Technical Advices > Ignition Systems
8.1 Ignition Systems
The ignition system sends an extremely high voltage to the spark plug in each cylinder when the piston is at the top of its compression stroke. The tip of each spark plug contains a gap that the voltage must jump across in order to reach ground. That is where the spark occurs.
The voltage that is available to the spark plug is somewhere between 20,000 volts and 50,000 volts or better. The job of the ignition system is to produce that high voltage from a 12 volt source and get it to each cylinder in a specific order, at exactly the right time.
Let's see how this is done.
The ignition system has two tasks to perform. First, it must create a voltage high enough (20,000+) to arc across the gap of a spark plug, thus creating a spark strong enough to ignite the air/fuel mixture for combustion. Second, it must control the timing of that the spark so it occurs at the exact right time and send it to the correct cylinder.
The ignition system is divided into two sections, the primary circuit and the secondary circuit. The low voltage primary circuit operates at battery voltage (12 to 14.5 volts) and is responsible for generating the signal to fire the spark plug at the exact right time and sending that signal to the ignition coil. The ignition coil is the component that converts the 12 volt signal into the high 20,000+ volt charge. Once the voltage is stepped up, it goes to the secondary circuit which then directs the charge to the correct spark plug at the right time. Here is a diagram of such a system.
Without exception, almost all ignition systems were “mechanical” systems, up to the early 70s. Engines, to operate efficiently, require the spark to fire at some point BEFORE the piston reaches TDC.
This is to allow the explosion to build enough pressure (push) on the top of the piston, at just the right time, to provide optimum power. If it is started too soon (advanced) then this explosion reaches piston while it's still traveling upward and you lose power, (trying to push the piston the wrong way) waste energy, and create heat in the combustion chamber area (and usually knocking or detonation from an explosion instead of a nice smooth flame traveling from the upper cylinder to the piston top). If started too late (retarded) then you loose power because the piston is already traveling downward, before the flame explosion can "push" it. This also creates heat in the surrounding combustion chamber because remember, heat is energy. This energy, if not used to push the piston, is released either into the surrounding water jacket or the exhaust manifold instead of powering your vehicle. Both are inefficient as far as maximum power is concerned, but it makes an effective heater! As the engine RPM's increase, given that the flame propagation speed remains the SAME, then the combustion cycle needs to be started earlier to achieve the desired "push" on the top of the piston. Also, as the pressure (more fuel/air) inside the cylinder increases, then the less advance the engine can handle at a lower RPM (bigger explosion). So as you can see it depends upon the speed (RPM) of the engine, AND the amount of air/fuel mixture (throttle position) that the engine is operating at. OK, elementary internal combustion education is out of the way.
These systems all used an “advance” mechanism to alter the timing of the spark pulse to the cylinder. A good starting point for setting the initial timing (static timing) is 10 BTDC. You might call this “idle advance” at 1200 RPM or below. As the engine RPM increase, the amount of time available for ignition decreases, so the initiation of spark must occur earlier. There is a limit to the amount of advance, but for standard engines this would be around 28 degrees.
So how do we get from the 10 degrees at idle to the 28 degrees at higher RPM?
Above is a good illustration of the standard mechanical advance mechanism. The point plate in the distributor has two advance weights on separate pivot pins. The advance weights are connected to the advance cam via small coiled springs. These springs provided tension to keep the cams against the cam. As the distributor spins, the centrifugal forces generated cause the weights to want to move away from the centerline of the distributor, however the springs provide a resistance to this centrifugal effect. As the weights swing outward the points plate is rotated so that the “points opening event” occurs earlier. By varying the cam design, and the tension of the advance springs, it is possible to change the advance characteristics of the distributor. In most competition engines, using a mechanical advance mechanism, the distributor is generally fully advanced by 3000 RPM.
