Could anyone recommend a primer on designing plasma ignition, i.e. very high energy spark ignition systems? My idea is to seriously improve the tired old ignition system on a 2-cylinder 4-stroke motorcycle, and I'd like to experimentally create the fiercest spark that's reasonably possible..
I notice that a few plasma ignition experimenters appear to be chasing after running traditional IC engines on water, given that plasma sparkplugs do make a heck of a crack when you squirt water vapour past them. Not me in this case, I'm still entirely petrol/gasoline based here for now.
> I notice that a few plasma ignition experimenters appear to be chasing
> after running traditional IC engines on water, given that plasma
> sparkplugs do make a heck of a crack when you squirt water vapour past
> them.
They can chase all they wish.
Unless the spark has more energy than is output by the engine they
will not catch anything.
That said, it may make an interesting "electric motor". Spark produces
O2 + H2 which are then burned. Electrical to mechanical converter. Low
oddsof being of much use, but fun.
On 04/04/2011 12:10, RussellMc wrote:
>> I notice that a few plasma ignition experimenters appear to be chasing
>> after running traditional IC engines on water, given that plasma
>> sparkplugs do make a heck of a crack when you squirt water vapour past
>> them.
> They can chase all they wish.
> Unless the spark has more energy than is output by the engine they
> will not catch anything.
>
> That said, it may make an interesting "electric motor". Spark produces
> O2 + H2 which are then burned. Electrical to mechanical converter. Low
> oddsof being of much use, but fun.
>
>
> R
I'm NOT pursuing the water engine; this is for a real bike! I'd like the circuit to sustain an arc for several crankshaft degrees after the ignition mark, theoretically helping to ignite every last scrap of fuel mixture. I expect there will be a point beyond which beefing up the ignition circuit becomes a waste of electrical effort, taking more power than it manages to liberate from the combustion. But it would be fun to determine that point. I'm just having difficulty finding anything among the home-brew schematics for now
Matt Rhys-Roberts wrote:
> Could anyone recommend a primer on designing plasma ignition, i.e.
> very high energy spark ignition systems? My idea is to seriously
> improve the tired old ignition system on a 2-cylinder 4-stroke
> motorcycle, and I'd
> like to experimentally create the fiercest spark that's reasonably
> possible.
Then maybe you can explain why that matters. Obviously you need a spark in
that kind of engine to start combustion. I've heard people talk of stronger
sparks being better, but nobody has ever explained exactly why. What is the
advantage of a stronger spark past reliably starting combustion? Once some
combustion starts, why can't it be considered the "spark" for further
combustion until it runs to completion? No matter how strong the spark,
wouldn't it still only ignite a small portion of the mixture in the
cylinder, which means it's relying on the combustion chain reaction for most
of the burning regardless of how strong the spark is?
Em 4/4/2011 09:37, Olin Lathrop escreveu:
> Matt Rhys-Roberts wrote:
>> Could anyone recommend a primer on designing plasma ignition, i.e.
>> very high energy spark ignition systems? My idea is to seriously
>> improve the tired old ignition system on a 2-cylinder 4-stroke
>> motorcycle, and I'd
>> like to experimentally create the fiercest spark that's reasonably
>> possible.
> Then maybe you can explain why that matters. Obviously you need a spark in
> that kind of engine to start combustion. I've heard people talk of stronger
> sparks being better, but nobody has ever explained exactly why. What is the
> advantage of a stronger spark past reliably starting combustion? Once some
> combustion starts, why can't it be considered the "spark" for further
> combustion until it runs to completion? No matter how strong the spark,
> wouldn't it still only ignite a small portion of the mixture in the
> cylinder, which means it's relying on the combustion chain reaction for most
> of the burning regardless of how strong the spark is?
With a stronger spark you start by igniting a larger mass of mixture and
it takes less time for the whole mass inside the combustion chamber to burn..
This may not be important at low RPM, but at higher RPM, the faster the
combustion the most efficiency you get.
If the ignition takes too long, the pressure is not at the maximum when
it would be more effective, that is, in the first 90 degrees of the
down-stroke.
Isaac Marino Bavaresco wrote:
> This may not be important at low RPM, but at higher RPM, the faster
> the combustion the most efficiency you get.
