Internal-combustion engines – Igniters
Reexamination Certificate
2001-06-12
2003-05-27
Kwon, John (Department: 3747)
Internal-combustion engines
Igniters
C313S118000
Reexamination Certificate
active
06568362
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to spark ignition engines in general and more particularly to spark ignition systems.
BACKGROUND OF THE INVENTION
A conventional spark plug is adapted for insertion into an opening of an engine where an air-fuel mixture is present. This area is typically referred to as a cylinder or combustion chamber of the engine. Spark plugs are provided with an electrically insulating shell through which a high voltage electrode, also commonly referred to as the anode, extends into the combustion chamber. The high voltage electrode is connected to an ignition system which supplies a high voltage pulsating “DC signal” which is applied during each combustion cycle at a time when the piston is approaching the end of its upward motion and the valves are closed.
A second electrode is commonly referred to as the ground electrode or cathode. The ground electrode is typically a projection or protrusion extending inward from the shell of the spark plug and disposed in spaced apart relation with the high voltage electrode. The ground electrode is also disposed within the combustion chamber and is electrically common with the combustion chamber. The electrode separation distance is commonly referred to as an air gap or spark gap. The high voltage signal pulsating DC signal is sufficient to generate an electrical arc (or spark) across the air gap.
The spark generated quickly develops into a low impedance arc. The volume occupied by the arc is low, the reactivity of the arc is low and the electrode erosion rate is high. There is no external magnetic field or other device to cause the arc to move about or to otherwise increase in reactivity.
In systems well-known in the art, the spark gap is set prior to installation of the spark plug into a corresponding engine receptacle. Normally, the spark gap is adjusted to a distance to provide an arc having desired characteristics necessary for initiating proper combustion of the air-fuel mixture. Improper combustion can cause poor engine performance such as backfire and result in increased emissions of harmful pollutants such as NOx, unburned or partially oxidized hydrocarbons and CO.
Internal combustion engines which use spark plugs to ignite air-fuel mixtures are commonly referred to as spark ignition engines. Current spark ignition engines are commonly controlled to operate “lean” on fuel, operating at essentially the stoichiometric air/fuel ratio, in order to meet government imposed emission regulations. The stoichiometric ratio is the ratio of air/fuel required to completely combust the fuel. Most emissions generated by the combustion process are significantly reduced through use of a catalyst system positioned in the exhaust stream. The major role of the catalyst system is to reduce levels of NOx, unburned or partially oxidized hydrocarbons, and CO output by the combustion process. Thus, a careful control near the stoichiometric set-point is needed because the chemistry requires a reduction reaction to eliminate NOx while oxidation is required for elimination of unburned or partially oxidized hydrocarbons and CO.
An efficiency increase for internal combustion engines (estimated at up to 14-20%) could be realized if “lean-burn” engines could supplant the current stoichiometric air/fuel engine technology. As used herein, lean-burn is the term used to describe an air/fuel mixture having excess air above the stochiometric air/fuel ratio. A major barrier to lean-burn engine use in the United States is the inability to meet the California and Federal emission standards. In particular, lean-burn engine mixtures have been shown to be unable to sufficiently suppress the generation of NOx during the combustion process. Once produced by the combustion process, current catalyst systems can only reduce NOx levels modestly (<30%) from the levels generated from the combustion process.
Known strategies for reducing NOx formation in lean-burn engines include the use of exhaust gas recirculation. This method involves re-injecting combustion products back into the combustion chamber together with fresh air/fuel. A second strategy operates an engine very close to the lean-combustion misfire limit. The misfire limit occurs when combustion becomes erratic and generally incomplete.
Both of these strategies for reducing NOx formation during combustion are related. Both depend on dilution effects causing suppression of peak combustion temperatures. Thus, they could be used in combination. Pushing engine operation further into the lean regime permits greater potential efficiency gains. However, for lean-burn technology to become viable in view of strict emission standards, a method for suppressing emission of NOx and other environmentally harmful pollutants must be found.
Lean-burn mixtures can also result in ignition instability. The fuel injection and turbulent-mixing process inside the engine cylinders can create mixture stratification that can make ignition unreliable. This effect can become more pronounced for increasingly lean mixtures. Fluid volumes may be produced that are excessively lean to the point that flame propagation can become impeded. The fluid elements nearest the spark event can become particularly lean such that adequate flame kernel development is prevented even though the overall mixture stoichiometry is sufficient to otherwise sustain combustion.
Complete and partial misfires cause significant unburned fuel to be exhausted and engine performance to accordingly degrade. It estimated that up to 95% of the pollution emanating from a running combustion engine is generated during misfires. A misfire can also be followed by a relatively strong combustion event because the residual gases and recirculated gases contain unreacted fuel and oxygen. Thus, at a subsequent instant the air/fuel mixture may have more fuel and air than the engine set-point would otherwise allow. This stronger combustion event can result in a higher combustion temperature than is meant to occur and is likely to produce relatively high quantities of NOx. This general cycle-to-cycle variation in combustion events has been a major focus of engine research. Tolerable levels of misfire are generally accepted to be limited to 1-5 misfires per 1000 combustion events.
Some principles of high-pressure (10 bar) spark discharges are presented to aid in an understanding of the invention. High-pressure sparks have properties which differ from low-pressure (but still collision dominated) sparks. In low pressure sparks, a Townsend discharge may occur where ambient free electrons are accelerated by an electric field and ionize neighboring gas particles through collisions. This is known as electron impact ionization. Newly generated “secondary” electrons are themselves accelerated by the ambient electric field causing an avalanche of electron and positive ion production. In low pressure discharges, the avalanche grows at the electron drift velocity, while plasma densities and associated currents are relatively low and collisional diffusion is usually significant.
At high pressures, such as 10 bar, the plasma charge density may build up to much higher values compared to the charge density normally built up at low pressure (e.g. 1 bar). As a result, the mutual coulomb or space charge forces, are much stronger at high pressures than the vacuum electrostatic forces. Ionization in this case produces an almost perfectly space charge neutralized plasma. However, the coulomb forces due to the residual space charge still dominate the forces due to the applied fields. The resulting space charge shielding of the applied fields by the plasma causes the electrical fields within the forming spark to be quite low.
According to Gauss's Law, this charge configuration makes the electrical field between the emerging spark and the spark plug anode correspondingly higher. This process continues during the ionization avalanche with the electric field in the front of the plasma, commonly referred to as the plasma front, becoming progressively stronger with time. For example,
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Tsai Chin-Chi
Whealton John H.
Akerman & Senterfitt
Kwon John
Ut-Battelle LLC
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