Now say your cruising at 2000 RPM little load, again low cylinder pressure, optimum advance (30 deg) engines happy. Suddenly you snap open the throttle. Now you have maximum cylinder pressure, low engine speed and advance needs to be at say 12 deg to prevent detonation. If the advance were purely mechanical again, and set for optimum advance (30) at the no/low load condition, then we would have too much advance for this high load condition, and one unhappy engine because of detonation. However, during high load conditions, the intake manifold pressure drops to zero (equals outside manifold pressure or no vacuum). IF the mechanical timing were now optimized for high load, low speed conditions (12 deg@2000 RPM), then the vacuum unit can optimize the timing at light or no load conditions (30 deg) because it is in effect not operating at high load conditions, and the mechanical advance can be optimized for high cylinder pressure or maximum load conditions.
So in this case, when you stomp on the pedal, the timing (at 30 deg light load, relatively high vacuum) would drop back to 12 deg, because the vacuum is now not operating, as stated before, the manifold pressure increased (vacuum dropped to zero) and the diaphragm returned to it's no vacuum position. In this way, timing can be optimized for all engine conditions. For racing, and max power applications, you don't really need a system for controlling advance at low or no load conditions because these engine are operating at maximum power most if not all the time. (and is one reason why some tend to overheat at idle) Also, another reason that early emission systems with idle retard, or advance cutouts have a provision that during extended idle periods, when the engine begins to overheat, it restores PROPER advance to prevent that overheating! Note: High performance engines generally do not use vacuum advance mechanisms, as the vacuum generated is relatively low due to the longer camshaft duration and overlap.
This would be a good time to speak about ignition coils. To put it simply a coil is nothing more than a “step-up” transformer. It has a primary side (12V DC from the car’s power supply) and secondary side. Each side of the transformer has a number of windings, and the number will be sufficient to step up the 12V to between 20,000-50,000 Volts. It is this secondary voltage that causes the spark in the cylinder when it jumps from the center electrode of the spark plug to ground.
8.2 Points Type
Ignition points are a set of electrical contacts that switch the coil on and off at the proper time. The points are opened and closed by the mechanical action of the distributor shaft lobes pushing on them. The points have a tough job, switching up to eight amps of current many times per second at highway speed. Indeed, as engine speed increases the efficiency of your ignition system decreases, thanks to heating problems and fundamental electrical laws. This declining efficiency has a serious effect on your spark voltage and results in poor high-speed performance, incomplete combustion and other drivability problems.
Condenser: Those same principles of inductance create a kind of paradox, because when the points open and the magnetic field collapses it also induces a current in the primary as well. It's not very much because there are only a few windings in the primary, but it's enough to jump a small air-gap, such as the one between the just-opening points in the distributor. That tiny spark is enough to erode metal away from the points and you'll 'burn' the points. It prevents the points from arcing and prevents coil insulation breakdown by limiting the rate of voltage rise at the points.
Ballast Resistor: This is an electrical resistor that is switched in and out of the supply voltage to the ignition coil. The ballast resistor lowers voltage after the engine is started to reduce wear on ignition components. It also makes the engine much easier to start by effectively doubling the voltage provided to the ignition coil when the engine is being cranked. Not all car manufacturers used a ballast
A points type of distributor uses a set of spring loaded points to “initiate” the spark event. When the points close, current through the coil primary increases from zero to maximum in an exponential manner, rapidly at first, then slowing as the current reaches it's maximum value. At low engine speeds, the points are closed long enough to allow the current to reach a higher current level. At higher speeds, the points open before the current has time to reach this maximum level. In fact, at very high speeds, the current may not reach a level high enough to provide sufficient spark, and the engine will begin to miss. This current through the coil builds a magnetic field around the coil. When the points open, the current through the coil is disrupted, and the field collapses. The collapsing field tries to maintain the current through the coil. Without the Condenser, the voltage will rise to a very high value at the points, and arcing will occur. Another possible problem with points type distributors, particularly at higher RPMs, is that you may get “point bounce”. This would be and indication that the points spring is not strong enough to run at higher RPMs.
Setting the Dwell Angle - The Dwell-Angle is the number in degrees of rotation of the distributor-shaft, whereby the breaker-points are closed. (The same for an electronic ignition module, discussed immediately below), only here the breaker-points are replaced with a control-module and the lobes on the distributor-cam are replaced by a reluctor. The reluctor induces pulses which are past on by the ignition-signal sensor to the ignition-module. The ignition-module "tells" the power-transistor to turn the current through the primary-coil on or off. The time in which the power-transistor turns the current "on", is also expressed in degrees of rotation of the distributor-shaft.)