> If the ignition takes too long, the pressure is not at the maximum
> when
> it would be more effective, that is, in the first 90 degrees of the
> down-stroke.
If that's the only reason, why can't you just advance the timing to get the
same effect?
> Matt Rhys-Roberts wrote:
> > Could anyone recommend a primer on designing plasma ignition, i.e.
> > very high energy spark ignition systems? My idea is to seriously
> > improve the tired old ignition system on a 2-cylinder 4-stroke
> > motorcycle, and I'd
> > like to experimentally create the fiercest spark that's reasonably
> > possible.
> Then maybe you can explain why that matters. Obviously you need a spark in
> that kind of engine to start combustion. I've heard people talk of stronger
> sparks being better, but nobody has ever explained exactly why. ...
"Why" is an interesting question but is close to orthogonal to his
stated intent.
He is interested in "whether" and "how much" with the intention of
determining this experimentally. (More Edison than Tesla).
It may well be that he will arrive at 'yes indeed!" or "apparently
not" without having got too close to "why".
So, if "why" is asked in the spirit of "inquiring minds want to know",
then it may be a useful addition to the quest. IOf its asked in the
spirit of "that seems to be a pretty stupid idea" then it may not be
too useful at a;;, as it could squelch "worthwhile endeavor" or even
just 'harmless pursuit that happens to produce useful results." Or
not.
I prefer Tesla to Edison BUT Edison certainly has its place, and can
turn up many a lightbulb (moment).
Em 4/4/2011 10:09, Olin Lathrop escreveu:
> Isaac Marino Bavaresco wrote:
>> This may not be important at low RPM, but at higher RPM, the faster
>> the combustion the most efficiency you get.
>> If the ignition takes too long, the pressure is not at the maximum
>> when
>> it would be more effective, that is, in the first 90 degrees of the
>> down-stroke.
> If that's the only reason, why can't you just advance the timing to get the
> same effect?
Perhaps because you may end applying power against the engine rotation
and generate a knock.
There is a safe maximum advance limit.
RussellMc wrote:
> "Why" is an interesting question but is close to orthogonal to his
> stated intent.
So? I wasn't pretending to answer his question. At worst you can claim I
hijacked the thread, but I thought my question was sufficiently related to
not start a new thread.
Isaac Marino Bavaresco wrote:
>>> This may not be important at low RPM, but at higher RPM, the faster
>>> the combustion the most efficiency you get.
>>> If the ignition takes too long, the pressure is not at the maximum
>>> when
>>> it would be more effective, that is, in the first 90 degrees of the
>>> down-stroke.
>>
>> If that's the only reason, why can't you just advance the timing to
>> get the same effect?
>
> Perhaps because you may end applying power against the engine rotation
> and generate a knock. There is a safe maximum advance limit.
You can't have it both ways. Either the combustion is delayed due to a weak
spark or its not. If it's delayed, then advancing timing to compensate
shouldn't cause knocking since the actual combustion still happens when you
want it to. If a weak spark doesn't delay combustion, then we're back to
looking for a reason a stronger spark is necessary.
> > "Why" is an interesting question but is close to orthogonal to his
> > stated intent.
> So? I wasn't pretending to answer his question. At worst you can claim I
> hijacked the thread, but I thought my question was sufficiently related to
> not start a new thread.
What you said was potentially very fine.
(And even hijacking seems to be fine enough these days :-))
I was noting that the value of potentially depended on the mind set
that was in place when it was asked.
Viz
So, if "why" is asked in the spirit of "inquiring minds want to know",
then it may be a useful addition to the quest.
Why and whether would both be good to know.
But the subject is complex enough that Occam will probably prefer
Edison in this case.
On Mon, Apr 4, 2011 at 11:46 AM, Olin Lathrop <spam_OUTolin_piclistTakeThisOuTembedinc.com> wrote:
> Isaac Marino Bavaresco wrote:
>> Perhaps because you may end applying power against the engine rotation
>> and generate a knock. There is a safe maximum advance limit.
>
> You can't have it both ways. Either the combustion is delayed due to a weak
> spark or its not. If it's delayed, then advancing timing to compensate
> shouldn't cause knocking since the actual combustion still happens when you
> want it to. If a weak spark doesn't delay combustion, then we're back to
> looking for a reason a stronger spark is necessary.
