Patent Application
20100018962
The present invention relates to a device for heating and vaporizing liquid fuels; and more particularly, to an apparatus and method for monitoring and controlling heater element temperatures of the heating device.
Fuel-injected internal combustion engines fueled by liquid fuels, such as gasoline, diesel, and by alcohols, in part or in whole, such as ethanol, methanol, and the like, are well known. Internal combustion engines typically produce power by controllably combusting a compressed fuel/air mixture in a combustion cylinder. In a Diesel cycle engine, intake air is first compressed by a piston in a cylinder, then fuel is injected into the cylinder for combustion by the compressed, heated air. For spark-ignited engines, both fuel and air first enter the cylinder where an ignition source, such as a spark plug, ignites the fuel/air charge, typically just before the piston in the cylinder reaches top-dead-center of its compression stroke. In a spark ignited engine fueled by gasoline, ignition of the fuel/air charge readily occurs except at extremely low temperatures because of the relatively low flash point of gasoline. (The term “flash point” of a fuel is defined herein as the lowest temperature at which the fuel can form an ignitable mixture in air). However, in a spark ignited engine fueled by alcohols such as ethanol, or mixtures of ethanol and gasoline having a much higher flash point, ignition of the fuel/air charge may not occur at all under cooler climate conditions. For example, ethanol has a flashpoint of about 12.8° C. Thus, starting a spark-ignited engine fueled by ethanol can be difficult or impossible under cold ambient temperature conditions experienced seasonally in many parts of the world. The problem is further exacerbated by the presence of water in such mixtures, as ethanol typically distills as a 95/5% ethanol/water azeotrope.
In many geographic areas, it is highly desirable to provide some means for enhancing the cold starting capabilities of such spark-ignited engines fueled by ethanol or other blends of alcohol. There are currently several approaches to aid cold starting of such engines. For example, some engines are equipped with an auxiliary gasoline injection system for injecting gasoline into the fuel/air charge that is utilized under cold start conditions. The use of such auxiliary system adds cost to the vehicle and to the operation of the vehicle and may increase the maintenance required for the engine.
Another approach to aid cold starting of spark-ignited engines fueled by ethanol or other blends of alcohol is to pre-heat the fuel before being ignited in the combustion chamber. One method of pre-heating the fuel is to spray the fuel directly onto a heat source before being mixed with air, causing the liquid fuel to vaporize upon contact with the heat source before being ignited by the spark.
Yet another method of pre-heating the fuel is to provide a heat source on the outside surface of a fuel injector body proximate to the injector tip to pre-heat the fuel. The key to implementing either of the last two methods is having sufficient heater power and heater surface area to transfer heat to the fuel. One current approach includes applying a heater formed of an electrically resistive material, such as a thick film heater element, to the injector or the spray target. When electric current is passed through the electrically resistive material, heat is generated, which is passed through the wall of the fuel injector or at the target to elevate the temperature of the affected fuel. In a thick film heater element for example, with a constant voltage applied across the element, the current flow through the element is inversely proportional to the temperature of the element. Thus, as the temperature of the element increases, the resistance of the element also increases. Such a heater element, wherein as the temperature of the element increases the resistance of the element also increases, is referred to herein as a heater element having a “positive temperature coefficient”.
Various open-loop methods have been suggested or employed to control the temperature of a positive temperature coefficient heater element for generating fuel vapor. In one open-loop set up, the heater element circuit includes a fixed resistor wired in series with the element. Such open-loop methods cannot compensate for variations in engine intake air flows, heater element tolerances, ambient temperature, or voltage supplies and, subsequently, the amount of vapor generated will fluctuate.
Furthermore, the on-board-diagnostics standard in many countries requires the vehicle's control system to not only detect when a problem with a system component occurs but also to determine the fault location and cause. For example, in the case of the heated fuel injector, failure of the injector, heater, fuse, fuel supply, injector driver, or heater driver may all have the same effect—failure to start the engine. This is unsatisfactory for diagnostic purposes as all of these potentially failed items would have to be checked to isolate the failure. In addition, even though the vehicle may start and run with the failed condition, the tail pipe emissions may be above acceptable limits—a problem that needs to be more easily detectable.
What is needed in the art is a device and method to control the temperature of a heater element that inherently compensates for various tolerances as well as to enable determination of component failure and cause for diagnostic purposes.
