The Ignition Process
The ignition process is key to the operation of internal combustion engines. Four-cycle engines require external combustion, and this is accomplished by discharging electrical energy through a spark gap. Heat transfer and gas ionisation (splitting apart of gas molecules) caused by the spark discharge initiates a flame kernel. The flame kernel then grows into a flame front that spreads through the combustion chamber. The stages of a spark discharge include breakdown, arc, and glow discharges.
Most engines require nearly 12,000 volts to fire the spark gap. When this voltage level is reached, a very high current flows for an almost immeasurably short period of time, as energy stored in the capacitance of the coil, secondary wiring, and spark plug is rapidly discharged. Because the breakdown phase is so short, it accounts for only a small percentage of the total ignition energy.
The breakdown phase is followed by an arc discharge. An arc discharge requires a considerable current flowing in the gap, at least 0.2 amps. During the arc phase, the voltage across the gap drops to a few hundred volts. An arc discharge is very visible and high levels of energy are transferred to the flame kernel near the centre of the spark gap, as shown here.
A glow discharge is less intense. A good analogy to arc versus glow discharge is comparing an arc welder to a fluorescent lamp. Much of the energy in the glow discharge is lost heating the electrodes as shown here. Most ignition systems start with a breakdown and arc phase and then end with a glow discharge as the stored energy is used up. The trick is to maximize the duration of the arc discharge, since this transfers the most energy to the flame kernel.
Terms and Variables Used to Rate ignition Systems
Peak Gap Current - the maximum current flowing during arc or glow discharge. A critical measure of ignition performance.
Total Spark Duration - how long the spark will last. Duration is usually measured in microseconds. Long spark duration reduces lean surge at part throttle.
Available Voltage - also referred to as the maximum open gap voltage. Measured by opening the gap until no spark occurs. Usually much higher than the required firing voltage under actual operating conditions.
Millijoules Ignition Energy - The millijoule is a measurement of ignition energy. (Energy = voltage x current x time.) Primary energy is the stored energy in the ignition system. Only a fraction of this energy is delivered to the flame kernel during the spark discharge.
Cyclical Variation - the technical term for misfiring. Variations in engine output from cylinder to cylinder firing caused by differences in the rate of pressure rise after combustion initiation. An unstable flame front can increase cyclical variation. High output ignition systems with a long arc duration reduce cyclical variation by promoting rapid growth of the flame kernel into a stable flame front.
Selecting The Right Ignition System
Selection of an appropriate ignition system can unlock power by reducing cyclical variation. Cyclical variation ("misfire") can be reduced by ensuring a stable flame front. How can we ensure a stable flame front? First, available voltage must exceed required firing voltage so that the spark will fire across the gap. In conventional inductive ignition systems, available voltage drops off at high RPM. At some point, the engine will start to exhibit gross misfires and power loss. Even if the spark gap has been fired, the system must transfer enough energy to ensure that the flame kernel will rapidly grow and spread. A high gap current, sufficient to cause an arc discharge and a long spark duration, will ensure a stable, spreading flame front.
Our tests have shown that most race engines with power output rated between 500 to 1,000hp require at least 0.3 amps peak gap current and a minimum 100-150 microseconds spark duration. Engines in excess of 1,000hp may require a peak gap current of 0.5-1.5 amps and a spark duration of 300-500 microseconds.
For street engines, a long spark duration is critical. Cruising at part throttle under high manifold vacuum conditions can result in pockets of lean mixture within the combustion chamber. Insufficient spark duration will result in misfires, lean surges, increases in emissions and loss of fuel economy. At least 0.05 amps peak gap current and 1000 microseconds spark duration is required.
Types of Ignition Systems
1. Inductive Discharge Systems
Most original equipment (OE) ignitions are inductive discharge systems derived from Charles Kettering's principle first described in 1908. Inductive discharge systems operate on the principle of energy storage in the coil primary windings. A typical inductive discharge ignition system is shown here. When the switching device (points in early systems, transistors in electronic systems) closes, battery voltage is applied across the coil primary and current builds up. Stored energy is proportional to the coil inductance times the square of the current. By its nature, an inductor tries to maintain a constant current flow. When the switching device opens, the primary current flow is disrupted and current flow transfers to the secondary, firing the spark gap until the stored energy is used up.
Points systems and early electronic systems use ballast resistors to limit current. Later systems limit the current electronically via the transistor switch.
A disadvantage of inductive systems is that energy falls off at high RPM because insufficient time exists to charge the coil. OE systems are generally good up to about 5,000 RPM. Crane's FireBall high output inductive discharge systems such as the Hl-6S, XR700 and XR3000 extend the RPM range upwards of 8,000 RPM.
Most electronic inductive ignition systems have a peak gap current of about 0.04-0.06 amps. This gap current is sufficient for street and some race applications. It is insufficient to cause the long arc discharge required for peak performance in high-output race engine applications.
