THE IGNITION SYSTEM
by Double H
A distributorless car ignition system
The ignition system of a car is one of the most overlooked elements when it comes to engine modification and car tuning. Most people think that once their car modifications are done, all they need to do is get the ignition timing right, turn the ignition and they'll be on their way. But it's much more complicated than that. For one, the spark must be strong enough to ignite the air/fuel mixture. That might sound obvious, but what's not so obvious is that air molecules act as an insulator, and when you modify your car to get more air into the engine, the spark from the stock ignition system might be too weak to effectively ignite the air/fuel mixture, particularly if you're using a forced induction system. In fact poor spark quality can have as negative an effect on engine power as poor ignition timing. In addition, an air/fuel ratio of 11 parts air to one part fuel, which is a fuel rich mixture, is most conducive to spark ignition. However, the air/fuel ratio for the proper burning of the fuel is 14,7 parts air to one part fuel. Thus, the air/fuel mixture is not ideal for a spark ignition system, particularly during cold start conditions where fuel vaporization is not as effective.
Once the air/fuel mixture is ignited, the rate at which the flame passes through the combustion chamber becomes important if you want to unleash the maximum power from your engine. If the flame travels too fast, it would place too much load on the pistons, conrods and bearings; if the flame travels too slowly, not enough force would be generated to create maximum power at the wheels. There are three things that influence the rate at which the air/fuel mixture burns and the flame passes through the combustion chamber:
- The quality of the air/fuel mixture
- The movement or turbulence of the air/fuel mixture in the combustion chamber
- The design of the combustion chamber
Langer discussed the air/fuel ratio when he discussed the four strokes of the internal combustion engine; we discuss the movement of the fuel mixture in the combustion chamber and the design of the combustion chamber in our section on gas flowing and cylinder heads. What this leaves us with then, is to discuss the system that generates that flame, i.e., the ignition system. That is what we endeavor in this section where we'll discuss the different types of car ignition systems , including the inductive-discharge ignition system, which makes use of either a points breaker or a magnetic pickup, the capacitor discharge (CD) ignition system, and the distributorless ignition system, as well as their strengths and weaknesses, adjusting ignition timing for optimal performance, improving spark strength and quality by choosing the optimal spark plug and using the best spark plug gap, and techniques for modifying the car's ignition system to improve engine performance. But first we'll begin with getting a basic understanding of how the ignition system works in ignition system basics.
IGNITION SYSTEM BASICS
There are three basic types of ignition systems used on a car's engine:
- inductive discharge ignition systems, which are sometimes referred to as inductive-storage type ignition systems and include points type, electronic and high energy ignition (HEI) systems
- capacitor discharge ignition systems
- distributorless ignition systems, which are sometimes referred to multi-coil ignition systems
Each of these systems includes various components, some of which are common to all the systems. These include one or more ignition coils; and spark plugs. Most of them, with the exception of distributorless ignition systems, will have a distributor, while ignition leads will be common in most ignition systems, except those that use coil on plug (COP) systems.
Let's look at each of these components, other than the spark plugs in more detail. We'll look at the spark plugs in more detail when we discuss the spark plug heat range, and the spark plug gap.
THE IGNITION COIL
Although there are coils of different shape and sizes on the market, they all share a common basic structure, with three functional components: 100 to 150 primary windings of heavy copper wire, which are directly powered by the battery and are insulated to prevent voltage loss between loops; secondary windings, which are thinner and 100 times more numerous that the primary windings and are located within the primary windings; and a metal core around which both the primary and secondary windings are wrapped.
When current is supplied to the coil, i.e., when the switching mechanism, be it contact breaker points or a pickup coil, is closed, the charge in the primary windings builds up slowly and magnetizes the metal core and creates a magnetic field around the secondary windings. A point is reached when magnetism of the metal core cannot increase any further. When this point is reached, the coils is said to be saturated. When the switching mechanism opens and interrupts the current to the primary windings, the magnetic field collapses instantaneously. This sudden change in magnetism induces a high voltage spark in the secondary windings of the coil. This spark is fed through the coil tower, to the distributor and on to the appropriate spark plug.
