The original ignition systems for gasoline engines used large coils and magnets mounted to the flywheel. These non-controlled magneto ignition systems were inefficient, difficult to tune, and were quickly replaced by breaker point type ignition systems. A points ignition system uses a mechanical switch to turn a low voltage current supply to a coil on and off. The coil "fires" when current is first supplied to the coil, but also when current is interrupted (broken by the breaker points). The breaking of the low voltage current is used to initiate the combustion process because the opening of the points is more precise over a wide range of engine speeds.
Magnito Versus CDI
The Breaker Point Ignition
The Ballast Resistor
Magneto ignition systems that have no control circuitry have two sever problems that contribute to quit high driving power requirements. The spark from a non-controlled magneto is very long and builds from nothing to full power gradually over a period of perhaps five degrees of crankshaft rotation. This long spark requires many more times as much power than a short duration spark from a coil being crisply turned on and off. The Capacitor Discharge Ignition (CDI) is a new type of magneto ignition that also uses a flying magnet and a coil. The CDI however stores energy from the coil in a capacitor, and then discharges the capacitor into a step-up coil to provide a high power short duration spark. Because the energy from the magnet passing the coil is stored and then used essentially instantaneously the size of the magnet and coil can be much smaller, and much less driving power is required. The other problem with all magneto ignition systems is that the driving power required tends to go up with the square of engine speed. If the magneto is capable of producing enough spark energy to start the engine at 200RPM cranking speed then 100 times more driving power will be required at 2000RPM. It is actually not quite this bad because resistance in the high voltage circuit prevents the current from increasing linearly with the increasing voltage, but substantially more driving power is required as the engine speed is increased. The much lower power requirement of CDI ignitions makes this a much less severe problem, and the control circuitry can prevent more current than is required so that the driving power requirement goes up linearly with increases in engine speed up through the operational engine speed range. A CDI ignition system may require 1W of power at a 100RPM cranking speed, but this might go up to hardly more than 100W at 10,000RPM. A magneto ignition system might require 10W at a 200RPM cranking speed, but the current is going to go up quite a bit as engine speed increases. Up at 2000RPM the current is not going to be ten times higher, but it may be five times higher which would mean a 500W power requirement. If the engine speed is increased to 3000RPM then the magneto ignition will require over 1kW and at 4000RPM this goes up to 2KW. A significant reason why the old magneto ignition gasoline engines operated at very low engine speeds.
The reason that magneto ignition systems were difficult to tune was that they had an advancing effect due to the voltage building earlier at higher engine speeds. This advancing effect was desirable because no other means of providing an advance mechanism was possible, but it was an inconvenient non-linear advance curve that was difficult to make good use of on full flame front travel engines. The sophisticated control circuitry for CDI ignition systems on the other hand can be built or programmed with any shape of advance curve that is desired.
The first step in trying to get magneto ignition systems to work better was to attempt to control the power delivery with a switch. This meant turning the current flow on and off with a mechanical switch on a cam which eventually lead to the breaker point type ignition. Using breaker points on a magneto ignition provides a better means of controlling the spark timing, and also reduces the driving power requirement. The driving power requirement is however only somewhat reduced, and goes up dramatically at higher engine speeds. The next change was to use a battery to store electrical power. With a storage battery and a rectifier the magnets and coils only had to provide enough power to keep the battery charged at low idle speed of about 400 to 600RPM. This then was essentially a modern points ignition system, but the electrical generator still consisted of magnets on the flywheel and coils mounted around the bell housing. Because copper/copper oxide rectifying diodes were so inefficient and unreliable a switch to a belt driven brush type generator was inevitable.