For a mechanical system, the Dwell-angle is during operation a fixed number, about 50 degree for a 4-cylinder engine. This means that the current flows through the primary-coil for 50 degrees of distributor-shaft rotation, regardless the RPM. For example when running idle: 500 rpm, the crankshaft makes 1 revolution in 120ms. The distributor-shaft, at half speed, 240ms. It takes 4ms to charge the ignition-coil till saturation. The required Dwell-angle is ( 360° / 240ms ) x 4ms = 6°. In reality the ignition-coil is charged for 50° duration, 44° more than required. The 50° Dwell-angle is required if the crankshaft makes 1 revolution in (360°x 4ms) / (50x2) = 14.4 ms. This is 4,166 rpm, nearly full throttle!!! So above the 4,166 rpm the ignition coil is charged below saturation, and the spark intensity will therefore be less. Certainly at 9000 RPM we may be pushing the limit of the ability of the coil to reach sufficient saturation to support combustion for extended periods of time.
Electronic Ignition systems are not as complicated as they may first appear. In fact, they differ only slightly from conventional point ignition systems. Like conventional ignition systems, electronic systems have two circuits: a primary circuit and a secondary circuit. The entire secondary circuit is the same as in a conventional ignition system. In addition, the section of the primary circuit from the battery to the battery terminal at the coil is the same as in a conventional ignition system.
The primary circuit of the electronic ignition systems operate on full battery voltage which helps to develop a stronger spark. Whereas, Breaker point systems needed a resistor to reduce the operating voltage of the primary circuit in order to prolong the life of the points.
Electronic ignition systems differ from conventional ignition systems in the distributor component area. Instead of a distributor cam, breaker plate, points, and condenser, an electronic ignition system has an armature (called by various names such as a trigger wheel, reluctor, etc.), a pickup coil (stator, sensor, etc.), and an electronic control module.
Essentially, all electronic ignition systems operate in the following manner: With the ignition switch turned on, primary (battery) current flows from the battery through the ignition switch to the coil primary windings. Primary current is turned on and off by the action of the armature as it revolves past the pickup coil or sensor. As each tooth of the armature nears the pickup coil, it creates a voltage that signals the electronic module to turn off the coil primary current. A timing circuit in the module will turn the current on again after the coil field has collapsed.
So in the case of the Magneti Marelli “Marelliplex” system. The distributor still has a mechanical advance system, on which is mounted the magnetic pickup. Because the signal from this pickup is extremely low voltage, it has to be amplified, so that it is sufficiently strong enough to trigger the coil. In addition the amplifier module, located on the large aluminum heat sink (to which the coil is mounted) has the timing circuitry to turn off the coil primary current. The module used by Magneti Marelli is actually a standard unit made by Delco, and the coil is an ordinary coil without a voltage drop resistor.
It would therefore be perfectly feasible to build your own electronic ignition system from readily available parts. Here is a short shopping list.
|A) Standard Fiat 850/903/A112 distributor
|B) Pertronics (or equivalent) sensor module and reluctor
|3) Delco or Chrysler Ignition amplifier
|4) MSD Coil (without ballasts resistor)
8.4 Capacitive Discharge & Transistorized Ignition
An advantage of the capacitive discharge ignition system is that the energy storage and the voltage ‘step up' functions are accomplished by separate circuit elements allowing each one to be optimized for its job.
Capacitive discharge ignition systems work by storing energy in an external capacitor, which is then discharged into the ignition coil primary winding when required. This rate of discharge is much higher than that found in inductive systems, and causes a corresponding increase in the rate of voltage rise in the secondary coil winding. This faster voltage rise in the secondary winding creates a spark that can allow combustion in an engine that has excess oil or an over rich fuel air mixture in the combustion chamber. The high initial spark voltage avoids leakage across the spark plug insulator and electrodes caused by fouling, but leaves much less energy available for a sufficiently long spark duration; this may not be sufficient for complete combustion in a “lean burn” turbocharged engine resulting in misfiring and high exhaust emissions.
