h a l f b a k e r yQuis custodiet the custard?
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// The cylinder will always ignite at exactly top-
dead-center if the voltage is correct// |
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If you mean that, with the right voltage, a spark
will always happen only when the piston is a given
distance from the top, I would disagree. The
breakdown of an insulating medium (like air, or an
air/fuel mixture) depends to a considerable
extent on things such as humidity and on the
precise arrangement of particles (eg droplets or
dust) in it. |
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Thus, at a given fixed voltage, a spark may happen
when the gap is 1cm, or it may not happen until
the gap is 0.8cm. |
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Also, do you always want ignition at TDC? I
thought a lot of effort went into modifying the
timing according to the speed and load on the
engine? |
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That's not to say, though, that a long spark as you
suggest isn't a good idea. |
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You'll need to jack the amperage through the roof, since
the piston head, rod, crankshaft, engine block, etc. is all
one big conductive medium. The grounding element on a
conventional spark plug is tiny, so it doesn't take much
juice to build up the resistance needed for a snappy little
spark. Your proposed method would require components
and energy levels more akin to a small arc welder. |
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//A spark plug generates a tiny spark inside the
plug itself and the heat of
this discharge ignites the fuel.// |
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No, it doesn't. It lights the fuel/air mixture directly
by means of an electric
spark. |
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//The spark piston simply grounds the electrode of
the spark plug through the
crankshaft// |
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This is already the case. Take a look at a picture of
a spark plug. Notice how
one of the electrodes comes down from the part
that screws into the cylinder?
That's a ground connection right there. |
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//so that the electrode can be exposed in the
cylinder// |
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//The electricity arcs from the electrode through
the fuel air mixture to the
top of the piston.// |
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No, it follows the shortest possible path. Every
single part of the cylinder is
grounded, so it'll simply jump to the nearest part of
the cylinder—assuming
there's enough voltage to create any spark at all
given the increased
resistance. |
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I think you're greatly confused about how an
ignition system works here. |
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// On modern engines multiple plugs are used to burn as much fuel as possible // |
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Errr .. dual-plug (magneto) ignition in aero engines have been around for a very long time ... like about a hundred of your Earth years ... |
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// magnetos are just better// |
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^nothing about that statement was wrong though, was it? |
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What, about magnetos just being better? Yes, quite a lot,
but it seemed a bit unsporting to go there. |
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But since you asked… Magnetos have mostly disadvantages
compared to electronic ignition. The only real advantage
they have is reliability (although all told, you're probably
still better off using a dual ignition system, even with a
single spark plug). That's great for aviation. If your
primary concern does /not/ happen to be keeping yourself
from falling out of the sky, however, you're probably better
off using something a bit less… crude. |
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As someone who's had an alternator fail on the highway in my car (in the middle of effing nowhere), I would much prefer dual magnetos to electronic ignition, disadvantages by damned! My last comment was actually about the "modern" part though. Just because aviation engines have had dual ignition for decades doesn't mean that it isn't just now making a comeback in cars. |
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I also have to add that a taser manages to arc electricity over a long distance by using tens of thousands of volts, at very low amperage. |
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//As someone who's had an alternator fail on the highway
in my car (in the middle of effing nowhere), I would much
prefer dual magnetos to electronic ignition, disadvantages
by damned!// |
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The “disadvantages” here are a vast reduction in power
and fuel economy, and a significant increase in pollution
output. Is driving a slow, smoggy gas guzzler really worth
the added convenience of not having to call for a tow in
the unlikely event of one specific type of failure, among
the many that could happen regardless? |
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Incidentally, I gather that you made it back from your
mishap. If you had been flying, you might not have been so
lucky. /That/ is a justification for using magnetos. Not
having to carry a AAA card, on the other hand… |
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//I also have to add that a taser manages to arc electricity
over a long distance by using tens of thousands of volts, at
very low amperage.// |
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I've explained this here before, but suffice it to say that
you can't actually adjust volts and amps independently of
each other for a fixed resistance. What you're saying
doesn't make any sense. Regardless, a taser does not “arc
electricity over a long distance”. It actually shoots out a
pair of electrical leads, both of which must make physical
contact with the target for it to have any effect. |
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The bottom line is this: Electricity always follows the
shortest path to ground. Apart from removing the ground
on the cylinders (good luck) or insulating the inside of the
cylinder (theoretically possible, but hardly worth the
massive added cost), you can do nothing about the fact
that current will flow between the electrode and the
nearest path to ground, i.e., the nearest cylinder wall or
the piston itself. You simply can't get electricity to flow
where it wouldn't otherwise want to go. |
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Since the distance from the center of the piston to the cylinder wall will typically be larger than the distance between the spark plug and the top of the piston at tdc, the electricity should arc to the piston first, correct? The only thing that needs to be insulated is the cylinder head right beside the plug. See my taser link for an example of what I was talking about. |
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You'd still need a massively high voltage in order to do so.
