Spark Plug

Abstract

A spark plug for a combustion engine or a combustion chamber in which the electrodes have a coating of titanium dioxide thereon.

Description

This invention relates to spark plugs. More particularly, this invention relates to spark plugs having novel electrodes which provide better ignition of the fuel-air mixture under marginal conditions.

The modern vehicle engine, unlike its ancestors, must be all things to all men at all times. It must deliver greater power for a given size than heretofore achieved. It must operate at greater economy than heretofore realized. It must accomplish all this while emitting nothing more than carbon dioxide and water. Emissions such as oxides of nitrogen, unburned hydrocarbons, carbon monoxide, and lead are no longer viewed as "progress." Large sectors of the public now demand that vehicular engines operate as free of harmful emissions as is possible.

The route to emission-free engines has been extremely difficult. A large amount of research effort has been expended in attempting to create the "perfect" piston-type internal combustion engine. Many of the results of such efforts have been less than perfect.

The installation of "emission control" devices has solved many problems. However, such devices have also created problems. Engines equipped with such devices must be operated at a relatively low fuel-air ratio in order to maintain emissions of unburned hydrocarbons and carbon monoxide at low levels. A low fuel-air ratio (lean mixture) makes the engine hard to start and more prone to misfire.

Lead emissions can be curtailed by using nonleaded fuels or fuels containing low levels of lead. Such fuels, however, normally have lower octane ratings than lead-containing fuels. The result of the use of such fuels can be hard starting and misfiring. Until now, there has been no simple, low cost method or device which would allow engines to utilize lean fuel mixtures and/or low lead fuel with no sacrifice in performance.

It is, therefore, an object of this invention to provide a spark plug which will effect ignition of a fuel-air mixture under marginal conditions of fuel mixture and spark intensity.

It is another object to provide a spark plug in which at least the spark-producing areas of the electrodes are coated with titanium dioxide.

It is a further object to provide a spark plug in which the spark-producing areas of the electrodes are formed of a molecularly bonded coating of titanium dioxide.

Other aspects and objects will be apparent to those skilled in the art from the following disclosure, example, appended claims, and the annexed drawing in which:

FIG. 1 is a view, partly in section, of a spark plug showing a coating on the spark-producing areas.

FIG. 2 is an enlarged view, partly in section, of the lower end of the spark plug of FIG. 1 showing more clearly this invention.

By the present invention I have found that spark plugs having greater reliability under marginal conditions of fuel mixture and spark intensity are provided if there is employed therein an electrode having spark-producing areas of the electrode coated with titanium dioxide.

Referring now to the annexed drawing, numeral 2 designates a spark plug in its entirety and includes a metal shell or shank 4 having a threaded portion 6 by which the spark plug is screwed into a cylinder head. Hexagonal nut 8, an integral part of shank 4, provides a means for driving the spark plug into and withdrawing it from the cylinder head. The upper end of the spark plug 2 includes a cap 10 for attaching a conductor connected to the high voltage source, not shown. Cap 10 is positioned at the upper end of a first electrode 12 which is insulated from shank 4 by a porcelain insulator 14. The electrode 12 passes through the entire length of spark plug 2. The lower end of the spark plug is extended to form a skirt 22 which carries a second electrode 16. The skirt 22 and threaded portion 6 provide a conductive path with the engine block (not shown) to ground electrode 16. Electrodes 12 and 16 can be made of ferrous metals or ferroalloys. Alternatively, the electrodes can be titanium or a titanium alloy.

On the lower end of electrode 12, and on the upper surface of electrode 16 in the area facing electrode 12, there is provided a very thin coating 18 of titanium dioxide. The surfaces of the coatings 18 are spaced to form a gap 20 across which a spark is created when high voltage electrical charge is impressed across electrodes 12 and 16.

Various techniques can be employed for achieving the desired coating of the electrode. For example, the coating of the electrode can be applied by molecularly bonding titanium dioxide to ferrous metal or ferroalloy electrodes by a process known as "sputtering." The coating can also be prepared by heating, in the presence of oxygen, a titanium "button" which has previously been impressed into or otherwise affixed to ferrous metal or ferroalloy electrodes. Alternatively, the electrodes can be formed of titanium or an alloy of titanium. The outer surfaces of the electrodes are then changed to titanium dioxide by heating in the presence of oxygen.

In a preferred embodiment, the coatings 18 are deposited by a device known in the trade as a "sputterer." The process is known as sputtering. Briefly, the process involves depositing molecules dislodged from an electrode, known as the target, onto another surface, known as the substrate. A coating formed in this manner can have a thickness ranging from 1 to 5,000 Angstroms. As a practical matter, a coating having a thickness ranging from about 200 to about 1,200 Angstroms is preferred.