>
I am not an engine expert by any means, but could it not be true that
a stronger spark decreases the duration of the combustion? In other
words, there are actually two parameters: when the combustion starts
AND how long it takes. Advancing/retarding the spark will only change
the first of these two. Changing the spark energy might change the
second one.
Many moons ago (mid '80's) when I was at UBC they were working on plasma
ignition. They could shoot the plasma out the end the better part of 1 cm.
Made quite a crack. Very cool.
They were also working on different piston / cylinder head designs to shoot
(squeeze) the gas/air mixture to the center of the cylinder when the piston
approached TDC. This was in an effort to create high speed turbulence. IIRC
the conflagration in the cylinder didn't radiate outward from the ignition
source in a hemispherical shape but rather moved preferentially outward
along "threads" and expanded into the area between the threads.
Both things were examined to create a faster pressure rise in the cylinder.
>> Perhaps because you may end applying power against the engine rotation
>> and generate a knock. There is a safe maximum advance limit.
>
> Olin said:
> You can't have it both ways. Either the combustion is delayed due to a
> weak
> spark or its not. If it's delayed, then advancing timing to compensate
> shouldn't cause knocking since the actual combustion still happens when
> you
> want it to. If a weak spark doesn't delay combustion, then we're back to
> looking for a reason a stronger spark is necessary.
Perhaps we should view it like any other engineering problem and ask ourselves what the worst case values are. For example, if we advance the spark only to a point which avoids knock under certain conditions will that still work under different temperatures, atmospheric pressures, variations in fuel, phase of the moon, presence or absence of dead fish, etc, etc, etc..
Matt Rhys-Roberts wrote:
> > Could anyone recommend a primer on designing plasma ignition, i.e.
> > very high energy spark ignition systems? My idea is to seriously
> > improve the tired old ignition system on a 2-cylinder 4-stroke
> > motorcycle, and I'd
> > like to experimentally create the fiercest spark that's reasonably
> > possible.
>Olin wrote:
>Then maybe you can explain why that matters. Obviously you need a <spark in that kind of engine to start combustion. I've heard people >talk of stronger sparks being better, but nobody has ever explained >exactly why. What is the advantage of a stronger spark past reliably >starting combustion? Once some combustion starts, why can't it be >considered the "spark" for further combustion until it runs to >completion? No matter how strong the spark, wouldn't it still only >ignite a small portion of the mixture in the cylinder, which means >it's relying on the combustion chain reaction for most of the burning >regardless of how strong the spark is?
Olin,
Of interest but perhaps sideways to your question -
Once upon a time, a friend was taking me for a ride in a light plane. While still on the ground (I think), he said "watch this" and flipped a switch to engage the second, back up, spark system and the RPMs went up significantly, perhaps as much as 10% (VERY long time ago - not sure of the %). I don't know why, and this may not address your why question anyway as I would guess it creates a second flame front which is different than a longer spark. On the other hand, a bigger, fiercer spark (as apposed to a longer spark) may create a bigger faster flame front.
One thing I think I know is that the whole deal inside the combustion process is complicated and may not be amenable to armchair ponderings. But may well satisfactorily yield to experiment. I say "spark on, Matt, let us know what comes of your explorations"
That's a good point (about aircraft). The standard procedure is to run
with BOTH ignition systems on all the time except when testing them.
During the engine run-up on the ground, one of the steps is to switch
to only the LEFT system and make sure that the RPM doesn't change by
more than a set amount (perhaps 50 rpm out of 800 rpm). Then you
switch to only the RIGHT system and watch the rpm again. Finally, you
switch back to BOTH. It is considered very normal to see a slight dip
in the rpm while only one of the systems is on.
>
>
> Olin,
> Of interest but perhaps sideways to your question -
> Once upon a time, a friend was taking me for a ride in a light plane.
> While still on the ground (I think), he said "watch this" and flipped a
> switch to engage the second, back up, spark system and the RPMs went up
> significantly, perhaps as much as 10% (VERY long time ago - not sure of
> the %). I don't know why, and this may not address your why question
> anyway as I would guess it creates a second flame front which is
> different than a longer spark. On the other hand, a bigger, fiercer
> spark (as apposed to a longer spark) may create a bigger faster flame
> front.
