Pop quiz: Why do boosted engines need a more powerful spark?
- Thread starter James Snover
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Answer for opening the exhaust valve early, even though significant pressure remains in the cylinder:
Again, as the piston approaches BDC, there is little or no mechanical advantage to be had by the piston and connecting rod to act on the crankshaft. Since the gasses can't produce any more real power, and must be purged from the cylinder prior to the next intake, might as well open the valve a little early. This lets the exhaust gas use it's remaining energy to assist in purging itself from the cylinder. And at the right speeds (in rpm) for any given engine at any given balance of pressures across the intake/exhaust and atmosphere, the cylinder will actually be evacuated to negative pressures. All of which helps pull in the next intake charge.
Again, as the piston approaches BDC, there is little or no mechanical advantage to be had by the piston and connecting rod to act on the crankshaft. Since the gasses can't produce any more real power, and must be purged from the cylinder prior to the next intake, might as well open the valve a little early. This lets the exhaust gas use it's remaining energy to assist in purging itself from the cylinder. And at the right speeds (in rpm) for any given engine at any given balance of pressures across the intake/exhaust and atmosphere, the cylinder will actually be evacuated to negative pressures. All of which helps pull in the next intake charge.
I’m not sure I understand how spark does not blow out. If there is not enough current to push the spark is it not the pressure that keeps the current from not flowing being it getting “blown” out from some type of force “wind”.
Compressed air is more dense than naturally aspirated air. The molecules are closer together, and it takes more power to send a spark.
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Which I assume is the more power to move the molecules is because the air is pressurized causing more force to push through the atmosphere. I would assume is why you gap your spark plugs smaller so there is not as much energy to make the jump. IE spark blow out because you do not have enough power to push that current. I am not a scientist so vocabulary may be off.
One correction, a minor one: you don't push through the atmosphere. You ionize the air between the anode and cathode. Because ionized air is conductive, like a wire. The thicker the air, the more difficult it is to ionize a path, because there are more air molecules to be ionized, and it takes a certain minimum quantity of energy to ionize. More molecules, more energy required to ionize a path.
It doesn't blow out. It's even worse than that: it never forms a spark to begin with.
You can see, feel, and smell this effect during particularly nasty thunderstorms. That neat purple "haze" and the distinct ozone smell- sometimes you can even see the ionization path before lightning strikes. If you catch it just right, you actually see the lightning start at the ground sometimes.
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