Patent Application
20110126789
The invention relates to a vehicle ignition system with spark plugs configured to relieve excess demand voltage.
Spark plugs generally have a center electrode connected with an ignition system as well as a side electrode spaced from the center electrode to establish a gap. The center electrode is surrounded by a ceramic insulator. When voltage generated by the ignition coil causes electrons to flow to the center electrode, a voltage difference develops between the center and the side electrode. The air and gasses in the gap act as an insulator, preventing voltage flow from the center electrode to the side electrode until the voltage exceeds the dielectric strength of the gasses in the gap, allowing electrons to flow across the gap, causing the gasses to react with each other to burn. The voltage at which electrons flow across the gap is referred to as the demand voltage.
If an insulating object, such as water, oil or debris, is in the gap or covers the electrode or counter electrode at the gap, the demand voltage increases. The demand voltage may go up so high as to exceed the dielectric strength of the ceramic insulator, causing it to chip. Ceramic chips may scratch the cylinder bore and allow oil to escape past the piston rings, causing excessive oil consumption.
A vehicle ignition system is provided that alleviates the problems associated with excessive voltage demand by spark plugs. The ignition system has a spark plug with an electrode and a voltage relief feature operatively connected with the electrode. The voltage relief feature is operable to discharge current from the electrode at a predetermined voltage.
In one embodiment, the voltage relief feature is a varistor configured to discharge current when voltage in the center electrode reaches a predetermined voltage that is less than the dielectric strength of the ceramic insulator around the center electrode. The varistor may be positioned in the ceramic insulator, in the ignition coil, or in the spark plug wire between the ignition coil and the spark plug.
In another embodiment, the voltage relief feature is a thinned portion of the insulator. The thinned portion may completely or only partially circumscribe the electrode. The thinned portion acts to localize and control the area at which the insulator will break in response to excessive demand voltage. When the insulator breaks, the voltage is discharged from the center electrode at the broken thinned portion to a metal casing surrounding the insulator that acts as a ground electrode. The thinned portion is encased in the metal casing and surrounded by the remaining (unbroken) portion of the insulator; thus, no ceramic chips escape from the spark plug. By controlling the thickness of the thinned portion, the voltage at which discharge will occur is controlled.
In still another embodiment, the voltage relief feature is a portion of the ceramic insulator that has ceramic properties of a dielectric strength lower than the dielectric strength of the remainder of the insulator. Thus, voltage discharge will occur at the voltage relief feature. By controlling the dielectric strength, the voltage at which discharge will occur is controlled to a level that will avoid ceramic failure of the remainder of the insulator. The voltage relief feature portion of the ceramic insulator is surrounded by the remaining portion of the insulator and the metal casing. Thus, if the insulator breaks at the voltage relief feature, as designed, no ceramic chips escape from the spark plug, thereby preventing associated engine damage and oil consumption.
By preventing ceramic debris due to excessive demand voltage, the ignition system protects the structural integrity of the spark plug ceramic and reduces oil consumption. The various embodiments of the voltage relief features are all structurally based solutions to the excess demand voltage problem. No change is required to the controller. A more expensive, software control system with feedback of voltage at the spark plug to prevent excess voltage demand is not required. In fact, no software changes are required.
The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings.
Referring to the drawings, wherein like reference numbers refer to like components,
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A ceramic insulator 40 surrounds the center electrode 28 to electrically isolate it from the rest of the spark plug 24, so that current will leave the center electrode 28 only via the gap 32, when a predetermined voltage is created between the center electrode 28 and the counter electrode 30, creating a spark at the gap 32. However, if oil, water or debris is situated in the gap 32, the voltage differential between the center electrode 28 and the counter electrode 30 will rise to levels beyond that intended to create a spark in the gap 32. The voltage differential may be beyond the dielectric strength of the ceramic insulator 40. In a typical spark plug, this could cause some of the ceramic insulator near the tip of the electrode 28 to chip, potentially damaging the cylinder 26 and increasing oil consumption.
The spark plug 24 is configured to prevent ceramic debris in the event of undesirably high voltage differential between the electrodes 28, 30. The spark plug 24 is equipped with a voltage relief feature 42 that controls the level at which excess voltage will be relieved from the spark plug 24, and also controls the location within the spark plug at which the voltage will be relieved. There are several alternative embodiments of voltage relief features 42 that will accomplish these goals. In one embodiment, the voltage relief feature 42 is a varistor that is designed to have a change in resistance at a predetermined voltage level to relieve excess voltage from the center electrode 28. As is well understood by those skilled in the art of electronics, a varistor is a voltage dependent resistor that shunts the current created by high voltage. At a range of voltages, the varistor has a high resistance, drawing only a relatively small current. At a predetermined higher voltage, however, the varistor will have a relatively low resistance, allowing significantly increased current to flow across the varistor 42 to ground at the metal casing 34, thus relieving the excess voltage of the center electrode 28. One type of known varistor that may be used is a metal oxide varistor. The varistor is designed to cause the resistance change at a predetermined voltage, such as 30 kV. The predetermined voltage is selected to be below the dielectric strength of the ceramic insulator 40, so that voltage is relieved prior to any breakdown and chipping of the insulator 40.
Alternatively, the voltage relief feature 42 may be a portion (e.g., a second portion) of the ceramic insulator 40 that is configured to have a dielectric strength lower than that of the remaining portion (e.g., a first portion) of the insulator 40. This may be accomplished by designing the voltage relief feature to have more air bubbles than the remainder of the ceramic insulator so that the voltage relief feature will be more conductive than the remainder of the insulator 40, thus relieving excessive voltage before any of the insulator 40 breaks down. Alternatively, the voltage relief feature 40 may be a portion of the insulator 40 that has more conductive material particles than the remainder of the insulator 40 (i.e., has a lower dielectric strength than the remainder of the insulator 40), such as by adding particles of a semiconductor, and hence will relieve voltage from the center electrode 28. In
Referring to
Alternatively, another embodiment of a spark plug 124A is shown in
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While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.
