How Spark Ignitions Systems work.....more than you wanted to know
©
Copyright, 2011, R. Fleischer
ignitiontheory.htm-28
This is article ALSO contains, revised, the
now deleted old article: ignitionsystems.htm
This article contains SIMPLIFIED explanations in SOME areas.
Not only is the theory of ignition covered, but several types of systems,
and special information for your BMW airhead is included.
Near the very end of this article, I have
put a Totally Nerdy Section.....which is just exactly what that
implies.....highly technical, boring to many of you, but offers
real insider information.
PURPOSE:
The purpose of a spark ignition system is to provide, at the
correct moment in relation to piston travel and rpm, one or more electrical sparks such that the fuel-air mixture in the combustion
chamber may be ignited at one or more places by the HEAT of that
spark discharge. Almost
universally the spark(s) are produced in the combustion chamber
at a 'spark plug'.
In the stock BMW Airhead motorcycle ignition system, there is one spark plug per
cylinder, and both cylinders have an ignition spark at the same
time, but only one cylinder is under compression, the other has a
'wasted spark'. The 'sparks' themselves are of very high
temperature, and ignite the fuel mixture if that mixture is
reasonably proper.
The higher the pressure in the combustion chamber at the time of ignition, the higher the voltage needed to jump the spark plug gap. The electrical voltage needed to jump the spark plug gap can be VASTLY higher inside the cylinder, than if the spark plug is outside and you are looking at the gap.
There seems to be some confusion as to how 4 stroke cycle gasoline engine ignition systems operate. I wish to emphasize that the
following is a SIMPLIFIED explanation, although lengthy. Following these
explanations, I will get into the BMW Airhead system more deeply.
The earliest ignition systems that had any similarity to today's
were in the days of the earliest automobiles. At around the same time that
magneto's were in use, there were also wooden boxes with a
vibrating electrical contact which sent storage battery energy
into a coil of wire wound on an iron core.
The vibrating contacts were typically made to vibrate by how they
were connected to the battery, the contacts opening when the
magnetic energy produced in the iron core attracted (pulled) one
of the contacts away from the other, breaking the battery
circuit. Since the vibrating contact opened and closed the circuit rapidly, the coil
winding was interrupted from current flow quickly and often. Each time the contact closed,
the coil charged up magnetically, and each time the contact opened, the coil
transformed that energy into high voltage. How did it do that?
There was a separate wire winding
on that iron core, with many hundreds of times more turns, and those extra turns 'transformed' the lower voltage of the battery to a few
thousand volts or more, and this was applied to rather crude looking spark plugs. In many
very early cars, that high voltage was applied
continuously to the spark plug. The biggest problem with the vibrating contact system is
that the spark output is constantly flowing, and it is hard to 'time' the
initialization of the combustion event, even with a rotating contact distributor. This
was not a big problem with very low output engines with very low rpm
and compression ratios. These 'boxes' also did not
have more than maybe 5 or 6000 volts...some a bit more.....so
they were not adequate for high compression engines...but we did
not have high octane fuels either....and for other reasons these
boxes, always under 12,000 volts, worked fine. A typical
coil of the type described is still easily purchased brand-new;
for Ford Model T cars.
Magneto's are simply a permanent magnet method of using mechanical
rotational energy, instead of a battery, to produce the high
voltage for the spark plug. Magneto's are usually equipped with a set of points
(contact points plate), and the energy transfer from the small number of turns coil in the contacts plate circuit
primary circuit, to the secondary, many turn winding, is done at the
moment the points open, which is set to coincide with a particular
high energy alignment of the iron core with the wire in relationship
to the magnet (actually at the switching point of magnetic poles,
but that is a nerdy point here). Magneto output tends to rise with rpm.
Since the exact time the spark occurs can be controlled by
positioning the points, timing on magnetos can be precise.
The same preciseness is available with ignition coils and POINTS,
or some other triggering method besides points (some form of
switch). I will get into it later.
