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Missing at boost - the physics background Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Wed, 31 Oct 2001 13:28:42
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Note: This is for people who really have very little else useful to do.
A common problem for turbos is engine missing under boost. We all know that too big a spark plug gap causes this. Basically, it's harder for a spark to jump a big gap than a small one, but it's also harder for a spark to jump when the pressure (boost) is higher. Problems in the ignition system can cause missing at boost. Not only spark plug gap, but ignition wire leakage/resistance, or problems with the spark coil. Anything that reduces the spark voltage, or gives the spark an easier path to ground than through the plug.
A few posts back someone wondered why higher pressure makes it harder to spark - that it was just a dialectric issue. Here's the physics:
Sparks are plasmas - ionized gases, where there are a lot of electrons without atoms flying around, and lots of atoms missing an electron or two. In air, there may be a few free electrons, but not many, and air molecules are uncharged - no 'holes' for electrons to fill. Not ionized.
When you apply voltage to the spark plug, it creates an electric field between the tip (which is ground) and the core (central round thingy). Atoms in this electrical field get stretched - their electrons are drawn to the core (+), and the protons are drawn to the tip (-). But it takes a lot of energy to pull those electrons free. When one does pull free, it goes whizzing along until it slams into another air molecule. This impact knocks a few electrons free, and now there's more electrons flying around, slamming into other molecules like cars on an icy highway, knocking even more electrons free.
This is called the avalanche effect. At some point there is enough voltage to cause the electrons to rip free, and the number increases geometrically. You get a spark.
So where does air pressure come in?
Because they are in an electric field, the free electrons accelerate. The more they accelerate, the harder they hit the 'target', and the more electrons they knock off to hit other atoms. If is isn't going very fast, the electron won't have the speed (energy) to knock an electron off the target molecule. So the free electron needs a certain 'running start' distance to start the avalanche.
The technical term for the 'running start' distance is 'mean free path' - or how far you can go before hitting something, on average. We all know drivers who can't get out of a parking lot without hitting at least two cars and a pole. If the mean free path is too short, the electrons can't get going fast enough, so you need a higher voltage to get things to go.
In a gas, higher pressure means more molecules packed into a given volume. The more molecules, the more things to hit, so the chances of hitting something increase. The Mean Free Path is SHORTER. So the higher the pressure, the harder it is to start the avalanche, to get a spark. That's why things like flash tubes use a low pressure filling - it takes less pressure to get the ball rolling, as it were.
So I hope you've learned your lesson - never ask a question unless you're prepared to hear the answer.
posted by 140.157....
, Wed, 31 Oct 2001 13:28:42 <-- Viewing This Message
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