The high voltage power supply required for a capacitive discharge system can be a disadvantage, as this supply provides the power for all ignition firings and is liable to failure.
Ignition in lean fuel mixtures by capacitive discharge systems can sometimes only be accomplished by the use of multi-spark ignition, where the ignition system duplicates the prolonged spark of inductive spark systems by sparking a number of times during the cycle. The MSD unit is a good example of this. At engine RPMs below 3000 the MSD provided multiple spark events. Above this RPM it reverts to a single spark event.
Below is a link to a very good discussion of the aspects of electronic CDI ignition.
8.5 Crankshaft Triggered or Distributorless Ignition Systems (DIS)
The third type of ignition system is the distributorless ignition. The spark plugs are fired directly from the coils. The spark timing is controlled by an Ignition Control Unit (ICU) and the Engine Control Unit (ECU). The distributorless ignition system may have one coil per cylinder, or one coil for each pair of cylinders.
Some popular systems use one ignition coil per two cylinders. This type of system is often known as the waste spark distribution method. In this system, each cylinder is paired with the cylinder opposite it in the firing order (usually 1-4, 2-3 on 4-cylinder engines). The ends of each coil secondary leads are attached to spark plugs for the paired opposites. These two plugs are on companion cylinders, cylinders that are at Top Dead Center (TDC) at the same time. But, they are paired opposites, because they are always at opposing ends of the 4 stroke engine cycle. When one is at TDC of the compression stroke, the other is at TDC of the exhaust stroke. The one that is on compression is said to be the event cylinder and one on the exhaust stroke, the waste cylinder. When the coil discharges, both plugs fire at the same time to complete the series circuit.
Since the polarity of the primary and the secondary windings are fixed, one plug always fires in a forward direction and the other in reverse. This is different than a conventional system firing all plugs the same direction each time. Because of the demand for additional energy; the coil design, saturation time and primary current flow are also different. This redesign of the system allows higher energy to be available from the distributorless coils, greater than 40 kilovolts at all rpm ranges.
The Direct Ignition System (DIS) uses either a magnetic crankshaft sensor, camshaft position sensor, or both, to determine crankshaft position and engine speed. This signal is sent to the ignition control module or engine control module which then energizes the appropriate coil.
The advantages of no distributor, in theory, is:
- No timing adjustments
- No distributor cap and rotor
- No moving parts to wear out
- No distributor to accumulate moisture and cause starting problems
- No distributor to drive thus providing less engine drag
The major components of a distributorless ignition are:
- ECU or Engine Control
- Unit ICU or Ignition Control
- Unit Magnetic Triggering Device such as the Crankshaft Position Sensor and the Camshaft Position Sensor
- Coil Packs
8.6 Ignition Timing and Combustion
Under ideal conditions the common internal combustion engine burns the fuel/air mixture in the cylinder in an orderly and controlled fashion. The combustion is started by the spark plug some 5 to 40 crankshaft degrees prior to top dead center (TDC), depending on engine speed and load. This ignition advance allows time for the combustion process to develop peak pressure at the ideal time for maximum recovery of work from the expanding gases.
The spark across the spark plug's electrodes forms a small kernel of flame approximately the size of the spark plug gap. As it grows in size its heat output increases allowing it to grow at an accelerating rate, expanding rapidly through the combustion chamber. This growth is due to the travel of the flame front through the combustible fuel air mix itself and due to turbulence rapidly stretching the burning zone into a complex of fingers of burning gas that have a much greater surface area than a simple spherical ball of flame would have. In normal combustion, this flame front moves throughout the fuel/air mixture at a rate characteristic for the fuel/air mixture. Pressure rises smoothly to a peak, as nearly all the available fuel is consumed, then pressure falls as the piston descends. Maximum cylinder pressure is achieved a few crankshaft degrees after the piston passes TDC, so that the increasing pressure can give the piston a hard push when its speed and mechanical advantage on the crank shaft gives the best recovery of force from the expanding gases.
Detonation: A violent explosion; also called combustion knock. This usually occurs near the end of the combustion process when highly compressed, high-temperature end gases spontaneously ignite, radically increasing the cylinder pressure. This pressure spike moves at the speed of sound in the combustion chamber, and the pressure can cause damage to pistons, cylinder walls, and the head gasket.