With the points ignition running off of battery power only about 20W is required to power the ignition system, and this actually drops off to less than 10W at higher engines speeds. The reason that the power requirement drops off at higher engine speeds is that the coil does not have time to fully charge between firings, and this is related to the main limitation of points ignition systems. Points ignition on engines with more than two cylinders requires the use of a distributor. On a one or two cylinder engine the points can ride on a cam on the crankshaft, and the points open once each revolution of the crankshaft. This works up to any engine speed attainable. On higher cylinder count engines the distributor is driven by the camshaft at half of crankshaft speed, or on two stroke engines the distributor is driven at crankshaft speed. A four cylinder four stroke opens the points twice for each revolution of the crankshaft, and an eight cylinder four stroke opens the points four times for each revolution of the crankshaft. For a one or two cylinder engine up to 12,000RPM or a four cylinder engine up to 6,000RPM there is no difficulty at all with the coil charging fast enough. On an eight cylinder engine though the adjustment of the points and the shape of the cam that they ride on becomes critical for getting the ignition system to work up to high engine speeds. If the dwell angle is not long enough then the coil will not charge up sufficiently to fire the spark plug. A well adjusted distributor on an eight cylinder four stroke will work fine up to about 6,000 or 7,000RPM, but slight adjustment problems can essentially impose a rev limiter on the engine.
The traditional solution to these high speed problems on high cylinder count engines was the use of a dual points system. A dual points distributor uses one set of points to break the primary circuit and a second set of points to close the primary circuit. The reason that this is such an advantage is that there is then no problem with the points "floating" at higher engine speeds. With the two sets of points the distributor can be setup so that the primary circuit stays open for only a very short period of time, allowing the dwell angle to be close to 90 degrees of crankshaft rotation. Since an easily attainable 50 degrees of dwell angle on a single breaker point distributor works up to 6,000RPM on an eight cylinder four stroke the 80 degrees of dwell angle attainable with dual points distributor would be good up to about 10,000RPM. Single points distributors can also be setup to run with as much as about 70 degrees of dwell for 8,500RPM operation, but the adjustment becomes tricky when only the tips of the sharp cam lobes touch the follower on the points.
These speed limits imposed by points ignition systems on high cylinder count engines can also be extended by using a higher capacity coil. Just using an old 6V coil on a 12V system can give a bit of a boost to maximum engine speed. High capacity aftermarket coils have also been widely available for many decades. Getting more engine speed with bigger coils tends however to be a dead end endeavor because slight increases in engine speed require radically larger coils that draw much more power and are hard on points. With just a two amp maximum current the points never give any trouble. Slightly higher capacity coils that came stock on most engines draw three or four amps maximum, and don't cause much trouble with the points. Points ignition systems have been known to go for hundreds of hours without requiring adjustment. Increasing the coil capacity to where the ignition draws ten amps though means radically reduced service intervals for the points. Never allowing the engine to low idle at less than 3000RPM helps considerably with points life on racing engines using very high capacity aftermarket coils.
The condenser is the most widely misunderstood part of a points ignition system. The condenser is actually a spiral wound capacitor that bridges the breaker points. The importance of the condenser is universally recognized in the long standing habit of replacing the "points and condenser" as a set when ignition work is done on an engine. What is not so universally recognized is what the condenser actually does. The folk lore description was that the condenser "stores electrical energy while the points are open so that the points do not burn". This is total nonsense, and is indicative of a widespread lack of understanding of how coils are wired. With just three terminals on the coil one of those terminals is common to both the primary and secondary windings. The positive terminal for the primary winding is connected to the ignition switch, and the negative terminal for the primary winding is connected to the points. The third terminal is the high voltage spark plug wire terminal. The secondary winding must however also be grounded, so it is connected also to the negative terminal for the primary winding. The negative terminal for the primary winding is however only grounded while the points are closed. When the points open to fire the coil the secondary winding looses it's ground. It is the capacitor across the points (condenser) that provides the ground for the secondary winding to fire. When the points open the capacitor is already fully charged to 12V, so the flow of current through the primary winding stops immediately. As the secondary winding fires though current continues to flow into the capacitor as it charges up towards the 10,000 to 20,000V or so that the secondary winding operates at. For the rapidly firing high voltage secondary winding the capacitor is essentially invisible, and the winding fires as if it has a solid ground.
