The twin spark ignition system has been with us for many, many years now and has proven itself advantageous on several levels.
It was Alfa Romeo who first utilized the concept of twin spark plugs for each cylinder. They did so back in 1914, and even though others had played with the idea prior to that, it’s Alfa Romeo which as one of the bigger manufacturers were given the honour of the invention. Even to this day, they continue using and further developing the concept.
The fundamental idea behind the twin spark system was obviously extracting more power from the engine. Needless to say, engines with double ignition have two spark plugs per cylinder rather than the one which we usually settle for. These two spark plugs ignite simultaneously (well, almost simultaneously…), the logic being that two sparks will ensure that the mixture of air and fuel within each cylinder will burn quicker and more effectively. This is helped by deliberately placing the two spark plugs unsymmetrically in regards to the combustion chamber, thus ensuring that the ignition of gasses reaches even the furthest corners of the chamber. The result is a more efficient burn, which in turn allows more power to be extracted for the same amount of fuel. A secondary advantage is a better fuel economy too. Both play a role on modern twin spark engines, but previously the concept focused on boosting performance.
The aviation industry has equally made great use of double ignition systems on piston-engined aircraft. Their primary goal has always been redundancy to ensure that the aircraft could continue to fly even if one ignition system failed. But of course, the more efficient burn was an added bonus when airborne as well.
If we are to get a bit more technical about the topic, a spark plug has very little time to ignite and then perform the perfect burn of the air and fuel mixture within each cylinder: An engine spinning at 6000 rpm gives each spark plug only 1/200th of a second to perform its duties! In this very short time, the ignition of the gas must be as quick and as uniform throughout the combustion chamber as possible, while the whole mixture should ideally be burnt off before the exhaust valve opens, as the exhaust fumes should preferably retain minimal amounts of unburnt fuel and hydrocarbons.
This is of course the main – arguably even the sole – objective of the spark plug. On smaller engines with smaller combustion chambers and especially if the engine is not constructed for skyhigh revs, a single spark plug will usually suffice for the job at hand. But if the engine size is increased and with it the size of the combustion chamber, a single spark plug will not ignite the gasses within the combustion chamber uniformly. If it is a racing engine which is built to deliver its best at very high rpm, this is even further amplified. That’s where a second spark plug is required to aid the ignition of the air and fuel mixture. It’s complicated as the ignition must start when the piston is at top dead centre within the cylinder, and theoretically continue throughout the pistons travel to bottom dead centre at which point the gas mixture should be fully burnt. This is achieved more efficiently with a twin spark system as the second spark plugs ignites marginally after the first, thereby prolonging the burn.
In these modern times, the twin spark ignition system may well have met its match in multivalve heads with variable timing combined with the electronic ignition system. However, even today several manufacturers continue to utilise twin spark systems to gain power, efficiency and not least cleaner exhaust fumes. 85 years on and the twin spark technology is thankfully still with us…
And of course, there’s also the added bonus that twin spark engines are simply sexier than ordinary engines!