BMW used magneto's in the early models (pre-1970). Magneto's are
difficult
to keep down in size when you need high energy sparks. They
have rotating mass and inertia. Lean burning
engines
demand high energy sparks. Magneto's have some other problems, that
is why they are not now used on transportation vehicles. They do have increasing voltage
as they rotate faster, but that means lower output when you are
kick-starting the engine....and if anything is slightly marginal,
the engine may not start or not easily. Some older race engines still use
them, but most race engines now use sophisticated electronic
ignition. Magnetos ARE still used in many small engines, especially in things like lawnmowers. Some small motorcycle and scooter
engines may still use them. More on magnetos later herein.
Later BMW's from the end of 1969 (/5 and later models) used coil
ignition with a points contacts assembly
being driven off the camshaft. The points contacts are
nothing more than a switch. There is a capacitor, often called a
condenser, wired across the points. This system of contact
points, capacitor, and coil (or two coils on some airheads), all
being driven by a battery, was invented by, and still named for
Mr. Kettering. It is really just an improvement on
the vibrating contacts coil mentioned earlier.
The camshaft rotates at half the crankshaft speed, making the extra
cam on the
end of that camshaft, that drives these contact points, an
easy-to-design and
build part, as well as performing better at the lower rotational speed. For our Airheads, there are two lobes on that points cam.
For every full turn of the crankshaft, one gets two sparks.
The 1981 and later BMW motorcycles ignition design eliminates the points in favor of an
electronic triggering device, the two lobes now being a
butterfly-looking piece of metal that rotates. The amount of TIME,
per engine revolution, that the points stay closed (78°, 110°, and 120° has
been used), supplying electrical energy to charge up the coil(s) magnetically
(or the Hall element trigger and electronic circuitry allows whatever dwell time it is
designed for....104° actually!), varies with year and model....and
is a bit of nerdy information, since it is not very important but
to the engineers who design it. You need a certain
time for the current to flow to fully magnetize the coil(s) core.
That time is more critical as rpm rises quite high. If the
DWELL (official word for the degrees per rotation of the cam that
the ignition is being energized by battery current) is too long,
the coil(s) might overheat. Too short, and the high voltage
output will suffer too much at high rpm.
There are other types of ignition methods; diesel motors can use
glowing coils of wire; and there is a type of ignition for
gasoline motors called CDI which stands for Capacitor Discharge
Ignition. That type typically produces a VERY high voltage,
and the TIME from when the voltage starts to rise, until the
spark occurs, can be ULTRA SHORT. It can be so very short
that this type of ignition can actually fire spark plugs that are
moderately fouled (potentially partially shorted). However,
CDI ignition typically has a drawback, its SPARK DURATION is
almost always EXTREMELY short. It
takes a reasonable amount of spark DURATION to properly ignite a
fuel-air mixture, particularly when running lean.
Making the spark, & WHEN does
it occur?::
The ignition coil consists of an iron core (actually multiple
thin iron pieces) that has TWO separate windings. One winding,
called the PRIMARY, has a modest amount of turns of a relatively
thick wire as it will be handling a large current flow. The
push-on male spade terminals on your Airhead's coil are connected
to the respective two ends of this "primary" winding. Well
insulated from this winding, there is another winding, this
separate high voltage winding consists of thousands of turns of a
rather thin wire. This wire is thin because otherwise the
many turns would not fit into the case; and thin is OK, since
this winding has low current, but high voltage. In some ignition
coils, the two ends of this winding go to TWO high voltage
terminals (called TOWERS) on the single coil. In some coils, there is only one
tower, and the other end of the high voltage coil winding is connected to one of
the low voltage winding terminals, to act as a ground return path (it could have
connected to the case, if metal, some coils are like that, but then the metal case must be
securely grounded).