And it's not even clear that there'd be any advantage to it.
Spark plugs work best when the gap is of a very specific
size. Any larger or smaller and they become less efficient.
Making the effective gap larger is going to create a /less/
controlled burn, because you're not going to know exactly
how the spark will travel through the air/fuel mixture for
any given spark. Regularity and repeatability are the keys
to engine performance, not just making things bigger. |
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I'm still not sure what problem you're trying to solve. It seems from your original
post you were simply describing some nonexistent problems stemming from your
misunderstanding of ignition systems, and then attempting to “solve” them by
proposing an existing technology, i.e., a spark plug. |
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//See my taser link for an example of what I was talking
about.// |
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It's just a very high voltage. Higher voltage necessarily
equals higher amperage. You can't adjust those two
variables independently of each other. Saying “increase
the voltage but decrease the amperage” is illogical
(assuming a fixed resistance). |
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Tasers work (or don't work, depending on your point of
view) by reducing the duty cycle significantly. The voltage
(and thus the amperage) is very high, but only for a brief
enough period of time that (hopefully) it won't prove fatal. |
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So you'd have stalactites coming down from the cylinder head (and possibly the other ones up from the piston crown). Each spike would have to be insulated where it perforated the cylinder head, be insulated on the inside from same and, unless you want all your spark going through just one spike, be individually addressed by the ignition system. I'm not sure how it would be simpler than the current system, though perhaps you might have less wear and tear on 5-6 spikes than you would on 1 spark-plug electrode. |
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//Saying “increase the voltage but decrease the amperage” is illogical // |
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<Spock> Hey, don't pin that one on me dude</S> |
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// Higher voltage necessarily equals higher amperage.//
Explain, please. |
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// // Higher voltage necessarily equals higher amperage.//
Explain, please.// |
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Ohm's law: amperage = voltage / resistance |
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So, for example, doubling voltage without changing resistance doubles amperage. Doubling resistance without changing voltage halves amperage. |
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I've linked to an idea where I've explained this in greater
depth
(just look for the really long anno about halfway down the
page), but it boils down to what [CraigD] said: Ohm's Law.