> One thing I think I know is that the whole deal inside the combustion
> process is complicated and may not be amenable to armchair ponderings.
> But may well satisfactorily yield to experiment. I say "spark on, Matt,
> let us know what comes of your explorations"
>
> --
>
> Looking forward,
> Al Shinn
>
On 04/04/2011 19:37, Sean Breheny wrote:
> Hi Al,
>
> That's a good point (about aircraft). The standard procedure is to run
> with BOTH ignition systems on all the time except when testing them.
> During the engine run-up on the ground, one of the steps is to switch
> to only the LEFT system and make sure that the RPM doesn't change by
> more than a set amount (perhaps 50 rpm out of 800 rpm). Then you
> switch to only the RIGHT system and watch the rpm again. Finally, you
> switch back to BOTH. It is considered very normal to see a slight dip
> in the rpm while only one of the systems is on.
>
Okay, I have a question then. If the idea is to produce a bigger/longer spark, then why not simply use multiple sparkplugs in the bike also?
I realise there will only be "room" for one per cylinder in a standard engine, but maybe it would be possible to design a custom plug with two spark gaps. They could either be fired at the same time or slightly apart.
Probably the only advantage of a stronger spark would be its reliability which would mean no misfiring and emission issues. Misfiring tends to produce more hydrocarbons then regular spark regular activity. If the present system is CDI/TCI and there are no misfires then there shouldn't be a problem.
To simply substitute it with some other type "experimentally" is a matter of time on hands and.....curiosity or doing something different??? Entirely up to the individual..
As I understand it, the reason in an aircraft is pure reliability.
This is also the reason why the ignition system is totally isolated
from the DC electrical bus which runs the starter and the
electric-powered instruments (there are also vacuum-powered
instruments).
Having two spark plugs per cylinder allows you to create two totally
independent ignition systems, each using a magneto and no battery.
Both are required to be working for flight, hence why you check both
separately before every flight.
> On 04/04/2011 19:37, Sean Breheny wrote:
>> Hi Al,
>>
>> That's a good point (about aircraft). The standard procedure is to run
>> with BOTH ignition systems on all the time except when testing them.
>> During the engine run-up on the ground, one of the steps is to switch
>> to only the LEFT system and make sure that the RPM doesn't change by
>> more than a set amount (perhaps 50 rpm out of 800 rpm). Then you
>> switch to only the RIGHT system and watch the rpm again. Finally, you
>> switch back to BOTH. It is considered very normal to see a slight dip
>> in the rpm while only one of the systems is on.
>>
>
> Okay, I have a question then. If the idea is to produce a bigger/longer
> spark, then why not simply use multiple sparkplugs in the bike also?
> I realise there will only be "room" for one per cylinder in a standard
> engine, but maybe it would be possible to design a custom plug with two
> spark gaps. They could either be fired at the same time or slightly apart..
>
>
>
part 1 3142 bytes content-type:text/plain; charset="iso-8859-1" (decoded quoted-printable)
>the whole deal inside the combustion process is complicated and may not be
amenable to armchair ponderings.
Actually that is why most of the time plasma Ignition isn't used alone as an
improvement.
They use coils on each plug and also apply a DC voltage between sparks
and measure the current caused by those ~400 Volts. Its called Ion Sensing,
and it reveals clues on:
- weather the ignition was successful
-pressure and knock because detonation shock waves are reflected inside the
combustion chamber
-ion content of combustion products or fuel (possibly one could even try
voltametry?)
-temperature
of course deducting all of that from just one voltage current relation
sounds like art!
Let me see if the circuit proposal I attached goes trough...
>
>
> Matt Rhys-Roberts wrote:
> > > Could anyone recommend a primer on designing plasma ignition, i.e.
> > > very high energy spark ignition systems? My idea is to seriously
> > > improve the tired old ignition system on a 2-cylinder 4-stroke
> > > motorcycle, and I'd
> > > like to experimentally create the fiercest spark that's reasonably
> > > possible.
> >Olin wrote:
> >Then maybe you can explain why that matters. Obviously you need a
> <spark in that kind of engine to start combustion. I've heard people
> >talk of stronger sparks being better, but nobody has ever explained
> >exactly why. What is the advantage of a stronger spark past reliably
> >starting combustion? Once some combustion starts, why can't it be
> >considered the "spark" for further combustion until it runs to
> >completion? No matter how strong the spark, wouldn't it still only
> >ignite a small portion of the mixture in the cylinder, which means
> >it's relying on the combustion chain reaction for most of the burning
> >regardless of how strong the spark is?