Assuming the system has mechanical points and a capacitor
(condenser), here is a simplified explanation of how the system
works:
1. Battery voltage through the ignition switch is
applied to one primary winding terminal on a coil. The other primary winding
terminal on the
coil connects to the ignition points insulated contact. On your airhead,
these push-on terminals are usually marked 1 and 15, and maybe + and
-. These terminals are so marked so that the connection to the battery is made such that the high
voltage output is negative, which adds to the negative electrons given off by
a spark plug center electrode when it is hot...and so this also helps with making the
spark occur more easily. If you have a SINGLE
dual-output coil, one spark plug always gets the negative
voltage. If you have TWO coils, the coil primaries are connected
in series in such a way that BOTH spark plugs always receive the
negative voltage.
2. The other points contact is permanently connected to the
grounded metal plate of the points contact assembly, and thereby
completes the circuit to the
battery negative terminal, through the engine casing, etc.
3. During engine operation the points are closed for a much longer
period of time of rotation, compared to the time the points are
open. This is what is
meant by "dwell" angle, as a portion of a 360° rotation.
In actuality, for our
airheads, we mean the ratio for EACH of the TWO lobes on the
points cam, there are two, at 180°. It is hardly
necessary to understand the technical details of crankshaft,
camshaft, and lobes dwell. Just understand that there IS
such a thing as dwell angle, which is used by some folks in
setting points gap. YOU do NOT need to do that, it is hardly
critical on our Airheads.
During the points closed period of time, the current
from the battery flows through the coil, or two coils, and back to the battery via
the points.
With the battery connected to one side of the
coil primary winding, and the points closed, current flow does
NOT instantaneously rise to maximum like it would with the coil
of wire removed from the iron core and just stretched out.
Magnetic theory is involved.. Instead, the current starts flowing at a low level,
and the current increases smoothly, and if the battery is connected long enough,
the current reaches the value calculated by Ohms law, that is, the maximum
current is calculated as the applied voltage divided by the resistance of the coil(s) (in ohms). If one had 2 ohms of total primary resistance, and the full 14 volts
was available, the current could reach 7 amperes...equivalent to 98 watts needed
from the battery and alternator. Without getting into the
mathematics of the shape of the curve if one was to plot the current input to
the coil over TIME, and without getting into the theory deeper
otherwise....suffice it to say that the coil is likely going to reach
peak current in perhaps 15 milliseconds. If too many sparks are
required (by too high an rpm), then the charging time is INsufficient to fully
charge the coil magnetically, and the output for the spark plugs will be lower, until at some rpm the voltage is insufficient for proper ignition of
the mixture. In the stock BMW Airhead motorcycle, the rpm that
CAN be reached is insufficient to require
fancy sophisticated ignitions systems. The points type
ignition for pre-1981; and
simple electronics type for 1981 and later, with the type of
components used, is quite sufficient.
Note that earlier in this article I stated that the MAGNETOS
used on pre-1970 models had low output during cranking.
The OPPOSITE is true for coil ignition. Thus, coil ignition
can give a good hot (high energy) spark for easier starting, and also the hot
spark enables higher engine tuning for increased power.
NERDY:
The amount of energy the coil is capable of producing is a function of
one of its
electrical properties called INDUCTANCE. One needs a lot of special
iron laminations in the core, and a very carefully designed windings, etc., to
get the highest performance.
4. As noted...during the time that the points are CLOSED, the iron core is
building up a magnetic field. Given enough time, the magnetic field
rises to the maximum the battery supply can make happen. If the
engine is rotating relatively slowly, the design is always such that
this maximum does occur. Note that the points cam rotates in our
BMW engines because it is connected to the camshaft, which is in
itself driven by a chain from the crankshaft. When the desired
position of the particular piston on its COMPRESSION STROKE is
reached, the points are designed to JUST BARELY begin to separate.
At this exact instant, where the points circuit is broken, the coil
cannot accept
any more current from the battery because the circuit has opened. More importantly, at this exact
instant, the
coil is 'loaded' with magnetic energy, which has to go someplace
(don't ask why, that is beyond this posting). When the
magnetic field collapses, that is when the high voltage is
developed in the secondary, many thousands of turns winding.
I'll stop here briefly.