This
is the fundamental law that governs electricity, and it's
written
as V=IR, or voltage equals current times resistance. |
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Since it's an equation with three variables, only two of
those
can be independent, and the third would be calculated
from
the other two. In the case where the resistance is not
adjustable, such as in this idea, it means that only voltage
or
amperage can be adjusted independently. So increasing
the
voltage will lead to an increase in amperage, and vice
versa. |
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The main point is that voltage is actually a calculation
rather
than a specific property, that indicates how much current
will
flow if a specific material with given electrochemical
properties is connected to a specified resistance. So, it
can't
really be adjusted independently of current, because it
both
defines and is defined by the rate of current flow. |
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The Tesla coil, like the Van de Graaff generator, is a
constant current device that is capable of generating a
high voltage. This means that the voltage is determined
by the resistance, unlike most other devices which are
constant voltage, such that the current is determined by
the resistance. Increase the resistance to a Tesla coil and
the output voltage will go up to keep the current at the
same level, until the maximum voltage of the coil is
reached, at which point current flow would begin to
decrease. |
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This is an idea I toyed with a lot a few years ago. The problems that really stuck it for me were: |
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1) The variations in ignition timing would dramatically change the spark plug gap. Like millimeters change. |
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2) To keep the gap clean would require it to be "swept" by inlet or exhaust gas, not happening in this scenario. |
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3) Oil is a good enough insulator that an additional conductor would need to be added somewhere on the piston/con rod/crank. or maybe you would like the spark to find a nice place to arc to the bore. No Bueno. |
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4) Gap fouls/erodes, a bitch if it cant be easily replaced. |
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On the plus side, the spark involved would be
huge, many times greater than the improvement
in present day multi-point sparkplugs and
increased voltage ignitions. |
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Several previous notes have called out major
minus. I think it may not have occurred to the
poster how challenging it would be to make only
points on the cylinder and piston head, not its
walls, be electrodes. You’d have to electrically
insulate the cylinders from the rest of the engine,
or all of the engine from the electric ground,
including from the piston, which in every large
engine I’ve ever seen, is sealed with conductive
piston rings. The only approach that comes to
mind as possibly feasible is to line the cylinder
walls and heads with some durable, heat-tolerant
non-conductor, likely a ceramic, and insulate the
cylinder head from the rest of the engine Since
electrical insulators are generally good thermal
insulators, this would necessitate nearly all
conductive cooling be done via head (and possibly
the piston). |
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Then there’d be the problem of how to get the
sparks to spark across their huge gap some sort of
well-timed way, which other already notes. |
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All in all, this sounds like a huge engine redesign
challenge, though not beyond the means of a
dedicated tinkerer. Would it be worth it, for a
huge spark? Could it be made to work at all?
Tinkering seems the only practical way to find out. |
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// Would it be worth it, for a huge spark? // |
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Not really, since you don't need a huge spark. You're not
trying to set off a daisy-cutter here, just ignite a tiny bit
of fuel in aerosol form. High-tech spark plugs are actually
centered around the concept of delivering smaller, faster
sparks. |
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But again, the question remains—why bother? An
engine in good condition emits maybe 1-2% of its
input fuel as unburnt hydrocarbons. You're talking
about an unnecessarily complicated design that may
or may not improve fuel burn (and would likely
decrease it, if anything), all for a marginal
improvement at best—and it turns out that it's not
really an improvement anyway. |
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You can achieve essentially the same improvement
simply by running the engine a bit lean. The key
reason for not doing this is it greatly increases
temperatures in the combustion chamber, as the
extra fuel helps to carry out heat. So there's really
no benefit to more complete burning after all.
Incomplete burning is actually a feature, not a design
flaw. |
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//Ohm's law: amperage = voltage / resistance// |
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Fine, except that an electrical discharge through gas does not remotely follow Ohm's law. The voltage depends strongly on the length of the spark, but is not directly proportional to current. In fact, for some regions, voltage strongly decreases as current increases. |
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//Fine, except that an electrical discharge through gas
does not remotely follow Ohm's law.// |
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Of course it does. It's a mathematical relationship. Volts
only exist as an abstraction to describe the relationship
between current and resistive load. |
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//The voltage depends strongly on the length of the spark,
but is not directly proportional to current. In fact, for some
regions, voltage strongly decreases as current increases.// |
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The only way that could be the case is if the resistance is
dropping as well. It is possible that as the gas burns or
electricity passes through it, its resistance changes, and
modeling that can prove tricky, but at any given instant the
properties of the circuit obey Ohm's law. To say otherwise
is equivalent to saying 2x2=5. |
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^this is, in fact, a fundamental operating principle of arc
welding. Without a known composition of flux gases, the
current of the arc fluctuates too wildly to control the
puddle. (Flux gases serve several other purposes as well,
including preventing oxygen from getting into the molten
metal and spoiling everyone's day.) |
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Sorry, I should have said that an electrical discharge in a gas does not remotely resemble an ohmic resistance. The voltage of an arc *decreases* with current - which is why arc welders and discharge lights need ballasts - but yes, V = I * R at any instant. |
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gotta love how me mixing up transformer law with Ohm's law produces 4 pages of annotations. |
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