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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, 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'.

The higher the pressure in the combustion chamber at the time of ignition, the higher the voltage needed to jump the spark plug 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.

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. 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. 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.

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 time it is designed for....104° actually!), varies with year and model.

    Making The Spark, & WHEN Does It Occur?

The coil consists of an iron core (actually multiple thin iron pieces) that has TWO separate windings. One winding, called the PRIMARY, has a few dozen or so turns of a relatively thick wire. The two two push-on (spade) terminals on your coil are connected to the ends of this winding. Well insulated from this winding, the separate high voltage winding consists of thousands of turns of a rather thin wire. This wire is thin because otherwise the wire would not fit into the case. In some ignition coils, the two ends of this winding each go to a high voltage terminal on the coil. In some coils, there is only one high voltage terminal, 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, some coils are like that, or have both connections, but then the case must be securely grounded).

Assuming the system has mechanical points and a capacitor, here is how the system operates, simplified, for a one coil system:

1. Battery voltage through the ignition switch is applied to on primary winding terminal on the coil. The other primary winding terminal on the coil connects to the NON-grounded points contact. On your airhead, these push-on terminals are usually marked 1 and 15, and maybe + and -. Connection to the battery is made such that the single coil high voltage output is negative, which adds to the electrons given off by a hot spark plug center electrode.

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 rotation. In actuality, for our airheads, we mean the ratio for EACH of the two lobes on the points cam. During the points closed period of time, the current from the battery flows through the coil, and back to the battery via the points.

4. 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, the points circuit is broken, and the coil cannot accept any more current from the battery. 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 this with the full electronic ignitions with Hall device.

As the engine is slowly rotated by hand, the static ("S") timing point (supposedly the same as very low rpm, such as zero to idle rpm), 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 (it will go 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 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 longish 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, that mentioned "S" timing point, the capacitor now suddenly receives...well, you can think of it this way I guess... the residual voltage left by the magnetic field in the coil. 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 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 several times as it dies down, as 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.

While there are, as mentioned, some effects in the secondary winding due to the magnetic field itself, the primary effect is that the approximately 12 volts of the battery across the coil is now
transformed by the RATIO of the number of turns between the two
windings....and we get a big voltage to fire the spark plug.

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 TWO functions!

7. Restating an earlier theme: In a system as described, the high voltage output of the coil has a rather complex type of waveform, but it is 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 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.

SPECIAL NOTE: For those of you with dual plug ignition conversions, there are TWO 6 volt dual output coils with the primaries connected in series. The high voltage secondary outputs have a positive and negative output at any one particular coil pair of terminals. 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 also 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 going to produce power, you want the other end of that coil's output to be a spark plug that has LOW cylinder pressure, so ITS spark gap is EASILY jumped. 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. We don't need to discuss red hot electrode tips here.

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.

8. In our BMW electronic ignition models (basically after 1980), the points are replaced by some semiconductor parts that act similarly to points, but are 'fired' by the proximity of a specially shaped rotating magnetic plate (this is very simplified explanation). The electrical signal from these "Hall" devices is very small, and the signal is applied to the electronic current amplifying circuit in the module under the gas tank. These triggering devices are VERY sensitive to stray magnetic and electric fields. 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 reapplying 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...then the canister must be disassembled for repair.

9. No matter what the method, 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. 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. Another reason is that 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 as rpm INcreases.   Common usage has EARLIER meaning ADVANCED, LATER meaning RETARDED.

BMW has used a mechanical advance device, in all its models. This mechanical device is simply a pair of shaped metal weights, with 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, this works out OK.   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 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 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 to a much lesser extent the chain sprockets and automatic chain tensioner wear....even with the automatic chain tensioner in the engine...the valve and ignition timing will change, and the ignition timing needs to be reset. When the chain is worn to the very sloppy point, the timing will be affected quite adversely as you move the throttle more open and as you back off the throttle; there will also definitely be more instability. Additionally, 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.

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 electronics. Points have one big advantage: They are exceptionally simple, and can almost always be 'fixed' by the side of the road.

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), 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 units (Boyer, for example), and there is even a crankshaft triggered type from Germany. 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.

Prompted by an inquiry on the Airheads LIST, I offer this, about "points amplifiers":

****NOTE, AGAIN!... that the system can be damaged if the spark plug wires are removed from the spark plug with the engine running...or being cranked... if the caps are not grounded. This damage may not show up for some time. The damage can occur to the Hall devices, the module, or the coil(s). The system may be damaged if the spark plug caps are not the correct 5000 ohm types.

****Additionally, ANY coil can be damaged, in any ignition system, if the spark plug wires are disconnected, see above. REPEAT: Even in the points models, the coils can be internally damaged from lifting the spark plug caps without grounding the inner contact in them.

Eliminating the points may be a good thing if you hate servicing the points, and a bad thing if the electronic conversion fails on the road in the middle of the night. Other types of ignition system, including one coil at each spark plug, etc., used on some cars, are not pertinent to this discussion.

Revisions:
04/15/2003: MAJOR revision. Combine ignitiontheory.htm and ignitionsystems.htm, and major editing.
11/28/2006: add paragraph explaining points amplifiers