Some have asked about making measurements at the points to 'static
time' the engine. This is acceptable to initiate timing at some close to correct
value, but the
correct method is to time the engine at maximum advance, at high
rpm. But, yes, you can use an ohmmeter or a voltmeter, or a piece
of the very thin 'cigarette' rolling paper. If you use an
ohmmeter, leave the ignition off or you might burn out the meter.
Either type meter is connected across the points.
***DO NOT try to measure anything in the canister ignition
from 1981 and later....it will injure the ignition, even with
power off.
Back to POINTS ignition:
As the engine is VERY slowly rotated by hand, so as to see the
exact place this happens, at the flywheel-marked static ("S") timing
point, the ohmmeter indication will suddenly increase (more
ohms). If using a voltmeter, turn on the ignition,
and the timing is when the voltage rises suddenly from zero to approximately
battery voltage. If using cigarette rolling paper, which is
extremely thin, the timing point is when the cigarette paper can
just barely be pulled through without grabbing. You must rotate the
engine in the forward normal direction VERY SLOWLY, degree by degree, when
finding the exact Static timing point.
5. Remember that capacitor/condenser? During
the relatively long time the points were closed (compared to
being open), and the coil charging up in magnetic energy, that
capacitor is SHORTED by the points. At the instant the points
separate the tiniest amount, the coil 'tries' to charge the capacitor. Since it takes TIME for the
electrons to move to the capacitor, the coil really "sees"
..INITIALLY....mostly a dead short at the capacitor, said dead
short slowly rising to less and less of a short, as the coil
dumps energy into the capacitor and the capacitor 'charges up'.
The dumping process reverses the magnetic field and current flow
directions in the coil, allowing a transforming effect from
primary to secondary. This is a complex process, and actually the current reverses numerous times as
it dies down, and is shortened when the spark occurs at the spark plug. We call it a
decaying or damped waveform, if seen on an oscilloscope...a type of TV-like
device that will display the electrical waveform. The points contact winding (Primary Winding) also sees an
increase in voltage, although much lower in value...perhaps a couple hundred volts at best.
The energy that is in the primary winding is 'transformed' by the
secondary winding. Voltage and current in
a ignition coil is not constant as the cam points open and close,
making the process very difficult to properly understand unless
you are an electrical engineer or understand calculus.
The main thing for you to know is that
the primary winding has a lot of current flowing, the points
open, the secondary winding develops a VERY high voltage, which
we hope goes to a spark plug that now has a spark jumping its
spark-gap. IF you pulled the spark plug cap off
the spark plug (a VERY bad thing to do), the secondary voltage
can rise up much higher than the insulation inside the coil
allows safely. Also, the voltage at the condenser and
points can rise up rather high too. Thus, your
pulling the spark plug cap off can damage the components.
When the spark plug fires, the whole magnetic filed
collapses....and things repeat for the next spark.
Here is another way of looking at it condensers (capacitors) in
the ignition circuit: These have a secondary function of reducing arcing at the points. Because the capacitor is shorted during coil charging,
at the moment the points break connection, that shorted capacitor
tends to reduce the electrical arc (sparking) at the contacts
that would otherwise occur, giving them a much longer life. So,
the capacitor has multiple functions!, but also augments the
rapid collapse of the magnetic field, providing what an engineer would call a
Low Impedance discharge path. Thus, the performance of the coil, let alone
points deterioration from arcing, would be affected if the condenser was
eliminated. One final thing about this condenser. The
value of it is not overly critical, but it must be there, not
open, not shorted. The value was selected by Bosch/BMW for
a compromise between protecting the points from arcing, and
characteristics of the voltage and current that develop in the spark....and
the spark DURATION. Another nerdy engineering point:
if the capacitor OPENS, not only is the spark vastly less
powerful, but the ignition timing changes quite a bit...it will
retard. The combination of the capacitor (condenser)
characteristics and the ignition coil characteristics, has a
substantial effect on the ignition system operation.
6. In a Kettering system as described, the high
voltage output of the coil has a rather complex type of waveform,
but it is ALSO definitely polarized. That is why the two spade terminals
on the coil must be connected to the correct wires. They are marked
+ and -, or 1 and 15, or both. While the coil might well fire the
spark plugs if the coil primary is connected backwards, the
effective energy at the spark could be reduced.
