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Pop quiz: Why do boosted engines need a more powerful spark?
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<blockquote data-quote="James Snover" data-source="post: 16195077" data-attributes="member: 67454"><p>Not just close, you're dead-on right! I'd expected this to take a lot longer, it usually does when I try it at parties.</p><p></p><p>Air is a great insulator. The more of it there is, the more difficult it is to form a spark. In WWII, they quickly found that as airplanes flew higher and higher, they had all kinds of arcing problems in the ignition systems. Below 5,000 feet, the engine ran like a bat out of hell; above 25,000 and the engine ran like a heart-patient on one lung. The answer was ... pressurized ignition harnesses! Taking a feed right off the supercharger, increasing the pressure surrounding the wires fixed the arcing.</p><p></p><p>Add in that gasoline (and alcohol) are very forgiving fuels. It takes a spark to make them burn, in most ordinary cases. No spark, no burn. (Unless it's the Chrysler dieseling gas-burning four bangers from the '80's, but that's a different story about poor combustion chamber engineering and ineffective cooling).</p><p></p><p>So, in a boosted engine, when the air-fuel charge is being compressed and then ignited, the pressure in the cylinder is now many times atmospheric pressure, and if you don't have a powerful enough ignition ... the spark is not able to even form. It doesn't get "blown out." It never even forms!</p><p></p><p>Another observation: it is even more difficult to make a spark in a fluid. There is a kind of medical machine called a lithotripter. The old ones used banks of gigantic, water-cooled capacitors as big an average man's chest (I kid you not), to make a spark form in water, framed against a specially-shaped parabloic dish, which converted that energy into a focused blast of a particular frequency that _just happened_ to be the resonant frequency of ... kidney stones! And also by luck, that particular burst of energy was largely (but not entirely!) unreactive to soft tissue of the kidneys. So they could break up kidney stones without surgery and let the body remove them on its own. Like xrays, it did do some harm to surrounding tissue, but nothing on the level of surgery! Modern lithotripters use staggering high voltage and high frequencies enabled by modern electronics to do the same thing. And I'm glad of it. Those huge old caps scared me. There were ways to handle them safely. But one mistake, one moment's inattention, and you'd be dead before you even felt any pain.</p><p></p><p>ANYHOW: That's why boosted engines need a more powerful spark, despite the fact the air-fuel charge in the combustion chamber wants to burn a hundred times more than when you're stuck in stop n' go traffic, idling. Without a spark, it just can't. And without a big shot of power, it can't make a spark.</p></blockquote><p></p>
[QUOTE="James Snover, post: 16195077, member: 67454"] Not just close, you're dead-on right! I'd expected this to take a lot longer, it usually does when I try it at parties. Air is a great insulator. The more of it there is, the more difficult it is to form a spark. In WWII, they quickly found that as airplanes flew higher and higher, they had all kinds of arcing problems in the ignition systems. Below 5,000 feet, the engine ran like a bat out of hell; above 25,000 and the engine ran like a heart-patient on one lung. The answer was ... pressurized ignition harnesses! Taking a feed right off the supercharger, increasing the pressure surrounding the wires fixed the arcing. Add in that gasoline (and alcohol) are very forgiving fuels. It takes a spark to make them burn, in most ordinary cases. No spark, no burn. (Unless it's the Chrysler dieseling gas-burning four bangers from the '80's, but that's a different story about poor combustion chamber engineering and ineffective cooling). So, in a boosted engine, when the air-fuel charge is being compressed and then ignited, the pressure in the cylinder is now many times atmospheric pressure, and if you don't have a powerful enough ignition ... the spark is not able to even form. It doesn't get "blown out." It never even forms! Another observation: it is even more difficult to make a spark in a fluid. There is a kind of medical machine called a lithotripter. The old ones used banks of gigantic, water-cooled capacitors as big an average man's chest (I kid you not), to make a spark form in water, framed against a specially-shaped parabloic dish, which converted that energy into a focused blast of a particular frequency that _just happened_ to be the resonant frequency of ... kidney stones! And also by luck, that particular burst of energy was largely (but not entirely!) unreactive to soft tissue of the kidneys. So they could break up kidney stones without surgery and let the body remove them on its own. Like xrays, it did do some harm to surrounding tissue, but nothing on the level of surgery! Modern lithotripters use staggering high voltage and high frequencies enabled by modern electronics to do the same thing. And I'm glad of it. Those huge old caps scared me. There were ways to handle them safely. But one mistake, one moment's inattention, and you'd be dead before you even felt any pain. ANYHOW: That's why boosted engines need a more powerful spark, despite the fact the air-fuel charge in the combustion chamber wants to burn a hundred times more than when you're stuck in stop n' go traffic, idling. Without a spark, it just can't. And without a big shot of power, it can't make a spark. [/QUOTE]
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