The ballast resistor is a wire wound ceramic power resistor of about one to two ohms and rated for about 20W of power dissipating capability. The ballast resistor is placed somewhere in the primary wiring, and it's purpose is to allow older 6V coils to be used on 12V systems without excessive current flowing through the points at low idle or when the engine is stopped. Ballast resistors reduce the overall efficiency of the coil, but they are not as much of a hindrance to efficient operation as might be expected. During a short circuit situation when the engine is not running a six volt coil that drew 3A on six volts would draw 6A on 12V, endangering the longevity of the points and possibly overheating the coil if it was left connected long enough. Adding a two ohm resistor drops the short circuit current of the coil to 3A where the points give no trouble. This appears to also make the coil half as efficient since the ballast resistor is then dissipating half of the electrical power. The reality though is that the coil only operates slightly less efficiently because the current drops off so much at higher engine speed. With the coil drawing an average of just one amp at high engine speed the ballast resistor is dissipating only 2W while the coil uses 10W. A six volt coil with a ballast resistor is not quite as efficient as a coil designed specifically for a 12V system, but the difference is so slight that ballast resistors continued to be used on gasoline engines for many decades after 6V systems were a thing of the past.
Gasoline engines can be made to work acceptably well for some applications with a fixed spark timing value, but for most applications performance and efficiency improves dramatically with the use of some type of advance mechanism. The original magneto ignition systems provided a certain measure of speed dependant advance and because the spark energy ramped up gradually there was even a certain measure of load dependant advancing effect. The way that the load dependant advance worked on a magneto ignition system was that the spark plug fired more easily at a lower voltage, and therefore slightly sooner, with less air and fuel in the combustion chamber under lighter engine loads. This was however of little benefit for full flame front travel engines, which generally did not need earlier spark timing under lighter loads.
Once distributors and breaker points where used for ignition systems more possibilities were available for controlling the spark timing. The most important of these devices was the centrifical advance in the distributor. A centrifical advance is a system of springs and weights that rotates the cam for the points as the engine speed increases. The advancing effect provided by a single stage centrifical advance is roughly linear, but some downward curve can be provided for at higher engine speeds with the use of elongated weights. Low compression ratio gasoline engines in the 8:1 to 10:1 range tend to require quite a lot of engine speed dependant advance from low idle speed of 1000 or 1,500RPM up to 2,500RPM where late compression ignition typically first takes place. This could be as much as 20 degrees of crankshaft rotation earlier spark timing at 2,500RPM for a four inch bore 8:1 engine. Then from 2,500 up to 4,000RPM a smaller amount of advance is required, perhaps another 10 degrees of crankshaft rotation for the four inch bore 8:1 engine. From 4,000 to 6,500RPM many engines get away with no additional advance, but another 5 degrees of crankshaft rotation can be very useful for lower compression ratio engines. The reason that less advance is required from 4,000RPM up is that this is where gasoline engines begin to run well. With less of the heat of combustion going into the cooling jacket there is more energy available in the exhaust to drive exhaust gas scavenging, and most good running gasoline engines do get some beneficial exhaust gas scavenging from the exhaust system somewhere between 4,000 and 8,000RPM. Intake runner length and diameter, valve timing and even the higher engine speed itself can all contribute to easier popping off on late compression ignition in as the engine speed is increased within the 4,000 to 8,000RPM range meaning that less additional advance than might be expected is actually required.
The other reason that simple mechanical advances normally topped out at somewhere between 3,000 and 4,000RPM is that they were intended to be used without load dependant advance mechanisms. With no load dependant advance mechanism it is necessary for the spark timing to be early enough for the engine to get into late compression ignition mode under reduced loads at 2,500 to 4,000RPM. Then from 4,000RPM up to the maximum engine speed the spark timing only needs to be early enough so that the engine will run under a full load. All light load operation is provided for at less than 4,500RPM where there is plenty of spark advance for reduced load operation. This general description of greater than 4,000RPM operation only being used under a full load mostly applies to longer stroke engines in the three and a quarter to three and three quarter inch range. Shorter stroke engines, or three and a quarter inch stroke engines with very lightweight pistons and rods, can run under light loads at 4,000 to 6,000RPM provided that the ignition system is up to the task of providing the correct spark timing and the compression ratio is well matched to the fuel being used.
