The Quarter-Wave Coaxial Cavity Resonator (QWCCR) Plasma Igniter is a novel use of a microwave plasma source as a spark plug improvement for internal combustion engines. Initially developed in 1992 by the Center for Industrial Research Applications (CIRA) at West Virginia University (WVU), the QWCCR is a highly efficient impedance transformation device that can step up the electrical potential of highly reactive microwave energy to potentials that are capable of heating and ionizing gases, and igniting air fuel mixtures.
Benefits of plasma ignition for combustion engines include:
- Allowing the ignition of problematic, multi-fuel mixtures and gaseous fuels, and is particularly suitable for ultra-lean burn and stratified charge engine environments.
- Providing a more complete combustion, allowing for the reduction of regulated emissions such as NOx and unspent hydrocarbons, and may also have other environmental benefits.
- Increasing the energized ignition volume compared to a conventional spark plug spark, thus reducing the thermal point loading that results from a spark gap, improving its efficiency over traditional spark ignition systems and reducing NOx formation.
- Pumping microwave energy into the cylinder cavity prior to the ignition as a way to energize the fuel/air mixture and to aid in combustion.
- Serving as an onboard diagnostic tool to tailor ignition characteristics, ramped spark, multi-spark, etc.
The plasma plug uses a microwave energy source to generate a hemispherical ignition surface at the face of the plug. The high energy delivery potential of this technology, its controllability and the indirect creation of reactive chemical species make microwave ignition an attractive candidate for a lean burn as well as gaseous and multi-fuel ignition source.
Significant development of the plasma plug and control system has already been performed. A prototype plasma plug, fabricated from a commercial off-the-shelf microwave source (magnetometer) was coupled to a brass plug and run with gasoline, kerosene, diesel, and Jet A fuels in a Briggs and Stratton 8hp, 305cc 4-stroke engine.
The geometry of the plasma plug is fabricated to be exactly ¼ of the microwave wavelength. Thus, the input voltage at one end is small while the output voltage at the tip is electrically very large. This step-up transformation is a simple, efficient way to achieve large voltages. The plasma that results from this process is a highly energized cloud of pulsing, reactive particles and ions.
The igniting of the effective surface area of this plasma has been shown to be considerably larger than would be created by an electrical spark discharge, and also at a lower contact temperature, thus reducing NOx formation. In the case of an engine with a hard-to-ignite mixture, such as a very lean or dirty fuel or multi-fuel, if the small-dispersed droplets of fuel are obstructed and not in the immediate vicinity of the spark, then complete ignition propagation will not occur. The unspent fuel will be expelled as un-burnt hydrocarbons; fuel economy and emissions performance will suffer.
In comparison, the microwave plasma which extends well beyond the unobstructed electrode tip will ignite the fuel thereby extending the lean limit thus improving fuel economy. It is also believed that a considerably greater portion of energy can contribute to the ignition process, whereas a traditional spark expends as much as 75% of its energy in losses during the glow discharge phase.
The current traditional spark plug technology has reached its limit for delivering more ignition power. The plasma plug has potential to extend this capacity by as much as an order of magnitude or more. Laboratory tests of the QWCCR have demonstrated repeated cylinder ignition of an internal combustion engine, however, precise ignition timing and associated microwave electronics suitable for incorporation into application specific engines needs development, the emphasis of the next step of this program. A high temperature, high pressure QWCCR plug constructed of commercial materials, such as low expansion ceramics, needs to be designed, fabricated and integrated into an engine cylinder for a specific end-use application.
Complete engine integration and subsequent field testing of a plasma ignition system will provide a commercially viable system that will enhance the future value of the global engine market in addition to the application-specific needs addressed.