7.
Note: If your airhead uses two coils, single plug ignition, these
are two each 6 volt coils connected in series, and all the foregoing
description still applies. If you have a dual-output 12 volt
single coil, as on R80ST, GS, and some other later airheads, there is still no
difference in the basic operation, except that after 1980, points
were eliminated in favor of electronic triggering of the coil primary winding....and,
that with ONE coil, the primary terminals may have NO polarity
markings, because they are unimportant.....one terminal tower is
always -, the other is always +.
For those of you with dual plug ignition conversions,
the normal setup is that there are TWO 6 volt primary, dual output coils, with the primaries connected
in series. The high voltage secondary outputs have positive and
negative outputs at any one particular coil pair of terminals;
exactly the same as BMW's ONE coil system. The
area of a spark plug that has the spark itself, that center
electrode, gets quite hot. Due to complex theory on
movement of electrons, you want a certain polarity of spark. You want
the strongest spark under the conditions of the high compression
pressures
in the combustion chamber just before the mixture is ignited by the
spark
plugs. To get the strongest spark at the ONE cylinder that is, at any one
combustion time, going to produce power, you want the other output tower of that
coil to be to a spark plug that has
LOW cylinder pressure, so ITS spark gap is EASILY jumped, although not igniting
a mixture. What all
this chit-chat boils down to, is
you want ONE of the ignition coils
with the two high voltage terminals, to connect to both
cylinders.
Reasoning: ONE cylinder will NOT be at compression pressures at the time
of the spark, so the spark will jump THAT spark plug quite a bit easier. Higher
the pressure, harder for the spark to jump.
AND....for those that are actually THINKING here...yes...it is TRUE
that there are theoretical advantages to the later dual output,
single coil, that BMW used on all models (and on some earlier one's
too...such as the GS and ST). Yes, that advantage....if the coil
design is correct....is the 'wasted spark' jumping more easily at the
cylinder NOT at compression stroke. Of course, an argument could be
then made about the two coils used on other models, and how they are arranged to
always have the correct output polarity, and could have plenty of energy/volts.
Kind of an exercise in arguing, really.
8. In our BMW electronic ignition models (those after
1980), the points are replaced by some semiconductor parts
located in the ignition module, under the fuel tank. This
module is 'triggered' by the proximity of a specially shaped
rotating magnetic plate (simplified explanation). The
sort of butterfly-shaped plate passes by a semiconductor part
that is magnetically sensitive, and is called a Hall Element. The
electrical signal from these "Hall" devices is very small, and
the small signal is applied to an
electronic current amplifying
circuit in the module under the gas tank. Hall devices
are VERY sensitive to stray magnetic and electric fields and
electrical spikes on wires connected to it. That is ONE MORE
REASON
why the spark plug wires must NOT be disconnected if the engine is
running or not, with the ignition key ON. This
system is very reliable and requires no regular maintenance, with
the exception of checking the timing at the maximum timing point ("Z")
every 10K or
so, and cleaning and re-applying heat conducting grease to the module under the
tank every few years. I also suggest one drop of
oil every year or so on the outrigger bearing. A moderately rare condition occurs where
the automatic advance parts tend to stick, often from a tiny amount of a part
swelling or hardened grease...then the canister must be disassembled for repair;
a tricky job.
The Hall device is damaged from stray electric currents, so the spark plug caps
must always be provided a method of being grounded, through the spark plugs or a
shorting wire. The spark plug caps must be of
the 5000 ohm style; the very early Airhead 1000 ohm caps are NOT
OK! The system will be damaged, and the damage
might not show up immediately, if you have a spark plug cap off,
or the caps are 1000 ohms. Do not use resistor
spark plugs, they add to the cap resistance and lower the spark
intensity. Do not use resistor plugs with no-resistance
caps, you cannot depend on the next person using those plugs,
nor, that the resistance in the plugs is really a resistance, it
can be another device.