>
> Olin,
> Of interest but perhaps sideways to your question -
> Once upon a time, a friend was taking me for a ride in a light plane.
> While still on the ground (I think), he said "watch this" and flipped a
> switch to engage the second, back up, spark system and the RPMs went up
> significantly, perhaps as much as 10% (VERY long time ago - not sure of
> the %). I don't know why, and this may not address your why question
> anyway as I would guess it creates a second flame front which is
> different than a longer spark. On the other hand, a bigger, fiercer
> spark (as apposed to a longer spark) may create a bigger faster flame
> front.
> One thing I think I know is that the whole deal inside the combustion
> process is complicated and may not be amenable to armchair ponderings.
> But may well satisfactorily yield to experiment. I say "spark on, Matt,
> let us know what comes of your explorations"
>
> --
>
> Looking forward,
> Al Shinn
>
There are several reasons for a longer and/or stronger spark. In most cases,
a short, weak spark will light the mixture, but sometimes it will not
causing misfires. If everything is perfect with the engine a normal spark
will reliably light the mixture. Things are rarely perfect in the combustion
chamber of an engine that is changing its operating conditions (fast/slow,
accelerating, using a lean mixture to get better mileage, using a rich
mixture to get more power, hot/cold, high/low humidity, high/low altitude,
alcohol in the fuel, etc). A stationary engine such as a generator runs at a
relatively constant speed and load which is much easier to deal with.
There are several reasons and conditions that can cause the mixture to be
difficult to ignite.
The mixture may be too rich or too lean. Anything too far outside the proper
ratio is harder to light.
Given that this is an old motorcycle, it is probably carbureted (likely
poorly), so there will often be significant rich and lean mixtures in the
combustion chamber.
Even if the overall charge is at a proper ratio overall, it may not be mixed
well. At high RPM there is little time for atomization and full evaporation
of the fuel and little time to mix with the air (particularly the 18% that
is O2), but there is lots of turbulence to help. At low RPM there is more
time, but lower velocities in the intake passages and less turbulence in the
combustion chamber to help it to mix.
With a short spark duration, the spark may only be there in the instant when
an un-burnable mixture is in the spark plug gap. With a longer spark there
is a much better chance that a good mixture will be in the gap while the
spark is arcing. With a stronger spark, there is more energy available to
impart to the mixture to get the molecules over the energy step required to
initiate the exothermic reaction.
Once the reaction has started it is likely to continue as the reaction is
very exothermic and once it starts both the pressure and temperature rapidly
rise which makes it easier to continue. I suppose it is possible that only a
few isolated molecules are burnt and the reaction stops. In that case is
would be doubly important to have a long spark so there is time for the
combustion chamber turbulence to blow the burnt mixture away and bring some
new mixture near the spark.
The mixture may also be far less than optimal because of gasses left over
from the previous power stroke. In a perfect world, a 10:1 compression
engine only gets 90% of the burnt gasses out of the combustion chamber. This
is very affected by engine speed and valve timing. At some speeds, almost
all of the burnt gases will be forced out (a tuned intake and exhaust system
running at resonance), but at other speeds, especially low RPM with long and
overlapped valve timing as used in a high performance engine, it may only
get half or less of the burnt gases out. That leaves the new fuel having a
much harder time getting near the O2 molecules its needs to ignite. A long
and powerful spark will more likely be there when the fuel and O2 molecules
are there as well.
The higher the cylinder pressure before ignition, the harder it is to get a
spark to light it up. I'm not sure of the reasons, but a couple of educated
guesses are: With a very compressed mixture (maybe 12:1 with a 100%
volumetric efficiency) you are looking at 175 lb/sqin or more of pressure
that is filled with an air/fuel mixture (gasoline molecules are pretty big
and heavy). That has a significant thermal mass and a small portion of it
must be raised in temperature to start the ignition. The second guess is
that maybe the dense air fuel mixture is a better insulator (or more
difficult to ionize) so more voltage is required to initiate the spark.
