Introduction

Internal combustion ignition has slowly evolved over the past century. In 2003, Robert Bosch’s original ignition patent is 100 years old. The evolution of ignition systems has progressed with a focus on minimizing cost while increasing durability and longevity. The ignition capabilities of conventional ignition systems seem to have been sufficient for a long time and, in many cases, still meet the needs of engines. However, requirements for increased efficiency and reduced emissions are now challenged by continually increasing standards for drivability. It is becoming increasingly expensive to design engines to meet the new specifications given limitations of conventional ignition systems. Engine manufacturers are considering incorporating higher performance, higher cost ignition systems to create the lowest total cost solution to meet tougher demands.

There have been many approaches to improve upon conventional ignition systems. This document considers two classes of new ignition systems: Pulsed Direct Current (DC) and Exotic. Knite’s Kinetic Spark Ignition™ System (KSI™) is a Pulsed DC solution that is the most practical approach to implementing an increased performance ignition system – simplicity, low cost, and significant ignition performance improvement.

Conventional Ignition Systems

The conventional ignition comes in two basic forms, each with many derivatives: Kettering and Capacitive Discharge Ignition (CDI).

The Kettering system is the current industry standard and the least expensive system presently available. Until today’s efficiency and emissions challenges, it has been sufficient for most applications. The system stores energy in the magnetic field of an ignition transformer. When the circuit is opened, the magnetic field collapses, generating a sufficiently high voltage in the secondary of the transformer to ionize the gas between the electrodes of the spark plug. The spark consists of a sub-microsecond, high intensity initial breakdown which is followed by a low current, long duration (multi-millisecond) glow discharge. The Kettering system’s ignition capabilities are quite limited, but the parameters are well-known because of its long history.

The Capacitive Discharge system stores energy in the electric field of a capacitor. When the circuit is triggered, the capacitor is discharged into the primary winding of an ignition transformer, generating a sufficiently fast-rising high voltage in its secondary winding to ionize the gas between the electrodes of the spark plug. This initial breakdown is followed by a high current, short duration spark (sub-millisecond). The CDI technology is beneficial for engines with low efficiency (such as two-strokes), because its fast rising, high intensity spark resists fouling. (Spark plug fouling is due to combustion products, fuel, or oil, which collects on the plug's insulator if the plug is not sufficiently hot enough to burn off the deposits.) The benefits come at a high price because a CDI system has substantially more expensive electronics and shorter igniter longevity and, this, its application is limited.

Both Kettering and CDI systems provide igniter life that is acceptable for their particular applications. Although many Kettering igniters can survive for 100,000 miles or more, notable performance degradation occurs after only a few thousand miles. The sharp edges of the electrodes of a new plug create higher electromagnetic field strength to create an intense spark, but as they wear, the field is reduced and the spark is weakened.

Many of the variations of conventional ignition systems have addressed igniter geometry with a desire to achieve some the following benefits:

Increased resistance to fouling;

Sustained ignition performance for a longer period of the igniter’s lifespan by having more edges;

Improved ignition system performance by raising the electric field concentrations at the discharge region (usually done with narrower electrodes).

Modifying the igniter’s longevity by introducing more edges (e.g., SplitFire®), provides no improvement in performance over an ordinary j-gap igniter, but may sustain their maximum performance for a longer period of time.

The peaking capacitor is a technology that is used with derivatives of conventional ignition systems and can be installed as an aftermarket application. It consists of a small capacitor (picofarad) mounted adjacent to the igniter. The capacitor adds to the normal stray capacitance of the ignition system, which is then charged to breakdown voltage. The effect is for more energy to be delivered during the initial, breakdown phase of the discharge and a larger spark formation. The advantage is increased ignition system performance for a small increase in cost. However, the peaking capacitor system is flawed. At low load, when a more potent ignition is desirable, the system delivers minimal improvement because of the low breakdown voltage. At high load, where enhanced ignition is usually not required, the capacitor is charged to high voltage and delivers significant, unnecessary energy. Additionally, the peaking capacitor causes rapid electrode wear (due to the high currents passing through a stationary arc) – substantially faster than CDI.

Pulsed Direct Current Ignition Systems

In conventional ignitions systems, there is an impedance mismatch caused by energy flowing through the high voltage coil in order to sustain a discharge once the arc is formed. Pulsed Direct Current ignition systems use a high voltage discharge as a switch to allow a capacitor to deliver energy directly into the discharge, bypassing the coil. This bypass discharge energy eliminates the inefficiency and limitations caused by the impedance mismatch of conventional ignition systems. Knite’s KSI™ System is a Pulsed DC ignition system. Other similar endeavors and technologies include Adrenaline Research’s Smartfire®, the University of Texas’ Railplug, and Plasma Jets.

Pulsed DC ignition systems have a pulse duration that is much shorter than that of a CDI system. The short duration provides precise ignition timing. It creates an excellent opportunity for the inclusion of ion sensing. The timing control enabled by the precise short duration pulse also means that the engine speed is only tied to the rate at which the ignition system can be charged (and any restrictions due to ignition discharge parameters are eliminated).

