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In 1986 a company called Alfa Romeo introduced the first ever twin spark engine. Instead of having one spark plug in each cylinder Alfa Romeo decided to double things up and install two spark plugs in each cylinder. This resulted in improved combustion inside the engine which lead to increased power and efficiency and to this day twin spark engines are the absolute pinnacle of internal combustion design, often imitated but never duplicated….
Did you buy all this? Well some of it is actually true...but the rest is absolute nonsense. Do you know which part is true and which is not?

1986 ..nope that's a big lie. The first functional dual ignition engine was introduced in 1914 on this car. Was it an Alfa....yes it was! This right here is the 1914 ALFA 40/60 Grand Prix. The car was a creation of Giuseppe Merosi who not only had an incredible mustache but also created an engine that was light-years ahead of it's time. It was a 4.5 liters inline with double overhead cams, four valves per cylinder, 90 degree valve angle and twin spark ignition. In 1914. Although the overall engine architecture was inspired by the Peugeot engines from 1912 and 1913 whose design is claimed by Swiss engineer Ernest Henry, the twin sparks were definitely an Alfa original creation.

Once initiated the combustion flame front travels from the spark plug outward until it covers the combustion chamber and obviously this takes time. The time required to complete a combustion depends on the flame speed of the combustion which depends on the type of fuel, octane rating, compression ratio, how well air and fuel are mixed together and what is the ratio of the air to fuel. But in general as the rpms increase the speed of the piston will outrun the speed of the combustion.
This means that we must rely on ignition advance to fire the spark plug before the piston reaches top dead center in order to give the combustion enough time to spread and build up maximum combustion pressure by the time the piston reaches just a bit past top dead center so that maximum pressure is exerted onto the piston at the correct time leading to maximum power and efficiency.

But there's a limit to ignition advance. Too much ignition advance eventually results in the spark plug being fired too early and creating combustion as the piston still moves upward which is essentially pre-ignition and can damage the engine because it forces the piston to work against the combustion exposing it to massive heat and mechanical stress.
So if we run out of ignition advance and still want to rev the engine to high rpm our only other choice is the increase the speed of the combustion and we can do this by initiating combustion at two different points. If we install two spark plugs and fire them at the same time the travel path for the flame front becomes much shorter. By installing two spark plugs we're not increasing the flame speed, we're simply reducing the travel path of the flame front which obviously decreases combustion time making it possible to fully cover the combustion chamber even at high rpm.

So if twin spark plugs offer the benefits of better combustion which leads to more power potential, improved economy and reduced emissions this must make them very desirable and one would expect to see dual ignition on all car engines on the road today. In reality the opposite is true and as many of you know twin spark plugs are a pretty rare occurrence on car engines. Dual ignition did pop on the engines of various manufacturers through the years. Nissan had it in 1978 on their NAPS-Z and NAPS-X cars. Ford also had it in the 80s and early 90s on their four cylinder Ford Rangers and Mustangs m. Honda also had intelligent and dual sequential ignition on their I-DSI engines found in their smaller cars in the early 2000s. But today there's virtually zero mass produced cars that have twin sparks per cylinder. In fact starting with around 2010 or so twin spark plugs are virtually extinct on car engines.

CHRYSLER HEMI ENGINE
So why do Hemi engines have dual sparks? Well first of all the hemi engine isn't a true hemi anymore, the combustion chambers aren't really hemispherical, instead they have a more complex oval shape. But more importantly than this the modern hemi is still a bit of a 2 valve dinosaur. Some versions do have cam phasing but that's pretty much it. There's no direct or dual injection, no variable intakes, no variable valve lift meaning that the Hemi needs all the help it can get meet recent emissions and economy standards.

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#d4a #twinspark

00:00 Intro
02:43 Combustion speed
06:35 2 and 4 valves
11:37 Bikes and planes