Turbulence in the combustion chamber is generally a good thing as it helps
air/fuel mixing, but it can also blow out the spark. The spark needs enough
potential across the gap to ionize the gasses that are there. Once they are
ionized, the electrons are free and the resistance drops, so a large current
can flow. The large current provides the energy to start the reaction. With
lots of turbulence, the partially ionized gasses may sometimes be blown away
before the spark can start. That can result in a misfire.
The spark plug works somewhat like a neon lamp. It is very high impedance
until the gasses are ionized, after which the resistance becomes very low
and a large current can flow. One of the reasons for resistive spark plug
wires is to increase the duration of a spark by inserting a resistance in
the circuit lowering the current and allowing the energy from the coil to be
dissipated over a longer time period. Lowering the peak spark current also
reduces the EMF emissions.
The bottom line is that longer and more powerful sparks can help out in all
sorts of edge conditions that might otherwise lead to a misfire. If you have
a simple engine that runs at a single speed under a single load at a steady
temperature with high quality and consistent fuel, there is probably little
benefit as the engine can be easily tuned to operate very well under its
single operating condition. In the real world, a powerful and long spark can
certainly help getting the fuel to start burning.
> -----Original Message-----
> From: EraseMEpiclist-bouncesspam_OUTTakeThisOuTmit.edu [piclist-bouncesspam_OUTmit.edu] On
Behalf
> Of Oli Glaser
> Sent: 04 April 2011 19:55
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] seeking introductory notes on DIY plasma ignition
>
> On 04/04/2011 19:37, Sean Breheny wrote:
> > Hi Al,
> >
> > That's a good point (about aircraft). The standard procedure is to
run
> > with BOTH ignition systems on all the time except when testing them.
> > During the engine run-up on the ground, one of the steps is to
switch
> > to only the LEFT system and make sure that the RPM doesn't change by
> > more than a set amount (perhaps 50 rpm out of 800 rpm). Then you
> > switch to only the RIGHT system and watch the rpm again. Finally,
you
> > switch back to BOTH. It is considered very normal to see a slight
dip
> > in the rpm while only one of the systems is on.
> >
>
> Okay, I have a question then. If the idea is to produce a
bigger/longer
> spark, then why not simply use multiple sparkplugs in the bike also?
> I realise there will only be "room" for one per cylinder in a standard
> engine, but maybe it would be possible to design a custom plug with
two
> spark gaps. They could either be fired at the same time or slightly
apart.
If you had two spark gaps in one plug they would be separated only by a
very short distance, so any advantage of dual flame fronts would be
lost. The point of a twin spark system (apart from redundancy in
aircraft applications) is to reduce the time taken to burn the charge by
starting two flame fronts, meaning less ignition advance is required
(which is always a good thing).
Regards
Mike
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On 05/04/2011 02:56, Sean Breheny wrote:
> As I understand it, the reason in an aircraft is pure reliability.
> This is also the reason why the ignition system is totally isolated
> from the DC electrical bus which runs the starter and the
> electric-powered instruments (there are also vacuum-powered
> instruments).
>
> Having two spark plugs per cylinder allows you to create two totally
> independent ignition systems, each using a magneto and no battery.
> Both are required to be working for flight, hence why you check both
> separately before every flight.
>
I see, that makes sense, thanks. I guess the ignition system is not something you would be particularly pleased to lose at 10,000 feet :-)
> I see, that makes sense, thanks. I guess the ignition system is not
> something you would be particularly pleased to lose at 10,000 feet :-)
> If it happens - then give me 10,000 feet any day (about 8 minutes find a field) rather than 1000 feet...
On 05/04/2011 09:57, Michael Rigby-Jones wrote:
> If you had two spark gaps in one plug they would be separated only by a
> very short distance, so any advantage of dual flame fronts would be
> lost. The point of a twin spark system (apart from redundancy in
> aircraft applications) is to reduce the time taken to burn the charge by
> starting two flame fronts, meaning less ignition advance is required
> (which is always a good thing).
>
Okay, thanks for the explanation - I can see how greater separation might be preferable. However, the OP was talking about creating the fiercest (single) spark possible - surely two sparks would be "fiercer" than one?
I'm not saying this would actually improve performance, I don't know enough about internal combustion to make a call, but it seems it might achieve the OPs goal of creating a longer/fiercer spark.
