Pop quiz: Why do boosted engines need a more powerful spark?

Smooth said:

Because physics?

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earico said:

I'll stab at it. I know how lightning works and I'm assuming it's similar albeit a much smaller scale. Takes more current to ionize the air between the electrode and ground strap when under boost pressure. Higher the pressure the more current needed. Am I close?

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James Snover said:

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.

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James Snover said:

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.

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James Snover said:

Bonus question:

On the compression stroke, combustion occurs*, the piston is approaching TDC, and both valves are closed. Temperatures and pressures are at the very highest. But the amount of power the engine is making is ... almost nothing. In fact, at that point the power the engine is making is so low, despite temps and pressures through the roof, that if it were left to itself, the piston would run backwards down the bore.

1.) How can it be the engine is making almost no power at this point when the process on which it derives power, is at its absolute peak?

2.) And ... considering that, what lets it run, at all?

* because having read the preceding post, you were smart enough to design your ignition around a discarded lithotripter capacitor. You have no trunk space, and it scares dogs and attracts lightning, but you do not lose races for failure to ignite!

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Coiled03 said:

It's not on the power stroke, that's why.

Temperature and pressure mean jack squat until the mixture is ignited, at which point the energy is transferred to the piston, which then travels a distance down the bore. Power is work done or energy transferred over a distance. Any time before it reaches TDC no energy has been transferred, thus no work has been done and no power has been made.

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James Snover said:

Correct ... but _why_ is no work being done at this point? What ... principle ... prevents it? Prohibits it, in fact?

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James Snover said:

Correct ... but _why_ is no work being done at this point? What ... principle ... prevents it? Prohibits it, in fact?

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lexustech48 said:

Liberals and Socialism, thats why.

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Double"O" said:

No fuel...just air on that stroke?

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03cobra#694 said:

Someone put that weird science thing up with Kelly LeBrock.

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James Snover said:

Correct ... but _why_ is no work being done at this point? What ... principle ... prevents it? Prohibits it, in fact?

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