2. Capacitive Discharge Ignition
Capacitive discharge (CD) systems are widely used in racing and overcome the limitations of inductive systems at high RPM. A typical Crane multi-spark CD system is shown here. A capacitor is charged up to about 475 volts. The stored energy is proportional to the capacitance times the square of the voltage. An electronic switching device such as an IGBT (insulated gate bipolar transistor) then discharges this stored energy into the coil primary. A high voltage pulse appears on the coil secondary and fires the spark gap. CD systems are more complex and expensive than conventional inductive discharge systems because a high voltage power supply is required to charge the capacitor.
Single spark CD systems create high gap current well above 0.2 amps but are characterized by a short spark duration of 150-500 microseconds. The high gap current can create a true arc discharge ideal for high RPM race engines running rich mixtures. CD systems can help reduce plug fouling. CD systems can also operate at engine speeds in excess of 10,000 RPM without energy loss, but the short spark duration can create driveability problems on the street, such as lean surge at part throttle. Thus the need for multiple spark technology
Multiple spark technology is required for street CD systems. Multiple spark CD systems have a specially designed power supply that quickly recharges the storage capacitor allowing multiple sparks to be fired from cranking speed up to 3,000 RPM.
Above 3,000 RPM all multiple spark systems revert back to a single spark. The multiple spark fired at the lower RPM levels typical of street driving results in a much longer effective spark duration. The benefits include quicker cold weather starting, smoother idle, and reduced lean surge during cruise. The Crane Hl-6 multiple spark CD system delivers up to 12 sparks per sequence.
3. Resonant Converter Ignition
State-of-the-art resonant converter technology drives the coil primary with high voltage alternating current. The Crane Hl-8 resonant converter system is shown here. There is no storage capacitor that needs to be recharged between cylinders firing. Resonant converter systems can theoretically generate a continuous spark. The spark duration is limited only by practical considerations such as cross firing within the distributor. The HI-8 resonant converter ignition system achieves a peak gap current in excess of 1 .5 amps with a spark duration of 450 microseconds. Big block engines with highly turbulent combustion chambers and engines running alcohol have gained 10-15hp based on documented dyno tests with resonant converter ignitions such as the FireBall Hl-8.
The ignition coil plays a key role in determining system performance. Original equipment coils usually have a turns ratio of 100 secondary turns to every primary turn. Secondary current, which is the same as the spark gap current, is about 1/100 the peak primary current. Available secondary voltage is 100 times the primary voltage. Some aftermarket coils have higher turns ratios to increase the available voltage. However, the available voltage need only exceed the required firing voltage, which is generally not more than 12,000-15,000 volts. Tests have shown that most engines running plug gap sizes around 1.1mm are better off with a lower turns ratio, in the 60:1 range. This gives a considerable increase in gap current.
Coil construction also effects ignition energy. Early coil designs were based on an open core and packaged in a can filled with oil. Newer designs use a transformer style construction with a closed core in the shape of an "E", hence the name "E" core coils. The closed core results in a closed magnetic path and less "leakage" of magnetic flux. The technical term is leakage inductance and the effect is shown here. Less leakage inductance results in more energy transfer to the spark gap.
The Voltage Myth
Nothing has caused more confusion than the issue of available voltage. Some ignition brands have compromised their ignition system designs so that they can claim very high available voltage figures, such as 40,000 or even 50,000 volts (40 or 50kV). Actual tests on race engines show that the required firing voltage is in the 8,000 to 12,000 volt range at wide open throttle and high RPM. Starting a cold engine is the worst case and may require as much as 15,000 volts to fire the plugs. Once the spark is fired, the arc sustaining voltage drops to a very low value - no more than a few hundred volts.
For a given ignition system, available voltage and spark gap current are inversely proportional. An ignition system designed to provide a high available voltage will sacrifice gap current. The high available voltage figure may look impressive, but actual engine performance suffers.
The Energy Paradox
Energy is the product of voltage, current, and time. The electric company sends us a bill based on the energy we use. In this case, the voltage stays constant. If we turn on more lights or leave them on longer, the energy consumed goes up and so does our bill. Things are not that simple with ignition systems. The problem is with the voltage across the spark gap. Here we are talking about the arc sustaining voltage once the spark has started. The arc phenomenon is very non-linear. If the gap current is low, as in an OE inductive ignition, the gases in the gap will be weakly ionised (glow discharge). The resistance across the gap remains high, causing a high voltage of about 1,000 volts and a high calculated energy figure. Race ignition systems with high gap current levels cause the gases to become strongly ionised (arc discharge). The resistance across the spark gap becomes very low and the voltage may drop to as low as 150 volts. This leads to the paradox that the calculated energy figure for the race ignition may be less than that of the OE system. Millijoules energy figures can be misleading.
Comparisons of gap current and spark duration are more useful and tend to correlate more closely with observed engine performance. Because of the paradox of low spark gap energy for race ignition systems, vendors often give primary energy specifications. For CD systems this is the energy stored in the capacitor. Only a fraction of this energy ever reaches the spark gap. Since factors such as coil turns ratio, leakage inductance, winding and plug wire resistance all effect the energy transfer, primary energy figures are almost meaningless. Again, gap current and spark duration provide a more accurate basis of comparison between ignition systems, since these parameters directly affect flame kernel growth.