Using the same energy input, a KSI™ system generates an “ignition kernel” dramatically larger than a conventional ignition system spark, which produces major improvements in engine operation.

CURRENT IGNITION SYSTEMS AND THEIR LIMITATIONS

History

Robert Bosch’s original ignition patent was issued in 1903. Since then, ignition systems have evolved only slightly, with a focus on minimizing cost, while increasing durability and longevity. The basic function has remained unchanged for over 70 years.

The common current systems convert about 10 percent of the input energy into a small, stationary spark across a one-millimeter gap. Current systems include the Kettering system and the Capacitive Discharge system. In addition, there are a number of other potential systems discussed further below. All have significant limitations in performance, and/or cost, and/or igniter life.

Common Current Systems

The Kettering system is the least expensive system currently available and is today’s industry standard. Until the recent challenges to improve fuel efficiency and reduce polluting emissions, it has been sufficient for most applications.

When the primary circuit of an ignition transformer, or spark coil, is opened, the sudden change in current generates a sufficiently high voltage in the secondary winding to ionize the gas between the electrodes of a spark plug.

The initial spark in a Kettering system lasts for less than a microsecond, followed by a low current, longer duration (multi-millisecond) glow discharge, which is mostly wasted energy.

The initial spark is small and stationary. It ignites a small volume of the air/fuel mixture next to the spark plug. The combustion then expands with increasing velocity into the rest of the cylinder.

Additional compression of the air/fuel mixture gives more power to the engine, but makes it more difficult to ignite the mixture or requires more expensive, higher octane gas. On the other hand, a very lean mixture (less fuel, more air), which tends to reduce polluting emissions, becomes more difficult to ignite.

Although many Kettering plugs can survive for 100,000 miles, performance degrades after only a few thousand miles, as the sharp edges of the electrodes wear and the spark is weakened.

Introducing more edges to the electrodes (e.g. SplitFire®) does not extend the plugs’ useful lifespan, but it does sustain the maximum performance somewhat longer.

The capacitive discharge ignition system stores energy in 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 a spark plug, followed by a high-current, sub-millisecond spark.

This produces a somewhat larger spark than a Kettering system and more energy delivered during the initial, breakdown phase of the discharge.

This fast-rising, high-intensity spark is beneficial for low-efficiency engines, such as two-cycle engines, because it reduces fouling.

On the other hand, a capacitive discharge ignition system has more expensive electronics than a Kettering system and a shorter igniter (sparkplug) life.

Alternative Systems

A small peaking capacitor mounted next to the plug can be added to a conventional ignition system as an aftermarket application. This will deliver more energy during the initial breakdown phase of the discharge and a larger spark formation. This increases ignition system performance for a small increase in cost.

At low load, however, when more potent ignition is needed to ignite the air/fuel mixture, the system shows little improvement, because it produces a low breakdown voltage. At high load, when enhanced ignition is not usually required, the capacitor is charged to high voltage and delivers significant unnecessary energy. In addition, the peaking capacitor causes rapid electrode wear, due to the high currents passing through a stationary arc.

New Concepts

New concepts, such as Pulsed Direct Current Ignition (PDCI) systems include Adrenaline Research’s Smartfire®, Plasma Jets, the University of Texas’ Railplug and the KSI system. The first four of these are described below in this section. KSI is described in the next section.

Pulsed direct current ignition systems provide a high voltage discharge that acts as a switch to allow a capacitor to deliver energy directly into the discharge, bypassing the coil and eliminating the inefficiency and limitations caused by the impedance mismatch of conventional systems. PDCI systems have a much shorter pulse duration than capacitive discharge ignition system, which provides more precise ignition timing.

Adrenaline Research’s Smartfire® system uses an igniter with a surface discharge design and a protruded-tip center electrode. When the coil bypass discharges into the igniter, it produces an elongated, stationary arc. This stationary arc causes high localized wear, which must be mitigated by limiting the discharge energy to that now used in conventional ignition systems and limiting the bypass energy to occasional use. As a result, this system requires a complex and expensive control system. The advantages of a Smartfire system over a conventional system are equivalent to the advantages of a peaking capacitor system (described above), but with better plug life.

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 gases 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. This localized high-energy in the cavity causes the electrodes to vaporize rapidly, limiting limited igniter life.

The Plasma Jet uses many times the energy of a conventional system or a KSI system to achieve a comparable improvement in ignition. Thus most of the energy savings in engine efficiency are consumed by the needs of the ignition system.

The University of Texas’ Railplug system propels a plasma deep into the cylinder in order to create turbulence. Turbulence is useful in mixing the air/fuel mixture to achieve complete and even combustion. This is useful in cylinders with limited turbulence. However, virtually all engines today already have sufficient turbulence, so the effects of the Railplug turbulence are minimal.

The improved ignition performance of the Railplug is similar to that of the KSI system, but the energy requirements are much greater and the pulse shape limits the longevity of the igniters.

“Exotic” Ignition Systems

RF ignition systems transmit electromagnetic waves of radio frequency into the combustion chamber to create a very large and energetic ignition. However, the efficiency is highly sensitive to pressure and generally poor at high pressure. RF ignition is academically compelling, but it would be difficult and expensive to make it work over a large operating range and the system would be significantly larger and require more energy than a conventional or a KSI system.

In a Laser-Induced Ignition System, a pulsed, high-power laser beam is focused through a window into the combustion chamber to ignite the air/fuel mixture. The ignition kernel can be located anywhere within the combustion chamber.

However, the window must be protected so it does not lose transparency. To be practical, the system would require a small, rugged and efficient solid-state laser. However, the only lasers that could reliably light the air/fuel mixture are large, precise and fragile. Such lasers cannot tolerate vibration, dirt or changes in atmospheric conditions. The efficiency is only a few percent, so the power supply would have to be very large. Therefore, these systems are impractical for all but research applications.