9. No matter what the method, point contacts or electronics, some means
must be
available on performance engines to change the timing of the spark
in relation to the piston stroke, as the rpm increases. One needs a
somewhat retarded spark to enable starting; that is, the spark must not occur
too early, compared to when the spark is needed at rpm considerably above idle
rpm. If the spark occurred
too early, the engine might try to rotate backwards, and/or not ignite
the mixture as it was not compressed enough. As rpm increases, there is less and less TIME for the ignited
mixture flame to completely burn, so the spark must occur EARLIER in
the process...again, this is as rpm INcreases. Common usage has EARLIER
meaning ADVANCED, LATER meaning RETARDED.
BMW has used a mechanical advance device, in all its Airheads
models; and, in fact, used them before the Airheads. This
mechanical device is simply a pair of shaped metal weights, with
calibrated springs attached, that move from an at-rest position to farther
and farther outward as rpm's increase, up to their preset stops.
The force is called centripetal (centrifugal does not exist).
The moving weights are mechanically linked to the rotating points
cam (or butterfly trigger metal in the electronics models). By
carefully designing the weights for shape, rpm and springs, the factory is able to give the optimum...or nearly
so...'advance curve'...so that the proper timing occurs at the
rpm found best by dynamometer and road tests. It is by no means
perfect, especially since no device is
added...such as the automotive type of vacuum advance/retard,
which can be set up for compensating for throttle amount, which
is somewhat allied to effective cylinder pressures. On a
practical basis, BMW's system works fine. Early /5
airhead models used about 2200 rpm for the maximum advance
amount, and later models used about
3000, mostly to compensate for
the lousy gasoline's that became more common. Use of the early advance
units (2200 rpm) is often impractical, causing pinging with today's
low octane gasoline's, but can give modified engines on premium high
octane gasoline slightly better throttle response, or, better
stated, slightly more low end acceleration. More intimate
details on the advance unit are posted elsewhere's on this site.
This mechanical device is part of the cam assembly that is free to
move upon, and driven by, the camshaft in the engine that controls
the valves. That engine camshaft is chain driven by the
crankshaft. As the chain stretches and the chain sprockets and automatic chain tensioner
wear, the valve and ignition
timing will change, and the ignition timing relationship to the
crankshaft position needs to be reset. When
the chain and sprockets and guides are worn to the very sloppy chain movement point, the timing will be
affected quite
adversely as you move the throttle more open and as you back off the
throttle;
and, there will also definitely be more instability.
As these parts wear, the timing in relationship to the the camshaft operating the valves, versus the piston, will change. So, you get a decrease in power from that, which you cannot 'adjust'.
10. In the standard system, and in the dual coil dual plug system,
both coils fire at the same time, although one cylinder is not on
the firing stroke (BMW calls this a 'wasted spark').
11. Points have many disadvantages: they have a rubbing block, which
must be kept faintly
lubricated, which is constantly wearing, closing the gap setting of
the
points, and that wear ALSO changes the angle slightly at the cam, thus changing the timing AND charging of the coil; not to mention there is
the slow but sure erosion of the points. Points also have problems
in some instances with the points not following the points cam at
very high rpm's...and also, being mechanical devices, tend to
rattle around and vibrate a bit, these are the reasons BMW went
to a special coupling arrangement in the canister points models,
which was produced only in 1979 and 1980, just before the 1981 change to full electronic
ignition. Points
have one big advantage: They are exceptionally simple, and can
almost always be 'fixed' by the side of the road. They also can be fitted
with a simple booster, or points amplifier, which GREATLY reduces the current
through the points, and hence they can have a very much longer life
than they normally would......and
maintenance is also reduced....but, do keep that cam slightly greased,
the felt too if you
have one (points canisters do not)....and check timing and gap now and then
(5000 mile intervals, such as when valve clearances are checked, seems right).
BMW HAS SHIPPED FAULTILY MADE POINTS, WITH THE RUBBING BLOCKS TOO
LONG, AND THEY CANNOT BE ADJUSTED PROPERLY. WATCH FOR THIS.
Norris points from such as Beemershop, are OK.
