At a time when the new 1½-litre Formula 1 engines will be required to rev much higher than the previous 2½-litre engines, Joseph Lucas have announced a patented ignition system for racing purposes in which the high tension voltage is produced entirely by means of an electronic circuit.
The development of multi-cylinder, high-compression-ratio, high-speed engines, both car and motorcycle, such as the 1½-litre V8 Coventry-Climax unit, with estimated speeds of 8,000-12,000 r.p.m., means that existing coil and magneto systems may be unable to meet the increasingly critical ignition requirements of sparking speed and timing accuracy, imposed by these engines. Mechanical considerations of contact breaker design and other factors restrict operating speeds of the existing systems to about 400 sparks per second in the case of coil ignition and 500 sparks per second for magnetos. In addition, timing accuracy may be affected to a small degree by backlash or torsional oscillation in the drive mechanism, and also by wear to which the moving parts are subject during their working lives.
In the new system, Lucas claim that a spark rate of 1,000 per second is a practical proposition, equivalent to an eight-cylinder engine at 15,000 r.p.m., and spark voltage is constant over the entire speed range. Ignition timing is determined by an electromagnetic triggering device associated with the engine flywheel, this being the engine member least affected by torsional oscillations and wear, so that extreme accuracy of timing is maintained. Automatic timing controls responsive to both engine speed and load can be incorporated when required.
The Lucas system comprises an electromagnetic pickup associated with pole pieces attached to the engine flywheel (or on a light wheel fitted to the opposite end of the crankshaft for convenience), a trigger amplifier, a spark generator unit and a high tension distributor. As the engine rotates, a voltage impulse is produced at the pickup each time one of the pole pieces passes within its field. This pulse is applied to the trigger amplifier, which can be considered simply as a normally-closed switch allowing current to flow through the primary winding of a trigger transformer. The voltage pulse results in this “switch” being opened, so that current flow through the transformer primary ceases,
The energy released by the resulting collapse of current induces a voltage in the trigger transformer secondary winding, and this in turn causes a current to flow in the base circuit of the spark generator unit. An associated transistor thereby becomes conducting, so that current flows in the primary winding of a high voltage transformer. A regenerative oscillation is initiated, resulting in a very rapid increase in primary current, which gives rise to an induced voltage in the transformer secondary of over 20 kilo-volts. This is fed to a rotor arm and is distributed to the spark plugs in the normal way. Regeneration ceases when the transformer is saturated, and the transistor again becomes nonconducting. The complete cycle time for regeneration is less than 200 microseconds. With the cessation of the voltage pulse at the pickup, conduction again commences in the trigger amplifier circuit in readiness for the cycle to be repeated at the next pickup pulse.
The current consumption is directly proportional to speed, rising from about 0.25 amp. at 50 sparks per second to 2.5 amps. at 500 sparks per second. This is the inverse of the conventional coil ignition system. At low engine speeds, when the dynamo is probably not charging, the electronic system takes only a very small current from the battery. At higher speeds, when the system requires more current, the dynamo will be generating an output depending on the electrical load in use.
This system is still under development to meet the needs of future high-revving racing engines and it is not envisaged by Lucas that it will be employed on normal production engines. However, it is possible that in future ignition systems on production cars the present coil and distributor could be supplemented by a transistor which will assist the contact breaker by breaking the primary current, the contacts then interrupting only the small base current of the transistor. Unfortunately transistors are not yet commercially available which are capable of withstanding the high reverse voltages generated across the coil primary. As a transistor can handle much higher currents than tungsten contacts the primary inductance can be reduced and high-speed performance consequently improved, and it is certainly to be hoped that a suitable transistor will become available in due course.