The KSI technology utilizes Lorentz force and thermal forces derived from the geometry of the igniter’s electrodes and the pulse shape provided by the discharge electronics. This results in a very large ignition kernel for a given input energy. In turn, the system provides the maximum gain in operating parameters without using excessive amounts of ignition energy. Additionally, KSI’s Lorentz force causes the spark to move along the electrodes. The moving plasma reduces electrode erosion by having the discharge spread its energy deposition along the surface of the electrodes, thereby enabling significantly greater longevity than other high current solutions. It is also known that the moving plasma propels a significant amount of ions beyond the ignition region, which likely has a positive effect on the fuel ahead of the flame front.

The KSI system delivers an ignition region that is over 100 times larger in volume than the Kettering system. The large volume improves tolerance to conditions like poor fuel preparation, mixture inconsistency, and dilution.

KSI’s electronics have highly evolved from coil bypass electronics used in other pulsed DC applications. Efficiency is improved because KSI has eliminated diodes from its electronics circuits.

The Adrenaline Research Smartfire® system uses an igniter with a surface discharge design and a protruded-tip center electrode. When the coil bypass electronics discharge into the igniter, an elongated stationary arc results. This stationary arc causes a high wear rate which must be mitigated by limiting the discharge energy to what is presently used in conventional ignition systems. In order to preserve the electrodes, the bypass discharge energy can only be used occasionally. This result is that the ignition system requires a very complex and expensive control system. The gains from this ignition system are equivalent to that of the peaking capacitor system, but with a plug life that is more acceptable.

A Plasma Jet ignition system delivers the energy of the coil bypass electronics to a small discharge cavity located in the tip of the igniter. When a discharge is initiated, the gasses within the cavity are ionized and expand. The pressure increase caused by the expanding ionized gas emits a jet of plasma from a small opening in the cavity. The high energy deposition within the cavity causes the electrodes to vaporize quickly and results in limited igniter life.

The Plasma Jet uses many times the energy of KSI to achieve a comparable improvement in ignition capabilities. The volume of the ignition region of the Plasma Jet is smaller than that of KSI for a given energy because it relies solely on thermal forces. Therefore, any engine gains achieved by Plasma Jet ignition are consumed by the ignition system’s significant power requirements. The net gain is minimal.

Similar to KSI, the Railplug developed at the University of Texas uses a Lorentz force to propel the spark. However, the Railplug propels plasma deep into the chamber in order to create turbulence. The resulting turbulence is effective in cylinders with limited or no turbulence, because it generates faster and more complete combustion. However, virtually all engines already have sufficient turbulence and the effects of the Railplug turbulence are minimized. The system utilizes substantially more Lorentz force than thermal force and, therefore, requires very high levels of energy, similar to that of Plasma Jets. The ignition performance gains of the Railplug system are similar to KSI, but its increased energy reduces the net gains. Additionally, the high-intensity energy required for the extreme acceleration of the plasma and the Railplug’s pulse shape limit plug longevity.

“Exotic” Ignition Systems

There are a number of alternative ignition concepts ranging from mundane to exotic. RF ignition and Laser-Induced ignition are examples of advanced alternative technologies.

RF ignition generates Electromagnetic (EM) waves of radio-frequency (RF) that are transmitted into the combustion chamber. A small waveguide is used or the waves can be generated directly into the chamber. The goal is to create a very large volume energetic ignition. In addition, the direct injection of EM waves into the combustion chamber may have beneficial effects on fuel prior to combustion. However, the efficiency is highly sensitive to pressure and is generally poor at high pressure. RF ignition is academically compelling, but it will be very difficult and expensive to make work over a large operating range. Deployed in an engine, it would be significantly larger and require much more energy than a Pulsed DC system like KSI. Although a number of scientific papers have been written on RF ignition, there is a minimal amount of published data demonstrating its feasibility across the broad operating conditions of an engine.

In a Laser-Induced ignition system, a pulsed high power laser generates a beam that is focused through a window into the combustion chamber in order to ignite the air/fuel mixture. Laser Ignition is capable of highly precise timing and location. The laser beam can be focused at any distance from the window, placing the ignition kernel at almost any location within the combustion chamber. It has the ability to ignite the combustion mixture at a relatively large distance from the combustion chamber walls.

For practical applications of laser ignition in an engine, the window in the combustion chamber would have to be protected so as to not lose transparency. The type and size of the laser is another important consideration. A small and efficient solid state device would be required for integration into an engine. However, the only lasers that can reliably light the air/fuel mixture are large, precise, and fragile devices. Such lasers cannot tolerate vibration, dirt, and changes in atmospheric conditions and the power supply for the laser would be as large as an entire automotive engine. Finally, the efficiency for transfer of the electric energy into laser beam energy is only a few percent. Therefore, the required electric energy would be very significant. Laser-induced ignition is impractical for all but research applications.

Conclusion

Conventional ignition systems, such as Kettering and CDI, have been a cost-effective solution for engine ignition for many years. However, engines are being pushed to be cleaner and more efficient. Overall engine cost may climb dramatically as other engine systems are designed to compensate for the limitations of a conventional ignition. An ignition system with greater capability may slightly increase the cost of the ignition system, but will significantly lower the overall cost of the engine solution by reducing cost and effort in making the engine meet specifications.

Of the new generation of ignition systems, the Pulsed DC ignition technologies are cost-effective and practical, and are closest to meeting the engine manufacturers’ implementation requirements. Knite’s KSI™ System is the Pulsed DC solution that has been demonstrated to provide the greatest benefit with minimal costs – small energy requirements generating a large ignition region with excellent gains, an uncomplicated and manufacturable architecture, and no technological barriers to delivering longevity.