To enhance reliability and performance, aftermarket companies such, as Lumenition and Piranha, offered conversion kits that ridded them of the original-fit ignition system’s shortcomings of original-fit ignition components. Many of these kits replaced, or supplemented, the contact breaker points with transistors, which switched the primary voltage in the coil on and off, with no contact breaker point erosion and greater precision.
While transistorised ignition appeared on production cars from the mid-1970s, demand for greater fuel efficiency and fewer pollutants meant that the ignition system had to communicate with other parts of the engine, plus the cooling, fuel and exhaust systems. Ultimately, these essential engine components would be equipped with sensors, the digital signals of which would be interpreted by a single electronic control unit (ECU), one of the first iterations of which was the Bosch Motronic, a brand name that was introduced first on the BMW 732i of 1979 and on many other models into the mid-2010s. Today, virtually all modern passenger cars’ ignition systems are solid-state, controlled by the engine management system, which emphasises again the critical role that electronics have played in motor car development.