Points amplifiers, sometimes
call points boosters, are made by a number of
manufacturer's. They will GREATLY increase points life.
Numerous makers.
Accell and Dyna are
two of the popular makers.
NOTE: http://www.qkits.com/ www.apogeekits.com
and maybe others...
Velleman is probably the
actual maker of a number of kits sold by others, using the model
number K2543, this is a kit.
It it is rated at 4 amperes, but with the heat sink that
comes with it, I think it will handle MORE, if placed in a
relatively cool place
on the motorcycle.
http://www.vellemanusa.com
A problem can occur if you have coils that draw more amperes than
the points boosters/amplifiers are rated for. Many have used them
in this somewhat overloaded condition, if they are kept
reasonably cool. The Velleman seems to hold up.
12. As engine speed increases the coil(s) itself has less and less
time to
charge magnetically. At some extreme rpm, the coil output will
begin to decrease, eventually to the point of not firing the spark
plugs correctly. For our stock 2 cylinder engines, that rpm is NOT
attainable. These same types of coils were used on V-8 engines that
attained over 5000 rpm....which means they fired 8 times times
more than the Airhead. If you question why 8, and not
4 more times, perhaps you need to think a bit about how the
airhead coil fires. I will be happy to explain that, simply, if
you cannot figure it out.
13. There are other types of ignition systems for 4
stroke engines, one popular high performance type is called a
'capacitor discharge ignition'. If carefully designed, the spark
can occur so fast that it will fire somewhat fouled spark plugs.
They are NOT needed, on our BMW airheads. There are other types
of electronics ignition conversions for the BMW Airheads. One
type lowers the current through the points (Dyna amplifier as
example, as mentioned above), slowing the replacement time for
the points...although that effect is offset to a considerable
degree if the points cam is not kept lightly lubricated with
grease. Several types eliminate the points (including versions of
the Dyna and Boyer), some have built-in advance (Boyer, for
example), and there are even crankshaft
triggered types. None of these help much as far as
actual spark plug firing on a stock or dual-plugged engine, and are
only slightly helpful for a moderately modified engine. However,
they can eliminate or reduce points problems....so long as they do
not fail. Some have characteristics that do NOT match the Airhead engine.
14. MORE on
"points amplifiers or points boosters":
NOTE!!....some
models of BMW Airhead motorcycles use especially low
resistance coil primary windings. Be sure your points amplifier will
handle the current. Usually, the points amplifier maker will specify in
OHMS what the MINimum coil(s) resistance must be, that is, the coils must not be
lower than that amount. Many HAVE gotten away with
using higher current coils, with the amplifiers. Usually
this is on the Velleman product.
****NOTE, AGAIN!... that the Ignition
System can be damaged if the spark plug wires are removed from
the spark plug with the engine running...or being cranked or
other wise operated if the caps are not grounded properly. Damage may not show up for some time. The damage can occur to the Hall
devices, the module, or the coil(s), or any combination. The system may be damaged if the spark plug caps are
not the correct 5000 ohm types on the 1981+ models. 1000 ohm caps
CAN be used on 1980 and earlier models, you do not have to use 5000 on the
points models.
Even with the points models, the coils can be internally damaged from
lifting the spark plug caps without grounding the inner contact in them. If
you see otherwise in some literature, the LITERATURE IS WRONG!
Totally Nerdy Section:
This is a rambling, highly technical section, for the
intensely curious!
(1) The spark plug can be thought of as a
capacitor. A capacitor is two plates, separated by an
insulator. The ground leg of the spark plug and the central
electrode can be thought of as a very tiny value capacitor, the
"insulator" being the air and some water vapor and a bit of
exhaust gases that were not completely removed during the
previous exhaust stroke. In the spark plug electrode
GAP, the high voltage coming from the ignition coil creates an
electrostatic field in the "gap insulation". That electrostatic
field rises very rapidly in intensity as the voltage is applied.
When the forces applied to the molecules in the gap reach the
required level, electrons are ripped loose from the outer shells
of the atoms, creating atoms with a net positive charge.