Out of interest, how far apart are the spark gaps in a twin spark system? I googled but all I got was pages on Alfa Romeos, of which I looked at the Wiki page and it doesn't go into too much technical detail.
On 05/04/2011 11:27, Geo wrote:
> Oli Glaser wrote:
>
>> I see, that makes sense, thanks. I guess the ignition system is not
>> something you would be particularly pleased to lose at 10,000 feet :-)
>>
> If it happens - then give me 10,000 feet any day (about 8 minutes find a
> field) rather than 1000 feet...
>
Yes, I guess 1000 feet would be more of a problem.
However, I think both situations would comfortably fit into the "not particularly pleasing" category though.. :-)
> Yes, I guess 1000 feet would be more of a problem.
> However, I think both situations would comfortably fit into the "not
> particularly pleasing" category though.. :-)
> Agreed - the amount of paperwork required....
The double spark makes little difference to the RPM but it is only checked on the ground - you do not switch off one mag in flight (ok you do if the engine starts misfiring as one mag could have lost sync) so I am not sure if a /single/ mag failure would be noticed.
On 05/04/2011 12:58, Olin Lathrop wrote:
> Fred Langley wrote:
>> There are several reasons for a longer and/or stronger spark.
>> ...
> Thanks. That was a very informative post.
Agreed, and thankyou all very much (so far) for giving me plenty more to think about while tinkering with coils, oscillators etc. Glad my original post triggered such interest. Sparked, even.
On 05/04/2011 09:57, Michael Rigby-Jones wrote:
>>
>> If you had two spark gaps in one plug they would be separated only
>> by a very short distance, so any advantage of dual flame fronts
>> would be lost.
Only engine of which I have first hand knowledge is my Lycoming
O-360 (360 cubic inch, 4 cylinder, horizontally opposed) engine.
Bore is 5.125" (130mm). Sparkplugs come in diagonally from the
top & bottom edges of cylinder heads. Given the angle, I estimate
the tips of the 2 sparkplugs are 3" to 3-1/2" (75-90mm) apart.
Aircraft sparkplugs come in massive and fine wire versions. The
massive electrode version has 2 arc-shaped ground pieces around
a cylindrical center electrode. Spark can arc from center to one
or the other or both ground pieces. I've never paid the premium
for fine wire aircraft sparkplugs so I can't comment on them.
> The double spark makes little difference to the RPM but it is only
> checked on the ground - you do not switch off one mag in flight (ok
> you do if the engine starts misfiring as one mag could have lost
> sync) so I am not sure if a /single/ mag failure would be noticed.
Depends on the instrumentation you have installed. If you are doing
it "by ear" then you may quite likely not notice early on.
If you have a multi-probe EGT gauge, a misfiring cylinder shows up
clearly as a change in exhaust gas temperature; particularly during
the run-up check when you only have 1 sparkplug firing per cylinder.
I prefer a bar graph style EGT gauge. I feel exact temperatures are
not as important as relative values and trends (easier to watch the
bar heights). A magneto failure shows up as a more subtle change in
all cylinders.
Note: exhaust gas temperatures are NOT the same for all cylinders.
Each one has a characteristic temperature that you learn to expect
in various flight regimes. Variation is due to differences in length
& shape of each intake manifold runner, mixture distribution, mixing
of air-fuel, cylinder cooling (cooling air flow varies by cylinder
location), etc. That's why I like a bar graph. Humans easily learn
a "picture" to expect when at cruise -- if the picture changes, you
perk right up. Hopefully before you become a glider pilot. :-)
There are several reasons for a longer and/or stronger spark. In most cases,
a short, weak spark will light the mixture, but sometimes it will not
causing misfires. If everything is perfect with the engine a normal spark
will reliably light the mixture. Things are rarely perfect in the combustion
chamber of an engine that is changing its operating conditions (fast/slow,
accelerating, using a lean mixture to get better mileage, using a rich
mixture to get more power, hot/cold, high/low humidity, high/low altitude,
alcohol in the fuel, etc). A stationary engine such as a generator runs at a
relatively constant speed and load which is much easier to deal with.
There are several reasons and conditions that can cause the mixture to be
difficult to ignite.
The mixture may be too rich or too lean. Anything too far outside the proper
ratio is harder to light.
