Many consumers prefer gas cooking appliances over electric cooking appliances for a variety of reasons. For example, the gas flame of a gas cooking appliance can deliver heat nearly immediately, while electric cooking appliances usually require at least some delay to bring a resistive heating element up to operating temperature. A gas flame can also provide better visual feedback regarding temperature and heat delivery during cooking compared to an electric cooking appliance with a resistive heating element. Although either type of cooking appliance can deliver very good performance, many consumers simply prefer gas, especially in the field of premium and high end cooking appliances sold to discerning consumers.
Automatic ignition systems are well known in gas cooking appliances. Early systems that include a continuous pilot flame have largely been replaced with electronic ignition systems. A typical electronic ignition includes a burner electrically connected to ground, and an electrode placed near the burner and electrically connected to a source of relatively high voltage, for example 10-20 kV. The source of relatively high voltage can be, for example, a transformer that receives normal household power (120 VAC or 240 VAC at 60 Hz) and steps that voltage up to produce a relatively high output voltage, for example in the range of 10-20 kv. Because the transformer is typically configured to deliver this relatively high output voltage at a relatively low current, for example in the range of milliamps, the high output voltage generally does not present any unusual hazard.
To provide ignition, this relatively high voltage is applied to the electrode, and the resulting difference in electric potential between the high-voltage electrode and the burner (which is electrically connected to ground) causes a spark to jump the gap between the electrode and the burner. Assuming that gas is flowing from the burner when the spark occurs, the spark thereby ignites the gas to produce a gas flame which will ordinarily continue burning until the flow of gas is stopped.
Automatic flame detection systems are also well known. An automatic flame detection system can be used to automatically shut off the flow of gas to a burner if no flame is present, for example if the ignition system fails initially, or if the flame is accidentally blown out after successful ignition. Instead of stopping the flow of gas, an automatic flame detection system can be used to trigger ignition when gas begins to flow, or to trigger re-ignition after flame loss. An automatic flame detection system can also be used for a combination of these purposes, for example by attempting ignition for a period of time after the gas begins to flow, and then shutting off the flow of gas if ignition is not achieved within some finite period of time.
Many commercial flame detection systems take advantage of electrical properties of the flame, in particular the fact that a flame includes electrically charged particles that can conduct electricity. For example, when a flame produced by a gas burner extends outwardly from the burner to touch at least part of an electrode, the flame forms an electrically conductive path between the burner and the electrode. When the flame goes out, the electrically conductive path between the burner and the electrode disappears. By measuring the presence or absence of the electrically conductive path between the burner and the electrode, the presence or absence of the flame from the burner can be detected.
Systems which utilize a single electrode for flame detection and flame ignition are also known, for example as taught in U.S. Pat. No. 3,614,280.
Because of the wide variety of foods that can be prepared using any cooking appliance, the optimum rate of heat production can also vary widely. For example, to boil a large kettle of water a cook may wish to apply a large quantity of heat to the kettle over a short period of time. In contrast, to melt chocolate or keep a sauce simmering at serving temperature, a cook may wish to apply a relatively low level of heat over a long period of time. Thus, a cook may desire a cooking appliance capable of delivering both low levels of heat over a long period of time, and high levels of heat over a short period of time.
For this reason, both electrical and gas cooking appliances are often provided with a plurality of burners, with each burner specially adapted to provide a low level of heat or a high level of heat. For example, some burners on a gas range (“high output” burners) may be adapted to deliver high levels of heat in a short period of time, for example by including a large number of gas ports of a relatively large size. Other burners (“simmer” or “low output” burners) may be adapted to deliver low levels of heat over a long period of time, for example by including a relatively small number of gas ports of a relatively small size.
In practice, the actual heat output of either a high output burner or a simmer burner can be modulated over a usable range by adjusting the gas flow to the burner. However, the upper and lower limits of the usable range of heat delivery from a particular burner are generally determined by the construction of the burner itself. For example, when the gas ports from a simmer burner are saturated with gas, the resulting heat output represents the maximum heat output that can be produced by a simmer burner of that particular construction. Similarly, when the flow of gas to a high output burner is adjusted downward to reduce the heat output of that burner, a minimum level of gas flow will be reached that will sustain a flame on a high output burner of that construction.
Because of the limited surface area of a typical gas cooking appliance, the total number of burners that can be accommodated on a single cooking appliance is also limited. For example, a typical gas cooking appliance might contain two simmer burners and two high output burners. The mix of simmer burners and high output burners used on a particular gas appliance is preferably chosen to provide the most appropriate set of burners according to the needs of the owner of that appliance.
However, even with a suitable mix of simmer and high output burners on a particular gas cooking appliance, it is sometimes the case that additional simmer burner capacity may be needed when only high output burners are available, or vice versa. For this reason, “dual stage” burners have been developed that include both high output and simmer features, for example as taught by U.S. Pat. No. 6,322,354, which is owned by the assignee of this application.
A typical dual stage gas burner includes a first main burner and a second simmer burner. The main burner and the simmer burner are each typically formed as a ring, with the radius of the main burner somewhat larger than the radius of the simmer burner, and with the main burner stacked on top of the simmer burner (or vice-versa). Combined flame detection and ignition electrodes have been used with dual stage gas burners, however existing electrodes used for this purpose are known to have several practical limitations. One manifestation of these limitations is “nuisance sparking,” where initial ignition attempts are repeated unnecessarily when the flame detection circuitry falsely reports that no flame has been ignited when the flame has already been lit.
First, because either the main burner or the simmer burner of a dual stage gas burner can be in use at any given time, a combined flame detection and ignition electrode must be able to sense flame from either the main burner or the simmer burner. To reliably detect the presence of a flame from the main burner, a flame detection electrode should ideally be placed at a location reached by the outer portion of the flame from the main burner. To reliably detect the presence of a flame from the simmer burner, a flame detection electrode should ideally be placed at a location reached by the outer portion of the flame from the simmer burner. Because the flame produced by the main burner is typically much larger than the flame produced by the simmer burner, it has been found that electrode locations that work well in detecting flame from the main burner may not work well in detecting flame from the simmer burner, and vice-versa.
Second, when used with a dual stage burner, a combined flame detection and ignition electrode must be able to ignite gas flowing from either the main burner or the simmer burner. To reliably ignite gas flowing from the main burner, a flame ignition electrode should ideally be placed at a location where the spark from the electrode will pass through the gas flowing from the main burner. To reliably ignite gas flowing from the simmer burner, a flame ignition electrode should ideally be placed at a location where the spark from the electrode will pass through the gas flowing from the simmer burner. Because the main burner and simmer burner are typically stacked on top of each other, it has been found that electrode locations that work well in igniting flame from the main burner may not work well in igniting flame from the simmer burner, and vice-versa.
Thus, finding a location for a conventional flame detection electrode that will reliably detect flame from both the simmer and main burners is problematic. Finding a location for a conventional flame ignition electrode that will reliably ignite both the simmer and main burners is also problematic. These problems are compounded when the same electrode is used both for ignition and for flame detection.
What is needed is a flame detection electrode that can be positioned to reliably detect flame from both the simmer and main burners. What is further needed is a flame ignition electrode that can be positioned to reliably ignite flame from both the simmer and main burners. What is further needed is a combined flame detection and ignition electrode that can be positioned to reliably detect and ignite flame from both the simmer and main burners. What is further needed is a dual stage gas burner system including a flame detection and ignition electrode that will reliably detect and ignite both the simmer and main burners. What is further needed is a gas cooking appliance with a dual stage gas burner system including a flame detection and ignition electrode that will reliably detect and ignite both the simmer and main burners.