The other type of advance mechanism is the load dependant advance mechanism. On points ignition systems the load dependant advance mechanism was usually in the form of a vacuum advance. What the vacuum advance actually did was to pull in more advance when the throttle plate on the carburetor was cracked open just a small amount. These vacuum advance mechanisms were very often miss applied to production engines, and usually did not work very well. When working well a vacuum advance would provide earlier spark timing so that an engine could get into late compression ignition mode under reduced loads at around 2,500 to 3,500RPM. Getting a vacuum advance to work well though required that it be matched to a particular carburetor and tuned for the compression ratio of the engine and the fuel that was to be run. Most vacuum advances provided too much advance at too low of an engine speed and were simply disconnected to prevent premature engine wear. A sever limitation of the vacuum advances was that even under the best of circumstances they only worked over a narrow range of engine speeds and loads. As the engine speed is increased the throttle plate has to be opened farther even for reduced load operation, and little or no advance is provided.
A better functioning form of load dependant advance that was used on points ignition systems was the boost dependant advance used with forced induction engines. A boost dependant advance provides earlier spark timing when no boost is produced, and as more boost is produced the timing is incrementally made later and later. In this way a turbocharged or blown gasoline engine could run with just enough spark advance to stay in late compression ignition mode over a wide range of engine loads. This of course required that the centrifical advance be well matched to the boost dependant timing mechanism, and this meant increased advance from the centrifical advance all the way up to maximum engine speed. Another reason that boost dependant advance mechanisms on forced induction engines worked so much better than the vacuum advance mechanisms used on normally aspirated engines is that the compression ratios of those normally aspirated engines were far too low. A turbocharged or supercharged engine with a 6:1 compression ratio that runs up to fifteen pounds (15psi) of boost might have an effective compression ratio of more than 11:1, and likewise a 7:1 engine that runs seven pounds of boost might have an effective compression ratio of 10:1. Either of these engines is going to have a much wider range of loads over which late compression ignition will work well than will an 8:1 normally aspirated engine.
Carburetors themselves can provide some compensation for the absence of spark advance mechanisms, but this tends to work best for gasoline engines with no advance mechanisms of any kind. The best thing that a carburetor can do to help control the time of late compression ignition is to run very lean under low throttle openings and then richen up at wide open throttle. This can be accomplished with butterfly valve type carburetors, but is much more effective on slide type carburetors with needle valves. A needle valve carburetor can be setup to be lean at low throttle openings, but then become progressively richer as the throttle valve is opened. What this is useful for is allowing the operator to deliver however much fuel is required to keep the engine in late compression ignition mode as the engine speed is increased. At low engine speeds where there is plenty of spark advance the engine is always run with rather small throttle openings for a very lean mixture. This lean mixture of course is what is required to get a gasoline engine to run as efficiently as it can under light loads. As the engine speed is increased the throttle valve can be opened more to richen the mixture and keep the engine in late compression ignition mode. Just as with a centrifical advance this mixture control advance makes for an engine that runs best under light loads at 2,500 to 3,500RPM, then runs best under medium loads from 3,500 to 5,000RPM and will only run under a full load from 5,000RPM up to maximum engine speed. The engine can be run under heavier loads at 3,500 to 5,000RPM, but it will be louder harsher and less efficient than an engine with a competent ignition system.
Butterfly valve carburetors have sometimes been setup to take the place of load dependant advance mechanisms, but this is a bad idea and never works very well. This type of butterfly valve carburetor would be rich at very small throttle openings and then get leaner as the throttle valve is opened towards wide open throttle, and would have to be used with a centrifical advance mechanism. What this backwards carburetor setup can accomplish is keeping the engine in late compression ignition mode over a range of engine loads at some engine speed. Under a lighter load the mixture is rich so that late compression ignition will take place even though the engine is running with substantial vacuum. As the throttle plate is opened further the vacuum drops off and a much larger charge of air is introduced into the cylinders. This larger charge of air would normally cause the time of late compression ignition to become earlier, but because the mixture has leaned out the time of late compression ignition can stay as late as possible. This does work for getting the engine to run smoothly over a range of engine loads, but it is horrible for efficiency and performance. With the engine running rich under light loads fuel consumption and exhaust emissions are higher than for any other type of gasoline engine. Then under a full load the mixture is so lean that the engine cannot make nearly as much power as would be expected for the displacement and engine speed.