IGNITION LEADS
The ignition leads should be able to transmit at least 20,000 to 50,000 volts of electricity without leakage and should have very little resistance of less than 5,000 ohms. Ignition leads with a solid wire core have the least resistance but they cause radio interference and are susceptible to cross fire between adjacent leads, which results in misfiring and lack of power. In addition, solid wires tend to vibrate in the insulating sheath, which leads to deterioration of the leads and a subsequent deterioration in performance. A good alternative to solid ignition wires is an ignition lead with a fine wire spiral wrapped around the core. This type of lead not only suppresses radio interference and cross fire, it is also immune to harmonic vibration, making them more reliable and longer lasting.
Regardless of which type of ignition lead you use, you should ensure that there is at least a 1 inch gap between adjacent wires and never run ignition wires or two consecutive firing cylinders next to each other, especially when using ignition leads with a solid wire core.
THE DISTRIBUTOR
The distributor is the controlling element in the ignition system. It consists of a distributor shaft that is driven by either the camshaft or the crank shaft, an ignition timing advance mechanism that can be electronic or mechanical and is responsible for advancing the ignition timing at higher RPM, a switching mechanism that switches the current to the primary windings of the coil, a rotor that distributes the spark, and a distributor cap.
There are two things to watch out for in terms of the distributor shaft: sprocket climb, which occurs when the distributor shaft rides up the sprocket gear that drives it; and distributor rattle, which occurs when the bearings that holds the distributor shaft in place are worn and allows the shaft to move fore and aft. Both would cause less than optimal ignition timing. You can check for distributor rattle by pushing and pulling on the distributor shaft but sprocket climb is harder to detect.
The mechanical timing advance mechanism usually consists of a vacuum advance feed from the intake manifold and two centrifugal weights each of which acts against a spring. Usually, one spring is weaker than the other, or one weight is smaller than the other to allow for a two stage advance. At higher RPM, the centrifugal weights overcome the tension in the spring and turn the distributor's base plate on which the switching mechanism is mounted, effectively advancing the ignition timing. When the springs become worn, or dirt and grime prevent the proper movement of the weights, ignition timing would be affected. Electronic ignition advance is much more accurate.
If it is at all possible, you should opt for a distributorless ignition system (DIS) as it eliminates the ignition timing errors and failures that can occur when using a distributor. We'll discuss distributorless ignition systems, as well as inductive discharge ignition systems and capacitor discharge ignition systems in the next few pages. We'll also discuss spark plugs, spark plug selection, and the spark plug gap a little later.
SPARK PLUGS AND HEAT RANGE
An important aspect of a spark plug is its heat range, which refers to the rate at which heat is drawn away from the spark plug's central electrode. A spark plug with a short central electrode is a cold spark plug because heat has a short distance to travel to the water jacket in the cylinder head. A spark plug with a long central electrode is a hot spark plug because the heat takes longer to dissipate into the water jacket.
What makes the heat range of the spark plug important is the reliability and longevity of the spark plug. A spark plug that is too hot will fracture due to excessive heat and, more critically, will become a hot spot in the combustion chamber that will cause pre-ignition and detonation, sooner rather than later. However, a certain amount of heat is required to prevent the spark plug from fouling. A cold spark plug will be prone to carbon deposits and fouling and once the sparkplug is fouled, it will become less effective and its spark quality will tail off. Therefore, it is best to use a spark plug that is hot enough to prevent fouling, but is not so hot that it will fracture or become a hot spot.
SELECTING THE CORRECT SPARK PLUG
As you probably realized already, different driving conditions that result in different temperatures in the combustion chamber will require spark plugs of different heat ranges. Fortunately a spark plug that meets most driving conditions for a stock production car has already been identified by the manufacturer. However, the situation is different for modified cars, where the difference in driving conditions will be more extreme.
On modified cars you can check if you are using spark plugs of the correct heat range by inspecting the spark plugs after driving in different conditions, such as stop-starting, cruising, and full throttle racing. After driving the car under one driving condition, remove and inspect the spark plugs. If the spark plug electrodes are covered by soft sooty black deposits, then the spark plug is too cold. However, the soft sooty black deposits can also indicate that your air/fuel mixture is too rich; and if the deposits are moist, it means that oil is finding its way into the combustion chamber. If the porcelain insulator of the central electrode is white or brittle, and/or there is excessive erosion of the electrodes, then the spark plug is too hot; though this can also be an indication that your ignition timing is advanced too far, that you air/fuel mixture is too lean, or that there is a leak on your intake manifold. If the spark plug electrodes exhibit grayish to light brownish deposits then spark plug is of the correct heat range.
Of course, if the spark plug is too hot, you need to change to a spark plug with the lower heat range, and if the spark plug is too cold, you need to change to a spark plug with the hotter heat range. Anytime that you change to spark plugs of a different heat range, you must to test all driving conditions again to ensure that the spark plug is appropriate for all conditions.
SPARK PLUG GAP
The Spark Plug Gap.
In our previous section we looked at the spark plug heat range and how to determine the most appropriate heat range for your particular engine. Now it's time to turn to spark plug gapping, which is simply a matter of bending the ground electrode so that it is closer to or further from the central electrode. On a spark plug with a single ground electrode, this can be accomplished quite easily with a gap gauge to widen the gap or a light tap on a hard surface to close the gap.
The spark plug gap, along with the combustion chamber pressure and the ignition timing has a direct bearing on the amount of voltage you require from the ignition system. The bigger the spark plug gap, the more air/fuel mixture will come into contact with the spark and the easier it will be to ignite the air/fuel mixture. However, a bigger spark plug gap requires more voltage from the ignition coil to create a spark that can arc across the gap between the central electrode, that is connected to the coil via the HT leads, and the ground electrode. Similarly, when the combustion chamber pressure is increased more voltage is required to arc the spark plug gap. Insufficient voltage will result in a failure to create a spark across the gap and may be noticeable as a misfire. However, misfires are not always noticeable, especially at high rpm but it will have an adverse effect fuel consumption and on engine power and performance.
A narrow spark plug gap would require less voltage to spark, but the spark might be too small and weak to ignite and consume the fuel-air mixture in the time available for the ignition phase of the Otto cycle. The result is a failure to effectively convert the chemical energy in the fuel-mixture to mechanical energy, which is how engine power is produced. Thus, engine power and engine performance will not be optimized. However, it's not simply a matter of increasing the spark plug gap and the output voltage from the ignition coil to improve power as firstly, there is a limit to the amount of voltage the ignition system can handle and, secondly, there is an optimal spark plug gap that will best suite the performance of your engine and your driving style.
As a rule, a properly gapped spark plug will burn hot without being too wide at high rpm to cause a misfire. Ironically, the car manufacturer's recommended spark plug gap is not optimal! The recommended spark plug gap is designed to be adequate for cold starting and smooth driving on a car that is in need of an engine tune up. If you drive your car normally and tune the engine regularly, you can increase the spark plug gap by about 0.010" for better performance and better fuel economy. However, if you drive at full throttle most of the time, you should reduce the gap by about 0.010" for better performance. The spark plug itself, and the residue that forms on it, would indicate whether the gap is too big or too small. A light brownish discoloration of the tip of to porcelain insulator indicates the proper operation of the spark plugs with the gap being ideal or close to ideal for the most recent engine speeds. Thus, to check the spark plug gap at high engine speeds, you'd need to run at full throttle and immediately turn the ignition off without allowing the engine to idle. But ultimately, you'd need to run your car on a dynamometer to find the best spark plug gap, and the right ignition timing for your engine.
Remember that when you increase the spark plug gap you need more voltage from the ignition coil to create a spark across the spark plug gap. We'll discuss ignition voltage at a later stage. When a greater voltage is required to create a spark, cold starting and firing fouled spark plugs become more difficult. Therefore you should ensure that your secondary, high-tension ignition wiring is at least 8 mm in diameter, and that it is always clean, dry and in peak condition. Also note that it is not advised to adjust the gap on a multi-electrode spark plug as this will affect the proper operation of the spark plug.
INDUCTIVE DISCHARGE IGNITION SYSTEM
Points type and transistor high-energy ignition (HEI) ignition systems are both inductive discharge ignition systems. Points type ignition systems relay on contact breaker points for spark timing and distribution while transistor HEI ignition systems use magnetic pulses and electronic circuitry instead of contact breaker points. For this reason, transistor HEI ignition systems are often referred to as electronic ignition systems. Both points type and transistor HEI ignition systems use timing advance mechanisms.
The contact breaker points and the distributor cam in points type ignition systems provide the switching trigger for the primary, low tension circuit. When the points are closed, the current from the primary windings in the coil flow through the points and is earthed. This causes a magnetic field to be formed around the coil's secondary windings. When the points open, the current can no longer flow to earth and the magnetic field around the coil's secondary windings collapses. This causes a high voltage current of 30,000 to 40,000 volts to be induced in the coil's secondary windings. This voltage is strong enough to jump the spark plug gap and is directed to the appropriate spark plug via the rotor in the distributor.
This system works well is stock engines but are not optimal for modified and tuned engines as at higher RPM there is less time for the current to fully saturate the coil and form the magnetic field around the coil's secondary windings. Generally, the points type ignition system is good for up to 18,000 quality sparks per minute. This is sufficient for engine speeds of up to 9,000 RPM on a 4-cylinder engine; 6,000 RPM on a 6-cylinder engine; and 4,500 RPM on a V8 engine.
A transistor high-energy ignition system.
TRANSISTOR HEI IGNITION
The distributor in a transistor HEI ignition system has a pulse generator rotor that turns inside a permanent magnet. The pulse generator induces an electric current in a pickup coil that flows through an ignition module. The ignition module is located in the distributor and functions much like a contact points breaker; when a current is induced in the pickup coil, the current from the primary windings in the coil flow through the ignition module and is earthed. This in turn causes a magnetic field to be formed around the coil's secondary windings. When the current in the pickup coil is disrupted, the magnetic field around the coil's secondary windings collapses and the high voltage current required to jump the spark plug gap is induced in the coil's secondary windings.
Transistor HEI ignition systems are superior to points type ignition systems because they do not have tungsten points that can pit or burn. This makes it possible to use a stronger current in the primary circuit to saturate the coil. As a result, transistor HEI ignitions systems require less time to saturate the coil, allowing a stock HEI ignition system to produce 20,000 quality sparks per minute as opposed to the 18,000 of the points type ignition system. A competition HEI ignition system, on the other hand, is good for up to 30,000 quality sparks per minute. This is sufficient for engine speeds of up to 15,000 RPM on a 4-cylinder engine; 10,000 RPM on a 6-cylinder engine; and 7,500 RPM on a V8 engine!
THE CAPACITOR DISCHARGE IGNITION SYSTEM
A Crane Cams Capacitor Discharge
Ignition System
In our previous section we had a look at inductive storage-type ignition systems. In this section we'll move on to the next type of ignition system: the capacitor discharge (CD) ignition system.
Capacitor discharge (CD) ignition systems differ from inductive storage-type ignition systems in that they store ignition energy in a capacitor rather than in an ignition coil. They still use an ignition coil, but the coil is used as a pulse transformer to quickly step up the current.
In a CD ignition system, the primary circuit powers a mini-oscillator or a transformer which charges a capacitor to about 400 to 600 volts and relies on the distributor to trigger the system. The distributor can have either a magnetic triggering system, as in a transistor HEI ignition system, or a light-emitting diode (LED) triggering system. If it has a LED triggering system, a tiny infra-red light beam between a LED and a photo transistor is interrupted by a rotor to produce the triggering signal. When the signal is triggered, the capacitor delivers its stored energy to the coil's primary winding. The coil then acts as a pulse transformer and steps up the current from the capacitor to the 30,000 to 40,000 volts that is required to create a spark across the plug gap.
ADVANTAGES AND DISADVANTAGES
CD ignition systems have a major advantage over inductive storage-type ignition systems in that it is far quicker to charge a capacitor than to saturate a coil. In fact, only 20 microseconds are required to fully charge a capacitor! Theoretically, a CD ignition system should be good for up to 3,000,000 quality sparks per minute. However, CD ignition systems produce a short-duration spark and rely on multiple ignition strikes to effectively extend the spark duration. This reduces the number of quality sparks a CD ignition system can produce to 20,000 RPM on a 4-cylinder engine and 10,000 RPM on a V8 engine, which is still quite a lot!
DISTRIBUTORLESS IGNITION SYSTEM
A distributorless ignition system
Electronic advance control greatly improve ignition advance as it eliminates the need for bob weights and rotating plates. In fact, electronic engine management completely eliminates the need for a distributor and, with it, the distributor cap! The distributor cap is a major problem on high revving race cars when you increase the spark plug gap and the secondary ignition voltage. With higher voltages and bigger spark plug gaps, flash over, or cross firing, is more likely, especially between adjacent terminal posts in the distributor cap. And the terminals posts in the distributor are even close together when your engine has eight or more cylinders.