Pre-ignition: The onset of combustion before the spark plug fires. This is generally caused by some type of glowing ignition source such as a hot exhaust valve, too-hot spark plug, or carbon residue. Pre-ignition is especially damaging to engine components like pistons and head gaskets, since excessive cylinder pressures can occur even before the piston reaches top dead center (TDC).
These are the classic definitions of detonation and pre-ignition. Perhaps a more fun definition of detonation would be to imagine the piston screaming up to TDC while you whack that piston as hard as you can with a 10-pound sledgehammer. The clang that you would hear is the same noise that occurs when your engine goes into detonation. Even if detonation doesn’t break any parts, as soon as an engine experiences detonation, the power drops way off. If you ever have a situation where at a certain point, when you give it more gas, the car physically slows, then STOP.
If you get the idea that detonation and pre-ignition are bad, that’s good. Of all the things that can kill an engine, detonation should be right at the top of your Public Enemy Number One list. The quickest and easiest way to cure detonation is to use a high-quality, higher-octane gasoline
Perhaps the easiest and least- expensive way to reduce an engine’s sensitivity to detonation is to cool the engine-inlet air. Not only is cooler air more dense, which makes more power, but cooler air is also less prone to detonate. The classic performance rule-of-thumb is that for every 10 degrees you reduce the inlet air temperature, the engine makes 1 percent more power. This is why drag racers use ice to cool the intake manifold and why all those cold-air inlet systems work on late-model cars. Forcing your engine to breathe hot underhood air will also make it more prone to detonate, so make sure your carburetors have ready access to air that is at least at ambient temperature. In addition, keep your fuel as cool as possible as well.
Ignition timing is another cheap and easy area to work on. If your engine detonates at low engine speeds at part throttle, consider retarding the initial timing by 2 or 3 degrees and then adding that amount back into the total by increasing the mechanical-advance curve. For example, let’s say you have 10 degrees initial timing with a total of 30 degrees and your engine rattles a little at part throttle, especially right off idle. You could cut the initial back to 8 degrees and add 2 degrees to the mechanical advance. The total remains at 30, but now the engine doesn’t death rattle every time you let the clutch out..
Camshaft timing also plays a huge role in dynamic cylinder pressure, especially with street-driven performance engines. As you increase intake duration, this means the intake valve now closes later than it does with a shorter-duration cam. This later-closing intake valve bleeds some cylinder pressure back into the intake manifold at lower engine speeds. The longer the duration of the cam, the later the intake closes. This reduces cylinder pressure at lower engine speeds, which reduces the tendency for the engine to detonate.
Late closing of the intake can also be accomplished by retarding the camshaft’s installed point. For example, many small-block Chevy cams are installed with the intake centerline at 106 degrees after top dead center (ATDC). This tends to close the intake valve sooner, which improves low-speed torque by increasing cylinder pressure at low speeds. But if the engine rattles at low speeds, retarding the closing point of the intake valve can by 3 or 4 degrees (from 106 to 110 degrees ATDC) softens the engine’s need for higher-octane fuel.
Obviously, this is a little more difficult to do than playing with ignition timing but may pay off by allowing you to run a lower-octane fuel. If you do retard the cam, it’s important to go back and perhaps add a degree or two of initial ignition timing.
You can also experiment with camshaft overlap. Unfortunately, this requires a new camshaft. Tightening the lobe separation angle, from 114 degrees to 110 degrees, for example, increases the amount of overlap since the exhaust valve closes slightly later and the intake valve opens a little sooner. This tends to bleed off cylinder pressure at lower engine speeds, which could be beneficial since this is a little like built-in exhaust gas recirculation (EGR) in the intake manifold.
There are several other ideas that you can try to reduce your engine’s sensitivity to detonation and allow it to live on lower-octane fuel. Any kind of oil contamination in the combustion chamber is bad news. Oil is a great breeding ground for creating detonation. The best way to avoid this is to ensure your combustion space enjoys the benefits of tight valve-to-guide clearances and good leak-free valve guide seals. Of course, you want to seal up that intake so it doesn’t suck oil into the cylinders, and your short-block should be in good shape.
