How Spark Ignitions Systems work.....more than you wanted to know
©
ignitiontheory.htm-28
This is the article that ALSO contains, revised, the old, deleted,
ignitionsystems.htm
The following are SIMPLIFIED explanations in some areas. Also note
that not only is the theory of ignition covered, but several types of systems,
and special information for your BMW airhead is included later in this lengthy article.
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. Most commonly, the sparks
are produced at the combustion chamber end of a 'spark plug'.
In the stock Airhead 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 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 wooden boxes with a vibrating electrical
contact which sent storage battery energy into a coil of wire, said wire wound on an iron core.
Since the vibrating contact opened and closed the circuit rapidly, the coil
INPUT winding was nearly continuously interrupted. 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 spark was applied
continuously. The biggest problem with the vibrating contact system is
that the spark output is constantly flowing, and it is hard to 'time' 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.
Magneto's are simply a permanent magnet method of using mechanical
rotational energy, instead of a battery, to produce the high
voltage. 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, the opposite of coil
and battery ignition. Since the exact time the spark occurs can be
controlled by positioning the points, timing on magnetos can be just as precise
as points-coil-battery type ignition.
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. Lean burning
engines
demand high energy sparks. Magneto's have some other problems, that
is why they are not now used. 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 the
type used 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 on) used coil ignition with the contact assembly
being driven off the camshaft, like it is normally on most other
vehicles that use that system. The 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. More later.
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 speed. For our Airheads, there are two lobes on that points cam.
For every two turns of the crankshaft, one gets two sparks.
The 1981 and later BMW 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.
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,
their is another winding, the separate high voltage winding consisting of thousands of turns of
a rather thin wire. This wire is thin because otherwise the wire 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 the TOWER) on the 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 are closed, current starts flowing at a low level,
and the current increases smoothly, and if the battery is connected long enough,
the current reaches a value that is 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, 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 a rpm), then the charging time is INsufficient to fully
charge the coil magnetically, and the output for the spark plugs will be lower
and lower, until at some rpm the voltage is insufficient for proper ignition of
the mixture. In the stock Airhead, the rpm reached is insufficient to require
fancy sophisticated ignitions systems....that is, the points type
ignition for pre-1981; and
simple electronics type for 1981 and later, is quite sufficient.
Note that earlier in this article I stated that the MAGNETOES
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 spark for easier starting, and also the hot
spark enables higher engine tuning for increased power.
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 note...during this 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).
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's. 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 (that is, hot
points lead and chassis).
***DO NOT try that with the full electronic ignition with Hall
device (1981+).
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. It is at this point that I will introduce a tad of technical
information for
those who might be interested. Remember that capacitor/condenser? During
the relatively long time the points were closed, 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
capacitor
now suddenly receives the residual voltage left by the magnetic field in the
coil. That is a very crude and inaccurate way of
putting it, but good enough at this point. At this instant, the coil 'tries' to back-charge the capacitor
with its energy. Since it takes TIME for the capacitor to 'charge
up', the coil really "sees" 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'. Another way of saying this is that
the coil 'sees' a reverse current and essentially dumps its magnetic field,
'transforming' the field into high voltage in the winding with the thousands of
turns. The dumping process basically reverses the field/current flow in the
coil, allowing a transforming effect from primary to secondary...as well as a
reversal of secondary induced current due to the secondary current also being
reversed. This is a complex process, and actually the current reverses numerous times as
it dies down, and is shortened if 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.
Here is another way of looking at it condensers (capacitors) in the ignition
circuit: These not only reduce arcing at the points, but also augment 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, but also
for 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.
One other thing: The 12 volts of the battery across the coil is,
upon the magnetic field collapsing, "transformed" by the RATIO of the number of turns between the two
windings....and we get a big voltage to fire the spark
plug. There is more to this transformation, but I will leave it at
this point.
6. 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!
7. 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.
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 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 the UPPER plugs,
and the other similar coil, connected to the BOTTOM plugs.
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 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 (this is
very simplified explanation), that passes as it rotates, 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 the
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
why the spark plug wires must NOT be disconnected if the engine is
running....or, if the ignition key is on, engine rotating. 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. A moderately rare condition occurs when
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. Besides that, the spark plug caps must be of
the 5000 ohm style; the very early Airhead 1000 ohm caps are NOT
OK!
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 the at-rest position to farther
and farther outward as rpm's increase, up to their preset stops.
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 and rpm and springs
also for rpm, 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, or even nearly so, especially since no device is
added...such as the automotive type of vacuum advance/retard,
which can be set up for compensating somewhat for throttle
amount, which is somewhat allied to effective cylinder pressures.
On a practical basis, BMW's system works fine.
Early airhead models used about 2200 rpm for the maximum advance amount, but 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. You set the ignition timing for the spark to occur at some crankshaft point. But, the valves could LAG their specified operation. So, if the spark occurs too early, particularly with low octane fuels, the pressures in the cylinder may rise radically fast and dangerously, doing damage. This particular effect is often called 'pinging or pinking' from the mechanical noise it makes. As the chain and sprockets and guide(s) wear, there is NO adjustment for the valve timing change (with regards to the crankshaft). As the camshaft operated valve timing lags more and more, engine performance will reduce.
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 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 in only 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 simply 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.
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 many more times per
second than the Airhead requires.
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":
****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 in the points models, the coils can be internally damaged from
lifting the spark plug caps without grounding the inner contact in them.
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.