Given that this is an old motorcycle, it is probably carbureted (likely
poorly), so there will often be significant rich and lean mixtures in the
combustion chamber.
Even if the overall charge is at a proper ratio overall, it may not be mixed
well. At high RPM there is little time for atomization and full evaporation
of the fuel and little time to mix with the air (particularly the 18% that
is O2), but there is lots of turbulence to help. At low RPM there is more
time, but lower velocities in the intake passages and less turbulence in the
combustion chamber to help it to mix.
With a short spark duration, the spark may only be there in the instant when
an un-burnable mixture is in the spark plug gap. With a longer spark there
is a much better chance that a good mixture will be in the gap while the
spark is arcing. With a stronger spark, there is more energy available to
impart to the mixture to get the molecules over the energy step required to
initiate the exothermic reaction.
Once the reaction has started it is likely to continue as the reaction is
very exothermic and once it starts both the pressure and temperature rapidly
rise which makes it easier to continue. I suppose it is possible that only a
few isolated molecules are burnt and the reaction stops. In that case is
would be doubly important to have a long spark so there is time for the
combustion chamber turbulence to blow the burnt mixture away and bring some
new mixture near the spark.
The mixture may also be far less than optimal because of gasses left over
from the previous power stroke. In a perfect world, a 10:1 compression
engine only gets 90% of the burnt gasses out of the combustion chamber. This
is very affected by engine speed and valve timing. At some speeds, almost
all of the burnt gases will be forced out (a tuned intake and exhaust system
running at resonance), but at other speeds, especially low RPM with long and
overlapped valve timing as used in a high performance engine, it may only
get half or less of the burnt gases out. That leaves the new fuel having a
much harder time getting near the O2 molecules its needs to ignite. A long
and powerful spark will more likely be there when the fuel and O2 molecules
are there as well.
The higher the cylinder pressure before ignition, the harder it is to get a
spark to light it up. I'm not sure of the reasons, but a couple of educated
guesses are: With a very compressed mixture (maybe 12:1 with a 100%
volumetric efficiency) you are looking at 175 lb/sqin or more of pressure
that is filled with an air/fuel mixture (gasoline molecules are pretty big
and heavy). That has a significant thermal mass and a small portion of it
must be raised in temperature to start the ignition. The second guess is
that maybe the dense air fuel mixture is a better insulator (or more
difficult to ionize) so more voltage is required to initiate the spark.
Turbulence in the combustion chamber is generally a good thing as it helps
air/fuel mixing, but it can also blow out the spark. The spark needs enough
potential across the gap to ionize the gasses that are there. Once they are
ionized, the electrons are free and the resistance drops, so a large current
can flow. The large current provides the energy to start the reaction. With
lots of turbulence, the partially ionized gasses may sometimes be blown away
before the spark can start. That can result in a misfire.
The spark plug works somewhat like a neon lamp. It is very high impedance
until the gasses are ionized, after which the resistance becomes very low
and a large current can flow. One of the reasons for resistive spark plug
wires is to increase the duration of a spark by inserting a resistance in
the circuit lowering the current and allowing the energy from the coil to be
dissipated over a longer time period. Lowering the peak spark current also
reduces the EMF emissions.
The bottom line is that longer and more powerful sparks can help out in all
sorts of edge conditions that might otherwise lead to a misfire. If you have
a simple engine that runs at a single speed under a single load at a steady
temperature with high quality and consistent fuel, there is probably little
benefit as the engine can be easily tuned to operate very well under its
single operating condition. In the real world, a powerful and long spark can
certainly help getting the fuel to start burning.
Fred Langley wrote:
> One of the reasons for resistive spark plug wires is to increase the
> duration of a spark by inserting a resistance in the circuit lowering the
> current and allowing the energy from the coil to be dissipated over a longer
> time period. Lowering the peak spark current also reduces the EMF emissions.
You were doing fine up to this point. In an L-R circuit, the time constant is
L/R -- in other words, adding resistance makes the time constant shorter, not
longer. Also, any resistance in the wire forms a voltage divider with the
resistance of the arc, reducing the available energy at the arc. AFAIK,
resistance wire is used only for EMI suppression, as early car radios were
poor at rejecting it.
embedinc.com> wrote:
