More power! It is something every hot rodder wants from their vehicle. From the beginnings of hot rod culture in the 1930s Southern California car scene, to the sophisticated electronically controlled vehicles of today, more power has always been a sought-after prize.
To make power, automotive engines mix air and fuel, then add spark to ignite the mixture in a controlled burn. This burning of the mixture creates pressure and forces the piston down the bore, turning the crankshaft. Because engines are nothing more than big air pumps, it makes sense that the more air and fuel we can force through engines, the more power we make.
There is only one problem with this simple theory: we must ignite all that air and fuel.
Coaxing a spark to jump across a spark plug gap, especially with the increased cylinder pressures modern engines produce, requires a lot of voltage. Upwards of 40,000 volts push current through the spark plug wires to create an arc that initiates the combustion process. The actual amount of voltage required depends upon variables such as compression, engine speed, shape and condition of electrodes and spark plug gap. The more current pushed across the plug gap, the better the chance for more complete combustion and more power.
Points Type Inductive Ignition
Up until 1975 or so, most domestic vehicles used a Points Type Inductive Ignition system, whose basic workings date back to 1908 and were developed by American inventor and Delco Electronics founder Charles Kettering. In addition to Inductive Ignition, Kettering also invented electric starting and lighting systems for automobiles and the first practical engine driven generator.
The basic Inductive Ignition system consists of the Primary Circuit (low voltage), consisting of the battery, the primary ignition coil windings, ignition breaker points and condenser. The Secondary Circuit (high voltage) consists of the secondary ignition coil windings, distributor (cap and rotor), ignition wires and spark plugs.
PAGE 2The ignition coil plays a pivotal role in the system as it transforms the low voltage of the battery into the high voltage necessary for ignition. The coil consists of two sets of windings made up of insulated wires that surround an iron core. The primary windings generally consist of several hundred turns of a heavy wire, while the secondary windings consist of a much smaller gauge wiring with thousands of turns.
Ignition breaker points are a set of electrical contacts that switch the coil on and off at the proper time. The mechanical action of the distributor shaft lobe pushes the points open and allows them to close. A condenser protects the metal from eroding on the points due to arcing; this is known in hot-rod parlance as "burning the points".
The distributor and ignition wires provide the all important path that links the high-voltage current to the spark plugs.
The key to how an Inductive Ignition system works lies in the physical reaction of the current as it runs through the system. When current flows through a conductor (a material that contains electrons), it generates a magnetic field around the conductor. In our case, current from the battery flows through primary windings of the coil when the points are closed. This creates a magnetic field that builds in strength due to the iron core of the coil. When the points open, the flow of current is broken and the magnetic field collapses. This collapse induces voltage in the secondary winding, which travels down the spark plug wires via the distributor and jumps the spark plug gap. Because the secondary windings have, on average, 100 times more windings than the primary, the voltage induced is magnified many times over the 12-14 volts the primary started with.
The most significant advantage of Inductive Ignition is that inductive coils are efficient and can provide long spark duration to ensure complete combustion. The ability to provide long spark duration is due to the inductive coil providing only enough energy to cross the spark gap; the remaining energy from the ignition coil maintains the spark.
Inductive systems are relatively simple and with the introduction of electronic points are easy to maintain. The main disadvantage of the system is that it requires time to charge the coil up to its maximum capacity (known as dwell). As engine speed increases, there is less time for the coil to charge, resulting in reduced spark voltage; not good if you are driving a fast car with a high winding engine. After all, there was only one coil to provide the spark to all eight cylinders. That continuous demand led to coil failure from overheating, especially as plug gaps wore larger, adding to the demand for firing Kv (kilovolts). Other disadvantages were related to the use of mechanical breaker points that allowed timing to change as they wore or even erratic timing at high rpms as they "bounced" on and off the distributor lobes.
Capacitive Discharge (CD) Ignition
The history of the Capacitive Discharge Ignition (CDI) system can be traced back to the 1890s when it is believed that Nikola Tesla, who’s work formed the basis of modern Alternating Current (AC) electric power systems, was the first to propose such an ignition system.
In an automotive CDI system, a charging circuit, powered by the alternator, charges a high voltage capacitor. When signaled by an electronic switch (traditional or electronic points) in the distributor, the system stops charging the capacitor, allowing the capacitor to discharge its output to the ignition coil and on to the spark plugs.
The spark generated by this system is much different from an inductive discharge spark. The duration of the arc generated across the spark plug is extremely short, limiting the amount of energy that the spark can deliver. However, because a CD system can recharge the capacitor very quickly, it has the capability to deliver multiple sparks at low engine speeds, overcoming the high-rpm limitations of the inductive-discharge system.
With modern supercharged, turbocharged and nitrous oxide injected engines creating tremendous cylinder pressures that place a tremendous load on the ignition when igniting the spark, CD ignitions have a tremendous advantage over inductive ignitions by producing a strong spark all the way up to 10,000 rpm. In our primitive example of an engine as an air pump, this provides almost unlimited potential for power.
In addition to allowing for almost unlimited rpm, CD ignitions allow for improved combustion efficiency by using an increased spark-plug gap. While normal spark plug gaps range from 0.25 to 0.35 inch, CD systems can handle larger plug gaps of up to 0.045 to 0.060 inch, resulting in flamethrower-like sparks.
However, increased gap comes at the cost of greater demands on the ignition system, requiring a higher voltage spark, which can ignite the mixture throughout the rpm range. Inadequate ignition system performance under maximum, high-rpm load can result in violent misfires by increased cylinder pressure.
Split Evolution
Auto manufacturers like Mercedes-Benz and Porsche have used capacitive discharge ignitions in the past, yet most modern auto manufacturers do not choose CDI systems because they are more complex, require more components, and can be more expensive. For example, an original equipment Bosch capacitive discharge ignition box for a Porsche 911 costs approximately $1,800. CDI ignitions also take up valuable space underhood.
However, there has been a significant growth in the number of aftermarket CD ignition systems from which to choose. This trend follows the unprecedented growth of the automotive aftermarket industry in the last decade. Increases in engine performance afforded by larger turbochargers, higher flow fuel and exhaust systems, and the cheap horsepower of nitrous oxide have spurred the advancement of performance ignitions. The most popular of these systems is a CDI ignition commonly referred to as Multiple Spark Discharge (MSD).
Two engineers working on a lean burn fuel system to help fuel economy of new and older automobiles founded Autotronic Controls Corporation in 1970. As they perfected the new fuel system, the air/fuel mixture became so lean that it was difficult for conventional breaker points and early electronic ignitions to ignite it. The high-energy spark of the CDI design combined with multiple sparks resulted in a potent ignition. Not only did the multiple spark discharge ignite the lean fuel mixture, it made overall improvements in the engine’s performance. Professional racers learned of the new ignition and quickly spread the word throughout the racing world. MSD Ignition was born.
While CD ignitions have all but dominated the aftermarket, auto manufacturers have concentrated on developing Inductive Ignitions to their next level. As we stated earlier, one problem with Inductive Ignitions is dwell or the time it takes to charge the coil up to its maximum capacity. Dwell limits the power of the system as rpm rises. However, if you allow one coil per cylinder such as in modern distributorless ignition systems, the disadvantage disappears.
The current-generation Chevy LS6 engine is a prime example. Eight individual coils mounted on the LS6’s aluminum rocker covers to supply spark for each cylinder. Crankshaft and camshaft position sensors that provide highly accurate ignition timing and misfire detection trigger electronic spark timing signals for the coil. The use of individual coils allows for a revised firing sequence that improves idle quality and reduces engine vibration.
No matter if you are running a single or multiple coils, induction or CDI type ignition, if you do not ignite the fuel you can not make more power.