The coatings 18 are deposited directly on the metal of the electrode without the need for an intermediate bonding agent, such as brazing and the like. In the preferred practice of this invention, the metal oxide coating is deposited into the pores of the electrodes and will not chip or peel.

Radio frequency sputtering is the most commonly used of all sputtering techniques. The article to be coated is placed in a vacuum chamber, which is pumped down to remove contaminants. An inert gas, usually argon, is then introduced into the chamber. Radio frequency power excites the gas molecules until they form a plasma discharge. This discharge bombards the target, dislodging atoms which deposit on the substrate. By introducing a small amount of oxygen into the chamber along with the inert gas, the coating is modified to form oxides of the target material.

In another embodiment, coatings can be formed by impressing solid "buttons" of titanium or a titanium alloy into the upper and lower electrodes. The "buttons" can be attached to the electrodes by any means known in the art, such as by spot welding and the like. A coating of titanium dioxide is formed on the exposed surfaces of the "buttons" by heating the button-containing electrodes in an oxygen atmosphere for a suitable period of time. A coating having a thickness of from about 500 to about 2,000 Angstroms can be formed by heating said electrodes for a period of from 12 to 36 hours at a temperature ranging from about 350.degree.C. to about 550.degree. C. The oxygen source can be air.

In a further embodiment, coatings 18 can be formed by heating solid titanium electrodes in the presence of oxygen in similar fashion.

In a yet further embodiment, electrode tips can be fabricated from any machinable metal, then coated with titanium dioxide. The coated tips can then be inserted into the upper and lower electrodes of the spark plug.

The following example illustrates the present invention, which is not to be considered as limited thereby:

EXAMPLE

Spark plug electrode tips were fabricated from stainless steel screws in the following manner:

The screw heads were machined to provide flat spark-producing areas. The thus-machined screws were attached to the anode of a sputtering device. The vacuum chamber was sealed, then evaluated to a pressure of about 10.sup.-.sup.9 torr. Pure oxygen was then admitted to the chamber to raise the pressure to about 10.sup.-.sup.5 torr. Titanium was then sputtered from the cathode to the screw heads. Treatment was continued about 2 weeks. The oxygen chemically combined with the freshly sputtered titanium.

The presence of the coating on the screw heads in the form of titanium dioxide was established by Induced Electron Emission Spectroscopy, X-ray diffraction and by an energy dispersive X-ray analyzer (EDAX) attachment to a Stereoscan Electron Microscope.

The coated electrode tips were inserted into drilled and tapped holes in the ground and center electrodes of standard spark plugs. The titanium dioxide-treated spark plugs were compared with untreated spark plugs under the following engine test conditions:

Engine Speed 1000 rpm Compression Ratio 6.00 Throttle 90.0% Spark Timing set for maximum power Fuel Mixture Temperature 125.degree. F. Engine Coolant Temperature 150.degree. F. Fuel 91 octane, lead free

The test results are expressed in terms of "Lean Misfire Limits (LML)." LML is defined as the leanest fuel-air mixture at which consistent firing of the mixture can be detected with the spark timing adjusted for maximum power. The fuel-air mixture is expressed in terms of "percent stoichiometric fuel," which is the ratio of the quantity of fuel actually present in the fuel-air mixture to the quantity theoretically required to produce a stoichiometric mixture with the same quantity of air.

Results of the tests are as follows:

Titanium Dioxide- Coated Plug Uncoated Plug ______________________________________ Lean Misfire Limits (average) 74.1% 81.4% ______________________________________

The tests show that a significant improvement in engine combustion characteristics can be effected by use of the spark plugs of the present invention.

Modifications and variations of the above disclosure will be apparent to those skilled in the art in light of the foregoing disclosure.

Claims

I claim:

1. A spark plug for a combustion engine comprising in combination a metal shell, an insulator positioned within said shell, a first electrode passing through said insulator and projecting from the lower end of said insulator to form a spark-producing area, a cap attached to the upper end of said first electrode, and a second electrode attached to said metal shell in juxtaposition with said first electrode to form a spark-gap and a second spark-producing area, wherein said spark-producing areas of said electrodes consist of a button having thereon a coating of titanium dioxide.

2. The spark plug of claim 1 wherein said electrodes are produced with at least one hole in said spark-producing area and wherein said button is impressed into said hole in said electrodes.

3. The spark plug of claim 1 wherein said electrodes are provided with at least one hole in said spark-producing area and wherein said button has a threaded portion, with said threaded portion having been inserted into said hole in said electrodes.