The first electronics upgrade for ignition systems was simply replacing the points with a contactless sending unit. These sending units originally went in the same distributors with the same centrifical and vacuum advance mechanisms, and were called hall effect sensors when no spinning magnets were use in the sensor. The purpose of replacing the points with a hall effect sensor was to remove the maintenance problems of the points, but more precise timing and crisper switching of the primary current also improved performance. Aftermarket transistorized ignition systems also became available that used the original points as the sending unit, but had most of the same advantages of the early stock type electronic ignition systems. The next step of course was to replace the centrifical advance mechanism with a programmed advance curve in the ignition control module. A programmed advance curve not only could somewhat better match the requirements of the engine, but also did not wear out, loosen up or stick as centrifical advance mechanisms sometimes did. Again the major advantage of the electronic advance curve was simplicity and reliability, although some performance improvements were also realized.
For many years the vacuum advance mechanisms were absent from engines with electronic ignition systems, which was a mixed bag. Reliability and functionality improved, but the total absence of any type of load dependant advance mechanism meant that there was no way to tune an engine to run better or use less fuel. Eventually more sophisticated electronic controls crept into existence, first in the form of electronically controlled carburetors with power valves and idle kickers and later as fully electronically controlled engine management systems. The huge leap forward in performance and efficiency realized with electronically controlled engine management systems was mostly the result of good speed and load dependant spark timing being included. Certainly electronic fuel injection itself did improve both efficiency and performance by allowing both precise fuel metering under all conditions as well as radically improved top end power output through the use of larger throttle bodies and port injection which got the fuel through the intake valves without interfering as much with intake air flow. The really big improvements in engine efficiency over a wide range of speeds and loads was however due to the spark timing being much better matched to the requirements of the engine. At the same time compression ratios were increased from 8:1 up to 9:1 or even 9.5:1, and this also helped immensely with getting gasoline engines to run better over a wider range of loads.
A 9.5:1 carbureted engine with both a centrifical advance and a vacuum advance really can do just as well as EFI port injected gasoline engines if both the carburetor and the ignition system are well matched to the application. This however requires both a well designed engine system and an operator that well understands the limitations of mechanical control and is able to make the required adjustments both in operating procedures and in engine tuning to get the most out of the engine. The electronically controlled engine management systems brought this level of perfection to casual users of gasoline engines without the necessity of periodic adjustments or compensations. The level of sophistication to get electronic engine management systems to work well was substantial though. The key ingredients are a means of measuring or predicting the precise amount of air ingested by the engine at any particular time as well as an oxygen sensor to check to make sure that the measurements and predictions are actually working out to the desired performance result. What this required was a coolant temperature sensor, an intake air temperature sensor and some type of sensor for measuring the amount of air flowing into the engine. The hot wire mass air flow sensors (MAF) tended to work best, but manifold absolute pressure (MAP) sensors were also made to work perfectly as long as nothing about how the engine ran was changed. Once information about the engine speed, load and temperature were available an ideal fuel delivery and spark advance combination was calculated from tables, or maps, stored in the electronic control unit. This is all that is really required to get an electronic engine management system for a gasoline engine to work perfectly, but the addition of the oxygen sensor allowed the engine management system to compensate for variations in the fuel as well as variations in engine performance due to wear and manufacturing defects.
The oxygen sensor was widely lauded as being used for perfectly tuning the air/fuel mixture to attain the highest possible efficiency under very light loads. Although this was one function of the oxygen sensors the really important thing that was able to be done with the oxygen sensors was to make a measurement of whether the engine was running in full flame front travel mode or in late compression ignition mode. Not all EFI systems made good use of this important capability of the oxygen sensor, but the best functioning EFI systems were able to adjust spark timing to keep the engine in late compression ignition mode even when the combustion properties of the fuel changed.