This is called IONIZATION. Once an ionization path is
created from one electrode to another, current will flow.
This current, flowing through the
resistance of the gap, creates the heat that starts the
combustion process.
The sudden flow of current, combined
with that tiny capacitance (and other small capacitances in the
ignition system)....create high-frequency
oscillations that want to radiate from the spark gap.
These oscillations cover a very broad band of frequencies.
It is these oscillations that cause the radio frequency
interference from the ignition system.
If the electrical 'noise' was allowed to radiate back towards the
ignition coil, it could radiate from the motorcycle and cause
problems with such as radios, nearby TV's run off antennas, cause
confusion with your bike's electronic tachometer, bike's engine
control computer, etc. When such radiation gets into
the bike's WIRING, it can cause what is called Standing Waves.
These can, under some circumstances, reach very high voltages.
The coil could be damaged....as well as the electronic module,
Hall device in the canister....and on points models even the
points capacitor/condenser.
The first line of defense is the spark plug
itself. In cars and trucks, a resistor is mounted inside the spark plug.
Since the spark plug has capacitance TO GROUND besides the gap
capacitance mentioned earlier, the noise will find it easier to
go to ground, rather than back through that resistor towards the
ignition coil, via the ignition wire. Keep in mind
what I have said earlier about the other uses of the resistor,
regarding the length of time the spark lasts, and lowering the
current and having the electrodes last longer.
In CARS/TRUCKS, the wire from the plug to the coil is also a
resistor...relatively evenly distributed along the high voltage
wire's length. Typically this wire has a core of a Aramide
fiber (like used in tires) covered in a somewhat conductive
graphite mixture. Typically the wire resistance is about
5,000 ohms per FOOT. This wire is a fairly poor radiator of
spark energy, just what is needed. The wire is
sometimes called Suppressor Wire. Some very modern cars are using even more sophisticated methods
of Radio Frequency Interference (RFI) reduction, due to the very
sensitive nature of the computers, etc., in newer cars.
Some have one coil mounted AT EACH spark plug. Some have a
very special lead from the coil, or distributor, to the spark
plug. There may be a ferrite rod in the middle of the spark
plug wire, and a spring coil, the combination of which 'chokes
out' RFI. The coil turns and design and the ferrite
(concentrates magnetic fields) are critically designed. The
parts are frequency sensitive. Did you
know that Stealth Airplanes have been made, covered in ferrite?
Here's a bit more on these graphite core wires: these act
as a plate of a capacitor, while the engine block acts as the
other plate. yes, true. So, the resistor cable
is not only a poor antenna, but it is a capacitor that shunts off
the high frequency noise to ground, rather than allow it to
travel through the car/truck.
The use of these methods is effective, and
avoids the expense and problems of fully shielding the ignition
components.....which is the approach used in piston engine
airplanes, where the shielding needs to be 100%.
There is usually, in cars, an additional bypass capacitor, of
substantial capacitance, on the battery power lead to the coil.
This shunts to ground any RFI that might get that far.
(2) Your Airhead motorcycle ignition does NOT use resistor wire nor resistor spark plugs.
(3) The output of the Ignition Coil is very much effected not only by the number of turns of wire on both primary and secondary, the type of iron core, and many other small details, but also the CAPACITANCE in the windings, most especially the secondary coil windings. Whilst one can substitute coils that are not the same as the original type's specifications, and the ignition system may seem to work OK, it may be marginal. The ignition system was designed as a total system, and if you make changes, they can have results that are found out about immediately, or not until much later. The Kettering ignition is hardly as simple as it appears.
Revisions:
04/15/2003: MAJOR revision. Combine ignitiontheory.htm and
ignitionsystems.htm, and major editing.
11/28/2006: add paragraph explaining points amplifiers
01/07/2009: revise entire article for clarity
10/03/2009: Again revise for more clarity, and more
technical details this time too.
01/25/2010: add Nerdy section
01/28/2010: expand a bit on CDI
06/13 /2011: Revise for clarity
© Copyright, 2011, R. Fleischer