It is a principal object of the present invention to provide a simple and inexpensive closed-loop method that enables controlling the temperature of a heater element, provides over heating protection for such an element, and can be utilized for on-board-diagnostics.
Briefly described, a heater element having a positive temperature coefficient, is incorporated into a simple Wheatstone bridge circuit. The heater element may be, for example, a thick film heater.
The values of the bridge resistances are selected such that a pre-selected target temperature of the heater element results in a balanced bridge. If the temperature of the heater element is below the target temperature, then the imbalance of the Wheatstone bridge turns on a transistor, thereby applying power to the heater element to increase its temperature. If the heater element temperature is above the target temperature, then the transistor is turned off, preventing a further supply of power to the heater, thereby reducing the temperature of the element. Consequently, the analog Wheatstone bridge and its control integrated circuit self-regulate the target temperature of the heater element in a closed-loop.
By integrating the resistance of the heater element itself into the Wheatstone bridge and by providing a closed-loop circuit, the temperature of the heater element can be controlled to a pre-selected value independent of battery voltage and fuel flow. The temperature feedback of the heated element is inherently part of the Wheatstone bridge balance, and eliminates the need for a separate, discrete temperature sensor, as utilized in the known prior art.
In one aspect of the invention, a thermistor selected according to the proper bridge resistance is integrated into the Wheatstone bridge such that the bridge can be temperature compensated for ambient temperatures.
In another aspect of the invention, software algorithms are utilized in conjunction with the heater temperature control circuit making it possible to detect and localize various component failures of an engine.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Corresponding reference characters indicate corresponding parts throughout the diagrams. The exemplifications set out herein illustrate various possible embodiments of the invention, including one preferred embodiment in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.
Referring to
Bridge circuit 20 is preferably a basic Wheatstone bridge circuit. Amplifier integrated circuit 40 is an operational amplifier, such as a differential amplifier. Transistor 50 may be, for example, a bipolar or field effect transistor.
First branch 12 includes a first resistor 22 having a first resistance R1 and a heater element 28 having a fourth resistance Rheater. Second branch 14 includes a second resistor 24 having a second resistance R2 and a third resistor 26 having a third resistance R3. Supply voltage node 30 is positioned between second resistor 24 and heater element 28, and ground node 32 is positioned between first resistor 22 and third resistor 26. Heater element 28 may be, for example, a thick film heater or any other heater element that has a positive temperature coefficient. Thick film heater elements can be made to close manufacturing tolerances, having a part to part resistance variation, for example, of less than 5%. Because of this, it is possible to use the measured resistance Rheater of a thick film heater element to estimate the temperature of the device 28.
When bridge circuit 20 is balanced, the ratio of the resistances (Rheater/R1) of the first branch 12 is equal to the ratio of the resistances (R2/R3) of the second branch 14. As a result, a node voltage V1 36 of the first branch measured at the node between heater element 28 and first resistor 22 is equal to a node voltage V2 38 of the second branch measured at the node between second resistor 24 and third resistor 26.
In one aspect of the invention, bridge circuit 20 is designed such that a pre-selected target temperature of heater element 28 and a corresponding resistance Rheater results in a balanced bridge circuit 20, where node voltage V1 36 is equal to node voltage V2 38. In this case, the resultant bridge voltage ΔV is zero volts: ΔV=V1−V2=0.
As shown in
If the temperature of the heater element 28 is below the pre-selected target temperature, the corresponding resistance Rheater decreases, which results in node voltage V1 36 being closer in value to input voltage Vsupply 34 and greater than node voltage V2 38. This imbalance of bridge circuit 20 results in a positive resultant bridge voltage ΔV and causes amplifier integrated circuit 40 to apply a positive voltage to transistor 50. As a result, transistor 50 is turned on and input voltage Vsupply 34 is supplied to bridge circuit 20 and, consequently, to heater element 28, causing the temperature of heater element 28 to increase to the pre-selected target temperature.
If the temperature of heater element 28 is above the pre-selected target temperature, the corresponding resistance Rheater 28 increases, which results in node voltage V1 36 being closer in value to ground and less than node voltage V2 38. This imbalance of bridge circuit 20 results in a negative resultant bridge voltage ΔV and causes amplifier integrated circuit 40 to apply a zero voltage to transistor 50. As a result, transistor 50 is turned off and no input voltage Vsupply 34 is supplied to bridge circuit 20 and, consequently, to the heater element causing the temperature of the heater element to decrease to the pre-selected target temperature.