8.7 Spark Plugs
Spark plugs provide one of the elements, without which an engine simply will not run. Well, at least a non -diesel engine any way. Most of us that dabble in cars would recognize that the primary purpose of spark plugs is to ignite the air/fuel mixture that enters the cylinder.
Spark plugs provide you with a window into the combustion chambers and also provide you with an evidence trail of what is going on in there. Spark plugs and their condition are one of the most important diagnostic tools. They tell you what happened in the cylinder. They will help you in tracking down what the root cause is for many problems and to maximize the air/fuel ratio
The two things that spark plugs do are:
1) Ignite the air/fuel mixture
2) REMOVE heat out of the combustion chamber.
If a sufficient amount of voltage is applied to the spark plug, so that the resultant spark spans the gap between the electrode and ground, then it is said to have sufficient electrical performance. In addition, the temperature of the spark plug's working end must be kept low enough to prevent pre-ignition, yet not too low so as to permit fouling. This is often referred to as the thermal performance of the hear range selected.
One popular misconception is that spark plugs CREATE HEAT. This is absolutely incorrect, and in point of fact spark plugs can only REMOVE HEAT. The spark plug is like a radiator, taking heat out of the combustion chamber and transferring it to the engine's cooling system. Therefore the heat range of a particular plug indicates the plug's ability to to shed heat. The rate at which a plug shed's heat to the cooling system is determined by:
1) The insulator nose length
2) The volume of gas round the insulator nose
3) Materials/construction of the center electrode and porcelain insulator.
The hear range of a particular spark plug has no relationship to the actual voltage transferred through the spark plug. As stated earlier, the heat range is simply an indicator of the spark plug's ability to remove heat from the combustion chamber. This heat transfer effectiveness is a function of ceramic insulator nose length and material composition of the insulator and center electrode.
The insulator nose length is the distance from the firing tip to the point where the insulator meets the metal shell of the spark plug. The insulator tip is the hottest part of the spark plug and therefore plays a crucial and primary role in both pre-ignition and fouling. No matter what the application, from lawn mower to race car, the tip temperature must remain between 450-850 degrees Centigrade. If the temperature is below 450C, the plug will not burn off carbon and combustion chamber deposits, including lead deposits if high performance fuels are used. This will lead to a misfire. On the other hand if the combustion temperatures are over 850C then the ceramic tip will overheat and fracture and the cause the electrode to melt. Once pre-ignition sets in, major damage can be done to an engine in a very short period of time. I know this to be the case, as I have personally experienced both ends of this scale.
Presuming you use the spark plugs with the same electrical/mechanical characteristics, moving one heat range colder will allow the plug to remove between 70-100 degrees from the combustion chamber.
Projected tip plugs run about 10-20 C degrees hotter than standard plugs. By the same token retracted tip plugs MAY runs somewhat cooler, but much will depend on their construction.
The optimum is to find a plug heart range that will work in the crossover range between fouling and optimum operating range. This is sometimes referred to as the "self-cleaning" range and is somewhere around 600 degrees Centigrade. Here there is little chance of pre-ignition or detonation, yet there is little buildup of carbon and combustion chamber deposits.
Let's get back to the length of the insulator, as this sometimes causes much confusion. The longer the distance between the tip and the spot where the insulator meets the spark plug body, the longer the heat of the combustion chamber has to travel before it can be dissipated to the cooling system. Hence this would be a "HOT" plug. The shorter this path, therefore the colder the plug and the colder plug will remove heat more quickly and reduce the chance of pre-ignition/detonation. A good rule of thumb is to use the coldest plug that is available that does not foul. THen you can work with carburetor jetting to get the air/fuel mixture to where it produces maximum results.
WARNING: Each plug manufacturer has their own way of denoting heat ranges. By example NGK uses a low number (2) to denote a HOT plug, whereas a number 10 would be a very COLD plug. Quite the opposite, Champion uses low numbers to denote a cold plug and high numbers for hot plugs. Make up your own cross reference chart so as not to get confused.
Credits: Some of the information on this page was derived from NGK literature.
Some content attributed to the following authors.
Vincent T. Ciulla