When you eliminate the distributor, you need another system to generate a signal for the engine control unit (ECU) to know when each piston reaches top dead center (TDC). This can be accomplished by taking a signal from a pulse generator attached to either the front pulley on the crankshaft, or the circumference of the flywheel. The ECU can then use this signal to determine the correct ignition timing and advance firing angle for each cylinder, and can switch the low voltage primary circuit on and off at the correct moment.
The important thing on a distributorless ignition system is to ensure that the pulse trigger is correctly mounted. This actually entails three things you need to check: the firing pin diameter, the firing pin position, and the air gap.
THE FIRING PIN
The firing pins must be precisely positioned and must be of the correct diameter. Obviously, for accurate timing, the firing pins must be accurately aligned with the magnetic pick-up when each piston reaches TDC. To get a reliable signal, the firing pins must be of the same diameter are the magnetic pick-up. Using a larger or smaller diameter firing pin than the magnetic pick-up will lead to inaccurate signal generation, and inaccurate ignition timing! The result could be catastrophic!
THE AIR GAP
The air gap between the firing pin and the magnetic pick-up must also be adjusted to the gap specified to by the manufacturer of the distributorless ignition system. Most manufacturers specify an air gap of 0.020-0.040". If the air gap is too large, the signal will be too weak and will result in miss firing and lack of power. If the air gap is too small, the signal may be generate too soon and your ignition timing will also be out.
IGNITION VOLTAGE
There are two types of voltage that is important for the ignition system; input voltage from the battery side, and output voltage from the ignition coil. The input voltage affects the output voltage in most ignition systems, other than capacitor discharge ignition systems but there are limitations to the amount of output voltage that the ignition leads and the distributor cap can handle. Modern motor cars have a number of electronic devices, such as electronic fuel injection, electric fuel pumps, electronic cooling fans, etc that reduces the amount of current that is available to the ignition system. For this reason, you may want to modify your ignition system by increasing the battery voltage, or by improving your car's wiring system.
IGNITION WIRING
Transistor High Energy Ignition (HEI) ignition systems draw quite a bit of current when the system first switches the primary, low-tension circuit to charge the coil. If the HEI ignition system does not get sufficient voltage feed from the battery, when it charges the coil, the system will misfire and you will lose power momentarily. For this reason you should ensure that no other electrical devices can draw current on the wiring from the ignition switch to the transistor HEI ignition system. You can do this by checking that no other device is connected to this part of the ignition circuit. You should also ensure that a minimum 10 gauge wire is used for this part of the ignition circuit.
You should do the same for points type ignition systems, but it is not necessary if you're using a capacitor discharge (CD) ignition system.
You can also improve the output voltage that is fed to the spark plugs by increasing the battery voltage. A general increase in the battery voltage will result in an increase in secondary coil energy, which will result in better spark quality overall, as well as at higher RPM. This works well on HEI ignition systems, but not as well on points-type or capacitor discharge (CD) ignition systems.
A HEI system with a 12.6 volt battery will provide good spark at up to 7,500 RPM on a V8 engine while a HEI system running on a 16.8 volt battery will provide good spark up to 8,800 RPM on the same engine! However, the HEI system must be designed to operate at higher voltages. Also, your other V8 electrical components, with the exception of the starter motor, are not designed to operate for too long above 13 volts. Fortunately, there are high voltage batteries with an additional 12 volt terminal on the market. These will allow you to run a high voltage HEI ignition system and your other electronic components on the 12 volt terminal.
Increasing the battery voltage on a CD ignition system should have no effect on the output voltage as a CD ignition system produces a constant spark regardless of whether the feed voltage is 10 volts, or 16 volts. If you do see in increase in spark quality when you increase the battery voltage, then you probably have a problem in your primary circuit. You either have a poor connection in your circuit, or the wire gauge is too thin to supply the required current.
You also cannot increase the battery voltage on a points-type ignition system as the higher output voltage will cause the contact points to burn and pit. The eventual result will be a weaker rather than a stronger spark, or no spark at all once the points are too badly burnt.
(Credit to Custom Car US)
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