If the temperature of the heater element 28 falls back below the set target temperature, transistor 50 is turned back on and reconnects bridge circuit 20 to a power source providing voltage Vbattery 52. Basically, whenever the temperature of heater element 28 is below the pre-selected target temperature, transistor 50 turns on and allows current to flow into bridge circuit 20 and whenever the temperature of heater element 28 is above the pre-selected target temperature, transistor 50 turns off and current stops flowing to bridge circuit 20. Heater temperature control circuit 10 toggles between these two states to maintain resistance Rheater of heater element 28 and, therefore, to maintain the pre-selected target temperature of heater element 28. Consequently, the analog bridge circuit 20 and its control amplifier integrated circuit 40 operates in closed-loop to maintain the pre-selected target temperature of heater element 28. As can be seen, the temperature of heater element 28 can be controlled by heater temperature control circuit 10 to a pre-selected value independent of variation of operational parameters including, for example, the battery voltage and the fuel flow.
Resistor 53 in branch 55 permits a low level of current (insignificant to heat heater element 28) to the bridge so that amplifier integrated circuit 40 can sense node voltage V1 36 and node voltage V2 38 when transistor 50 is off and not supplying current to the bridge. When transistor 50 is on, its internal resistance is much lower than the resistance of resistor 53. Transistor 50 thereby supplies a sufficiently higher current to heat heater element 28.
Still referring to
Referring to
While heater temperature control circuit 10 provides a simple and inexpensive method to control the temperature of any heater element having a positive temperature coefficient and to provide overheat protection for such a heater element, heater temperature control circuit 10 may be especially useful for applications in the automotive industry. For example, control circuit 10 may be used to control the temperature of a heated fuel injector or of a heated target used to generate vapor from sprayed, liquid fuel. Control circuit 10 may also protect such heater elements from over heating.
In addition, control circuit 10 may be utilized for On Board Diagnostics (OBD) of a motor vehicle to detect when component failure or malfunction occurs and to identify the fault location. In the case of a heated injector, a failure of the injector, the heater element 28, the fuse, the fuel supply, the injector driver, the heater driver can all have the same general failure effect—failure to start the engine. Detection of such a general failure is of little use for diagnostic purposes as numerous components would have to be checked in order to isolate the cause of the failure. In another case, the vehicle may start and run but the tailpipe emissions may, nevertheless, exceed emission limits.
By utilizing software algorithms in conjunction with heater temperature control circuit 10 as described above, it is possible to more precisely identify and isolate such failures. For example, in the case where the heater element is integrated in an injector, if the heater element 28 on one of the injectors fails, the resistance Rheater would be read by the diagnostic system as being infinite and transistor 50 would turn off voltage to the heater element. The diagnostic system, reading this, could pin point the problem to a specific failed injector, and identify the failure cause. In another example, if fuel pressure to the injector is reduced, for example due to a pump malfunction, the temperature of all of the heater elements 28 would increase from a reduction of fuel flow through the injectors and a corresponding decrease in the heat transfer rate away from the heater elements. The increase of the heater element temperatures above a set target temperature would lead to an abnormally increased resistance Rheater in all of the heater temperature control circuits 10. The resistance increase in all heaters would be sensed by the diagnostic system thereby triggering the necessary warning or remedial action. In still another example, in some cases where heated injectors are used to reduce emissions regardless of ambient temperatures and fuel flash points, the heater element diagnostic system may be linked with the vehicle's engine sensors/emission control devices to alert the driver of an emission failure.
While switching device 49, and more specifically transistor 50, is exemplified as an NPN transistor, it is understood that the transistor may be, for example, a PNP, an FET or an IGBT transistor. Moreover, any type of switching device suitable for the application other than a transistor is contemplated by the invention.
While the above examples demonstrate a useful application of the invention for internal combustion engines, the application of heater temperature control circuit 10 is not limited to such applications. Heater temperature control circuit 10 may be useful for controlling, monitoring, and protecting any heater element 28 that has a positive temperature coefficient.
It should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described, including but not limited to other configurations, materials, and locations of vaporization elements. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.
